>EPA
United States Office of
Environmental Protection Drinking Water
Agency WH-550E
Washington, DC 20460
EPA 570/9-87-004
August, 1988
Technical Assistance Manual:
Formation Testing, Procedures,
Applications, Equipment and
Specifications Related to
Injection Wells
/\
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EPA 570/9-87-004
AUGUST, 1988
TECHNICAL ASSISTANCE DOCUMENT:
FORMATION TESTING, PROCEDURES,
APPLICATION, EQUIPMENT AND
SPECIFICATIONS RELATED TO
INJECTION WELLS.
Project Manager:
Mr. Mario Salazar, Environmental Engineer
Chairman QA Workgroup, UIC Branch
Office of Drinking Water
State Program Division, WH-550-E
U.S. Environmental Protection Agency
401 M St., SW
Washington, DC 20460
Final editing done by:
Viking Systems International, Inc.
11480 Sunset Hills Road,Reston, VA. 22090,
Document Reference # 8422-040-94002
Tel. No. (703) 471-0883
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DISCLAIMER
This report has been assembled and presented for informa-
tional purposes only. Mention of specific companies, trade name
or commercial products does not constitute endorsement or
recommendation for use.
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CONTENTS
Page
FIGURES i
TABLES ii
INTRODUCTION 1
PUMPING OR AQUIFER TEST 8
Definitions & Introductions 8
General Procedure 8
Data Analysis 9
Test Design 9
SLUG TESTS > 10
DRILL-STEM TESTING 14
Introduction 14
Surface Equipment 14
Procedure 22
Results . 23
Qualitative Chart Analysis 27
WIRELINE FORMATION TESTING 34
SAMPLING OF FORMATION COMPONENTS 37
Formation Matrix Samples 37
Formation Fluid Samples 40
INJECTIVITY 43
FRACTURE GRADIENTS 46
GEOPHYSICAL LOGGING 48
Introduction 48
Resistivity Logs 48
Acoustical Transmission Logs 49
Gamma-Gamma Logs 50
Neutron Logging 50
Nuclear Magnetic Logs 50
Spinner Logs 51
Spontaneous Potential 51
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CONTENTS
(continued)
REFERENCES
APPENDICES
APPENDIX A Aquifer Tests
APPENDIX B Determination of Transmissibility by Using the
Slug Test Method
o Chemical Resistance of the SE-1000 Pressure
Transducer (Page B-7)
o Performance Information for the Model 570
Transducer (pages B-8 through B-11)
APPENDIX C Example of Evaluation of Constants
APPENDIX D Interpretation of Formation Test Pressure Charts
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FIGURES
Elgure Page
1 Schematic arrangement of surface equipment
for a drill stem test (after Gatlin 1960) 16
2 Drill-stem testing tools (after EPA, 1982) 17
3 Coil spring pressure recorder (after Welge,
1981) 20
•
4 Bourdon tube pressure recorder (after Welge,
1981) 21
5 Fluid passage during a formation test
(Welge, 1981) 24
6 Schematic diagram of a drill-stem test
(EPA, 1982) 25
o Pressure - Time Graphs for Various Positions
of Recorder and Packers (Fig. 6A-6E) 28
7 Schematic diagram of a wireline formation-
testing pressure curve (after Smolen, 1977). 36
8 Step-rate injectivity data plot (Guerard,
Publication M1 3) 44
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TABLES
lable Page
1 EPA Classification and Types of
Injection Wells 2
2A Comparison of Formation Testing Methods 3
2B Comparison of Borehole Geophysical
Formation Testing Methods 5
3 Typical Pressure'Transducer Sampling
Schedule 12
4 Methods for Determining Formation Test
Validity Through Qualitative Examination
of Pressure Charts 31
5 Comparison of Formation Component
Sampling Methods 38
ii
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ACKNOWLEDGEMENTS
The first draft of this document was prepared by Mr. John
Mentz, Mr. Michael Edelman, Mr. Chris Agoglia, and Ms. Ruth Ryan
of SMC Martin Inc., Valley Forge, Pennsylvania, under EPA
Contract No. 68-01-6288. The data has been extensively revised
by the EPA appointed UIC Quality Assurance (UIC-QA) Work Group
Members. The authors would like to acknowledge the invaluable
input provided by Mr. Charles Koch of the North Dakota Oil and
Gas Division; Mr. Paul Osborne of EPA Region VIII, Denver,
Colorado; and Wycon Chemical Corporation, Houston, Texas. The
UIC-QA members include:
Philip Baca* (505) 827-5812
New Mexico Oil Conservation Division
P. 0. Box 2088
Santa Fe, NM 87501
Gene Coker (404) 881-3866
U.S. EPA Region IV, GWS, WSB
345 Courtland* Street, NE
Atlanta, GA 30365
John Creech*
Dupont Company
Box 3269
Beaumont, TX 77704
Richard Ginn* (512) 445-1227
Railroad Commission of Texas
P. 0. Drawer 12967
Capital Station
Austin, TX 78711
Fred Hille (601) 961-5171
Bureau of Pollution Control
Mississippi Department of Natural
Resources
P. 0. Box 10385
Jackson, MS 39209.
Juanita Hillman (303) 236-5065
U.S. EPA Region VIII, 8ES
i860 Lincoln Street
Denver, CO 80295
/ Linda Kirkland* (214) 767-9791
EPA Region VI,
Office of Quality Assurance
1201 Elm Street
Dallas, TX 75270
* No longer in workgroup
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ACKNOWLEDGEMENTS
(continued)
Charles A. Koch**
North Dakota Industrial Commission
900 East Blvd
Bismarck, ND 58505
Bernie Orenstein*
EPA Region V, 5WD
230 South Dearborn Street
Chicago, IL 60604
Paul Osborne**
U.S. EPA Region VIII,
Water Division (WM-DW)
i860 Lincoln Street
Denver, CO 80295
Irwin Pomerantz*
Criteria and Standards
Division, ODW (WH-550)
EPA Headquarters
401 M Street, SW
Washington, D C 20460.
Joseph Roesler
U. S. EPA EMSL
26 W. St. Clair
Cincinnati, OH 45268
Mario Salazar
U. S. EPA Headquarters
401 M Street
Washington, DC 20460
Jeff VanEe
U. S. EPA, EMSL-LV, AMD, AMW
P. 0. Box 10527
Las Vegas, NV 89114
Ron Van Wyk*
U. S. EPA Region VI, WSB (6W-SG)
1201 Elm Street
Dallas, TX 75270.
(701) 224-2969
(312) 886-1500
(303) 564-1418
(202) 382-3026
(513) 684-7268
(202) 382-5561
(702) 798-2367
(214) 767-2774
* No longer in workgroup
** Mainly responsible for TAD corrections
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INTRODUCTION
The Underground Injection Control (UIC) program was deve-
loped by the U. S. Environmental Protection Agency to protect
Underground Sources of Drinking Water (USDW) from contamination
due to injection operations. In order to regulate injection
wells efficiently, EPA divided all injection practices into 5
classes. A description of each class of well is given in
Table 1.
Regardless of the type of fluid injected into a particular
formation, an important consideration is the ability of the
formation to accept the fluids. Factors that must be considered
when injecting into a formation include chemical compatibility
between formation and injection fluids, propagation of existing
fractures caused by increasing injection pressure, migration of
injected fluids into hydraulically connected zones, and fluid
migration within the injection formation. Knowledge of the
properties of the formation is needed in order to assess these
elements.
The development of both petroleum and water resources has
resulted in the development of tests or techniques which can be
used to provide knowledge of injection formation characteristics.
These characteristics can be defined by use of the tests
described in Table 2A, comparison of Formation Testing Methods,
and 2B, Comparison of Borehole Geophysical Formation Testing
Methods.
