&EPA
            United Stat.t
            Environmental Protection
            Agency
           Environment*! Monitoring
           Sveterra Laboratory
           P.O. Box 93478
           Lee VegeeNV 89193-3478
EPA/HXV4-90/029
October 1980
            Re«eefch and D«v»lopm»nt
Evaluation of Selected
Borehole Geophysical
Methods for Hazardous
Waste Site Investigations
and Monitoring
    EJBD
    ARCHIVE
    EPA
    600-
    4-
    90-
    029
'//Si

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M-                              US EPA
^ 6 -               Headquarters and Chemical Libraries
                        EPA West Bldg Room 3340
                             Mailcode 3404T
                        1301 Constitution Ave NW
                          Washington DC 20004
                              202-566-0556
                            - FINAL REPORT -
        EVALUATION OF SELECTED BOREHOLE GEOPHYSICAL METHODS

       FOR HAZARDOUS WASTE SITE INVESTIGATIONS AND MONITORING
                                   by


                              Kendnck Taylor
                                John  Hess
                              Steve Wheatcraft
                                   t
                           Water Resources Center
                           Desert Research Institute
                       Las Vegas and Reno, Nevada 89S06
                      Cooperative Agreement No. CR812713
                                                Repository Material
                                               Permanent Collection
                              Aldo Mazzella
                   Environmental Monitoring Systems Laboratory
                          Las Vegas, NV 89193-3478
            ENVIRONMENTAL MONITORING SYSTEMS LABORATORY
                  OFFICE OF RESEARCH AND DEVELOPMENT
                 U.S. ENVIRONMENTAL PROTECTION AGENCY
                        LAS VEGAS, NEVADA 89193-3478

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 NOTICE
      The information in this document has been funded wholly or in part by the United States Environmen-
 tal Protection Agency under Cooperative Agreement #CR812713 to the Desert Research Institute, Universi-
 ty of Nevada System. It has been subject to the Agency's peer and administrative review, and it has been
 approved for publication as an EPA document. Mendon of trade names or commercial products does not
 constitute endorsement or recommendation for use.
                                           ABSTRACT
      Borehole geophysical methods can be used to provide valuable information about the hydrogeology of
hazardous waste sites. This information enables remedial action programs to be designed in a more cost and
time efficient manner. Borehole geophysical methods lend themselves to addressing two classes of problems
which were addressed in this project: 1) characterizing the hydraulic properties of the subsurface; and 2)
locating electrically anomalous contaminants. Three methods were tried to determine the vertical distribu-
tion of hydraulic conductivity with depth: 1) Stoneley wave attenuation: 2) a method based on natural flow
through a well; and 3) a single-well electrical tracer test. Only the single-well electrical tracer-test method
proved effective in the unconsolidated formations typical of hazardous  waste sites. Contaminants that alter
the electrical conductivity of the formation can be detected with electrical conducuvity logs, however, the
influence  of clays on the  logs needs to be taken into consideration during the interpretation process.
     This report $as submitted in fulfillment of Cooperative Agreement No. CR812713 by Desert Research
Institute. Water Resources' Center under  the partial sponsorship of the U.S. Environmental Protection
Agency.

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                                        CONTENTS

ABSTRACT 	
                          	  M
FIGURES	

TABLES	           ...    u,
ACKNOWLEDGMENTS	

    1.  INTRODUCTION  	        ,

    2.  METHODS TO DETERMINE FORMATION HYDRAULIC PROPERTIES	2
          Background	           2
          Sonic Methods	             2
                 Overview - Stoneiey Wave Attenuation	 4
                 Previous Work - Stoneiey Wave Attenuation	  4
                 Field Evaluation - Stoneiey Wave Attenuation	  5
                 Discussion - Stoneiey Wave Attenuation	8
          Overview - Natural Flow Methods	                     5
          Theoretical Development   	'.  . .                      3
          Sensitivity of Hydraulic Conductivity Equations to Kr	                 II
          Measurement of Thermal  Pulse Advective Velocity in a Saturated
              Porous Medium	            ! 4
          Modified Thermal Flowmeter Experiments   	  16
          Modified Borehole-Dilution Technique for  Measuring Flow  Velocity
              Through Porous  Media	  18
          Factors That Influence Pore-Water Flow Within  Packed Borehole
              Measuring Volume	                 •? 1
          Borehole  Packer Porous Materials	                  22
          Flowchamber Construction and Characteristics	        .              .23
          Vormal and Modified Borehole-Dilution Experiments   	         '            25
          Experimental Apparatus	            25
          Discussion - Natural Flow Methods	   35
          Single-Well Electrical Tracer (SWET) Method	    38
                 Field Application	  38
                 Theory of Analysis  	    39
                    Assumptions	  39
                    Porosity Calculation 	        41
                    Calculation of Radius of Invasion  	       ...       ...  -12
                                            in

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                      Calculation of Hydraulic Conductivity        	             .  .  43
                      Checks on the Analysis Method	      44
                  Discussion of Results	               45
                      Well E-3	'.'.'.'."".'.'..  ....        "    45
                      Well E-6	                    45
                      Well E-7	    '..'..'..''.      '..50
                      General Comments - Single-Well Electrical Tracer Method               50

    3.  METHODS TO LOCATE CONTAMINANTS ...        	             5!
           Background	            5 j
           Evaluation of a Slim Hole Induction Logger	51
                  Background	             < 1
                  Theory of Operation	    52
                  Calibration	             52
                  Time Constant	             53
                  Temperature Effects	             53
                  Vertical Response    	               55
                  Borehole Influence	                  55
                  Comparison of Units	     .        .                     57
                  Discussion -  Evaluation of Induction  Tool   	               58
                                                                                          •_
    4.  APPLICATION TO DETECTION OF CONTAMINANTS	63
           Background	           53
                  Method 1 - Borehole Geophysical Logging	   63
                  Method 2 - Pumping of Discrete Intervals	  67
                  Method 3 - Dilution Sampling	       69

    5.  INDUCED  POLARIZATION 	            72
           Background	               72
           Instrumentation  	            73
           Field Effort 	         73
           Discussion	'  	             73

    6.  CONCLUSIONS	    79
           Notice  	        79

REFERENCES   	               so
                                            IV

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                                          FIGURES
 1.   Sonic Logging Tool	      	    3

 2.   Sonic Waveform in 10-cm Diameter Hole	   6

 3    Sonic Waveform in 15-cm Diameter Hole	                6

 4.   Full Waveform Log in  15-cm Diameter Hole	7

 5.   Response of a Uniform Flow Field to the Presence of a Permeable Cylinder.
     Shown is Map View, Flow in X-Direction.  Cylinder Radius = rp	   10

 6.   Sensitivity of Hydraulic Conductivity Equations to the Parameter Kr.  The
     Parameter can be thought of as Dimensionless Borehole Pore- Water Speed.  ...     ..       13

 7.   K-V Associates Thermal  Flowmeter Downhole Probe Geometry	15

 8.   Thermal Flowmeter Experimental Configuration	       17

9    Results of Thermal Flowmeter Experiments Showing Conductive  Heat
     Breakthrough Curves at Varied Flow Velocities, with  Predicted Advective
     Heat-Pulse Arrival Times Indicated	     .   ...      .       13

 10.  Explanation  of Modified Borehole-Dilution Geometry	20

 11   Idealized Diagram of the  Aquifer-Simulating Flowchamber	      ....        .    24

 12.  Diagram  of Laboratory Apparatus. Showing Location  of Mixing Chamber   .  .      ....   26

 13.  Diagram  of Downhole Tool used in Hydraulic Conductivity Experiments	     27

14.  Results of Repeatability  Experiments. Laboratory  Apparatus Sensitivity to
    Varied Flowchamber Pore-Water Velocity is Clearly Illustrated	  28

15. Theoretical Output of Laboratory Apparatus for Flowchamber Pore-Water
    Velocity = 3.72 m/day	        	       . . 29

16. Theoretical and Experimental Response for  the First Vaned-Kr Experiments,
    Plotted at Same Scale for Comparison	      	30

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 17. Theoretical Versus Experimental Change in Semiloganthmic Slope as a Function
     of Kr, for the First Vaned-Kr Experiments. Note the Lack of Systematic Change
     in the Experimental Results	              31

 18  Least-Squares Plot of Second Vaned-K, Experimental Results. Flowchamber
     Pore-Water Velocity = 1 84  m/day   	     . .               33

 19  Theoretical Versus Experimental Change in Semiloganthmic Slope as a Function
     of Kr. for the Second Vaned-Kr Experiments. Apparent Random Fluctuation in
     Experimental Results is Illustrated	   34

 20. Idealized Representation of Flow Field Distribution  Due to Packed Borehole Section	35

 21. Idealized Representation of Flow Field after Increasing Packed Borehole Section.             36

 22. Theoretical Versus Experimental Change in Semiloganthmic Slope as a Function
     of Kr. for the Third Vaned-Kr Experiments  Apparent Random Fluctuation in
     Experimental Results is Illustrated	      .                      37

 23. Experimental Setup  for Single-Well Electrical Tracer Test	     	  40

 24. Radial Response of EM-39	   43

 25. Induction  Logs of Well E-3 at Different Times After Injection	46

 26. Induction  Logs of Well E-6 at Different Times After Injection	      .  .  46

 27.  Porosity Log of Well E-6	         ...  47

 28.  Matrix Conductivity  and Natural Gamma Log of Well E-6.      .             .    .          47

 29   H\draulic Conductivity Logs of Well E-6	       .        ...   49

 30.  Hydraulic Conductivity Logs and Packer Slug Test Results in Well E-6.      	     49

 31.  Induction Logs of Well E-7	      ...     .   50

 32   Calibration of EM-39	      	      .         .      53

 33.  Logging Speed-Induced Shifts  on Induction Logs	                             54

 34.  Vertical Shift  as a Function of Logging Speed for EM-39	            55

 35   Temperature Drift of EM-39	      	56
                                                                              4

36.  Vertical Averaging Function of EM-39	57

 37.  Response of EM-39 to Abrupt Contact.   .      	53

3S   Response of EM-39 to Layers of Varying Thickness. Layer Thickness  in Meiers
     is Indicated	           	      .  .      59

39.  Response Versus La\er Thickness for EM-39       	            .    . .     60
                                               VI

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 40.  Example of Ambiguous Interpretations of Induction Logs	      .               f>\
 41.  Comparison of Logs from Two Different EM-39 Units	       .    .    .           52
 42.  Induction Logs at Henderson, Nevada	        55
 43  Natural  Gamma Logs at Henderson,  Nevada	   66
 44  Experimental  Setup for Tracer Test	                 gg
 45  Tracer Concentration Versus Time	69
 46.  Dilution Curves Versus  Time for Various Depths for Dilution Sampling Method	70
 47.  Comparison of Electrical Conductivity of the Pore Fluid and Formation in Well 635     .   .   71
 48.  Log Suite  for  Well  633A	             75
 49  Log Suite  for  Well  633D	       	76
 50.  Induction  Logs Before and After Injection	                       77
 51.  Induced Polarization Logs Before and After Injection	77
                                           TABLES
1.   Theoretical and Experimental Glass Bead Hydraulic Conductivity Values        .  .   . .      .23
2.   Induced Polarization Time Windows	       .   .        74
                                             Vll

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                                     ACKNOWLEDGMENTS

     Special thanks goes to Les McMillion who saw the need for this project and took the initiative to start
it. Joel Hayworth, Tom Morris, and Scott Lewis also deserve recognition for the long hours of field work.
Thanks also goes to Barbara Nauroth and Deborah Wilson who contributed their electronic publishing skills.
                                              vin

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                                          SECTION 1

                                        INTRODUCTION
     Ground-water  contamination is a serious and widespread environmental  problem. As  such, the
United States Environmental Protection Agency (EPA) has identified a need to develop advanced methods
to detect and remediate ground-water contamination. Borehole geophysical methods hold considerable
promise for providing needed information about the location of contaminants and the properties of the.
subsurface  through which the contaminants move. The purpose of this report is to present the results of a
cooperative agreement between the EPA and the Desert Research Institute to evaluate borehole geophysical
methods that can be used to obtain needed information about the subsurface. Development of these meth--
ods makes  possible a more detailed characterization of the subsurface which permits remedial action pro-
grams to be conducted in a  more efficient manner.
     Borehole geophysical methods are well suited for two applications of interest in ground-water contami-
nate investigations which were addressed in this project: 1) detection of contaminants which alter the electri-
cal properties of the ground water; and 2) characterization of formation  hydraulic properties which control
the movement of the pore fluids. Although borehole geophysical methods have been used by the petroleum
industry for many decades, their application in shallow ground-water monitoring investigations of small sites
is not as well developed. Borehole geophysical logging operations for ground-water contamination investiga-
tions are commonly conducted in conditions that are sufficiently different from what is encountered in the
petroleum industry, that many of the borehole geophysical methods developed by the petroleum industry
need to be modified for use in ground-water contamination investigations. For  example, in contrast  to
petroleum investigations, ground-water contamination investigations are frequently in unconsolidated and
shallow formations. These formations tend to collapse quickly,  requiring that the borehole be cased immedi-
ately after drilling. In addition, the study sites frequently have  an abundance of existing cased boreholes in
which it is desirable to conduct logging operations. Legal restriction severely restricts the use of radioactive
sources in water wells. Hence,  non-radioactive borehole geophysical methods that can be used in cased
wells in unconsolidated formations are of particular interest in ground-water contamination investigations
and were the focus of this cooperative agreement.

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                                            SECTION 2

             METHODS TO DETERMINE FORMATION HYDRAULIC PROPERTIES

 BACKGROUND
      In ihe past, it was usually sufficient to consider that the hydraulic properties of a  formation were
 uniform. This assumption is usually acceptable when dealing with ground-water quantity issues in porous
 media. However, in ground-water contamination issues,  accurate transport models must take into account
 the spatial variability of the hydraulic properties of ihe formation. As more accurate transport models are
 developed, it has become clear that a major stumbling block in the development of advanced models is the
 difficulty of obtaining  information on the spatial variability of hydraulic conductivity and porosity
      Hydraulic conductivity is a property of a porous medium which controls the rate at which water flows
 through the porous medium under the influence of a given driving force. Determining the spatial variability
 of  hydraulic conductivity in unconsolidated formations is particularly difficult. Because it varies by more
 than ten orders of magnitude in natural formations, it is the property of a formation that has the most impact
 on  the transport of fluids through the formation. Although many methods exist to determine  the hydraulic
 conductivity of a formation, they all have significant limitations. Porosity is an important hydraulic parame-
 ter  because it also controls the movement of fluid through the subsurface, although to a lesser degree than
 hydraulic conductivity, and because  it is a measure of the fluid stored in a  saturated  formation.
      An overview of borehole methods and their associated limitations, available for determining the spatial
 variability of formation hydraulic properties, was presented by Taylor (1989). In the course of this coopera-
 tive agreement, two new methods of determining the spatial variability of hydraulic conductivity were consid-
 ered and an effort was made to adapt an existing method to unconsolidated environments. One new method
 to determine the spatial variability of porosity was also developed. Because of severe legal restrictions on the
 use of active source logging tools in water wells, methods  that utilize a radioactive  source were not consid-
 ered in the course of this study.

 SONIC METHODS
     Sonic methods relate  the way induced  acoustic waves propagate in the formation to the hydraulic
properties of the formation. Typically, sonic methods utilize a downhole tool having a desien shown in

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 Figure 1. The tool has an acoustic source, which is usually a piezoelectric cylinder, and two receivers. When
 the transmitter pulses, numerous different types of acoustic waves are generated. The optimal spacing be-
 tween the source and between the receivers, and the frequency of the source, is dependent on the objective
 of the  survey being conducted.

     Sonic velocity logging is commonly used in the petroleum industry to determine the porosity of consoli-
 dated formations. The velocity of a refracted compressional wave is determined by measuring the time the
 compressional wave requires to travel between the  two receivers. To obtain the desired vertical resolution,
the method is usually performed with an acoustic source with a frequency of 20 kHz and a source and
receiver spacing of less than 1 m.  Wyllie et al. (1962) has shown that the compressional wave velocity in the
formation is a function of the total porosity and the compressional wave velocity in the  fluid and formation
matrix. If the lithology and the type of pore fluid are known, the compressional wave velocity in the fluid and
            TRANSMITTER
             RECEIVERS
               FORMATION
                                                             ., STONELEY WAVE PATH
                                                                      REFRACTED
                                                                   COMPRESSIONAL
                                                                      WAVE PATH
                                                           FORMATION
                               FIGURE  1.  Sonic Logging Tool.