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TABLE 1
EPA CLASSIFICATIONS AND TYPES OF INJECTION WELLS
Well Code
CLASS I
11
1M
1W
1X
CLASS II
2A
2D
2H
2R
2X
CLASS III
3G
3S
3U
3X
CLASS IV
4H
CLASS
5A
5B
5D
5F
5G
51
5N
5R
5S
5T
5W
5X
5Z
V
Primary Function
of Injection Well
INDUSTRIAL and MUNICIPAL DISPOSAL WELLS THAT INJECT
BELOW DEEPEST UNDERGROUND SOURCE OF DRINKING WATER.
Nonhazardous Industrial Disposal Well
Nonhazardous Municipal Disposal Well
Hazardous Waste Disposal Well
Other Class I Wells
OIL AND GAS PRODUCTION AND STORAGE-RELATED INJECTION
WELLS
Annular Injection Well
Produced Fluid Disposal Well
Hydrocarbon Storage Well
Enhanced Recovery Injection Well
Other Class II Wells
SPECIAL PROCESS INJECTION WELLS
Solution Mining Well
Sulfate Mining Well by Frasch Process
Uranium Mine Well
Other Class III Wells
WELLS THAT INJECT HAZARDOUS OR RADIOACTIVE WASTE
INTO, ABOVE OR BETWEEN UNDERGROUND SOURCES OF
DRINKING WATER (BANNED AS OF MAY 1985 OR BEFORE)
Hazardous or Radioactive Waste Injection Well
ALL OTHER WELLS THAT INJECT INTO OR ABOVE AN UNDER-
GROUND SOURCE OF DRINKING WATER.
Air Conditioning/Cooling Water Return Well
Salinity Barrier Well
Stormwater Drainage Well
Agricultural Drainage Well
Other Drainage Wells
In Situ Gasification Wells
Nuclear Waste Disposal or Storage Well
Recharge Well
Subsidence Control Well
Geothermal Well
Waste Disposal Well
Other Class V Wells
Industrial Wastewater Disposal Well
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TABLE 2A
COMPARISON OF FORMATION TESTING METHODS
Aqui-
fer
Test
Slug
Test
Measures Transmiss-
ivity (T) of forma-
tion. T is a mea-
sure of fluid rate
per unit thickness
in an aquifer and
is related to per-
meability. Can
also aid in eva-
luation of well
and pumping plant
efficiency. Gives
measure of storage
of the formation.
Provides formation
quality
information.
Measures T of a
small part of a
formation.
(See above.)
Drill- Provides:
Stem
Test
An estimate of
average effect-
ive formation
permeability.
A measure of
static reservoir
pressure.
A determination
of productiqn
rate and flftw
capacity.
An evaluation
of formation
damage near the
bore hole.
Advantages
Provides accurate
information on aq-
uifer characteris-
tics. The test
places stress on a
large volume of an
aquifer. This pro-
vides more knowled-
ge of the aquifer
than tests which
affect smaller
volumes.
Requires a short
period of time
(a'few hours).
Test does not
produce water
to discharge.
Requires a mode-
rate amount of time
(half day) to per-
form test. Yields
useful permeability
information before
well is completed.
Affects a large
volume of a forma-
tion yielding more
representative
values of the
formation on a
large scale. Pro-
vides a sample of
formation fluid.
Disadvantages
Produces a large
amount of water
which must be
prevented from
seeping back in
to surficial
aquifer. Requires
a large amount
of time (ca 96
hours in general)
and labor inten-
sive. Requires
construction of
observation wells
in some cases.
Highly suscepti-
ble to error.
Provides limited
information on a
small portion of
an aquifer. No
storage capacity
information.
Subject to in-
terference by
mudcake on the
wellbore. This
results in a
lowered permea-
bility estimate.
Fluid samples
may be contami-
nated by drill-
ing muds.
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TABLE 2A
COMPARISON OF FORMATION TESTING METHODS
(Continued)
Method Uses
Advantages
Disadvantages
Wire- Can be used to es-
line timate formation
Forma- permeability by
tion measuring format-
Test ion pressure
build-up similar
to drill-stem
testing. Provides
formation quality
information.
Not susceptible to
mudcake interference,
Capable of measuring
pressure build-up in
many formations dur-
ing one trip in the
well. Can take at
least two fluid
samples during a
single trip. Is
quick and in-
expensive and can
be a good alterna-
tive to DST.
More complicated
interpretation
than drill stem
tests. Measures
permeability of
a small volume
of the formation
in contrast to
drill-stem
tests.
- Interpretat-
ions of data
are semi-
qualitative
- Information
is inferior
to DST.
- Estimation of
permeability
is uncertain.
- Skin factor
cannot be
estimated.
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TABLE 2B
COMPARISON OF BOREHOLE GEOPHYSICAL FORMATION TESTING METHODS
Log Name Advantages
Resisti-
vity
Accousti-
cal Trans-
mission
Logs
(sonic
ampli-
tude
Logs)
Allows the de-
termination of
boundaries be-
tween permeable
formations and
permits estimate
of hole size and
mud cake. Best
for direct meas-
urement of forma-
tion resistivity
and bed thickness
in uncased wells.
Provides infor-
mation on 'forma-
tion lithology,
formation fluid
resistivity,
degree of forma-
tion saturation
and porosity.
Determine the
porosity and
Lithology of the
formation when
used in conjunc-
tion with neutron
and density logs.
Provide accurate
porosity data in
rugose bore hole.
Locate zones of
abnormal press-
ures. Determine
secondary
porosity when
used with other
logging tools.
Provide a cali-
bration of
seismic records
by providing a
time versus depth
calibration.
Disadvantages
- Not useful in
cased holes.
- well size
could affect
accuracy of
Measurements.
- Not very use-
ful in dry
beds or non-
conductive
fluids.
Bunning Time
A) Half hour set-
up time.
B) 10-25 ft/min.
Dependent upon
rock matrix,
formation fluid
and porosity.
Cycle skipping
may occur in
well where the
diameter is a
problem (wash
outs, large
vugs, etc.)
A) Half hour set-
up time
B) Run slower than
electrical or
radiation logs.
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TABLE 2B
(Continued)
COMPARISON OF BOREHOLE GEOPHYSICAL FORMATION TESTING METHODS
Name Advantages
Gamma- Measures appa-
Gamma rent density of
Logs formations adja-
(Density cent to borehole.
Logs) Determines litho-
logy and porosity,
Determines boun-
daries between
permeable and non
permeable beds.
Useful in cased
and uncased
holes. In' cased
holes is used
to identify
potentially
productive zones
that cannot be
detected by
other logs.
Neutron Measures hydro-
gen concentra-
tion in the
formation.
Determines
lithogy and
porosity when
used in combina-
tion with other
tools. Can
estimate quali-
tatively percent-
age of shale and
ratio of sand to
shale.
Pulse Able to provide
Neutron quantitative
information of
fluid movement
behind the
casing.
Disadvantages
Dependent on
lithology, poro-
sity and forma-
tion fluid.
A) Half hour set-
up time.
B) 10-25 ft/min.
A) Half hour set-
up time.
B) 20 ft/min.
A) Stationary
B) 3-5 minutes at
each measuring
point.
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TABLE 2B
(Continued)
COMPARISON OF BOREHOLE GEOPHYSICAL FORMATION TESTING METHODS
Log Name Advantages
Spontan-
eous
Potential
Nuclear
Magnetic
Spinner
Log
Identifies per-
meable beds.
Determines boun-
daries between
permeable and
nonpermeable
beds.
Analysis of the
SP may yield an
approximation
of the value of
the formation
water salinity
(an important
parameter in
log analysis).
Used to estimate
qualitatively,
permeability and
porosity of sub-
surface forma-
tions. Measures
strength and
decay of induced
magnetic field.
Easily distin-
quishes between
hydro-carbons
and water.
Used to detect
leaks in casing,
tubing or packer.
Determines where
injected fluid
is lost into
borehole. Deter-
mines formation
receptivity to
injected fluids.
Disadvantages
Affected by the
clay content of
the formation.
Functions only
in uncased
holes.
Does not funct-
ion in dry beds
or beds con-
taining non-
conductive fluids
such as water
and oil.
Requires highly
sophisticated
equipment.
Running Iim§
A) Half hour set-
up time.
B) 10-25 ft/min.
No data
Cannot detect
fluid movement
adjacent to
wellbore.
Variable: Depends
on tool type and
number of zones to
be investigated.
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PUMPING OR AQUIFER TESTS
DEFINITIONS AND INTRODUCTION
In recent years, an increased interest in ground-water
resources throughout the United States has arisen from the
development of public and domestic uses of ground water, the
impact of mining and industrial activities on ground water, and
an increase in studies of already contaminated aquifers. In
order for the various public and private individuals involved in
decision making activities on ground water usages to assess the
supply available it is necessary to conduct studies of the
aquifers or pumping facilities to develop reliable information on
aquifer characteristics. The major types of studies usually
conducted are pump or aquifer tests.
This document outlines the types of planning, equipment, and
test procedures that should be followed in designing an accurate
aquifer test. If the proper equipment and the right procedures
are used, and if all the ordinary precautions are adopted, the
resulting data should be considered reliable. The aquifer
information obtained from analyzing the data will depend on the
method of analysis.
In general, pump type tests have been used for water well
development. More often, injectivity, slug and drill stem tests
are used to determine characteristics of "injection" formations.
GENERAL PROCEDURE
An aquifer test is the most common method of evaluating
formation characteristics. A properly conducted test involves
two stages: 1) a step test to determine the discharge rate for
the aquifer followed by a rest period during which the well is
allowed to come to equilibrium; and 2) a discharge period (when
the well is pumped) followed by a second rest or recovery period.
During the first stage of an aquifer test, the well is
pumped at a constant discharge rate. Water levels are measured
at selected observation wells at predetermined time intervals
during the test. These data are used to calculate the hydraulic
characteristics and to evaluate the hydrologic character of the
aquifer. During the second recovery period, water level recovery
readings are taken in the well at pre-determined time intervals.
These measurements provide important supplemental data as well as
a check on the pumping phase data. Usually, the coefficient
values calculated from the pumping and recovery data are
averaged.
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DATA ANALYSIS
Within the last two decades, hydrologists have developed a
number of graphical and mathematical methods to analyze pump test
data. Kruseman and Ridder (1979) give approximately 50 methods
for analyzing pump test data. In most cases, each method is
applicable to a specific set of hydro-geological circumstances.
The Theis logarithmic and the Jacob straight line semi-
logarithmic graphical solutions are used to interpret pump test
data. Although these methods are restricted in their use by
certain assumptions, the Theis and Jacob methods can be applied
in a wide variety of circumstances. Fetter, (1980) describes the
application of the Theis and Jacob methods for the analysis of
data obtained during the pumping and recovery phases of pump
tests.
TEST DESIGN
Adequate attention to the planning and design phase of the
aquifer/pump test will help assure that the effort and expense of
conducting a test will produce useful results. Information on
the construction and completion of the pumped well and the
observation well to be used in the test should be collected at
the beginning of the test design phase. In addition, all
available information on the aquifer itself such as saturation
thickness, locations of aquifer boundaries, locations of springs,
estimates of regional transmissivities, and other pertinent data
should be collected for use in selecting the final design.
Because costs of drilling new production wells and
observation wells expressly for an aquifer test can be expensive,
it may be advisable to use existing wells for conducting an
aquifer test when possible. However, many existing wells either
are not suited for use during an aquifer test, or are inappro-
priately located. Further, well logs and well completion data
for existing facilities are not always reliable. Existing data
should be verified wherever possible. Each well, whether
existing or to be drilled, must meet the criteria presented in
this document to be suitable for use during an aquifer test.
Appendix A gives details on aquifer (pump) tests.
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SLUG TESTS
A slug test is a short-term test that involves a sudden dis-
placement of fluid to obtain data on aquifer characteristics. It
is a relatively fast and inexpensive type of test but also less
accurate than an aquifer test. Slug tests can provide a stress
test at localized points within an aquifer. There are many types
of slug tests and before a type is selected knowledge of the
transmissivity is very important. If the transmissivity is low,
then the injectivity or block method should be used. For high
transmissivity, the pressure buildup method should be used.
Slug tests are generally performed by suddenly introducing
or displacing a known volume of water in a well, causing an
instantaneous change in water level, and then observing the
recovery of the water level with time. The tests are commonly
performed in small diameter monitoring wells (2 to 4 inches)
and/or in situations where secondary observation wells are not
available. A growing application of slug tests is in hydrologic
studies of contaminated aquifers, where pumping tests would
generate large volumes of contaminated water that must be either
disposed of or treated.
Two basic considerations are important in slug tests: 1) the
method of application of the stress to the aquifer; and 2) the
monitoring of water levels. As with pumping tests, the most
reliable data will be obtained with the largest, practicable
stress applied to the aquifer. Larger stresses will yield data
reflecting larger portions of the aquifer.
As with pumping tests, the initial parts of the slug tests
are the most critical in determining aquifer parameters. In
pumping tests the critical portion may be the first several
minutes to several tens of minutes; with the scaling down that
occurs in the slug test the critical time is generally in the
first tenths of seconds, depending on the size of the well,
volume of the displacer, and hydraulic conductivity of the
aquifer. In any event, it is important to obtain accurate
measurements of water levels during the critical portions of the
test.
One method of applying a stress to the aquifer for a slug
test is by lowering a weighted cylinder of known volume below the
water level in the well. When the cylinder is below the water
level, the water level will rise in proportion to the volume of
the cylinder. After reaching a static level, the water level is
measured and the cylinder is suddenly removed from the well. The
water level will fluctuate and return to a static level.
Measurements of the water levels are taken during the fluctuating
period until a static level is again reached.
10
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Another method used to apply stress to a permeable aquifer
for a slug test is the use of pressure. This method requires the
use of a cylinder of gas, usually nitrogen, and a pressure
transducer. The pressure transducer is placed at or near the
bottom of the well. The cylinder of gas is connected to the well
and the gas is released into the well causing a buildup of
pressure. As the pressure increases, the water level is
suppressed. The pressure is suddenly released by turning a valve
at the surface causing the water to fluctuate. The transducer
detects this change in pressure of the water column and sends an
electronic signal to a solid state controller at the surface.
The controller converts pressure readings to an equivalent height
of water column and stores the readings for each fluctuation in
the memory. State-of-the-art pressure transducers are capable of
making accurate readings to within 0.01 feet. Additionally,
transducers can detect rapid changes in water level, and provide
readings which are considered critical during the first few
seconds of the test (Table 3). For some details on the chemical
resistance of the SE-1000 Pressure Transducer, see page B-7. For
performance information for the model 570 transducer see pages
B-8 through B-11.
The analysis of the slug test data is similar to that for
pumping tests. Various methods of data analysis have been demon-
strated in the literature; most involve plotting the water level
change against a time function and either calculating the slope
or matching with type curves. These methods will yield estimates
of hydraulic conductivity and transmissivity and, in some cases,
the storage coefficient for the. aquifer immediately surrounding
the well where the test was performed. The area of influence
(effective radius) of the test for screened wells is dependent
on the magnitude of the stress applied to the aquifer, which in
turn is dependent on the size of the well and size of the
displacer. If slug tests are performed in a number of wells
throughout a site or region, average estimates of hydraulic
conductivity and transmissivity can be obtained for the aquifer
as a whole.