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  formation matrix can be adequately estimated and the total porosity determined. In unconsolidated forma-
  tions, the congressional wave velocity in the matrix cannot be accurately estimated, which restricts this
  method to consolidated formations. Another acoustic method which is under development by industry is the
  Stoneley wave attenuation method. The applicability of this method to shallow unconsolidated formations
  was investigated in this cooperative agreement.

  Overview - Stoneiey Wave Attenuation
      When an acoustical source emus a pulse in a fluid-filled borehole, several types of waves are pro-
  duced. One of these, the Stoneley wave, is a guided wave that exists only within the borehole and along the
  borehole-formation interface.  Based on the theoretical model of Biot (1956a and 1956b). Rosenbaum
  (1974) argued that the attenuation of the Stoneley wave is dependent on the hydraulic conductivity of the
  adjacent formation. This has been verified in the field by numerous researchers: among them are Schmitt et
  al. (1988); Cheng et al.  (1987); Williams et al. (1984); Hardm et al (1987); and Burns et al.  (1988) A
 conceptual model of the phenomenon is that as the hydraulic conductivity of the formation increases, more
 of the guided energy of the wave is lost from the borehole into the formation. This attenuates the Stoneley
 wave and results in a decreased amplitude of the wave at nearby  receivers. Attenuation of the Stoneley wavet
 is usually quantified by a  measure of the average amplitude or energy in a time window that contains the
 wave.

 Previous Work - Stoneley Wave Attenuation
      The concept of determining hydraulic conductivity with acoustical logging methods is appealing be-
 cause of the minimal logistics. In the references cued above, a full waveform log with the appropriate acqui-
 sition parameters was run and the data were processed to obtain a log of hydraulic conductivity. The hydrau-
 lic conductivity logs were usually qualitative in nature. It was not necessary to inject fluids or wait for natural
 fluid movement to occur. Core samples  were used to verify the results of the logging, but were not necessary
 for the interpretation of the  logs. Although  the method  has not matured to the extent that it is a standard
 method in the petroleum industry, it is attracting significant attention. All work to date has been conducted
 in open boreholes in consolidated formations with hydraulic conductivities less than 2 5 m/d. The adapta-
 tion of the method to screened wells in unconsolidated formations, which generally have a higher hydraulic
 conductivity, would be  a significant development for shallow ground-water investigations. Analysis of, the
 influence of a mudcake on the borehole wall (Schmitt et al.. 1988)  suggests that the method may work in
 screened wells.

     T\pically, a sonic  logging sonde similar to the one in Figure 1 is used. While the logger is raised in the
 well, the  transmitter produces an acoustic pulse. This generates several forms of propagating waves, one of
 which is the Stoneley wave. The waves are detected at the two receivers. The separation of the receivers and
 transmitter is adjustable. To provide better separation between the arrival times of the different waves, the
spacmgs are usually larger when the slower traveling Stoneley wave studies are being conducted than when
the more common faster compressional wave is being logged. The frequency content of the transmuted pulse
is important. At frequencies  above a critical frequency, the Stoneley wave  is not excited (Schmitt et al..

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  1988) because the higher frequency wave will not resonate. Most commercially available instruments oper-
  ate in the 20 to 30 kHz range.  To excite the Stoneley wave, m consolidated  formations, a transmitter
  frequency less than 10 kHz is usually required. The critical frequency, though, is a function of formation and
  fluid properties. Unfortunately, tools well suited for Stoneley wave studies are not commercially available.
  This study was conducted to see how well commercially available sonic logging equipment would perform in
  unconsolidated material.
 Field  Evaluation - Stoneley Wave Attenuation

       A site near Mobile. Alabama, was chosen to investigate the influence of changes in hydraulic conduc-
 tivity on Stoneley waves. The sue consists of Quaternary sands and clays. It has been used extensively for
 large-scale tracer tests (Molz et al.. 1986a. 1986b.  1988)  and testing of borehole methods for the determi-
 nation of the spatial variability of hydraulic conductivity (Taylor et al., 1988; Molz et al., 1988). The major-
 ity of these studies have been conducted in an aquifer located  at a depth of 40 to 60 m. This unit has an
 average porosity of 35 percent and an average hydraulic conductivity of 54 m/d (Molz et al., 1986a). Wells
 are available m this interval that are completed with either a 10-cm PVC or 15-cm steel screen. Both types
 of wells were drilled with the mud rotary method. The acoustic logging tool used in the study had a transmit-'"
 ter frequency of 29 kHz. The distance from the transmitter to the closest receiver was selectable to either
 0.91 or 3.57 m. The distance between the  receivers  was 0.3 m.

      Figure 2 shows the waveform in a 10-cm well at a distance of 3.57 and 3.87 m from the source. At
 these distances, the arrival time of the  Stoneley wave is well separated  from the compressional wave. The
 relative amplitude of the two'arnvals is not meaningful in the plot because of different gams. The Stoneley
 wave should be propagating with a velocity less than the velocity of a pressure wave in the fluid. Since the
 well fluid is fresh water, this velocity is estimated to be 1.540 m/s. The wave should also be a sinusoidal
 packet. The pulse in Figure 2 has a velocity of 1.720 m/s and is believed to'be a refracted pressure wave that
 is observed through the well screen.  Results are similar for all the 10-cm wells with either a 0 91 or  3.57 m
 receiver-transmitter spacing. The Stoneley wave was poorly excited in the 10-cm diameter wells If a logging
 tool with  a lower pulse  frequency were  available, it might  be possible to excite  the Stoneley wave m these
 holes.

      Figure 3 shows the waveform in a 15-cm diameter well also at a distance of 3.57 and 3.87 m. Both the
 velocity (1.493 m/s) and the shape of the wavelet are consistent with a Stoneley wave and the wave associ-
 ated with the leaky P-mode. The leaky P-mode is a trapped compressional normal mode, the first energy of
 which travels at the velocity of a compressional wave in the fluid.  Hence,  the velocity of the leaky  P-mode is
 not a function of the formation.  Because the borehole fluid properties are constant as a function of depth.
 the velocity of the leaky P-mode should also be constant as a function of depth. This is in contrast to the
 Stoneley wave velocity, which is a function of formation properties and.  hence, changes as a function of
 depth.

      Figure 4 shows the waveform logs of a 15-cm diameter well. The  constant arrival time indicates the
velocity of the wave is a constant with respect to depth. This indicates that the observed wave is the leaky

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                                     T-R SEPARATION  3.57  M
  TIME (mS)
                                     T-R SEPARATION  3.88 M
 2.0
 2.2
 2.4
 2.6
 2.8
                                                            3.0
                  WELL DIAMETER  10 cm
              FIGURE 2. Sonic Waveform in 10-cm Diameter Hole.
                                    T-R SEPARATION  3.57 M
                                    T-R SEPARATION  3.88 M
 TIME (mS)
2.1
2.3
2.5
2.7
2.9
3.1
                  WELL  DIAMETER 15cm
             FIGURE 3. Sonic Waveform in 15-cm Diameter Hole.





                              6

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 40
                WELL  12

         T-R SEPARATION  3.57M

                 TIME  (mS)
          I	I	!	|_
54-
 FIGURE 4.  Full Waveform Log in 15-cm Diameter Hole.

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 P-mode. Additionally, the amplitude of the leaky P-mode should be larger in large-diameter wells than in
 smaller-diameter wells; for Stoneley waves, the opposite is true (Paillet.  1988). Because this wave has a
 larger amplitude in the larger well, it also indicates that the observed wave is the leaky P-mode.  The fre-
 quency of the sonic tool (29 kHz) is apparently too high to excite the Stoneley wave in this unconsolidated
 formation. For the Stoneley wave attenuation method to be  effective in soft formations, a lower frequency
 sonic tool must be constructed.
 Discussion - Stoneley Wave Attenuation
      The results suggest that it is not possible to determine the hydraulic conductivity of unconsolidated
 formations with Stoneley wave attenuation techniques in screened wells with commercially available equip-
 ment. If further work is conducted, a transmitter with a lower frequency should be built. Numerical model-
 ing will be required to determine what the optimal frequency is for a given set of field conditions. Even with a
 low frequency tool, with a transmitter pulse frequency of 5 kHz. the radius of investigation is not likely 10
 exceed 0.3 m. The issue of the influence of a well screen needs to be addressed

 OVERVIEW - NATURAL FLOW METHODS
      It is now widely recognized that in most field situations,  the hydrodynamic dispersion of solutes is
 insignificant compared to the dispersion caused by differential advection (Guven et al.. 1985; Sudicky et al ,
 1985; Smith and Schwartz.  1980; Pickens and Gnsak, 1981). Hydrodynamic dispersion is caused by  solute
 velocity fluctuations at the microscopic level owing to the irregular geometry of the void  space of the porous
 medium. Differential advection is dispersion that occurs at the macroscopic level owing to the heterogeneity
 of the aquifer hydraulic conductivity. As a result, it is very important to have a large amount of information
 on the spatial distribution of hydraulic conductivity.

      \Vheatcraft and Wmterberg (1985) developed solutions for flow around and through a cylinder of
 different permeability from the surrounding medium. Wheatcraft et  al. (1986) used these solutions 10
 develop a technique that allows the direct measurement of both the ground-water velocity (magnitude and
 direction) and the hydraulic conductivity as a function of depth in a borehole. The technique relies upon
 obtaining accurate information from a borehole thermal flowmeter or a  borehole point-dilution instrument.
 The overall goal of this study was to determine the feasibility of using these flowmeter techniques in conjunc-
 tion with the solution of Wheatcraft and Wmterberg (1985) to directly obtain velocity and hydraulic conduc-
 tivity  information.  First, the theoretical work is summarized, and then laboratory experiments with  the
 thermal flowmeter and the  borehole point-dilution techniques  are summarized.

THEORETICAL DEVELOPMENT
     The approach of this  method  is to obtain an accurate measurement of the horizontal pore-water
velocity through a section of a cylindrical borehole packed with a porous material of known  hydraulic con-
ductivity; then the hydraulic conductivity of the aquifer material surrounding the borehole can be deter-
mined, as well as (in  some  cases) the ambient ground-water flow rate and direction.

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       Consider a saturated, homogeneous and isotropic aquifer in which a steady-state, lateral flow field
 exists. It is assumed that the flow field obeys the two-dimensional Laplace Equation and that How within the
 aquifer is uniform. Embedded within this aquifer is a permeable cylinder of hydraulic conductivity Kp which
 differs from the aquifer hydraulic conductivity^. The condition of a homogeneous, isotropic aquifer exhib-
 iting Darcian flow requires that all perturbations experienced by the flow field be due to the presence of the
 permeable cylinder.  For a cylinder which is more permeable than the surrounding aquifer, the flow field is
 disrupted in such a way that fluid preferentially moves into the cylinder, increasing the pore-water velocity
 within the cylinder relative to the aquifer. Conversely, for a cylinder which is less permeable than the sur-
 rounding aquifer, the flow field is disrupted such that fluid tends to move around the cylinder, decreasing the
 pore-water velocity within the cylinder relative to the aquifer. These situations are shown in Figure  5. For a
 cylinder with hydraulic conductivity equal to that of the aquifer,  the flow field remains undisturbed.
      The response of the  flow field to the presence of the permeable cylinder has been examined theoreti-
 cally by Wheaicraft and Wmterberg  (1985). The solution, detailed in Wheatcraft et al. (1986), is in terms of
 complex stream functions. The important equations for this study are the total flow through  the cylinder
                                        n
                                        Qf
 where
      Qp   =  volumetric discharge per unit length through the permeable borehole packer (L3/T)
      la   =  Va^a = aquifer specific discharge  (L2/T)
              /c,
      rp   =  radius of cylinder
 Qp represents the volumetric discharge per unit length through the permeable cylinder, and can also be
 written as the product of the cylinder diameter and the cylinder effective pore-water velocity

                                           QP  = ^P IP                                      (2)
 where
      qp   =  vpnp = borehole packer specific discharge (L2/T)
      np   = borehole packer porosity
 Setting Eqs. (1) and (2)  equal and solving for qa gives

                                                     + K.~\
                                                                                              (3)
                                               <•  |_  "r  J
which can also be written
     The significance of Eq. (4) becomes apparent when applied to a particular problem. If it is assumed
that a cylindrical borehole, completed within a porous aquifer exhibiting Darcian flow and packed with a

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 3           2           1


  UNIFORM FLOW FIELDS
                                                 -1
-3
FIGURE 5.  Response of a Uniform Flow Field to the Presence of a Permeable Cylinder.
            Shown is a Map View, Flow in X-Direction.  Cylinder Radius = rD
            (modified after  Wheatcraft et al.. 1986).
                                10

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  material of known porosity and hydraulic conductivity approximates the conditions that validate Eq. (4),
  then it is theoretically possible to determine the aquifer specific discharge qa and hydraulic conductivity Ka
  by measuring the horizontal pore-water velocity through the packed borehole Vp. Application of this tech-
  nique requires measurement of borehole pore-water velocity through two different borehole packing materi-
  als. If Kp, is the hydraulic conductivity of the borehole packing material through which a specific discharge
  of qp,  is measured, and likewise Kp2, and qp2 are the same parameters for a different borehole packing
  material, then using Eq. (4)
                                                         K.
                                                          •pi
                                                                                              (5)
 and
                                                   /C. + /C,;
 If we divide both sides of Eqs. (5) and (6) by qp2 and let
                                                                                              (6)
                                            if   _
                                                 ~
 we can then equate Eqs. (5) and  (6)
                                                       K,
                                                           + 1
Solving for Ka,  we have
=  K,,
                                                                                              (7)
Hence, by making two independent measurements of the pore-water velocity through a borehole packed
with two different porous materials, each of known hydraulic conductivity,  it is theoretically possible to
determine  the hydraulic conductivity of the aquifer material directly surrounding the borehole

SENSITIVITY OF HYDRAULIC CONDUCTIVITY EQUATIONS TO Kr
      It is important to understand the sensitivity of the equations developed in the previous section to the
value of the cylinder-aquifer hydraulic conductivity ratio Kr.  Rearranging Eq. (3), let e be a dimensionless
constant such that
                                                11

-------
                                                        K-
                                                       I + K,)                                  W

  The constant E can be thought of as a dimensionless borehole pore-water velocity. If the borehole is packed
  with a porous material of zero permeability, then

                                             lun e  = 0

  and

                                              1r - 0
  Similarly, if a borehole  is packed  with a  porous material of "infinite" permeability  (that is, an open
  borehole), then

                                             lun c  = 2

 which yields

                                            1f  =  2 qa                                        (9)

 This result demonstrates that the maximum pore-water velocity through a borehole packed with a porous
 material of hydraulic conductivity Kp. embedded  in a porous aquifer of hydraulic conductivity Ka  (with
 pore-water flow  obeying  Darcy's Law),  is twice the aquifer pore-water velocity.