The advantages of performing a slug test rather than a pump
test are that slug tests are inexpensive and can be performed
quickly; i.e., most pump tests are run for several days but slug
tests can be performed within minutes. If a standard pump test
is run for a period of 96 hours at 130 gallons per minute (GPM),
a huge amount of outflow (in this case, 749,000 gallons) may
present a disposal problem. However, the speed and cost advant-
ages of using a slug test are exchanged for accuracy of data a'-nd
the amount of usable data obtainable with pump tests. ;,
/*
A major disadvantage to slug tests is the small area of the
aquifer which is affected by the displacement of water in the
well. Because the affected area is small, slug tests cannot be
used to determine the overall characteristics of the injection
formation. Also, the small test area increases the potential for
11
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TABLE 3
TYPICAL PRESSURE TRANSDUCER SAMPLING SCHEDULE
Elapsed
0-2 sec
2-20 sec
20-120 sec
2-10 min
10-100 min
100-1,000 min
1,000-10,000 min
(1 week)
1 wk-230 days
Sample Interval
0.2 sec
1 .0 sec
5.0 sec
0.5 min
2.0 min
10.0 min
100.0 min
500.0 min
Hfi* Qf Data Points
10
18
20
16
45
90
90
661
12
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error when estimating the hydrogeologic characteristics of an
aquifer, particularly when the size of the gravel pack is greater
than the size of the screen in a well. In this case, water
stored in the gravel pack causes a rapid recovery in water levels
so that there may be an overestimation of transmissivity.
Appendix B presents an example of step-test data analysis.
13
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DRILL-STEM TESTING
INTRODUCTION
Drill-stem testing is a technique that provides a means of
temporarily completing a well to allow formation fluids to flow
under natural formation pressures. While developed originally to
collect fluid samples, drill-stem testing is now used to deter-
mine a number of other characteristics including formation
pressures, average effective permeability, borehole damage, and
permeability changes or barriers (U. S. Environmental Protection
Agency, 1982).
Temporary completion of a well is accomplished through
expanding either a single packer or a number of packers attached
to the drill stem tester. A number of drill stem test tools are
available on the market to accomplish formation tests covering a
wide range of situations encountered during drilling.
Drill-stem testing can be accomplished in open-hole
operations and also in cased-hole operations. The Open Hole Test
(OHT), used to measure formation characteristics before a well is
cased, is the more economical of the two tests because it can
provide information on a formation's capacity before a well is
completed. The OHT is used when hard rock formations are
sufficiently consolidated that caving of adjacent formations can
be avoided. If the potential for water production from a
formation is in doubt, a drill-stem test will show it, thus
saving the cost of completing the well.
There are two types of OHTs, the first type using a single
packer to test a length of hole below the packer. The distance
below the packer can intercept a number of formations. The
second type of OHT is called a straddle test. Straddle tests are
used to test individual formations in a well by placing packers
above and below a formation, stradding it and resulting in the
testing of a single formation. A straddle test is more expensive
and operationally more difficult than a single packer test. It
is more difficult to set and seal two packers and the possibility
of unseating one of two packers, is greater. For this reason,
this type of test is not run unless absolutely necessary.
The Cased Hole (or Hook Wall) test is performed inside a
cased hole. Testing inside casing is used in many areas of the
country where the formation rocks are soft and where caving of
surrounding formation is possible. Formations behind the casing
are tested through perforations using the same procedure as an
OHT with different types of packers. The packers used have slips
which engage or grab the casing wall and support the compressive
load used to expand the packer element. The cased hole test is
14
-------
also performed to determine if the amount and quality of cement
provide total isolation of the in question zone. This type of
test is called a water shutoff test because it is designed to
determine if there is water movement behind the casing.
There are three types of water shutoff tests: the shoe, gun
perforating, and lap tests. In the shoe test, the well testing
tool is placed above the casing shoe. If a rise in pressure is
observed after the tool is set, the casing shoe is not sealed.
The gun perforating test is performed when the casing is shot or
perforated. This is a straddle packer test which is performed on
an area of casing which is suspected of having channeling, due to
loss of production or insufficient cement. The gun perforating
test can also be performed in a well drilled in material subject
to collapse. The test is performed to determine formation
characteristics. The testing tool is placed above the perfora-
tion, and if the pressure gauge in the tool is constant, there is
no water movement behind the casing. In the lap test, the well
testing tool is placed above the lap between two casing strings.
A constant pressure recording indicates that the casing strings
are cemented properly (Welge, 1981).
A drill stem (or formation) tester is usually made up of
components assembled into a larger piece of equipment called a
"tree". Drill stem testers are available in a variety of
different designs. This discussion will present a generic
description of drill stem equipment components and, when
available, diagrams or photographs of the equipment.
Surface Equipment
When a tester is initially opened to formation fluid
pressures, a "blow" may occur. A "blow" is a rapid rise in fluid
or gas level in the drill stem. It is necessary, therefore, to
have a surface control head on top of the drill pipe. The
surface control head is composed of a number of valves for
controlling fluid pressures and a manometer for measuring fluid
pressures. Figure 1 shows a schematic diagram of surface
equipment for a drill stem test.
Drill Stem Components
Figure 2 shows a diagram of three typical drill stem testing
assemblies. The components of the drill stem testing tools are
diagrammed in Figure 2 and described below:
Impact Reverse Sub - a valve which remains closed during the
entire test. After the final shutin, pressure is measured,
the reverse sub is opened allowing the sampled fluid to
drain out of the drill pipe to allow for easier removal of
the drill stem tester. Normally the reverse sub is in-
stalled 90 feet above the remainder of the tool so that a
sample of the formation fluid remains in the tool for
analysis.
15
-------
Flow
Legend
Drill pipe
Control head
Flow hose
Orifice well tester
Monometer
Small rubber hose
Water container
Valves
Figure 1. Schematic arrangement of'surface equipment for a drill stem test.
When the tool is opened, the rapid entry of fluid into the pipe
displaces air and causes the tube to bend such as indicated in F.
This bend is referred to as a "blow" (valves 1, 2 open; valves
3, 4 closed). The displaced air goes to a bubble bucket. The
bucket is present so that blows can be detected when the produced
gas bubbles out of the base. If the blow is excessive, valves
3 and 4 may be opened. This displaces the produced gases and
fluids to a safe distance from the well. If the produced fluid
surfaces, it is diverted through valve 4 if liquid, or metered
through E) if gas (after Gatlin 1960).
16
-------
Hook Wall Packer Test
• Tubing
• Impact Reverse
Sub (Optional)
• Tubing
Dual Closed In
• Pressure Valve
Reverse Circulation
SS Ports
• Handling Sub &
Choke Assembly
Hydrosprmg
• Tester
• By-Pass Ports
•BT Pressure
Recorder.
- Hydraulic Jar
• V R Safety Joint
• By-Pass Ports
- Hook Wall
Packer
-Collar
•Perforated Tailpipe
-BT Pressure
Recorder
(Blanked Oil)
• Thread Protector
Open Hole
Single Packer Test
Drill Pipe
Impact Reverse
Sub (Optional)
Dual Closed in
Pressure Valve
Reverse Circulation
Ports
Handling Sub A
Choke Assembly
By-Pass Ports
Expand Shoe
Packer Assembly
Anchor Pipe
Safety Joint
Flush Joint
Anchor
BT Pressure
Recorder
(Blanked Oil)
Open Hole
Straddle Packer Test
-Drill Pipe
• Impact Reverse
Sub (Optional)
- Drill Pipe
Dual Closed In
• Pressure Valve
• Reverse Circulation
Ports
• Handling Sub &
Choke Assembly
• Hydrospring
Tester
• By-Pass Ports
• B T Pressure
Recorder
• Hydraulic Jar
• V R Safety Joint
• By-Pass Ports
• Upper Body-
Pressure Equalizer
• Pressure Equalizer
Ports
-Expanding Shoe
Packer Assembly
Flush Joint
• Anchor
BT Pressure
Recorder
• (Blanked Off)
• Expanding Shoe
Packer Assembly
• Adapter
• Flush Joint
Anchor
• Anchor Shoe
ARROWS INDICATE TOP PART OF EACH ASSEMBLY
Figure 2. Drill-stem tools (Haliburton, Catalog 41)
17
-------
Handling Sub and Choke Assembly - an attachment point for
the elevators when lowering and raising the tool. The choke
attached to the handling sub, called the "top choke," is
generally used as a safety device to handle high flow rates
(Gatlin, 1960) and to minimize surface pressure in a casing.
This choke is optional and is often omitted from the
assembly.
Dual Closed in Pressure Valve (Dual CIP Valve) - a five
position valve which opens for two flow periods, closes for
two closed in pressure periods, and has a final position
which opens some parts for reversing the assembly out while
keeping some parts closed to preserve a formation fluid
sample.