      Note that calculation of Ka m Eq. (7) depends on two measurements of qp using two different values of
 Kp.  It is therefore important to  examine how rapidly  qp  changes as a function of  Kp. We can do this by
 differentiating Eq. (8) with respect to Kr. This tells us how  qp changes as a function of Kr. Differentiating Eq
 (8)  with respect  to Kr, we obtain

                                    de_   =     2         2Kr
                                   dK,
 Figure 6 is a graph of € and d e.ldKT as a function of Kr. The curve representing E clearly demonstrates the
 asymptotic nature of e. while the curve representing de.ldKr demonstrates the  sensitivity of Eq  (3)  to
 changes in Kr. As Figure 6 illustrates, e experiences us greatest change for values of Kr less than 1  For values
 of KT greater than  1. changes in E become very small. For values  of Kr greater  than 5, changes m E are
 insignificant.
                                                                                  •j
     The physical interpretation of the results is shown in Figure 6. The technique for determining m-situ
aquifer  hydraulic conductivity  in the  region surrounding  the  borehole requires  two  measurements of
borehole pore-water velocity through porous packing material of differing but known hydraulic conductivi-
ties. Intuition suggests that a large value of Kr will produce a "sink effect" such that a large portion of aquifer
pore-water flow will be funnelled into the packed borehole,  producing a large, measurable borehole pore-
water velocity Vp. However, Eq. (10) shows that when Kr is large, changes in e are relatively small. Converse-
ly, when Kr is small,  changes  in E are large. The change  in e is what is measured between the two experi-
                                               12

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                                                                                         VI
                                                                                        T3
                                                                                  — 0.8
             0.0
                                                                                   0.0
                                                 Kr
     FIGURE 6  Sensitivity of Hydraulic Conductivity Equations to the Parameter Kr. The Parameter
                 can be  thought of as Dimensionless Borehole Pore-Water Speed.
ments. hence we need to make our measurements in the range where e changes the fastest, but this is also
the range where the borehole pore-water velocity is smallest, and therefore hardest to measure accurately.
Thus, for values of Kr in this range, instrument sensitivity  to Vp will become a  critical factor
     Although theoretically correct, the experimental application of Eq. (7) requires a technique for direct-
ly measuring pore-water velocity through a borehole packed with a material of known hydraulic conductiv-
ity, a problem which until now has  not been addressed. Furthermore, the technique used to measure the
borehole pore-water velocity must  be sensitive enough to detect changes  in the amount of flow  moving
through the borehole packing material as a function of the hydraulic conductivity of this material Analysis
of the hydraulic conductivity equations  presented in this section suggests that the most promising results
should be obtained for values of Kr less than about 2. This report considers these  problems by investigating
two possible methods for direct borehole pore-water velocity measurement, a heat pulse advective transport
method and  a modified borehole-dilution method.
                                               13

-------
  MEASUREMENT OF THERMAL PULSE ADVECTIVE VELOCITY  IN A
     SATURATED POROUS MEDIUM
       K-V Associates, Inc.. have designed a single-well flo*meter which the manufacturer claims can de-
  tect aquifer  pore-water velocities in the range of 0.03 m/d to 3.0 m/d (Kerfoot.  1982). In general, the
  system operates by generating a heat pulse within a borehole section packed with a saturated porous me-
  dium. Temperature changes within this medium (due to the transported heat pulse) are then related to the
  aquifer pore-water velocity. Unfortunately, operating procedures require a laboratory calibration of ques-
  tionable reliability. Modifications to the flowmeier system were made in an attempt to directly measure the
  pore-water velocity through  the borehole porous packing material, thus allowing a value for the aquifer
  hydraulic  conductivity to  be  determined.
      Measurement of ground-water velocity with the K-V Associates flowmeter is based upon the transport
 of a thermal heat pulse within a porous medium, under  the influence of interstitial fluid  flow  (Kerfoot.
  1982). The  flowmeter  system consists of a downhole probe and an uphole electronics package. The
 downhole  probe contains a stainless steel heating probe symmetrically surrounded by heat-sensitive thermis-
 tors (Figure 7). Ideally, the downhole probe is buried in an aquifer with minimal disturbance to the porous
 medium. When operating the flowmeter. a transient, short-duration point-source of heat is generated at the
 heating probe tip, which is then transported radially away from the heat source. If interstitial fluid  flow does
 not exist, then  heat transport is by conduction, and the rate of heat transport is dependent on the thermal
 conductance of the porous material and the stationary pore water. If interstitial fluid flow does exist, then
 heat transport  is  by two mechanisms:  conduction through  the porous material, and advection  by  water
 moving through the porous medium. Therefore, for conductive heat transport alone, a symmetric  tempera-
 ture field is generated about the heat probe, and opposing thermistors are exposed to the same temperature.
 For conductive and advective heat transport, opposing thermistors are exposed to an asymmetric  tempera-
 ture field which is shifted  in the direction of pore-water movement. Theoretically, the magnitude of the
 as>mmetry between opposing thermistors is proportional to the thermal front advective velocity, and there-
 fore the  pore-water velocity.
      When operated as designed,  the flowmeter measures the difference in resistance between  opposing
 thermistor  pairs at a set time after the heat pulse is emitted. Thus, the flowmeter output is a relative number
 which must be  convened  to aquifer pore-water velocity.  K-V Associates  provides a small flowchamber
 which is used to empirically calibrate the flowmeter.  This is accomplished by filling the flowchamber with
 porous material representative of the aquifer material in which field measurements will be performed, and
 then embedding the flowmeter probe within this material. A constant-flow metering pump is used  to gener-
 ate a series of known flow  rates through the flowchamber, and the instrument response is  noted. Based on
 these measurements, an empirical calibration curve of flowmeter output versus flow rate is produced. This
 calibration  curve is then used in the field to convert  flowmeter response into aquifer pore-water  velocity.
     Wheatcraft et al. (1986) have thoroughly investigated the manufacturer's suggested calibration proce-
dures, and  found them to be of limited value. Specifically, the practice of using disturbed aquifer material as
a medium through which the flowmeter is calibrated is flawed, since this material probably has hydraulic
properties much different than the m-situ values found in the field. Thus, in designing experiments utilizing
                                               14

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pore-water direction
-t
f
1


S
1
uphole electronics cable
\

1
r0 = 1 60 cm
* 	 *



downgradient thermistor
"7
	 U_fc.v
                heat source
                             SIDE VIEW
 pore-water direction
                                                 [Section A - A']
   heat source
                                                   downgradient thermistor
                             TOP VIEW





FIGURE 7.  K-V Associates Thermal Flowmeter Downhole Probe Geometry.
                                 15

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  the flowmeter in determining the pore-water velocity through a borehole packed with a porous material of
  known hydraulic conductivity, it is very important to attempt to develop a method  to obtain pore-water
  velocities without using the empirical calibration procedure Equations (5) through (7) provide the theoreti-
  cal basis for obtaining pore-water velocity and the hydraulic conductivity of the aquifer.

  MODIFIED THERMAL FLOWMETER EXPERIMENTS
       The purpose of the experiments using the modified K-V Associates thermal flowmeter was to deter-
  mine the ability of the instrument to directly measure the advective velocity of a heat pulse generated within
  a borehole packed with a porous medium. If it is assumed that the heat-pulse velocity approximates the
  velocity of a conservative tracer, then this value is approximately equal to Vp, and theoretically can be used
  to determine Ka in Eq  (7).

       The modified thermal flowmeter experiments utilized the small calibration flowchamber provided by
  K-V Associates. A section of 5.08-cm diameter well screen (identical to that used in the large aquifer-simu-
  latmg flowchamber) was placed within the calibration flowchamber, and #16 sieve silica sand was packed
 around it. This porous medium was then saturated, and a constant-flow metering pump was used to establish!-
 a known pore-water velocity through the chamber. A mesh endcap filled with uniform-diameter glass beads
  (Kp = 3 93 x 10'3 m/s) was attached to the downhole probe, and then inserted into the  well. The experimen-
 tal configuration is shown in Figure 8.

      Modifications to the flowmeter circuitry were made so that the downgradient thermistor could be
 continuously monitored. This was accomplished by placing a  resistor in series with the downgradient ther-
 mistor, and providing this circuit with a constant voltage. One channel of a two-channel analog recorder was
 used to monitor the voltage across the resistor as a function of time. Thus, a heat-pulse  front is sensed by the
 downgradient thermistor, producing a proportional increase in voltage, which is recorded as a breakthrouch
 curve on the analog recorder. The second channel on the  recorder was used to monitor the current supplied
 to the heat-probe tip. allowing the starting time of an experiment to be recorded.

      The question as to whether the advective pore-water velocity could be determined using the proposed
 technique was answered by recording heat transport breakthrough curves at the downgradient thermistor.
 using different but known pore-water velocities. If the advective portion of the heat-transport velocity  was
 sufficiently large, then us arrival at the downgradient thermistor would appear as a separate peak, distin-
 guishable from the conductive heat-transport peak arrival. Additionally, the peak representing the advec-
 ti\e component  of the transported heat  pulse would occur at earlier  times for higher pore-water veloci-
 ties,while the conductive heat-transport peak arrival would remain relatively constant. Hence,  by knowing
 the horizontal distance between the heating probe tip and  the downgradient thermistor, the advective veloc-
 ity of the heat pulse within the porous packing material could be calculated. If it is assumed that the advec-
 me heat-pulse velocity approximates the  advective velocity of a conservative tracer, then this method could
 be used to determine Ka in  Eq. (7)

     Figure 9 gives the results of three experiments using  the modified K-V Associates thermal flowmeter
The time at which the advective component of the heat pulse should have  arrived at the downgradient

                                               16

-------
                                                   uphole  electronics cable
                                               V
                                                        downhole probe
                                                            slotted PVC screen
                  static water level
                  j pore-water direction
                         pore-water direction |
          #16 sieve silica
          sand
    input from constant-flow
    metering pump
mesh endcap filled with uniform-
diameter glass  beads
output to constant-flow
metering pump
                    FIGURE  8.  Thermal Flowmeter Experimental Configuration.
thermistor, calculated from a priori knowledge of the pore-water velocity and heat-pulse travel distance, is
indicated for each experiment. As this figure illustrates, the arrival of the advective component of the trans-
ported heat cannot be differentiated from the conduction-dominated heat-transport breakthrough curves.
In effect, conduction of heat through the porous material surrounding the downhole probe is a much more
efficient heat-transport mechanism than advection by pore water.
     The  flowmeter was no longer considered a  viable method to determine Ka because it was unable to
distinguish the advective component of the heat pulse at the downgradient thermistor. As a result, no further
experiments were conducted with the thermal flowmeter. Attention was instead focused on developing a
method to  measure Ka  using Eq. (7) and borehole-dilution techniques.
                                              17

-------
                 2J)
             O
             O)
             0)
             0>
             O)
             (0
             6
             O
             a
             I
             I
             JO
             
-------
 considered. This condition requires an elimination of all other flow components, or the derivation of correc-
 tion factors to compensate for their influence.

       Assuming an "instantaneous" mixing of an electrolytic tracer homogeneously distributed in the mea-
 suring volume, the time rate of change of the tracer concentration within the measuring volume is given by
                                           dC
                                           or  = -kc
 where
       C   =  tracer concentration within the borehole measuring volume
       t    =  residence time of tracer in measuring volume
       k    =  rate constant
       The constant * corresponds to the fraction of the total amount of tracer that leaves the measuring
 volume  in one  unit of time, thus
                                                                                            (12)
 where
      v
          =  borehole measuring volume
      A   =  vertical cross-sectional area of measuring volume
      Vb  =  apparent water velocity within borehole section
Substituting Eq. (12) into  Eq.  (11), integrating, and solving for, Vb yields

                                         1/4 - -  >£                                  03)
where
      C0  =  initial tracer concentration at t=0
Krolikowski (1965) has suggested that Eq. (13) be corrected for any background tracer concentration with-
in the measuring volume:
where
      Q,  =  background tracer concentration
      In applying borehole-dilution theory to the problem of measuring pore-water velocity through a sec-
tion of a borehole packed with a porous material of known hydraulic conductivity, the basic borehole-dilu-
tion equations were modified to account for the presence of the porous material within the mixing volume.
Additionally, the factors that influence flow within a packed borehole were examined.
      Figure 10 represents an isolated section of a cylindrical borehole embedded within a saturated  porous
aquifer  across which exists a significant hydraulic gradient. The borehole section is packed with a homoge-
neous porous material of known hydraulic conductivity Kp. It is further assumed that the porous material is
chemically nonreactive with both the normal borehole fluid and the electrolytic tracer Due to the presence

                                                19

-------
                                             borehole
        saturated
        aquifer
 pore-water flow
                                                 packed
                                                 borehole
                                                 section  hy-
                                                 draulic con-
                                                 ductivity  =
                                                 Kp
     Vertical cross-sectional area J_ to flow A=(x')(z')

     Measuring volume  V=,T  I —I  z1
FIGURE 10.  Explanation of Modified Borehole-Dilution Geometry.
                             20

-------
 of the porous packing material, the effective mixing volume is now the volume of the pore spaces within the
 borehole section
 where
      ve   =  effective  mixing volume of packed borehole
      v    =  volume of unpacked borehole section
      nf   =  effective  porosity of the borehole packing material
      Assuming an "instantaneous" mixing of an electrolytic tracer homogeneously distributed within the
 packed borehole section, then the general form of the  point-dilution equations holds, and the apparent
 velocity of the pore fluid within  the packed borehole section  becomes
 where
      Vp   =  pore-fluid velocity within the packed borehole
      Application of Eq. (15) is similar to the normal borehole-dilution technique, with the electrical con-
 ductance of the packed borehole pore fluid being measured as a function of time. By plotting the natural
 logarithm of the corrected electrical conductivity versus time, a linear relationship is produced which allows
 for the direct calculation of Vp
                                                   m ve
                                          V»	~^-                                    (16)
 where
      m    =  slope of the semilogarithmic  line
      Whereas, the aim of normal borehole-dilution theory is to determine the aquifer pore-water velocity
 Va from the value measured m an open borehole, the aim of the modified borehole-dilution technique is to
 determine at least two accurate values for the pore-water velocity using two packers  of different hydraulic
 conductivity. These two measurements of Vp will, through Eq. (7). allow determination of Ka, as well as Va.
 Ideally, it  is desirable that all components of flow within the packed  borehole not related to the  actual
 aquifer pore-water  velocity Va be negligible. This requires an understanding of the factors which influence
 pore-water flow within the packed borehole measuring volume, so that these factors can be  eliminated or
 compensated for.

 FACTORS THAT  INFLUENCE PORE-WATER FLOW  WITHIN PACKED
     BOREHOLE MEASURING VOLUME
      In general, the  factors which influence the measured value of the apparent pore-water velocity \'p
within the packed borehole measuring volume ve are similar to those which influence the borehole-water
velocity Vh within  an open borehole measuring  volume  v. These are  factors due  to hydraulic and
hydrogeologic conditions within the porous medium surrounding and within the packed borehole, borehole

                                               21

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 construction influences, instrument influences, and physical influences. The major difference between the
 modified borehole-dilution theory and the normal borehole-dilution theory is the magnitude of the influ-
 ence of these  various factors on Vp.

      The influence of hvdrauhc and hydrogeologic factors on the measured value of the packed borehole
 pore-water velocity Vp are  similar to their influence on the measured value of the open  borehole-water
 velocity Vh. For a borehole completed within a stratified aquifer, naturally occurring pressure gradients may
 generate vertical currents within the borehole. As in the open borehole case, the use of inflatable packers as
 a means of isolating the packed borehole measuring volume ve should sufficiently reduce the effects of
 artificial tracer dilution caused by vertical borehole  currents for most situations.

      As in normal borehole-dilution applications, the influence on the packed borehole pore-water veloc-
 ity Vp due to the presence of the borehole tool is minimal, provided the measuring volume ve can be accu-
 rately determined,  and provided the instrument does  not obstruct the measuring volume vertical cross-sec-
 tional area perpendicular to the aquifer  pore-water  flow direction.

      Analogous to normal borehole-dilution applications, the modified borehole-dilution method requires
 an instantaneous, homogeneous distribution of the tracer within the packed borehole measuring volume v
 However, achieving this tracer distribution within the packed measuring volume is very difficult, because
 m-situ  mechanical mixing within the packed borehole section is impossible. Thus, if an electrolytic tracer
 introduced into the packed borehole section  is not efficiently mixed with the background pore fluid and
 distributed throughout the measuring volume, then there is a strong likelihood that an unwanted borehole
 velocity component due to artificial mixing will be generated. This is likely due to the nature of the move-
 ment of the tracer,  which may migrate from the packed borehole as a high concentration mass rather than
 being continuously diluted by aquifer pore water moving through the  packed borehole section.

      Another unwanted borehole velocity component which may result from insufficient mixing and distri-
 bution of the tracer within the packed borehole section is  artificial outflow of the tracer due to density
 differences between the high-concentration tracer cloud and the surrounding background pore-water fluid.
     As in normal borehole-dilution theory, physical effects caused by tracer diffusion  within the measur-
 ing volume are  minimal for aquifer pore-water velocities greater than approximately 0.3 m/day. Similarly,
 ph\sical effects caused by temperature gradients resulting from temperature differences  between the tracer
 and the background borehole pore water are minimal for temperature differences  between the tracer fluid
 and background fluid of 2°C or less.