Reverse Circulation Ports - devices to perform the same
functions as the impact reverse sub. During open hole tests,
the reverse circulation ports are kept closed because packer
orientation does not permit rotation of the tool to open the
ports. Therefore, during open hole tests, reverse circula-
tion ports are only used in the event of impact sub
malfunction.
Tester Valve - serves two functions in the testing spring.
It can be used as a downhole master valve which is slow
opening and quick closing so that high pressures can be
controlled or as a bypass valve to aid in running-in and
retrieving tools. As the tools are run in the hole, the
tester valve's sliding valve is closed to prevent well bore
fluids from entering the empty drill pipe.
By-pass Ports - openings to permit water to pass around
testing equipment without entering the test instrument.
Pressure Recorders - the most important pieces of equipment
used in a drill stem test. Pressure recorders provide the
data for interpretation of formation characteristics. The
use of pressure recorders is preferred over manual measure-
ments because pressure recorders provide a continuous record
of the test. The following discussion of pressure recorders
was taken directly from Welge (1981).
"A pressure recorder makes a complete record of events that
occur during a test. The record is in the form of a
pressure versus time graph recorded on a chart. The
recording chart is essential for interpreting test results
accurately. Both metal and paper charts are available.
Metal charts are used most commonly and consist of a thin
sheet of brass coated with black oxide that can be marked
easily with a stylus.
Special clocks drive the charts while the stylus records
pressure changes. Clocks are available with 3 to 27-hour
18
-------
time cycles. Recorders are available with pressure ratings
ranging from 800 to 22,000 psi. Recorder carriers and
various subs are used with pressure recorders."
There are two types of pressure recorders used in drill stem
testers: the coil spring and the Bourdon tube recorder.
"The coil-spring recorder consists of a calibrated coil
spring, under tension or compression, attached to a piston
that is activated by external pressure (Figure 3)- The
pressure range of the recorder can be changed by using
different-sized pistons and springs. An increase in the
piston area lowers the range of the recorder. The diameter
of most pistons varies from 7/32 to 3/4 inch.
A stylus records the deflection of the calibrated spring on
a chart that is inserted in the clock and drum assembly.
Calibration tables or charts permit conversion of the spring
deflection from inches to pressure, and the tables are read
in pounds-per-square-inch. The accuracy of the coil-spring
recorder is within 5 to 10 percent of the maximum range.
In the Bourdon tube recorder, well pressure is transmitted
through a diaphragm to fluid inside a coiled tube that is wound
in the shape of a long helix (Figure 4). A stylus at the lower
end of the tube records the deflection of the coiled tube as the
pressure changes cause the tube to uncoil and coil. The Bourdon
tube recorder is extremely sensitive and records pressures
accurately to within 1/4 of 1 percent," (Welge, 1981).
During the early development of the drill stem tester, it
was impossible to determine if a lack of pressure in the drill
stem was due to a dry well or a malfunctioning packer. Downhole
pressure recorders, such as the ones described above, eliminate
this problem because they measure the pressures directly below
the packer and produce data which will demonstrate that the
packer was set properly.
Hydraulic Jar - a device not directly related to testing but
very useful if the tool string becomes jammed. The jar is a
joint which has telescoping capabilities. When the joint is
extended slowly upward, it allows the drill pipe to be
stretched. After the upper section of the tool has been
sufficiently extended, a bypass, located within the hydrau-
lic system, opens and allows the force stored in the
stretched drill pipe to travel to the upper end of the tool.
Inside the jar, a hammer strikes an anvil and loosens the
jammed tools below.
VR Safety Joint - a dual purpose part of the testing
string. The safety joint acts as a bypass when one is
running the tools down the hole or pulling them up out of
the hole. The safety joint also serves as a release
19
-------
«
1. CLOCK COLLAR
2. CLOCK RETAINER
3. CLOCK
4. SPRING SHIM
S. CHART CASE
S. SPRING
7. MIDDLE SUB
8. STYLUS
9. PISTON GLAND
10. PISTON
11. OIL TUBE
12. PISTON BELL
13. PISTON SUB
14. RUBBER DIAPHRAGM
15. DIAPHRAGM
Figure 3. Coil spring pressure recorder (after
Welge, 1981) .
20
-------
-BUMPER
•DIAPHRAGM
-HELIX TUBE
-STYLUS CARRIER
-STYLUS
-CHART DRUM
>CLOCK
-CLOCK RETAINER
Figure 4. Bourdon tube pressure recorder (after
Welge, 1981).
21
-------
mechanism so that the upper section of the tool string can
be removed from the hole if the packer or other parts become
jammed.
Expanding Packer Assembly - a packer is attached to the
drill stem and it expands in such a way that a seal is
formed between formations. Two packers are used during a
straddle test to isolate a particular formation. This
packer is ideal for use in soft sediment and open hole
tests.
Many packers require a certain amount of hydrostatic
pressure to expand. The hydroflate and other similar packers use
a system of internal pumps which force water into the packer
elements, causing the packer to expand forming a tight seal
against the walls of the hole.
Anchor Pipe Safety Joint - the joint that permits retraction
of the testing string below the packer to allow removing the
maximum amount . of the string. It is designed to be
compatible with the VR safety joint. The anchor pipe safety
joint is run along with the VR safety joint in order to
allow the testing string to be backed off at the lowest
possible point. The anchor pipe safety joint is normally
located just below the packer, with the VR safety joint just
above the packer.
Flush Joint Anchor - an extension below the tool which
supports the weight applied to set the packer. The anchor
pipe is perforated to permit water flow into the drill stem
(Edwards and Winn, 1974).
PROCEDURE
During a drill-stem test an assembly connected to the drill
stem containing a packer (or packers) to isolate a particular
formation is lowered into the borehole. A valve located in the
assembly, is closed to prevent drilling fluid from entering the
drill stem and thereby maintaining atmospheric pressure within
the drill-stem pipe as it is lowered into the borehole. Once the
packers are seated and the formation undergoing evaluation is
isolated, the valve is opened to allow drilling fluid and
formation fluids to flow into the drill stem.
The drilling fluid in the isolated zone of the borehole is
under the pressure exerted by the column of fluid before the
packers are seated. It will flow into the drill stem as the stem
is opened. If the formation is permeable, fluids will enter the
borehole and follow the drilling fluid up into the drill stem.
If left unchecked, the drilling fluid and formation fluid will
continue to enter the drill pipe until the weight of the column
of liquids equals the pressure exerted by the fluids in the
22
-------
formation. However, the valve is generally closed in a short
time and the pressure is allowed to build again as formation
fluids flow into the isolated portion of the borehole. The short
flow period removes any pressure exerted by the drilling fluid in
the well bore. Once the excess pressure is removed and the valve
is closed a second time, the pressure measurement is assumed to
be that of the formation fluids. This is called the "initial
shut-in pressure." The valve is usually opened a second time so
that an "initial and final flow pressure" can be measured. The
valve is then closed a third time and a "final shut-in pressure"
is recorded. A pressure gauge located in the assembly measures
changes in pressure over time. These pressure changes are
recorded downhole and the instrument and recording brought to the
surface after the test. After the pressure testing is completed,
the valve is closed and the packer(s) is released. The drill
stem and attached testing equipment are raised out of the hole,
bringing along the fluid samples contained in the drill stem.
Figure 5 shows a schematic diagram of the formation testing
process. The fluid sample, which may contain both drilling and
formation fluids, is collected for analysis (U. S. Environmental
Protection Agency, 1982).
•
RESULTS
Figure 6 shows a schematic diagram of a drill-stem test.
Important features of a test such as Initial Hydrostatic Mud
Pressure, Initial Shut-in Pressure, Initial Flow Pressure, Final
Flow Pressure, Final Shut-in Pressure, and Final Hydrostatic Mud
Pressure are labelled in the diagram. These test features
correspond to periods when the valve is opened and closed.