     Because of the significant influence of the insufficiently mixed tracer, a method for efficiently mixing
 and distributing the tracer within the packed  borehole measuring volume is critical to  the validity of the
 modified borehole-dilution technique.

 BOREHOLE PACKER  POROUS  MATERIALS
     The borehole-dilution experiments were conducted using a laboratory-scale  flowchamber (discussed
below) packed with  porous media designed to be large  enough to provide a uniform flow  field around a well

                                               22

-------
 emplaced in ihe center of the flowchamber. Packers were designed using glass beads that covered a range of
 hydraulic conductivity ratios of about 0.01 <: K, < 10. thus spanning two orders of magnitude. Actual hy-
 draulic conductivity values were determined by laboratory permeameter tests, and are given in Table 1. The
 permeameter tests were designed so that Reynolds numbers were kept to less than one, thus minimizing the
 possibility of nonlinear head losses.

          TABLE 1.  THEORETICAL AND EXPERIMENTAL GLASS BEAD HYDRAULIC
                                    CONDUCTIVITY VALUES
Bead Batch Size (mm)
2.8
2.0
1.4
1.18
0.85
0.71
0 61
0 25
0.18
0.15
- 2.
- 1.
- 1.
- 0.
- 0.
- 0.
- 0.
- 0.
- 0.
- 0.
0
4
0
85
60
50
425
180
125
106
Experimental Kp (mis)
nonlammar flow
2
1

3
2
1
3
1
9
.61
.72

.93
.31
.79
.16
.68
.59
x
x

X
X
X
X
X
X
10-2
10-2

10-3
10-3
10-3
10~4
io-4
10-5
K,

n/a
8
5
7 12
1
0
0
1.08
5.75
3.28
.94
.89


x 10-3
.34
.79
.61
x 11



D-I
x ID'2 ..
x 10-2
 FLOWCHAMBER CONSTRUCTION AND CHARACTERISTICS
      The aquifer-simulating flowchamber utilized in this study is designed to exhibit uniform flow through-
 out us vertical cross section. Figure 11 is an idealized drawing of the flowchamber; construction details are
 given in \Vheatcraft et al. (1986). The dimensions of the flowchamber are 1.2 m by  1.2 m (cross-sectional
 area perpendicular to flow direction) by 2 m (length of flow path). Plexiglass (3/8 in) is used to contain the
 flowchamber porous medium, which is composed of #16 sieve silica sand (1  19 mm geometric mean size).
 Water chambers are constructed at opposite ends of the flowchamber. and act as constant-head reservoirs.
 The height of these reservoirs can be adjusted with a  precision of 0.01 cm  Six fully penetrating,  2-mch
 diameter (5.08 cm) screened wells are emplaced within the flowchamber in two rows of three, evenly spaced
 parallel to  the direction of flow. Well screen slot openings are 0 05 cm, arranged m five vertical rows
 (Wheatcraft et al.,  1986).

      During construction of the flow chamber, care was taken to avoid stratification within the porous me-
 dium; hence, the flowchamber closely models a confined, homogeneous and isotropic aquifer. Hydraulic
 conductivity of the flowchamber medium  was determined using Darcy's  Law. written in the  form
                                                                                          (17)
                                                 J-UJJl
where
     Q   =  volumetric discharge through the flowchamber
     A    =  vertical cross-sectional area of flowchamber
     A/   =  length of flowpath
     A/I  =  hydraulic head across flowchamber (difference m reservoir heights)

                                              23

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                                              static water
                             plyboard confining
                             layer
                                                          2-inch diameter well
                                                               upgradient
                                                               head reservoir
             downgradient
             head reservoir
  potentiometric
  surface
                      4J-1-1J-3-I J J _l
                       JJ-J-IJ-1-iJJJ
                           -1JJJJJJJJJ
                       Jl J J.J Jj J _
                                                                                 potentiometric
                                                                                 surface
                                                                                  potentiometric
                                                                                  surface
                                                                 flow direction
                                              porous medium
            clear Plexiglas*
              FIGURE 11.  Idealized Diagram of the Aquifer-Simulating Flowchamber.
All of the parameters in Eq. (17) can be accurately measured. Based on Eq. (17), the flowchamber hydrau-
lic conductivity Ka was calculated to be  approximately 252 m/day.

     Accurate knowledge of the flowchamber pore-water velocity is of primary importance in this study.
This parameter can also be  calculated using Darcy's Law, here written as
                                                                                           (18)
where
Va  =
             flowchamber pore-water velocity
     na   =  effective porosity of the flowchamber porous medium

The  accuracy of Eq. (18) depends on an accurate measurement of na. This parameter was determined
through a conductive tracer experiment.  In this experiment, an electrically conductive tracer (NaCl) was
introduced into the upgradient head reservoir at time t=0.  Breakthrough curves of this tracer were then
                                              24

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 recorded at each well along the flow path. Based on the time-distance relationship for the peak arrival of the
 tracer front  at each well,  an average effective porosity for the flowchamber was determined to be 43.6
 percent.

 NORMAL AND  MODIFIED BOREHOLE-DILUTION  EXPERIMENTS
      Experiments utilizing the modified borehole-dilution technique were conducted in an attempt 10 di-
 rectly measure the horizontal pore-water speed Vp through a borehole with a porous material of known
 hydraulic conductivity. These experiments were conducted in the aquifer-simulating flowchamber, using a
 laboratory apparatus designed and constructed for this purpose. In general, the method of investigation was
 twofold. First, the laboratory apparatus was configured as a normal borehole-dilution device, and experi-
 ments were carried out to determine if the device could produce results in agreement with borehole-dilution
 theory. The mam reason for conducting these experiments was to verify that the laboratory apparatus could
 produce accurate and  repeatable dilution data. Having achieved this objective, the apparatus was reconfi-
 gured, and experiments were performed in an attempt to produce results which were in agreement with the
 m-situ hydraulic conductivity theory  (Eq.  7). Initial reconfiguration of the laboratory apparatus consisted
 mainly of introducing a porous material into the borehole measuring volume. Further modifications to the:-
 apparatus were applied as  experimentation proceeded. Basically, the modified borehole-dilution experi-
 ments consisted of measuring a value  for Vp as a function of Kr, while holding the flowchamber pore-water
 velocity Va constant. From Eq. (8) it follows that these data should produce a  curve which asymptotically
 approaches a value of 2. as  in Figure 6.

 EXPERIMENTAL APPARATUS
      The experimental setup is shown in Figure 12. with the downhole portion of the apparatus shown in
 Figure 13. The mixing chamber shown in Figure 12 is designed to optimizethe mixing process so that it most
 closely approximates the initial conditions of Eqs.  (11) and (14). Without such a mixing chamber, wild
 oscillations in concentration will be experienced in the system until the tracer slug has fully mixed, which can
 take several residence times, thus ruining the early time results of the experiments. The optimum volume for
 the mixing chamber was found through a set of experiments to be about 250 ml, which is about the volume of
 the total system without a mixing chamber Because it is virtually impossible to remove all early-time oscilla-
 tions, the first three minutes of the data were discarded for all experiments. Experience showed that three
 minutes was  sufficient  for the data to plot  as a straight line on semi-log graph paper.

      A series of experiments were conducted to determine the  repeatability  of data obtained  during a
dilution experiment. Three experiments were performed at two different flowchamber pore-water velocities
 (3.72m/day and 7.62 m/day), while holding all other test parameters constant.  Between each experiment.
the downhole tool was removed from the flowchamber well, and the glass beads were removed, dried, and
repacked. Care was taken to repack the glass beads in a systematic way, and to reposition the downhole tool
within the flouchamber well as consistently as possible. Figure 14 gives the results of the repeatability experi-
ments. Although there  is an obvious separation between the two sets of experiments performed at different
flowchamber  pore-water  velocities, it is clear that the experiments are not perfectly repeatable. If the data

                                               25

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                                                                           two-way valves
       peristaltic pump
         FIGURE  12.  Diagram of Laboratory Apparatus, Showing Location of Mixing Chamber
from these experiments uere used in Eq. (7) to calculate Ka, there would be a noticeable difference in some
cases.  The issue of nonrepeatability will be addressed further in  the next sections

     Figure 15 is a semiloganthmic plot  of the laboratory apparatus theoretical response, generated by
varying the parameter A',  Figure 15 follows from Eqs  (8). (15). and (16). which can be combined to give
where
                                               26

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                                         bladder inflation lines
              system output
system input
mixing volume
                                                              upper inflatable bladder
                                                            mesh sock containing
                                                            uniform glass beads
                                                                 mixing circuit
                                                               lower inflatable bladder
   FIGURE 13.  Diagram of Downhole Tool used in Hydraulic Conductivity Experiments.
                                         27

-------
 o
 LLJ
 T3
 
•o
I
O)
m
c
     -3-
    -4-
    -5-
           Flowchamber P.W. Vel.
          	 3.72 m/day
          	 3.72 m/day
          	 3.72 m/day
          	7.62 m/day
          	7.62 m/day
          	7.62 m/day
                          10
                                            20
                                      Time  (minutes)
                                                               I—
                                                              30
                                                                                 40
O
    -2-
8  -'
3   -4.
O)
0
(0
CD   —5-
   -6-
           Flowchamber P.W. Vel
           - 3.72 m/day
                     m/day
                     m/day
                     m/day
                     m/day
                3.72
                3.72
            -- 7.62
            -- 7.62
            -- 7.62  m/day
                         10
                                          —r~
                                           20
                                     Time (minutes)
                                                            —I—
                                                             30
FIGURE 14.  Results of Repeatibility Experiments. Laboratory Apparatus Sensitivity to Varied
             Flowchamber Pore-Water Velocity is Clearly Illustrated.
                                       28

-------
            o
            OJ
            •O
            I
             O
            m
            5
                                                           Theoretical Response
                                                    V V   >        "»
                                                    xv  v
                              10
                                             Time  (minutes)
       FIGURE  15.  Theoretical Output of Laboratory  Apparatus for  Flowchamber Pore-Water
                    Velocity = 3.72 m/day.
Figure 15 shows that by setting the flowchamber pore-water velocity Va constant and varying Kr, the slopes
change significantly (and therefore should be easy to detect experimentally) for Kr greater than about 0.5,
and Kr less than about 2.  Note that the curves for Kr greater than about 2 get increasingly close together, as
one would expect according to Eq. (10).
      A series of dilution  experiments were performed in an attempt to duplicate experimentally the results
of Figure 15. For these experiments, the flowchamber pore-water velocity was held constant at 3.72 m/day,
and the parameter Kr was varied from 0.61 to 8.94 (as given in Table 1). Special care  was taken to ensure
uniformity in experimental procedures, thus holding systematic error to a minimum.
      Figure 16 presents the results of the varied-A:,. experiments, along with the theoretical response that
would be expected. The experimental response has been plotted with a best-fit log-linear regression equa-
tion to aid comparison with the  theoretical response. The proportionality between Vp and Kp displayed by
the theoretical model (Figure 15) does not appear in the experimental data. Figure  17 is a comparison of the
                                               29

-------
 o
 UJ

 •o

 2
 o
 O
 i
 -o


 o


 3
 o
 
k.

b
O
 i


c

o
0

(O

CD
    -3-
    -5-
                             10
                                  Time (minutes)
                                                        Experimental Response
    Kr


	 0.61


• —  0.79


 -  - 2.44


	5.89


	8.94
                            10
                                  20
                                                                      —r~

                                                                      30
                                  Time (minutes)
 FIGURE  16.  Theoretical and Experimental Response for the First Varied-Kr Experiments,

              Plotted at Same Scale for.Comparison.
                                       30

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                 OJJ-
                             Theoretical Response
                             Experimental Response
                -ai —
            c
            O
            a.
            o
           55
               -0-4
                               Flowchamber Pore-Water Vel. = 3.72 m/day
                                                  ~r
                                                   a
                                                   Kr
10
    FIGURE 17. Theoretical Versus Experimental Change in Semiloganthmic Slope as a Function
                 of Kr,  for the First Varied-Kr Experiments. Note the Lack of Systematic Change
                 in the  Experimental Results.
theoretical verses experimental semilogarithmic slopes (determined from Figure 16) as a function of Kr. As
Figure 17 illustrates, the expected asymptotic behavior of flow through the packed borehole for increasing
Kr is not observed. On the contrary, the experimental results appear quite independent of Kr. As observed
from the theoretical portion of Figure 16, the slope of the dilution data (plotted semilogarithmically) should
become more negative as the parameter Kr increases. As indicated by the experimental plot, however, this is
not the case. Although there is a change in the experimental slope as a function Kr, this change does not
occur in a systematic way. Many factors could be responsible for this result.  Systematic error could have
been introduced by nonuniformity in experimental procedure. Random error due to fluctuation in physical
parameters over the course of an experiment, or due to other unknown phenomena, could be responsible.
Additionally, these  error sources could be operating  together in some unknown way.

     Because the proportionality between Vp and Kr is critical to the application of the modified borehole-
dilution technique in determining aquifer hydraulic conductivity Ka, a second set of varied-K, experiments
                                               31

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 were performed, taking great care to avoid introducing systematic error into the experiments. Furthermore.
 water temperature variation was closely monitored, under the assumption that significant changes in water
 temperature could affect the background electrical conductivity during the course of an experiment, and
 therefore bias the results. Additionally, significant water temperature variations could affect the viscosity of
 the flowchamber  water, possibly changing the fluid flow characteristics in some way.
      Figure 18 is a semiloganthmic plot of the second varied-*,, experiment results. For these experiments.
 the flowchamber pore-water velocity was held constant at 1.84 m/day. and the parameter Kr was varied
 from 1.34 to  8.94. Three experiments were performed at each value of Kr. Between each experiment, the
 downhole  tool glass beads were removed, dried, and repacked, as a means of determining the variability
 between experiments likely due to packing  arrangement.  Again,  the  proportionality between Vp and Kr
 predicted by the theoretical model is not observed in the experimental results. This condition  can be seen
 more clearly in Figure 19. which presents the theoretical and experimental slopes as a function of Kr. As
 noted for the  first  varied-*, experiments, there is a definite change in the slope of these experimental data
 for different Kr, however, this change appears to be random. The care with which these  experiments were
 performed, together with the successful normal point-dilution experiment results previously obtained, seem
 to rule out systematic experimental error as the cause of the negative results.                          '
      Four factors that most likely cause the  observed nonproportionality between the tracer dilution rate
 and the parameter Kr are: 1) the intrinsically random packing arrangement of the glass beads within the
 laboratory  apparatus downhole measuring volume; 2) the randomness associated  with positioning the
 downhole tool within the well; 3) the fundamental problem of obtaining an accurate measurement of the
 packed borehole pore-water velocity Vp in the region of maximum sensitivity to the parameter Kr: and 4) the
 probable existence of nonumform. three-dimensional flow  within the flowchamber  due to the  presence of
 the packed borehole. More than likely,  these factors are  operating in unison to produce the seemmcly
 random fluctuations observed in the experimental results.

     Of the four effects most likely responsible for the experimental results, the effect due  to the problem of
 obtaining an accurate value for Vp in the region of maximum sensitivity to the parameter Kr, coupled with the
 effect due to nonumform, three-dimensional flow within the flowchamber, may be the most significant. The
 first  of  these  effects has previously been discussed. The second effect, that of three-dimensional flow
 around  and through the packed borehole, can be conceptualized as follows. The solution of Wheatcraft and
 Wmterberg  (1985) requires that flow within the porous medium surrounding the packed borehole be hori-
 zontal and  laminar, and further that flow transitioning from this porous medium into the borehole porous
 medium and out again also obey these conditions. However, the presence and scale of the  packed borehole
section  may exert an influence on the flowchamber flow field such that a significant vertical component of
 flow exists in the region surrounding the packed borehole. Figure 20 is an idealized representation of this
situation. Although this phenomenon is difficult to quantify, it is reasonable to assume that this process could
introduce significant changes in the portion of flow moving through the packed borehole section, relative to
the predicted amount of flow through this section based on the theoretical results in Wheatcraft and Winter-
berg (1985). It is possible that by increasing the volume (that is, the length) of the packed downhole tool
relative  to the portion of the tool where the actual dilution process takes place, the randomness introduced

                                               32

-------
o
LU

T3
ID

a
 I
T3



O
o
(0
m
                                      Time (minutes)
  -4
                                     Time (minutes)
  FIGURE  18.  Least-Squares Plot of Second Varied-Kr Experimental Results.  Flow-

               chamber Pore-Water Velocity =1.84  m/day.
                                      33

-------
                 o.oo-
                         •+— Theoretical Response
                        — *-  Experimental Response
               -0.05-
           c
           C  -0.10-
           c7)
               -0.15 —
                                Flowchamber Pore-Water Vel. = 1.84 m/day
               -0.20-}-
                                       3456789     10
                                                   Kr
 FIGURE 19. Theoretical Versus Experimental Change in Semilogarithmic Slope as a Function of Kr,
              for the Second Vaned-Kr Experiments.  Apparent Random Fluctuation in Experimental
              Results is Illustrated.
into the experimental data due to three-dimensional flow can be removed (Figure 21). Reducing this source
of random error may provide the sensitivity needed to detect proportional changes in dilution  rate as a
function of Kr, for a given  flowchamber pore-water velocity.

      The significance of the length of the packed borehole section relative to the section where the dilution
measurement is made  was  investigated in the third varied-*, experiments. In these experiments, the
downhole tool was modified as in Figure 21 to reduce vertical flow components, and  three sets of three
dilution experiments were performed, using glass beads of Kr=2A4, 5.89, and 8.84. The flowchamber pore-
water velocity was held constant at 1.72 m/day.  All other experimental parameters were identical to the first
and  second varied-AT, experiments. Figure 22  is a plot of the experimental versus theoretical slopes as a
function of Kr.  As in the  first and second varied-^ experiments, the experimental slopes from the third
varied-A:r experiments appear to be independent of the parameter Kr. Hence, it is likely the case that the
effect due to the problem of obtaining an accurate measurement of Vp in the range of maximum sensitivity to
                                              34

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          [x-section
                                                              — borehole

                                                              Packed borehole section,
                                                              measuring volume.
                 flow lines
    FIGURE 20.  Idealized Representation of Flow Field Distribution Due to Packed Borehole Section.
Kr is the dominant factor responsible  for the apparent random experimental results. Unfortunately, this
effect cannot be controlled experimentally. Thus, it is reasonable to conclude that the modified borehole-
dilution method, as applied in these experiments, is an inappropriate method for investigating the in-situ
saturated hydraulic conductivity theory.


DISCUSSION - NATURAL FLOW METHODS
     The experiments performed to determine packed borehole pore-water velocity Vp by measuring the
arrival time of the advected portion of a heat pulse generated within a packed borehole were unsuccessful,
mainly because conduction of the heat  pulse was the dominant transport mechanism.  Hence, the arrival of
the advective heat front at the downgradient thermistor was completely masked by the arrival of the conduc-
tive heat front. It is possible that by utilizing a porous packing material less sensitive to  heat conduction, the
advected portion of the heat pulse may  be detectable, in which case the thermal-pulse technique for deter-
mining Vp  may be  applicable. Additionally, an effort was made to determine a method for utilizing the
measured differences in thermistor response between the upgradient and downgradient thermistors, howev-
                                              35

-------
                [x-section]
                                                               packed borehole section
                  flow lines
     FIGURE 21.  Idealized Representation of Flow Field after Increasing Packed Borehole Section.
 er, this method required empirical calibration of the instrument and therefore was determined inappropri-
 ate.
      Results of experiments utilizing the modified borehole-dilution technique to determine Vp were incon-
 clusive. Application of the technique using the laboratory apparatus has shown that first-order decay of a
 continuously diluted electrolytic tracer introduced into the borehole porous medium can be measured, and
 that the rate of dilution of this tracer is directly proportional to the pore-water velocity within the medium
 surrounding the borehole. However, experimental investigations of the  applicability of the modified bore-
 hole-dilution technique required that the dilution rate of the tracer be directly proportional to the parameter
Kr (the ratio of borehole to  flowchamber hydraulic conductivity), as measured at a  constant flowchamher
velocity. This proportionality, the  basis of the modified borehole-dilution technique, was not observed.
Many factors may be responsible for the apparent random relationship between tracer-dilution rate and Kr.
Additionally, one or more of these factors may exert a dominant influence on the data, and any of these
                                               36

-------
               -0.10 —
             c
             £
             o
            c/5
                             •Theoretical Response
                              Experimental Response
                                  Flowchamber Pore-Water Vel. = 1.76 m/day
               -035
                                 T
                                  2
T~
 a
 Kr
                                                                                    10
  FIGURE 22.  Theoretical Versus Experimental Change in Semilogarithmic Slope as a Function of Kr,
                for the Third Varied-Kr Experiments. Apparent Random Fluctuation in  Experimental
                Results is Illustrated.
factors may be operating in unison. Thus, understanding the cause of the randomness in the experimental
data is difficult.
      The factors least likely to have influenced the experimental results are systematic errors due to exper-
imental procedure prior to and during an experiment. As previously noted, much care was taken in selling
up and conducting a dilution experiment in a systematic way; hence, any systematic error introduced in this
process should tend to influence all experimental results in a similar fashion. This argument  also applies to
other potential sources of deterministic error, such as the calibration  experiment performed to determine
the relationship between the laboratory apparatus electrical response and the electrical conductivity of the
circulating borehole fluid.
      Influencing factors due to the introduction of random errors are the most likely cause of the nonpro-
portionality between tracer-dilution rate and Kr. Of these random errors, the least likely to affect the data
were instrument errors. All electrical equipment was checked for correct settings and sufficient charge (for

-------
 battery operated devices) between each experiment, and during an experiment, these instruments uere
 closely monitored  for any nonumform behavior  Additionally, mechanical vibrational noise and posvihle
 electrical noise was monitored. Any experimental data that may have been biased by these sources of error
 were  rejected.

       Another source of random error not likely to have influenced the experimental results is fluctuations m
 the flowchamber ambient water temperature. The temperature of water entering the flowchamber was es-
 sentially constant, and remained so throughout the course of an experiment. Also, water temperature unhm
 the flowchamber, as measured over the course of several experiments,  remained essentially constant
       The results of the first, second, and third varied-*, experiments suggest that the fundamental problem
 of obtaining an accurate measurement of the packed borehole pore-water velocity Vp within the region  of
 maximum sensitivity to the parameter K, is the dominant factor responsible for the random experimental
 results. This problem cannot be controlled experimentally using  the modified borehole-dilution method.

 SINGLE-NELL ELECTRICAL TRACER (SWET)  METHOD
       The most direct way to determine the hydraulic conductivity of a formation is to observe  the move- -.
 ment of fluids through the formation under the influence of a known driving force. In the method developed
 here, an electrically anomalous tracer (saltwater) is injected under steady-state conditions into a well. While
 injection of the tracer continues, the radius of invasion of the tracer is determined with a borehole induction
 tool. By repeatedly measuring the  radius of invasion at different  times, the rate of invasion can be deter-
 mined. The hydraulic head, which is a  measure  of the driving force required to inject the fluid, is also noted.
 The tracer will invade different intervals of the formation at different rates depending on the hydraulic
 properties of each interval of the formation. This information can be used to calculate a hydraulic conductiv-
 ity log of the formation. Because multiple induction logs are run,  the rate of invasion can be calculated  at
 several different radii of invasion. Hence, the hydraulic conductivity log of the formation can be calculated
 at several different radii of invasion. A porosity log can  also be calculated by using a  model of  formation
 electrical conductivity that accounts for variations in matrix conductivity and porosity.

 Field  Application
       The study area is located approximately  32 km north of Mobile. Alabama, and  has been the site of
 numerous hydrogeological studies (Molz et al.,  I986a and b). The surface geologic unit consists of Quater-
 nary mterbedded sands and clays to a depth of 61 m. The  wells used in this study are completed mm a
 confined aquifer between  40 and 60 m deep. The upper 40 m of the wells are cased,  the lower 20 m are
 screened Cores of the aquifer show that it consists of an unconsolidated, medium to fine sand, containing 1
 to 15 percent fines by weight. The potentiometnc head is 2 to 3 m below the land surface, natural vertical
 and horizontal hydraulic gradients are negligible  (Molz et al..  1986a and b).
     The first step m conducting the test was to establish a steady-state flow system while injecting into the
well. Injection rates varied from 2.5 to 4.4 x I0~s m3/s depending on the well. The injection fluid uas
obtained from a well completed into the same aquifer that was located 250 m away from the injection well.

                                               38

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 At each well, the injection was at a constant rate. When the head in the injection  well stabilized for 30
 minutes, the system was considered to be at steady-state. Heads were measured with  a pressure transducer
 to avoid errors due to turbulence from the  injection stream.
       After steady-state conditions had been achieved, several induction logs were run in the injection well.
 These logs provide background information on the formation electrical conductivity when the formation is
 saturated with native pore fluid. Logs were repeated several times to ensure that the induction tool was not
 drifting. The electrical conductivity of the injection fluid was also measured. An electrical fluid conductivity
 cell was secured at the top of the screened interval (40 m). This cell was used to determine the arrival time of
 the tracer. To minimize the chance of instrument drift, the induction tool was not removed from the well A
 Geonics EM-39 was used for the induction tool. Because the water level in the well remained below ground
 level during  injection, it was not necessary to use special equipment at the well head to log and inject fluid
 simultaneously. Figure 23 illustrates the experimental setup.
      A concentrated sodium  bromide solution was pumped into the injection line  with a constant rate
 metering pump. The rate of the metering pump had been previously adjusted so that the electrical conduc-
 tivity of the tracer solution would be three to five times greater than the 4600 jimhos/cm electrical conductiv-
 ity of the native pore water. The output of the fluid conductivity cell at the top of the screened interval uas:-
 recorded on an analog recorder. The time at which the electrical conductivity of the borehole fluid at the lop
 of the screened interval was halfway between the conductivity of the tracer and the conductivity of the native
 water  was considered to  be the amval time of the tracer at the top of the screened interval. The time
 required for the transition from native to tracer fluid at a given depth  within the well screen was on the order
 of one minute. Other tests demonstrated that the tracer reached the bottom of the screened interval (60 m)
 about one minute after the top of the  screened interval.
      Induction logs were run repeatedly while injection  was continuing. In addition  to digitally recording
 formation electrical conductivity as a function of depth, the time at which each measurement was made was
 also recorded. To complete the full quantitative analysis, the injection and logging continued until the radius
 of invasion was greater than the radius of investigation of the induction tool. When  this condition occurred,
 repeated logs of the well were identical. Sufficient time must elapse between running the two logs to ensure
 that this condition was met. One way to ensure this is to increase the interval of time  between running the
 logs by a factor of 2 after each log is run. With the equipment used in this study, it was necessary for the fluid
 to invade 2 to 3 m into the  formation to have all of the data necessary for the analysis method presenied
 here. With the injection rates and wells used m this study, the time required  to reach this condition was up to
 three to four hours.

Theory of Analysis
Assumptions
     Several assumptions are necessary for the following analysis. In general, the assumptions are realiMic
and can be tested. In  some cases, it is evident from the data when an assumption is violated.
     The first assumption is that well hydraulic losses are negligible compared to formation losses. Head loss
can be caused by an excessive injection rate for the well, and m this case will be evenlv distributed alonu the
                                                39

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ELECTROLYTE INJECTION
LINE
TO BOREHOLE LOGGER
                                              LAND SURFACE
      HIGH PERMEABILITY
      ZONES
           ELECTROLYTE
           FRONT
                                      INDUCTION TOOL
     FIGURE 23. Experimental Setup for Single-Well Electrical Tracer Test.
                                40

-------
 well. It can also be limited to short intervals of the well as will happen if the screen is damaged or plugged In
 a poorly completed well, drilling mud can also seal-off a portion of the formation from the well. If only a
 short interval of the well is in poor hydraulic connection with the formation, it is readily apparent in the dnia.
      The second assumption is that the formation is fully saturated. If air is introduced m the  formation
 during the injection process, the baseline logs will  not be accurate and an unrealistic increase m electrical
 conductivity will  be observed.

      The third assumption is that there are no chemical reactions between the injected fluid and the forma-
 tion. The possibility exists in some formations with clay that the salt solution can react with the formation and
 alter the hydraulic properties of the formation.

      The fourth assumption is that the tracer is perfect and moves radially away from  the well by advecuon
 The now will be radial because the injection rates are sufficiently high to cause an appreciable upconmc of
 the potentiometnc surface. Some dispersion takes place along the 2 to 3 m travel path of the study, but this
 dispersion does not influence the results.

 Porosity Calculation                                                                              '•-
      Archie's Rule (Hearst and Nelson, 1985)  can  be used to  calculate the electrical conductivity of a
 formation due to  the electrical conductivity of the  pore fluid. In  clay-free formations saturated with pore
 fluids that have a  high electrical conductivity, this will be the only significant mechanism for electrical con-
 duction and is a suitable model for formation conductivity. However,  if clays are present, there can be
 significant electrical conduction through the matrix of the formation. This is particularly true in formations
 saturated with  pore  fluid that has a low electrical  conductivity

      In many ground-water investigations, the influence of the electrical conductivity of the matrix cannot
 be ignored and must be taken into account. The following is the development of a model to accomplish this
 task.
The formation conductivity can  be represented by:

                                        Ofln  = Om  +  Opf                                   (|9)

where:
      Ofln =  formation electrical conductivity
      om  =  electrical conductivity of the formation due to conduction through the matrix
      Opf  =  electrical conductivity of the formation due to conduction through the pore fluid
From Archie's Rule we know:
                                        Opf  =  (of &")/a                                   (20)
      9  =  porosity  (\\ hich is a function of depth)
      Of  =  electrical conductivity of the pore fluid
      a   =  tortuosity
                                                41

-------
       m  =  cementation factor
       °P  =  electrical conductivity of the formation
 Jackson et al. (1978) have shown that for unconsolidated sands an appropriate value for tortuosity and ihe
 cementation factor is 1 . 0 and 1 . 4 , respectively. It is important to recognize that the use of the term tortuosn y
 in connection with Archie's  Rule is only indirectly related to the path traveled by a fluid in the formation.
      Substituting Eq. (19) into  Eq.  (20) yields:
                                                 +  (ar &")/a
      In the case of the single-well electrical tracer (SWET) test we know the formation electrical conductiv-
 ity when it is saturated « ith pore fluid of two different electrical conductivities. The electrical conductivity of
 the pore fluid, with and  without the tracer, is also known because it can be measured during the injection
 process. The electrical conductivity of the formation when it is not invaded by the tracer is determined by
 the baseline logs that are run prior to tracer injection. The electrical conductivity of the formation when it is
 invaded by the tracer is determined by the logs made after the tracer has invaded the formation a sufficient
 distance that only the invaded  zone is measured.
      Using the following notation1
      °j\   =  electrical conductivity of the pore  fluid without the tracer
      an   =  electrical conductivity of the pore  fluid with the tracer
      °flni  =  formation  electrical conductivity without the tracer
      Ofn2  =  formation  electrical conductivity with the tracer
 allows us to write  a  system of equations:
                                         =  am  +  (ofl &")/a
                                         -  am  +  (afl &")/a
which can be solved  for porosity and the  matrix electrical  conductivity.
                                    0  =
1/m

                             (21)
Calculation of Radius of Invasion
     To calculate the  radius of invasion of the tracer, it is necessary for us to introduce a model for ihe
measurement of electrical conductivity by the induction tool:
                                aa =  X(R)  OJM +  (1 -*(*))  G>,,                           (22)
     oa    =  apparent formation electrical conductivity (this is what is measured by the induction tool)
     R    =  radius of invasion
     X(R) =  radial response function of the induction tool
                                                42

-------
      The radial response function (Figure 24) was calculated using a knowledge of the coil geometry of the
 induction tool (McNeil,  1986) and the method described by Saito (1982).

      The only unknown in Eq. (22) is the radius of invasion. This is solved for numerically by first solving
 for the value of l-X(R), and using the radial response function to determine the  value of /?.