The following discussion on data analysis is reproduced
entirely from EPA Report # 68-01-5971 (U. S. Environmental
Protection Agency, 1982). Analysis of the transient pressure
response observed during drill-stem testing can be used to
quantify many of the tested formations hydraulic parameters. An
equation developed by Horner in 1951 (EPA, 1982) serves as a
basis for much of this analysis. The equation assumes radial
flow, homogeneous formation, steady-state conditions, infinite
acting reservoir, and single phase flow and shows that:
162.6 QpB T + 0
P = P log ( ) (1)
f r
Kh 0
where,
P = formation pressure during build-up T (psig)
f
P r actual reservoir pressure (psig)
r
Q = rate of flow (bbl/d before shut-in)
23
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REVERSE
CIRCULATING SUB
CLOSED-IN
PRESSURE VALVE
TESTER VALVE
BYPASS PORTS
INSIDE
PRESSURE RECORDE
BYPASS PORTS
PACKER -
PERFORATED
fAIL PIPE
OUTSIDE
PRESSURE RECORDE
PULLING OUT
Figure 5: Fluid passage during a formation test. (Welge, 1981)
24
-------
i! &
*• «u
k-
"5 $
'Z t>
'5 fe.
_u
o
-0
>> u.
JC Q.
LJ
5
J-
o
" 5!
-u
w
(U
4J
13
(4-1
O
CM
U 00
•H 0>
•»-> rH
43
U
CO
(1)
en
•H
Cu
25
-------
ju = fluid viscosity, centipoise
B = formation volume factor
K = permeability of formation, millidarcys
h = formation thickness, feet
T = time flow, minutes
0 r time of shut-in, minutes
Equation (1) may be rearranged to show that:
M = "162.6* QpB (P -P )
r f
= (2)
Kh T + 0
log ( )
0
* The constant 162.6 is arrived at by converting the variable
units from centimeter, gram, second to a set of practical
units, such as, barrel, centipoise, millidarcy,, feet and day
(see Appendix C).
The (M) value is' constant for steady-state flow. If the
pressure drawdown (P - P ) is plotted against log T + 0, a
0
straight line will result. The slope of this line is M. Such
plots are commonly done in drill-stem transient pressure
analysis. If extrapolated to infinity, the plot should provide
the static bottom hole pressure. By determining the slope of
this line, the permeability (K) or flow capacity (kh) can be
computed if the discharge rate, viscosity and the formation
volume factor are known. These measurements of permeability are
generally considered superior to those obtained from core
analysis because they represent dynamic flow average covering the
entire formation. These plots can also be used to identify
changes in formation permeability, or flow barriers, or fractures
since such conditions will result in an abrupt change in the
slope of the line. Similar analysis can be used to determine
loss of permeability in the formation near the borehole resulting
from well damage.
Sinha et al., (1976) listed several problems associated with
drill-stem test data collection. The problems include:
o occurrence of critical flow;
o presence of supercharge;
o usually short duration;
o questionable production data;
o mechanical problems;
o drastic variations in bottom-hole pressures;
o presence of more than one producing zone;
o lack of reservoir and fluid properties data at the time
of the test.
26
-------
These sources of error are magnified when the data are
incomplete. Reliable data analyses can be performed using curve-
matching techniques. Sinha et al (1976) discusses the application
of curve-matching methods.
QUALITATIVE CHART ANALYSIS
Qualitative analysis of a single pressure-time graph similar
to the schematic diagram in Figure 6 can provide evidence of test
validity. Many times two pressure recorders are used to deter-
mine test validity. One pressure recorder is placed outside of
the test system, usually below the bottom packer in a straddle
test, and one is placed between the two packers enabling it to
measure the test formation pressure. Comparison of the two
simultaneous pressure recorders can illustrate test failure or
success. Pressure-time graphs which indicate features in single
and double pressure recorder tests that show valid and invalid
tests are given at the end of this section (see Figs. 6A, 6B, 6C,
6D and 6E).
Single Pressure Charts
Halliburton Services Company has performed drill stem tests
for many years. Appendix D section contains excerpts from a
document prepared by Halliburton Services (Murphy, undated) which
describes many of the features of drill stem test records which
can be used to either prove or disprove the validity of drill
stem test results. A summary of this information is presented in
Table 4.
Welge (1981) described the use of two pressure recorders to
determine water shut-off (WSO) straddle" test validity. The
schematics and interpretations given here have been extracted
from Welge (1981).
27
-------
Figure 6A: A typical dry WSO test using straddle packers
(testing between packers). Lower recorder,
below bottom packer, shows: little drop in
pressure, indicating the bottom packer is
holdi ng.
(a) Recorder between packers.
(b) Recorder below bottom packer.
Figure 6B: Running straddle tests without bypass ports will
will cause a gradual decline in the pressure below
the bottom packer (b). Basically, this pressure
decline is due to the differences in the total
forces acting above and below the bottom packers.
However, so long as the pressures in (b) are
greater, respectively, than the pressures in (a),
it has been demonstrated that the bottom packer
seated properly and prevented fluid flow from
below the bottom packer into the test interval.
28
-------
(a) Recorder between packers.
(b) Recorder below bottom packer.
Figure 6C: The recorder below the bottom packer (b) indicates
the bottom packer was leaking fluid. The upper
recorder (a) indicates the leaking fluid is
entering the testing tool. However, these charts
can also indicate a wet test with the formation
below recorder (b) taking fluid.
(a) Inside recorder.
(b) Outside recorder.
Figure bD: While no pressure was recorded in (a) during
the test, (b) recorded a pressure buildup.
Jf only the (a) chart was available, a dry
test would be implied.
29
-------
(a) Inside recorder.
(b) Outside recorder.
Figure 6E: Because both (a) and (b) show identical behavior,
the perforated tail pipe did not plug. However,
the recovery of 15' of drilling mud would not
account for the pressure buildup shov%n on the
charts. The choke in the tool above the
perforated tail pipe plugged, and the recorders
merely recorded a shut-in pressure buildup.
30
-------
Table 4
METHODS FOR DETERMINING FORMATION TEST VALIDITY
THROUGH QUALITATIVE EXAMINATION OF PRESSURE CHARTS
Problem
Incorrect Base-
line
(see Fig. 1)*
Difficulty in
Lowering Tool
Due to Hole
Size Fluctuat-
ions.
(see Fig. 2)*
Pressure Gauge
Malfunction.
(see Fig. 3)*
Clock Stopped.
(see Fig. 4)*
Evidence on Pressure Chart Test y.alid.jLty.
Clock "Running
Away".
(see Fig 5)*
Leaking Closed-
Pressure Valve
(see Fig. 6)*
Supercharged
Initial
Build-Up.
(see Fig. ?a)*
Initial shut-in pressure
unequal to final shut-in
pressure.
Fluctuations in initial
hydrostatic pressure
readings.
Stair-step appearance
to build-up curve.
Creates overlap of curves,
Can mask build-up curve.
Confusing data.
Erratic pressure curve.
Initial pressure higher
than final pressure.
Quantitative analy-
sis can still be
performed by ad-
justing baseline.
Will not affect
test.
Test data should not
be used.
Test data can be
used in some cases
depending on when
the clock stopped
(i.e. if clock
stops during remo-
val of tool, test .
data can be used).
Data are useless.
Test is invalid.
Unless initial pres-
sure is measured
for a long enough
period for the
pressure to re-
equilibrate, test
is invalid.
* Refer to Appendix D
31
-------
Problem
Depleting
Liquid and/
or Gas
Reservoir.
(see Figs.
8 & 9)*
Barrier
Boundary.
(see Fig. 11)*
Low Reservoir
Pressure and
Low Permeabi-
lity.
(see Fig. 12)*
High Damages in
the Wellbore.
(see Figs.
15 & 16)*
S-Shaped
Build-Up
Curve.
(see Figs. 18,
19, and 20)*
TABLE 4
(continued)
Evidence on Pressure Chart Test Validity
High initial build up
curve compared to the
final build up curve.
Initial pressure less than
final pressure. However,
the rate of pressure
build up is less for the
final measurement. In
many' cases, the tool is
pulled before the final
pressure can be measured.
Rate of pressure increase
is low. Generally the
test is run without allow-
ing enough time for the.
two pressure build up
phases.
Features are as follows:
o Sharp rise after shut-in
o Short radius curve
o Reasonably flat slope
o High differential pres-
sure between closed in
and final flow pressure,
Complex reasons - text
Appendix E should be
consulted.
in
Equalized Pres- Second shut-in pressure
sure Due to
Extended Flow
Period.
(see Fig. 21)*
build-up is not observed
due to pressure reaching
static.
Extrapolated static
reservoir pressures
difference between
the initial and
final build up
curves indicate
loss in reservoir
pressure produced
during the second
flow period.
If test is run long
enough, test is
valid.,
Test must be rerun
allowing adequate
time for pressure
measurements.
Test valid.
See text of
Appendix B.
Test invalid,
*Refer to Appendix D
32
-------
TABLE 4
(Continued)
Evidence on Pressure Chart Jest Validity.
Well Plugging
(see Fig. 22)*
Swabbing During
Formation Test
(Swabbing is a
method of re-
moving fluids
from the well
bore).
Flowing of Two
Phases or
Flowing in
Heads
Erratic pressure readings
Erratic pressure readings
Test invalid
Test invalid
Erratic pressure readings Invalid.
* Refer to Appendix D
33
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WIRELINE FORMATION TESTING
In the 1950s, wireline formation testing tools were
developed to provide a quick and inexpensive means for obtaining
fluid samples and rudimentary pressure data. Schlumberger Well
Services, Dresser Atlas Inc., and Gearhart Industries Inc. have
developed similar tools. Schlumberger calls its tool a "repeat
formation tester" (RFT), because it can be used to evaluate
several formations during one run. The tool is run into the
borehole and is seated in formations for which data is required.
A typical wireline formation testing tool contains several small
chambers for collecting fluid samples; the number of samples
obtained is limited by the number of collection chambers
available. The tool is made up of a pad seated on an expanding
mechanism at the desired depth which pushes the tester into the
side of the borehole. A small valve on the side of the tool is
pushed into the formation to collect fluid samples. The
apparatus contains a pressure transducer with a surface recorder
(EPA, 1982).
In using a wireline formation tester, special care must be
taken to ensure that fluid samples are representative of true
formation fluids and not of the drilling-fluid filtrate which has
invaded the formation. To avoid collecting filtrate, fluid
samples are withdrawn during a pretest. This liquid is not
collected, but fluid pressure in the formation adjacent to the
borehole is monitored and recorded. This recording provides data
useful to formation evaluation. An unlimited number of pretest
pressure measurements can be made during a single trip downhole.
Although only one or two fluid samples can be obtained during the
same trip, a large amount of information can be obtained by using
the "pretest", mode of the wireline formation tester. The
pretest measurements can be used to determine which formations
are interesting enough to merit taking a fluid sample.
During the pretest, constant pressure measurements are made
as two consecutive fluid samples are removed from the formation.
As the first sample is removed, the pressure drops at the
formation face. When the flow is stopped, the pressure at the
face, of the formation will begin to rise. The second sample is
taken almost immediately after and at a higher rate than the
first. Smolen (1977) suggests that the ratio of the two rates
should be about 2.5-1.0. In the geographic areas where the
higher rate causes plugging of the second probe due to a large
pressure differential, the second pretest is omitted (Smolen,
1977). After the second fluid sample is taken, the pressure is
allowed to build up until equilibrium is reached. The final
pressure reading yields a good estimate of the formation pressure
because the factors which could alter the pressure readings, such
as increase in pressure due to packer expansion or decrease due
to probe extension and mud cake buildup, are removed during the
two flow periods. A typical RFT pressure profile is given in
34
-------
Figure 7. The pressure drops labelled P1 and P2 correspond to
fluid samples 1 and 2 taken during the pretest. The equation for
calculating the permeability of the formation, using a quasi
hemispherical model, was given by Smolen (1977) as:
K1 = Cuq qu
= 4388 x C —
2 Pr P
w
where,
4388 = coefficient for unit conversion
q = flow rate, cc/sec. (from curve)
u = viscosity, cp
P = pressure drawdown, psi or difference between the formation
pressure and the flowing bottom hole pressure
K = permeability, md
C = flow shape factor
C = 1 for hemispherical flow
C = .5 for spherical flow
C = .75 for flow into probe set in 8" borehole
r = radius of the well
w
Smolen (1977), Smolen and Litsey (1977), and Stewart et al.
(1981) discuss the practical applications of using pretest data
to determine the permeability of formations. The hemispherical
model is the simplest model which can be applied. However, the
hemispherical model generally gives permeability values which are
affected by the film or skin which forms on the face of a well
bore during drilling. Therefore, permeability measurements made
during the drawdown steps can be misleading. Analysis of
fractured formations is also questionable using the simple draw-
down analysis. Smolen (1977) suggests that analysis of buildup
curves during the final pressure readings can provide permea-
bility values more representative of the formation and not the
mudcake. Steward et al., (1981) also recommends pressure buildup
analysis for permeability measurements in naturally fractured
rocks. Other information on wireline testing methods for deter-
mining formation characteristics has been given by Moran and
Finklea, 1962 and Schultz et al., 1975.
Interpretation of wireline formation tester pressure data is
semi qualitative. The information obtained is generally inferior
to that of a normal DST. Permeability estimate by this method is
uncertain and the skin factor cannot be evaluated.
35
-------
Flow rate
Pressure
Hydrosfafic
pressure
t=0
Shut-in
\
Formation
pressure
Figure 7. Schematic diagram of a wireline formation-
testing pressure curve (after Smolen, 1977)
36
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SAMPLING OF FORMATION COMPONENTS
Stability of the chemical compounds in an injected fluid is
desirable. An unstable compound may precipitate to clog a
formation. In some instances, decreased permeability occuring
when injection fluid dissolves, formation matrix components could
result in severe structural collapse (Warner and Lehr, 1977). It
is, therefore, necessary to obtain a sample of the solid matrix
of a formation and also a sample of formation fluid to test the
compatibility of formation components and injection fluid. Table
5 compares the advantages and disadvantages of the sampling
techniques given below.
FORMATION MATRIX SAMPLE
There are three methods of collecting formation matrix
samples. The first method is called "sidewall coring". This
technique is used to take a series of samples from the walls of a
borehole. Sidewall coring is accomplished by running a wireline
coring device, which contains small hollow cylinders, into a
cased well. These hollow cylinders are driven into the formation
by an explosive charge. Data obtained from sidewall cores are
not entirely reliable because the process of sidewall coring
substantially disturbs the more permeable formations, altering
porosity and contaminating core samples with drilling fluids and
suspended solids. Another liability encountered when obtaining
sidewall core samples is that the small size of the core limits
interpretations of lateral geology.
The second method of obtaining a formation matrix sample is
collection of drill cuttings. Drill cuttings are produced when
hard rock formations are drilled. Cuttings produced during cable
tool drilling accumulate in the hole and are removed at intervals
by bailing (EPA, 1982). Examination of cuttings gives a more
representative average of formation characteristics than side
wall cores because cuttings are produced continuously. In
contrast, side-wall cores only sample a limited amount of a
formation. In Rotary drilling techniques, cuttings are con-
tinuously removed by drilling fluids. Drill cuttings are nor-
mally examined on site under a low-power magnification micro-
scope to identify rock type, grain size, color, and mineralogy.
Testing the samples with acid is used to determine carbonate
material, and exposing the cuttings to ultraviolet light is used
to identify organic and inorganic components. The organic
components will fluoresce under ultraviolet light, but an
additional test is needed to confirm the presence of organic
matter because some inorganic compounds will also fluoresce.