 Calculation of Hydraulic Conductivity

      The rate of injection into a segment of the well will be equal to the rate that the pore volume is invaded:


                                         Q = ^^~                                    <23>

      B   =  thickness of segment
      t    =  time since start of injection
      Q   =  porosity
      Q   =  rate of injection into well segment
      R   =  radius of invasion

The rate of injection into  the segment can also be described by Darcy's Law:
           1.0
          0.8
     LU   0.6
     co
     Z
     to
     UJ
     06   04
          0.2
          0.0
             0.0
0.5
1.0         1.5          2.0

       RADIUS  (M)
                                                                          2.5
                                                             3.0
                            FIGURE 24.  Radial Response of EM-39.
                                              43

-------
                                         Q  = IxrBK                                        (24)

      K   =  hydraulic conductivity
      h    =  potentiometric head
      r    =  radial coordinate
 In each segment of the well, Eqs. (23) and (24) are equal and can be solved for the hydraulic conductivity of
 the segment:
                                       *  = IST ln  l^J                                  <25>
      h    = head in the well
      r,    = radius of well (5  cm for this study)
      /2    = radius where the  head is 0  (radius of effect of the well)

      The only way to measure the radius of effect is to have a large number of wells at increasing distances
 from the injection well. In addition to this, the radius of effect will be different for each segment of the well.
 Fortunately, the equation is very insensitive to this variable so that it is sufficient to make a knowledgeable
 estimate of the value. In this study, it was known to be less than 5 m because a well 5 m away from  the
 injection well was not affected  by the injection.

      The radius of invasion (Eq. 22) and the porosity (Eq. 21) as determined by the preceding methods can
 be used in conjunction  with Eq.  (25) to calculate  the hydraulic conductivity.


 Checks on  the Analysis Method

      The hydraulic conductivity  log calculated from the above procedure  (Eq. 25) can be  averaged to
 determine an average hydraulic conductivity for the  well. This can be compared to the hydraulic conductiv-
 ity calculated from the Thiem equation (Thiem, 1906, or Kruseman and De Ridder,  1983)
                                       K  =
                                            2nBh
      A close agreement between the two methods does not mean they are correct in an absolute sense
because it is necessary to assume the same radius of effect in both procedures. A wide discrepancy between
the two methods, however, indicates that there is a problem with the data or the method of analysis.

      The analysis can also be done at many different times and hence, radii of invasion. Near the well, the
invasion may be influenced by the alteration of the formation during drilling and well development. In the
analysis, it is possible to only consider radii of invasion that are sufficiently far from the well  that drilling
induced disturbances are negligible. This distance was chosen to be 40 cm. The  error m calculating the
radius of invasion increases with increasing distance due to the shape of the response curve. To limit this
error, onlv invasions of less than  120 cm were considered.
                                               44

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       One check on the uniformity of this zone (40 to 120 cm) and the analysis method is to calculate the
 hydraulic conductivity log at several times when the radius of invasion is within this zone. The hydraulic
 conductivity logs calculated at the different times should agree closely.

        The matrix electrical conductivity log should be strongly influenced by clays. Some, but not all. cla\s
 will also have a response on the natural gamma log.  Hence, one would expect to see a correlation between
 these two logs.

 Discussion of Results
 Well  E-3

       One well (E-3) is unique at the site because it is not screened through the  entire aquifer. Instead,
 there are seven screened intervals each  1  m long separated by a 2 m nonperforated cased interval. When
 fluid is injected into this well, it can only enter the formation adjacent to the screened intervals. Figure 25
 shows the induction logs that were run in this well. The location of the screened intervals is quite apparent.
 Because of the vertical averaging of the induction tool, the logs run after the tracer has invaded the forma-
 tion do not exactly overlie the background log in the nonscreened intervals. The vertical averaging is onlv a -..
 significant effect where there are rapid vertical changes m the radius of invasion. Fortunately, this is noi ilie
 case in fully screened wells. This example does graphically illustrate that the method is capable of detecting
 invaded zones.  This well was not  analyzed quantitatively because of the partial screening.

 Well E-6
      The analysis method was applied to a fully screened well  referred to as E-6. The induction tool verti-
 cally averages over an interval of about 2 m. To obtain an accurate induction log. the tracer must enter the
 formation through the entire vertical interval measured by the induction tool. This implies the well must he
 screened at least 1 m above and below the  sonde. For this reason, valid data cannot be obtained in the top
 and bottom 1 m of the screened interval. These sections have been omitted from the analysis. The induction
 logs are shown in Figure 26.

      The porosity log was calculated by the method discussed above and is shown in Figure 27. The average
 value for the log is 31 percent,  which agrees well with the value of 35 percent from other studies (iMolz ei al.,
 1986b). Between 57 and 58 m, the porosity is in the 35 to 40 percent range which, although high, is still
 realistic. Porosity values of 40 percent have been observed in cores at the site. The porosity values calculated
 by this method are a radially weighted average of the formation around the well. The weighting function is
 the same as the radial response  of the induction tool which was shown in Figure 24.
     The matrix electrical conductivity log is plotted with the natural gamma log in Figure  28. The matrix
electrical conductivity is largely a function of the clay content of the formation. The natural gamma log will
also respond to clays that contain radioisotopes such as  Potassium-40. The two correlate  very well, both
indicating several zones containing increased amounts of clay. The exception is at  55 to 56 m. where ihe
matrix conducuvity log indicates clay, but the natural gamma log does not. This could occur if the type of
clay in this interval does not produce detectable amounts of gamma radiation but still has a significant matrix

                                                45

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  FORM.  ELEC.  CONDUCTIVITY

     WELL  E3  (MMHO/M)

20   40    60    80   100  120
                                                     FORM.  ELEC.  CONDUCTIVITY

                                                         WELL E6  (MMHO/M)
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FIGURE 25.  Induction Logs of Well E-3 at
            Different Times After Injection.
                                         FIGURE 26.  Induction Logs of Well  E-6 at
                                                     Different Times After Injection.

-------
                POROSITY
    0.20  0.25   0.20   0.35  0.40   0.45
    40i
   42
   46
   48
X
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   50
   52
   54
  56
  58
  60
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 FIGURE 27.  Porosity Log of Well E-6.
                                               MATRIX ELEC. CONDUCTIVITY (MMHO/M)



                                                      NATURAL GAMMA  (CPS)
                                                40
                                                42
       0      10     20     30     40
                                               44
                                               46
                                               48
  X

  Q.
                                               52
                                               54
                                               56
                                               58
                                               60
                /

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                                                           I
                                                          y
                                                          I
FIGURE 28. Matrix Conductivity and Natural
            Gamma Log of Well E-6.
                                      47

-------
 electrical conductivity. Because the well was drilled with the mud rotary method, the existence of two i\pes
 of clays  was not noted during drilling (Molz et al., 1986b).

       During injection, the head in a second well, located 6 m away, was observed. No change in head was
 observed in this well. This provided an upper bound on the radius of effect of the well. Slightly arbitrariK.  a
 value of 4 m was assumed for the radius of effect. Because numerous induction logs were run, it is possible 10
 calculate hydraulic conductivities with data collected at several different times after the start of mjecuon.
 Hydraulic conductivity logs were calculated with Eq. (25) and are shown in Figure 29. The range of scatter
 at each depth gives an indication of the precision of the method. The agreement is  excellent except at 57 10
 59 m where there is a spread between the logs. The reason for this discrepancy is not known.

      The hydraulic conductivity log can be vertically averaged to calculate an average hydraulic conductiv-
 ity for the well. As mentioned earlier, the data from the SWET test can also be used to calculate the hydrau-
 lic conductivity of the well by the traditional Thiem method (Thiem.  1906. or Kruseman and De Ridder,
 1983). The two methods should agree. The hydraulic conductivity calculated with the Thiem method is 6  7
 m/d, the  value from vertically averaging the SWET test is 7.9 m/d. The agreement between the SWET test.
 and the traditional Thiem method reflects favorably on the former.

      A possible  source  of  the  discrepancy between the  vertically averaged SWET test and  the Thiem
 method is in the choice of values for a and m in Eq. (21). The effect of using a different choice of a and m
 can be determined by using values of a and m determined by Winsauer et al. (1952)  for use in friable
 sandstones. These values are 0.62 and  2.15, respectively. Although these values are not wholly suitable for
 the unconsolidated sands characteristic of the site, they can be used to give an insight into the effect of a and
 m on the final results. From Eq. (21)  we can see that substituting these values will increase the  a venue
 porosity from 31 percent to 37 percent. This will cause an increase m  the calculated average hydraulic
 conductivity (Eq. 25) from  7.9 m/d to 9.4 m/d. Hence, for both porosity and hydraulic conductsity.  the
 range of uncertainty caused by the uncertainty in a and m is small, even when a  different lithology is as-
 sumed. The only other value in the analysis that was not directly measured is the radius of effect. If a 6 m,
 instead of 4  m, radius of effect had been assumed, there would still be a 20 percent difference between  the
 two methods,  but both methods would yield values that were 10 percent higher.

      Figure 30 compares the results of the SWET test to the measurements of normalized hydraulic con-
 ductivity made by performing slug tests in packed intervals (Molz et al., 1989). The agreement is quite good

     The hydraulic conductivities calculated  by the SWET test are representative of the formation for a
 radius around the well that is roughly equivalent to the radius of effect of the  well. This radius is larger than
the radius of invasion because the radius of invasion is not only influenced by the hydraulic properties of the
portion of the formation that is invaded, but is also influenced by the  portion of the formation into which the
displaced  native fluid is forced. In this example, this was on the order of 4 m. Because most wells have a
disturbed  zone around them, techniques that have a shallow radius  of investigation will be inaccurate; the
SWET test minimizes these  problems.
                                               48

-------
                                                 0.0
                 UAX  SLUG  TEST
                 0.4        0.8       1.2
   40
   42
     HYD.  CONDUCTIVITY  (M/D)
    .0    5   10  15   20   25
                      • 33 WIN
                      + 40 WIN
                      * 48 WIN
                        59 M!N
       HYD.  CONDUCTIVITY (M/D)
       0    5    !0   15   20  25    30
                                                                 •  23 MIN
                                                                 +  40 MIN
                                                                 *  48 MIN
                                                                 A  59 MIN
                                                                 — SLUG "3
FIGURE 29.  Hydraulic Conductivity Logs
            of Well E-6.
FIGURE 30.  Hydraulic  Conductivity  Logs  and
            Packer Slug Test Results in Well E-6.
                                        49

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     HYDRAUjJC ^CONDUCTIVITY (M/D)

       FORM.  ELCC. COND.  (mSAO
        10  20 30 40  50 60  70 80
    60
      BASELINE /   31  MIN
              6  MIN
FIGURE 31. Induction Logs of Well E-7.
 Well E-7
      Due to equipment problems, it was not possible
 to run the SWET test in a third well, referred to as
 E-7, for a sufficiently long time for the tracer to in-
 vade the formation further than the radius of investi-
 gation of the induction tool. This prevents the use of
 the quantitative  analysis method described here.
 However, a qualitative analysis can be made from the
 formation electrical conductivity logs shown in Figure
 31, which compares the induction logs to the results
 of straddle packer slug tests. It is clear from this fig-
 ure that  fluid is invading the formation slowly at 47 to
 49 m and is invading rapidly at 44 to 46, 49 to 52.
 and 55 to 58 m. If a qualitative interpretation such as
 this is adequate, then the volume of tracer needed to
 be injected can be reduced.                      ;

 General Comments - Single-Well Electrical
     Tracer Method
     The  SWET method yields both  porosity  and
 hydraulic conductivity  logs. The results are in good
 agreement with other  methods. It is significant to
 note that this method is applicable in wells that have a
 nonconductive  casing and a disturbed annulus.  No
 other geophysical  technique for determining hydrau-
 lic conductivity  has demonstrated this capability.  An-
 other advantage of the SWET test is that the entire
 well is subjected to the same hydraulic head. Straddle
packer tests pressurize only a portion of the well  and
will be in error if there is leakage around the packer.
It is quite common for this to  happen because of a
poor packer seal or by leaking through the disturbed
formation/gravel pack around the well.
                                           50

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                                           SECTION 3

                           METHODS TO LOCATE CONTAMINANTS

 BACKGROUND
      As the awareness of the complexity of ground-water contamination increases, it is apparent that meth-
 ods are needed to rapidly determine the vertical and horizontal distribution of ground-water contamination.
 Although sampling methods are available to do this, sampling can be costly, particularly when it is necessary
 to determine the vertical distribution of contaminants. One class of contaminants that lends itself to rapid'
 detection is that which significantly alters the electrical conductivity of the formation. Most commonly, ilus
 is the result of an increase in the concentration of dissolved ions that are associated with the contamination.
 Common examples of this class of contaminants are brines and metal processing wastes. Low concentrations
 of organic compounds do not alter the electrical conductivity of the ground water enough to result m a
 sufficient change to  be useful for detection  purposes.
      Unfortunately, there are no methods available to directly measure the electrical conductivity of the
 pore fluid. It is possible to measure  the electrical conductivity of the fluid in the well bore, but because of
 vertical mixing and the influences of well design, the fluid in a well at a particular depth may not be represen-
 tative of the pore fluid at that depth. Instrumentation is available to measure the electrical conductivity of
 the formation that surrounds the well which, when used in conjunction with conceptual models of the electri-
 cal conduction by natural formations (see the discussion in the single-well electrical tracer method section of
 this report), can  be  used to determine the pore-fluid electrical conductivity.
     Until recently,  it was not possible to measure formation electrical conductivity in  small-diameier,
 PVC-cased wells. Recently,  instrumentation to do this has become commercially available. Because of the
 opportunities this new instrumentation creates and concerns regarding the quality of the data that are pro-
 duced, a portion of the cooperative agreement was spent evaluating this instrumentation. Then the applica-
 tions and  limitations of using measurements of formation electrical conductivity measurements to  detect
 ground-water contaminants  were considered.

 EVALUATION  OF  A SLIM  HOLE INDUCTION LOGGER
 Background
     To measure the electrical conductivity of a formation surrounding a cased well, it is necessary to  u<:e an
induction tool. Currently, there is only one manufacturer in North America that offers a unit which can he
                                               51

-------
 used in 2-mch diameter wells. Because of the interest this unit has generated in the ground-water commu-
 nity, the performance of it was evaluated.
 Theory of Operation

      The induction tool has a transmitter coil that emits a continuous wave 39.2 kHz electromagnetic smnal.
 This produces a primary field in the formation surrounding the borehole. The primary field, in turn, pro-
 duces a secondary field that is sensed by the receiver coil. The strength of the secondary field is a function of
 the formation conducmity (Hearst and Nelson, 1985). Because the measurement is made by induction
 methods, the unit will operate in a slotted or unslotted PVC- or Teflon-cased well. It will also operate in an
 open hole. It is adversely  affected if there is  any  metal within approximately 3 m of the instrument

      Two receiver coils are  used in the EM-39 at distances of 25 and 50 cm  from the transmitter coil
 (McNeil, 1986). The receiver coil at 25 cm facilitates instrument design by canceling the primary field and
 focusing the horizontal response. The  focusing reduces the influence of the borehole fluid  on the instru-
 ment. The location of the  coil that is 25 cm from  the  transmitter is considered  to be the location of the
 measurement. Because of  minor instrument drift,  the  instrument must be zeroed before each use. This '
 simple task is accomplished by holding the tool high in the air, which is a medium of zero conductivity, and
 making an adjustment on  the front panel so that  the instrument indicates a conductivity of zero.


 Calibration

      The  instrument is first-order calibrated by the manufacturer. A second-order correction is also  re-
 quired at conductivities above 300 mS/m (McNeil,  1986) and is explained with the documentation accom-
 panying the  instrument. However, if accurate absolute values are  critical for a specific  application, the
 calibration should be checked. To do this, it is necessary to place the instrument in a medium of known
 conductivity. This medium  must be homogeneous for the approximately 4-m high and 3-m diameter c\lm-
 dncal volume that is sensed by the instrument. This calibration can be easily accomplished in large bodies of
 water such as lakes. The electrical conductivity of the lake water can be measured with a calibrated fluid
 electrical conductivity meter.  A vertical  profile  of fluid electrical conductivity should be run to ensure that
 the lake is not stratified near the tool. As an example of this procedure, Figure 32 plots measured versus
 actual conductivities for four different lakes in  Nevada.  In this example, the measured conductivities were
 low by about 30 percent. The manufacturer has since corrected this problem, the  unit used for the remain-
 der of the work meets the manufacturers claim  of +5  percent accuracy.

      A secondary calibration procedure is available if only relative changes in conductivity are important.
The manufacturer can provide a calibration coil which, when held in a prescribed geometry, will produce a
predictable response from the instrument. By ensuring that the magnitude of this  effect is the same before
each survey, one is assured  that the instrument calibration has not changed. This procedure should be used
if the instrument will be used to monitor changes in conductivity over a period of time. It can also be used to
verify that a  previous absolute calibration is still valid.
                                               52

-------
          1000
                                                                                PYRAMID
                                                                                LAKE. NV
                                           LAHONTAN RESERVOIR. NV
10            30            100            300
       TRUE CONDUCTIVITY (mmho/m)
                                                                                     1000
                               FIGURE 32.  Calibration of EM-39.   .
Time Constant
     The instrument a\erages readings over about one second; hence, as a hole is logged, the measured
value at a given depth is an average over a short interval through which the tool has just passed. This causes a
displacement in the log. Figure 33 is an example of a log run up and down a well. Both directions were run at
the same logging speed of 7 m/mm. Note that the two logs are vertically shifted from the correct position.
which is halfway between the two. By repeating this measurement at several logging speeds.,the magnitude of
the shift can be determined as a function  of logging speed (Figure 34). A logging speed less than 5 m/mm
results in a shift of less than 10 cm between the log and the formation. If vertical positioning is critical for a
given application and  logging speeds are  high, the  logs should be adjusted to correct for this shift.

Temperature Effects
     Induction tools are susceptible to temperature-induced errors because of thermal-induced variations
m coil geometry. To evaluate the magnitude of this error for the  EM-39. the instrument was allowed to
                                              53

-------
                                      CONDUCTIVITY (MMHO/M)

                           0   100 200 300 400 500  600  700  800  900 1000 1100
                   I
                   a
                   FIGURE  33.  Logging Speed-Induced Shifts on Induction Logs.
temperature equilibrate on the surface and was adjusted to read a conductivity of zero when held in the air.
It was then placed in a lake which was 7°C colder than the air temperature. Figure 35 shows the drift in
apparent conductivity caused by the temperature change. After 20 minutes, the instrument was returned to
the surface. After allowing 20 minutes for the temperature to equilibrate to surface temperatures, the instru-
ment reading had returned to zero. Figure 35 indicates that it takes about two minutes for the instrument to
respond to temperature changes and about  15 minutes for the instrument to stabilize at a slightly hither
value. The magnitude of this change,  less than 0.3 mS/m/°C, is so small that in most situations temperature-
induced errors will be insignificant. If there is concern that temperature drift may be a problem, such as
logging extremely low conductivity formations in a hot environment, the stability of the instrument should he
checked before logging.
                                              54

-------
                                              SPEED SHIFT
              20.0
                               Measured Values
                               Fitted Line
                                             SPEED  (M/MIN)
               FIGURE 34.  Vertical Shift as a Function of Logging Speed for EM-39.
Vertical Response

     Like all borehole instruments, the EM-39 averages over a vertical interval. This averaging function
was calculated by the method described by Saito (1982) and is shown in Figure 36. The asymmetry in the
response is an effect of the coil geometry used to focus the instrument. When this response is convolved with
the true conductivity log, the conductivity log that is measured by the EM-39 is obtained. Figure 37 com-
pares the calculated response to an abrupt contact to field data for the same situation. The surface of a lake
was used for the contact. The  agreement is good and shows that an abrupt contact will be smoothed over
about aim interval.

     The response of the instrument to thin beds can be calculated in a similar way. Figure 38 shows the
response to a layer of varying thickness arbitrarily located at a depth of 20 m. The log was normalized by
                                              55

-------
                                         TEMPERATURE RESPONSE
                                         7° C CHANGE AT TIME 0
               10
           E    SH
          >
          ^
                6-
          O
          O   4-J
               2-1
               *  >
                         24      6      8     10     12     14     16     18     20
                                                TIME (MIN)
                            FIGURE 35.  Temperature Drift of EM-39.
 showing the percentage of maximum response to the high conductivity middle layer. When the layer is less
 than 4 m thick, the instrument does not accurately measure the conductivity of the layer because adjoining
 beds are also averaged into the measurement. Figure 39 shows the relation between layer thickness and
 response. From this plot, it can be seen that beds less than 1 m thick are poorly defined. For example, a bed
 of 0.5  m thick would have an apparent conductivity difference above the surrounding material that is only
 half of the true conducm ity difference. Figure 40 demonstrates how this can lead to an ambiguous suuaiion.
 Two models are considered, both of which have a background conductivity of 10 mS/m. The first case (»(ihd
 line) has a 1-m thick la>er with a conductivity of 70 mS/m. The second case (dashed line) has a 0.5-m ihick
 layer with a conductivity of 100 mS/m. The logs of the two different cases would be difficult to differentiate
 in the presence of geologic noise. The ambiguity also exists for resistive layers. Because of the poor instru-
 ment response to thin lavers, the instrument is not suitable for detecting thin layers of floating hydrocarbons.

 Borehole Influence
     A field test was performed to  investigate the influence of the borehole fluid.  A 15-cm diameter and
4-m long PVC pipe was suspended in a lake and  filled with fluids that had higher conductivities than the
lake. This is analogous to a fluid-filled borehole in a saturated homogeneous formation. Even when the fluid
                                              56

-------
                                            VERTICAL RESPONSE
        iii
        2
        2
            0.06
            0.05-
           0.04-
           0.03H
           0.02-
           0.01-
           0.00
                                     -2-101       2
                                         VERTICAL DISTANCE (M)
                       FIGURE 36. Vertical Averaging Function of EM-39.
in the pipe had a conductivity 100 times the conductivity of the lake water, the difference between the lake
conductivity and the apparent conductivity of the lake measured with the EM-39 in the borehole was less
than 5 percent. This error is comparable to the overall  accuracy of the instrument. The position of ihe
instrument in the pipe, either in the center or off to the side, had no effect on the apparent conductivity  The
conductivity of the lake was 10 mS/m, the conductivity of fluid in the pipe ranged from 10 mS/m to 1.000
mS/m. This also demonstrates that the measurement is  insensitive to  drilling-induced formation distur-
bance.
Comparison of Units

     To determine if differences exist between commercially available units, one well was logged with two
different units. Care was taken to ensure both units were properly zeroed and had reached thermal equilib-
rium. The logs (Figure 41) from the two units agree within the manufacturer's specifications of +5 percent
                                              57

-------
                                    RESPONSE TO ABRUPT CONTACT
                                   CALCULATED
                               *  MEASURED
                                    23496789   10
                                  APPARENT CONDUCTIVITY (mS/m)
                       FIGURE 37.  Response of EM-39 to Abrupt Contact.
Discussion - Evaluation of Induction Tool
     The EM-39 can effectively measure formation conductivity. When proper attention is paid to calibra-
tion, these measurements are accurate to within +5 percent. In most situations, the instrument is not af-
fected by temperature variations. The minimum bed thickness required by the instrument to  accurately
measure  electrical conductivity is 4 m. Thin beds can be detected if the conductivity contrast between
adjacent  beds is large enough. For layers thinner than 4 m, an ambiguity exists in determining the layer
thickness and intrinsic conductivity. The effect of borehole fluid on the measurement is negligible for wells
up to at least 15 cm  in diameter.
                                             58

-------
                       NORMALIZED
                        RESPONSE
        Q.
        UJ
        o
FIGURE 38. Response of EM-39 to Layers of Varying Thickness.
           Layer Thickness in Meters is Indicated.
                         59

-------
                              THIN LAYER RESPONSE
LU

CO
CO
HI
tr
                       1               2


                              LAYER THICKNESS (M)
         FIGURE 39. Response Versus Layer Thickness for EM-39.
                                60

-------
           FORMATION
          CONDUCTIVITY
             (mS/m)
1 R
I O
1 7
18
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     APPARENT
   CONDUCTIVITY
      (mS/m)

20   40    60   80   100
                                         \\
                                          \
FIGURE -JO.  Example of Ambiguous Interpretations of Induction Logs
            (dashed line - model 1; solid line - model 2).
                            61

-------
                       CONDUCTIVITY  (mS/m)
           0   100  200  300  400  500 600 700 800  900  1000
0
4
K 6
Q.
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FIGURE 41. Comparison of Logs from Two Different EM-39 Units.
                            62

-------
                                           SECTION 4

                     APPLICATION TO DETECTION OF CONTAMINANTS
 BACKGROUND
      To evaluate the usefulness of induction tools to locate contaminated intervals, a field investigation was
 conducted. The Pittman Lateral in Henderson, Nevada, was used as the study site. The site has been used
 by the U.S. Environmental Protection Agency and the Desert Research Institute to evaluate a wide varieu of
 geophysical and ground-water sampling techniques. The source of the contamination is an industrial com-
 plex located 2 km upgradient of the study area. The complex has been used intermittently for the lasi  40
 years and has produced a plume of inorganic and organic contamination. The most contaminated water has
 a significantly elevated electrical conductivity of 20,000 nmho/cm, compared to less contaminated water at
 the site that has a conductivity of 5,800 nhmo/cm. In 1983, 24 wells were drilled at 61-m intervals along the
 Pittman Lateral. The Lateral is approximately perpendicular to the local ground-water gradient. The wells
 were drilled using the dual-tube reverse rotary air method. The 20-cm holes were completed with 10-cm
 diameter  PVC. A  slotted screen was placed throughout the saturated zone. No backfill was used in the
 saturated  zone because the formation tended to collapse around the casing. This resulted in at least a 5-cm
 disturbed  zone around the well. The presence of a 5-cm disturbed zone is common around monitoring wells
 unless special precautions are taken during the drilling and completion of the well. From the drilling, u is
 known that the site consists of alluvial deposits 5 to 15m thick composed of sand and gravel  overlying a
 massive silt and clay deposit. The silt unit is part of the Tertiary Muddy Creek formation and is  assumed to
 act as  an  aquitard. Depth to  water varies from  3 to 8 m along the Lateral.

 Method  1 - Borehole Geophysical Logging
     Many, but certainly not all, ground-water contamination problems are associated with a pore fluid that
has an anomalously high electrical conductivity. This will influence the electrical conductivity of the forma-
tion, which can be measured with an induction log. Induction tools can only measure the electrical conduc-
tivity of the formation. Although this is influenced by the conductivity of the pore fluid, there are oilier
significant factors that also  influence formation  electrical conductivity. A simple model of the electrical
conductivity of the formation was presented in  the section  on the single-well electrical tracer tesi. For
convenience. Eq. (20a) has been repeated here:
                                              63

-------
          a   =  tortuosiiy. approximately 1 for unconsolidated sands (Jackson et al..  1978)
          m   =  cementation factor, approximately 1.5 for unconsolidated sands (Jackson et al , 1978)
          af   =  electrical conductivity of the pore fluid
          9   =  porosity

      This shows that m addition to the electrical conductivity of the pore fluid, changes in porosity, tor-
 tuosity, and the conductivity of the formation matrix will affect the formation electrical conductivity and
 hence, the induction log.

      Because of the disturbed zone around the well, porosity tools that have a limited depth of mvesticanon,
 such as gamma density and neutron absorption, will not yield accurate information in these wells. In addition
 to this, Nevada and many other states severely restrict the use of borehole tools with radioactive sources in
 water wells. At this site, sonic velocity logs could not be used to determine the porosity because of the low
 sonic velocity and the variability of compaction typical of unconsolidated formations. Because of these
 difficulties, porosity logs could not be obtained.

      The electrical conductivity of the matrix is dependent in a complicated and not fully understood way
 on the  quantity,  type and distribution of clays, and interaction between the clays and the pore fluid. A few
 percent of a clay with a high cation exchange capacity can cause a factor of 5 change in formation electrical
 conductivity (Kean et al..  1984). Because such a !ow concentration of clay is not easily quantified and the
 complex way  in which clays influence the formation electrical conductivity, it is difficult  to account for the
 influence of clays. Natural gamma logging will detect some types of clays when they are m high enough
 concentrations, but it is not possible to use a natural gamma log to  remove the influence of clays on the
 induction  log unless significant amounts of other data on the cation exchange capacity of the  clavs are
 available  (Waxman and  Smits, 1968). Because of these difficulties, it was not generally possible to quantify
 the influence of clays on induction logs.

      Induction logs were run to determine the formation electrical conductivity (Figure 42). The increase in
 formation electrical conductivity due to the contamination is seen in Wells 635 and 637 The contamination
 causes an overall increase of the electrical conductivity in these wells.  These wells are known to be the most
 contaminated from chemical analysis which were run as part of another project. Several of the induction Iocs
 (Wells 629. 633,  635. 637. and 639) show significant vertical variation in  the formation electrical conductiv-
ity.
      The vertical variations could be caused by vertical stratification of the contaminants or by vertical
changes in the formation. Natural gamma logs were run to see if these variations could be correlated with
lithology (Figure  43). Natural gamma logs measure the naturally occurring radiation emitted by the forma-
tion. Because clays tend to retain naturally occurring radioisotopes, an increase in the natural gamma log is
usually associated with an increase m clay content. The natural gamma logs show considerable vertical and
horizontal variability. However, a relationship that is common to the saturated zone in all the wells is that the

                                                 64

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            FORMATION ELECTRICAL CONDUCTIVITY
621    623    625   627    629   631    633    635   637    639
                                                                         641
ON
    0
     8-
    10-
    15-
    to-
        0   1200
            HORIZONTAL SCALE (m)
        I
                  100
                       200
                                              MUDDY CREEK FORMATION
                          FIGURE 42.  Induction Logs at Henderson, Nevada.

-------
                    NATURAL  GAMMA RADIATION

621    623    625   627    629   631    633    635    637    639    641
  6-
*"*  <

'>-'



t 10-

o




 18-






 20-
 MATURAL QAIIMA (CP8)


  60   300


      HORIZONTAL SCALE (m)
   I
   0
                 too
                        200
                                                     MUDDY CREEK FORMATION
                    FIGURE 43. Natural Gamma Logs at Henderson. Nevada.

-------
 natural gamma log and the induction log are inversely  correlated. This suggests that there is a  strong
 lithologic influence on the induction logs. A likely interpretation is that as the clay content increases (indi-
 cated by an increase in the  natural gamma log), the porosity of the formation decreases due to small clay
 panicles filling the pores. The decrease in porosity will cause the formation conductivity (indicated  by the
 induction log) to decrease because there is less fluid to conduct the electrical current. Because of the  high
 electrical conductivity of the pore  fluid, conduction by the  pore  fluid has  more of an influence on the
 formation conductivity than the matrix conductivity, which probably increases with the  addition  of clay to
 the formation.

      Because of the complicated relationships between clay content, natural gamma emissions and forma-
 tion conductivity, it is difficult to use the natural gamma log to qualitatively remove the  effects of lithology
 from the induction logs. The vertically averaged formation conductivity is higher in Wells 635  and 637,
 which are in the contaminant plume, and can be used as an indicator of contamination. The vertical vari-
 ation shown on the logs is interpreted to be more of an indication of lithology than the pore-fluid conductiv-
 ity. To verify this interpretation, two other methods were considered to sample the pore  fluid from discrete
 zones.
 Method 2 - Pumping of Discrete Intervals
      In an attempt to determine the vertical vanation of pore-fluid chemistry, a pump with an inflatable
 straddle packer was used. The packer has two 10-cm bladders located 10 cm apart surrounding the pump
 intake. The ability of the bladders to restrict the vertical movement of fluid in the casing was demonstrated
 by the inability of the pump to deliver fluid from a nonperforated segment of casing which was submersed in
 a water bath. Because of the well construction, it was suspected that piping could occur along the outside of
 the casing. This would permit fluid from the main portion of the borehole to enter the pump even though
 that interval was  packed-off. To evaluate this possibility, a tracer test was performed.
      The packer/pump assembly was set in the well and a tracer (Rhodamme-WT) was introduced into the
 well above the packer (Figure 44). The borehole fluid above the packer was circulated with a pump on the
 surface to evenly distribute the tracer in the borehole. The fluid in the circulating circuit was monitored to
 determine when this occurred. After the tracer was evenly distributed, the packer isolated pump was oper-
 ated continuously to collect samples.
      The concentration of the tracer in the sample indicates how much of the sample is coming from the
 portion of the well above the packer. Presumably, an equal portion of the sample is coming from below the
packer where there is no tracer. Therefore, to determine how much of the sample is coming from the well, it
is necessary to compare the concentration of the tracer in the well to twice the concentration of the tracer in
the sample. This is shown in Figure 45. The results indicate that after two minutes of pumping. 40 percent of
the sample comes from the packed-off portion of the well. After seven minutes, essentially all of the sample
is coming from the well by movement through the disturbed zone. This suggests a channel of high hydraulic
conductivity around the well bore. This could easily occur because of the 5-cm disturbed zone around the
well. The same situauon  would occur if a gravel pack, which would presumably be of higher hydraulic
conductivity than the  formation, had been installed.
                                               67

-------
Flow Around Packer
         Sample
         Pump
         Inlet
                                               Packers
                                     Disturbed Zone
      FIGURE 44. Experimental Setup for Tracer Test.
                           68

-------
                                     SAMPLE FROM PACKED INTERVAL
            0.0
                                                  TIME (MIN)
                          FIGURE 45.  Tracer Concentration Versus Time.
      The results of the tracer test suggest that it is not possible to obtain samples of short discrete intervals
without some kind of a seal at the ends of the intervals between the casing and the formation. Since most
monitoring wells are not completed with sealed-off intervals, the method of pumping with a straddle packer
has limited application.

Method 3 -  Dilution Sampling
      A third technique was developed to investigate the possibility of stratified pore fluid. With the dilution
sampling method, a segment of the well is isolated with a straddle packer. An electrically anomalous tracer is
then injected into the packed-off segment.  The tracer concentration decreases as dilution and advective
flow of ground water replace the fluid in the segment. When the fluid electrical conductivity in the secment
stabilizes, the fluid in the segment is considered to be representative of  the adjacent pore fluid. The same
packer as used in the pumping from discrete zone  method was used.

      Because of the level of contamination associated with the ground water at the study site, fresh water
(low electrical conductivity) was used as a tracer. The method was employed on Well 635  because the
alternative to the previously discussed interpretation is that this well is stratified. Measurements were made
                                              69

-------
 starting below the water surface and continuing along the entire screened length, at intervals between 0.5
 and 1.0 m. Experimental procedure consisted of setting the downhole tool at the desired depth, inflating the
 bladders, and noting the background electrical conductivity. An experiment was initiated by pumping fresh
 water into the borehole segment until the electrical conductivity within this segment decreased by at least a
 factor of two. The dilution of the fresh water by native pore fluid was then monitored at  the surface by
 recording the electrical conductivity as a function of time. An experiment was considered complete when
 the electrical conductivity of the borehole fluid in the packed-off segment had  stabilized.  When this oc-
 curred, a final measurement of electrical conductivity was made. The downhole tool was then removed from
 the well,  and the electrical conductivity cell and meter were calibrated using prepared standards. Thus,
 calibration curves were generated for each dilution experiment as a means of removing the effects of instru-
 ment drift from the final measurement of borehole fluid conductivity.

      Figure  46  is a plot of the dilution curves at varied depths within the test well  before the cell was
 calibrated at each depth. Notice that the conductivity has stabilized at the end of each measurement. The
 different recovery rates are  due to different rates of transport of the  tracer from the borehole. Figure 47
 compares the pore-fluid electrical conductivity to the formation electrical conductivity. The  final value for
the pore-fluid electrical conductivity as measured at each depth interval is within +5 percent  of the average '
value. This is considered to be within the error range of the measurement technique. This indicates that: 1)
either the stratification of electrically anomalous pore fluid is minimal; or 2) that the movement of fluid
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^ S^ r ^ 	 	 —
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     FIGURE 46.  Dilution Curves Versus Time for Various Depths for Dilution Sampling Method.
                                               70

-------
                     2
                     I
                     111
                     Q
                         2-
                         4-
6-
                         8-
                        10-
                        12-
                                       ELECTRICAL CONDUCTIVITY
                                   200    400     600     800    1000    1200
                                          FORMATION (mS/m)
                                          PORE FLUID (>iS/m)
    FIGURE 47.  Comparison of Electrical Conductivity of the Pore Fluid and Formation in Well 635.
through the disturbed zone surrounding the borehole is such that any pore-fluid stratification is masked.
Given the short length of the wells and the geologic environment, it is considered unlikely that significant
vertical differences in head existed in the interval adjacent to the well. This reduces the possibility of vertical
mixing in the disturbed  zone. Hence, the pore fluid is not considered to have  significant stratification.
                                               71

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                                           SECTION 5

                                   INDUCED POLARIZATION

 BACKGROUND
      Due to the large influence of clays on log interpretation, it was desirable to investigate the use of an
 additional clay indicator log besides the natural gamma log.  Ideally, such a  log would have a depth of"
 investigation similar to the induction log and would be sensitive to the electrical influence of clays. The
 Induced Polarization (IP) log meets this criteria and, hence, was selected for consideration.

      The  IP effect is an electrical phenomenon that can be thought of as the ability of the  formation to
 temporarily store electrical energy. In environments that do not have metallic mineralization, the predomi-
 nate cause of the IP effect is membrane polarization. If electrical current is applied across a fluid-formation
 contact, the positive ions are attracted towards the contact and the negative  ions are repelled from the
 contact. After the current is turned off. the ions take a short amount of time to return to the initial condition.
 During this time,  a current is created in the formation  by the movement of the ions. The process is strongly
 dependent on the size of the pores through which the current is being conducted (Telford et al , 1976). The
 membrane IP effect is largest when clay minerals with small pore sizes are present (Keller and Fnschknecht,
 1966).  The IP effect is related to cation exchange capacity (Telford et al., 1976), which also controls the
 electrical conductivity of clay-bearing formations (Waxman and Smits. 1968). The relationship between
 clay content and IP is nonlinear, reaching a maximum value at some  percentage  clay content and then
 decreasing with additional clay  content. The percentage  of clay corresponding to the IP maximum is in-
 fluenced by the pore-fluid chemistry, and clay type and distribution  (Roy and  Elliott. 1980; Worthmgton
 and Collar. 1982; Vacquier et al., 1957). No model has been developed that can qualitatively account for all
 of these effects without the use of extensive measurements on core samples (Park and Dickey,  1989; Atwa-
 ter. 1986). Negative IP signatures occur when the current after turn-off flows in the opposite direction of the
 applied current and can occur when there are polarizable bodies with limited dimensions (Sumner. 1976;
 Benin.  1968; Nabighian and Elliot, 1976; Roy  and Elliot. 1980).

     The IP effect can be measured in the frequency domain by measuring  the formation  resistivity  at
several different frequencies. It can also be measured in the time domain by measuring the decay of current
after a direct current is abruptly  turned off. In the time domain, results are commonly expressed as charge-

                                              72

-------
 ability, which is the percentage of current remaining during a given time window after the abrupt turn-off  In
 theory, the two measurement methods give identical results, but due to practical limitations on the type and
 amount of data that can be collected, it is generally not possible to directly transform field data from one
 form to the other.

 INSTRUMENTATION                                          A':,.,
                                                                 • A f fty
      For the purposes of evaluating the borehole IP method for use in sfialtow contaminate investigations it
 was desirable for the instrumentation to have the following characteristics: 1) a depth of investigation com-
 parable to a readily as a liable induction  tool; and  2) nonpolanzmg electrodes. The first requirement was
 necessary  so that the  induction logs  and IP logs could be readily compared. To meet this in the highly
 conductive fluids (up  to 18.000  uS/cm) at the  Henderson study site  (see Section 4), it was desirable  to
 separate the electrodes  from one  another with an inflatable  packer  to block current flow through the
 borehole. The nonpolanzmg potential electrodes were desirable because of the large concentration of metal
 ions in the fluid at the Henderson study site and the uncertainty to their influence on traditional lead elec-
 trodes. Electrochemical effects across the porous electrodes were reduced by stacking the signal with alter-
 nating polarity.                                                                                   :-

      A review of commercially available IP borehole tools determined  that such a tool did  not exist. Com-
 mercially available tools had  lead electrodes with no way to block current  flow in the borehole.  It was
 determined that an IP tool could be constructed that met the desired criteria for  less than the cost of the
 commercially available tools; primarily because the commercial tools were designed for greater depths and
 pressures than required for this project.

      A tool was constructed that was based on a Wenner array with an "a" spacing of 50 cm. Nonpolanzmg
 porous pots (Cu-Cu2SO4) were used for the potential electrodes,  lead was used for the current electrodes.
 Electrical conduction through the borehole fluid was minimized by the use of inflatable packers  which
 isolated the electrodes. The wires to the potential  electrodes and the wires to the current electrodes were run
 in separate shielded leads which were held 6 cm apart by nylon spacers.  A Scmtrex IPC-9/200 time domain
 transmitter and IPR-10A receiver was used. To make a measurement,  it was first  necessary to ensure that
 the tool was located in a perforated section of casing. This was accomplished by use of a preliminary survey
 with a borehole television camera. The tool was lowered to the desired depth, the packers were inflated, and
 and the IP response was measured with the Scmtrex unit on the surface. Although this tool is not suitable for
 deep holes, it was felt that the use of the  packers and porous pots would diminish problems  related to
excessive current flow in the borehole  and electrode polarization. The time windows during which the char-
geability was measured are listed in Table  2.

 FIELD EFFORT
     Two wells (633A  and 633D) were selected  for study because they have long perforated intervals and
interesting stratigraphy. Natural gamma, induction,  and IP logs were run in both wells (Figures 48 and 49),
which are located 15 m apart.  Six IP logs, one for each time window, are plotted for each well. The major

                                                73

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                     TABLE 2.  INDUCED POLARIZATION TIME WINDOWS
                   Window                  Stan (ms)                 End (ms)
                                               130                       390
                                               390                       650
                                               650                       910
                                               910                      1170
                                              1170                      1430
                                              1430                      1690
 features on all the logs occur in both wells, indicating that measurements are representative of the formation.
 Minor differences between the logs in the two wells are  expected because of spatial variability.
      The natural gamma and induction logs clearly show the occurrence of separate units. On the IP log. it
 is more difficult  to discern separate units.  In addition, the IP log does not correlate well with either the
 induction or the natural gamma log. If it were not for the correlation of the IP log between the two wells, the:
 reliability of the  IP log would be in question.  Had additional funds been available, other electrode arrays
 would have been tried which may have affected the ability of the IP system to identify units.
      To investigate the influence of a zone with a different pore-fluid conductivity than the surrounding
 formation, an injection experiment was performed. A half of a cubic meter of water with an electrical
 conductivity of 490 jiS/cm was injected between two inflated packers in the interval 8.5 to 9.7 meters. The
 native pore fluid has an electrical conductivity on the order of 12,000 jiS/cm. Induction logs run before and
 after the injection (Figure 50) clearly show that the injected fluid has entered the formation and displaced
 the native pore fluid. This has resulted in the change in formation electrical conductivity, which is noted on
 the induction logs in this interval.  Some smearing of the  injection  zone has occurred on the logs due to
 vertical fluid flow in the formation during injection and vertical averaging by the induction tool.
      The IP log  was run three times before the injection  to assure that the measurement was repeatable
 (Figure 51). Some drift is evident, but the form of the log is consistent for all three runs. After the injection,
 the IP log was run again (Figure 51). The major difference  between the before and after injection logs is the
 development of a trough centered on the injection zone which is bounded by two peaks. A lower IP response
 was noted below 13 m after the injection, but this is attributed to instrument drift because the general shape
 of the log is unchanged. This is an analogous geometry to the IP effect of a surface Wenner array traversing a
 low conductivity vertical dike. This was modeled by Elliot (1971) and was found to produce the same effect
 observed in  this study of a trough  bordered by peaks.

      Both examples demonstrate the complexity of interpreting the  IP log. The injection example demon-
strates the significant dependence of IP on pore fluid conductivity.  This makes it difficult to separate the
effect of changes in clay content and type  from  changes  in pore-fluid chemistry. This difficulty is com-
pounded because of the nonlinear complex influence of clay and pore-fluid chemistry (Worthmgton and
Collar. 1982) Both of the above examples demonstrate that IP effect is also strongly influenced not just by
                                               74

-------
          CHARGEABILITY (Vs/Vp)


      0   10  20   30  40   50  60
    6
    10
    12
E   14
o.
ui
O   16
   18
   20.
   22-
   24-
Tlme Window

\\//7/
                                           NATURAL GAMMA (CPS)


                                           75  100  125  150  175  200
CONDUCTIVITY (mS/m)
                                             24-
                                                                                         100    200    300    400    500
                                                                                      24-
                                           FIGURE 48.   Log Suite for Well 633A.

-------
          CHARGEABILITY (Vs/Vp)

      0   10   20   30   40  50   60
    6
    10
    12
   14
t
LU
Q  16
   18-
   20-
   22-
   24-
NATURAL GAMMA (CPS)

75  100  125  150  175 200
     CONDUCTIVITY (mS/m)

100   200    300    400    500
                                                                                          6
                                                                                         10
                                                                                         12
                                              24-
                                I

                                EL
                                111
                                Q   16
                                                                                        18
                                                                                        20
                                                                                        22-
                                                                                        24-
                                           FIGURE 49.   Log Suite lor Well 63313.

-------
450'
400-
                      _L
                              _L
                                       _L
        	 PRE INJECTION
        - ~ 20 MINUTES AFTER INJECTION
        - - 120 MINUTES AFTER INJECTION
                     10
                                   DEPTH (m)

            FIGURE 50.  Induction Logs Before and After Injection.
40
           POST INJECTION
           PRE INJECTION (multiple runs)
10
                                                                        16
      FIGURE 51. Induced Polarization Logs Before and After Injection.
                                  77

-------
 the formation adjacent to the logging tool, but also by the contrast to units above and below the tool. This
 explains why the IP log does not indicate similar units as the induction and natural gamma log; instead, it is
 strongly influenced by the contrast of adjoining units.

 DISCUSSION
      This work has highlighted several significant shortcomings of the IP log m relation to shallow ground-
 water contamination investigations. A significant limitation is the need for a nonconductive perforated cas-
 ing or open hole. Most monitoring wells are quickly cased with short-perforated intervals, which restricts the
 utility of the borehole IP method. By  contrast, the induction and natural gamma logs can be  used m a
 nonperforated. nonconductive casing. The IP response of a formation is also dependent on a wide number
 of parameters (clay type, content and distribution; pore-fluid chemistry; adjacent units), which make a
 unique interpretation very difficult even when other logs are available. Some investigations have sorted out
 these effects for specific units by the use of extensive core studies, but it is not clear that this approach is cost
effective for the shallow wells typical  of ground-water  contamination investigations. Because ihe results of
this initial work were  not encouraged,  the  IP method was not pursued  further during  this project.
                                              78

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                                            SECTION 6

                                         CONCLUSIONS

      For the determination of hydraulic properties in unconsolidated formations, the borehole geoph>sical
 methods that are available are limited. The use of Stoneley wave attenuation methods to determine hydrau-
 lic conductivity is limited by the need to utilize a sonic tool with a source of considerably lower frequency
 than is commercially available. This method  is also very equipment intensive and the presence of casing
 creates additional difficulties. Methods that utilize the natural flow of fluid through the well to determine the
 hydraulic conductivity of the formation were  not successful because of the influence of near-hole drilling
 disturbance and  the difficulties of measuring small changes in  the flow through the well.

      The single-well electrical tracer method, which was developed as part of this cooperative agreement,
 offers several advantages over the other methods  that were considered, and other well known conventional
 methods as well. The single-well electrical tracer method has the ability to determine the  vertical distribution
 of both hydraulic conductivity and porosity, even in wells with a large disturbed annulus, and can be used in
 unconsolidated formations, even if clays are  present.

      It is difficult to determine the electrical conductivity of the pore fluid if the formation contains clays.
The combination  of the natural gamma and induction log can be used to suggest zones of varying pore fluid.
but does not yield quantitative values of the pore-fluid electrical conductivity. The induced polarization log
is of limited value because of the need for a nonconductive perforated casing or open hole, and because of
the many interdependent parameters which effect it. The dilution sampling method, although tedious, is an
effective method  to determine pore-fluid properties in a perforated or open hole.
                                               79

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