Combining the cuttings and injection fluid will provide useful
information regarding compatibility. The major drawback to
inter-preting formation characteristics from drill cuttings is
the question of how representative the cuttings are. During
37
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TABLE 5
COMPARISON OF FORMATION COMPONENT SAMPLING METHODS
Methods
Uses
Limitatio.ns
A. Formation Matrix Samples
Sidewall
Coring
Drill
Cuttings
Conven-
tional
Coring
Collects samples of
unconsolidated ma-
terial directly.
Provides samples of
hard rock material
as a well is
drilled.
Provides the most
information on
lithology and per-
meability of forma-
tions encountered
during drilling.
Formation Fluid Sampling
Drill
Stem
Tests
Provides a large
sample of formation
fluids. The test
also provides infor-
mation on formation
permeability.
Samples are often dropped
from the core gun when re-
trieving the tool from the
borehole. Small sample
size. High potential for
contamination by drilling
fluids. The process alters
the porosity of samples
collected.
As samples are brought to
the surface, cuttings become
mixed. Other correlation
problems can be encountered.
Continuous coring is prohi-
bitively expensive and is
used only when drilling in
formations of interest.
Fluid samples are easily
contaminated by drilling
fluids.
38
-------
TABLE 5
(Continued)
Methods
Uses
. Eormation Fluid Sampling (Continued)
Wireline
Formation
Tests
Swabbing
Bailing
Provides at least two
formation fluid samp-
les. Also provides
information on forma-
tion permeability.
A swab is lowered
into a well. As the
swab is lifted from
the well, it pushes
water up through the
casing to the surface,
Used in wells drilled
by cable-tool methods,
Water is bailed as
the hole is dug.
Lifl]itat.io,ns
Small fluid sample size
This test is usually used
in addition to a drill-
stem test. This increases
the cost and time used to
evaluate the well.
Samples become contami-
nated by other producing
formations.
39
-------
drilling, there is a considerable delay between the time the
cuttings leave the drill bit and the time they reach the
surface. During the lag period, a considerable amount of mixing
can occur between cuttings produced at different intervals,
between cuttings and sidewall chips, and between drilling fluid
and cuttings. This contamination of samples can result in
erroneous lithologic profiles and interpretation of formation
characteristics. However, borehole geophysical logs, such as
natural conductivity and resistivity logs, can be used to clarify
some of the data.
The third and most precise method of formation sampling is
called "geologic coring". Geologic cores taken while drilling
provide lithologic and hydrologic information superior to that
obtained from the analysis of drill cuttings (Warner and Lehr,
1977). Coring is accomplished through the use of a special
drilling bit and a coring barrel which is attached to the end of
the drill pipe. As the bit cuts into the rock, an inner core is
left intact and pushed into the core barrel. Visual examination
of cores can reveal fractures, bedding features, and solution
cavities. Laboratorytexamination can determine porosity, grain
size, permeability, 'mineralogy, and formation fluid quality.
This information can be used to determine potential interaction
between formation and injection fluids (EPA, 1982).
The major drawback to this method is that taking geologic
cores is expensive. For this reason, coring is usually employed
only when drilling formations of special interest.
FORMATION FLUID SAMPLES
There are five methods of formation fluid sampling commonly
used in the petroleum industry. These methods include drill-stem
testing, wireline formation testing, swabbing, bailing and air-
lifting. These methods can be used to sample all classes of
injection wells. The previous discussion of drill-stem testing
focused on the use of this method to determine formation
characteristics such as formation pressure and permeability.
Although the drill-stem test, (or formation test) is currently
used primarily to determine formation characteristics, the
original purpose of the formation .tester was to sample formation
fluids. Even though the potential for contaminating fluid
samples with drilling fluid is considerable, formation testers
are still used to collect water samples.
The drill-stem test begins by lowering the formation tester
into the well. During this time the pressure will increase,
showing up as a peak or trough on the chart recording. After the
packers are set, the formation tester is opened to the formation.
This will result in a rise of fluid into the drill-stem pipe.
Fluid is allowed to flow for some time to ensure that the water
sample retained in the drill-stem pipe is representative of the
formation fluids. The unwanted fluid entering the drill-stem
40
-------
pipe is fed into the well casing. After a substantial flow
period, the main valve is closed so that a final formation sample
is retained within the drill-stem. If a second flow period is
unnecessary, a final formation pressure is measured and the
drill-stem device is pulled out of the well. The drill-stem
tester can only provide a single formation water sample. Special
care must be taken to ensure that fluid is permitted to run into
the drill-stem for a sufficient time so that drilling fluids are
flushed from the pipe.
Wireline formation tests provide results similar to drill
stem tests. The advantages of a wireline test is that two water
samples can be collected during a single trip into a well and any
number of pressure tests can be run. Also, wireline formation
tests take less time to perform than drill-stem tests and avoid
contamination of the fluid samples by the mudcake.
The tool used in wireline formation tests is called a repeat
formation tester (RFT). During the test, a RFT is lowered into
the well until it reaches a formation to be tested. A probe on
the tester is set into the face of the formation. Formerly, a
pretest portion of the test was used to ensure the quality of the
formation sample by withdrawing two water samples and discarding
them prior to taking a sample for analysis. The pretest phase of
the test is currently considered the primary phase of the test
and is used, in a manner similar to drill stem tests, to
determine format-ion characteristics. The RFT has the capacity to
take at least two water samples.
Swabbing is a method of producing fluid similar to pumping
a well (Warner and Lehr, 1977). In swabbing, fluid is lifted
from the borehole through drill pipe casing, or tubing by a swab
that falls freely downward through the pipe and its contained
fluid, but which sets against the pipe walls on the up-stroke,
drawing a volume of fluid above it as it is raised. Swabbing is
preferable to drill-stem testing where unconsolidated formations
cause testing to be difficult. Swabbing may also be used in
conjunction with drill-stem testing to increase the volume of
fluid obtained. The advantage of swabbing is that it can be
continued until all drilling mud has been drawn from the pipe,
thus allowing the chemistry of the formation water sampled to
reach a steady state. This procedure helps to ensure that a
representative sample of formation water is obtained.
Fluid samples can be obtained by injecting gas under
pressure into a well. The pressure of gas forces the fluid in
the well to rise to the surface, thus the name gas-lift sampling.
In holes drilled by cable-tool, bailing may be used to
obtain formation water samples but care must be taken to ensure
that the water sample taken is representative of the formation of
interest and not of another formation also draining into the
borehole. This problem is reduced in holes in which casing is
41
-------
driven into the well bore since the casing acts to isolate the
lowest formation from the other water producing formations (U. S.
Environmental Protection Agency, 1982). The choice of bailer
material should be made carefully. The bailer should be composed
of unreactive material such as teflon or stainless steel.
Reactive material could substantially alter the chemistry of the
water sample.
42
-------
INJECTIVITY
Injectivity tests are methods of formation testing used for
defining reservoir characteristics. They are useful in deter-
mining the anticipated performance of the injection zone.
Injectivity tests can be performed during the final stages of
well construction or after well completion, but are invariably
performed prior to initiation of full scale operations. It
should be noted that injectivity tests are merely one way of
obtaining data on formation characteristics; this data should be
used to supplement other test data (e.g., drill stem tests,
formation logging, etc.).
Injectivity tests involve injecting fluids into the
formation in question, whereas productivity tests deal with
extracting fluid from the formation. The water used for
injectivity tests is either treated water from a surface or
ground water source or water that was produced from the
formation. Mobile pumps mounted on trucks are most commonly used
for these injectivity tests. The fluid compatibility for the
injection test must be considered prior to test initiation. The
water being pumped into the formation must be compatible with the
formation materials; the test fluid must also be representative
of the water to be injected during full scale injection.
Two basic methodologies are used for injectivity tests: step
and steady state (or pressure falloff) tests. Step tests are
performed in a manner similar to steady state tests, but the pump
rate is increased incrementally after each leveling off period.
Each step or increment is of equal duration. Low permeability
formations (K <5 md) should experience one-hour steps and step
tests for medium permeability formations (K >10 md) (Guerard, no
date) should be 36 minutes long. At the end of each step the
measured pressure is plotted against the injection rate as shown
in Figure 8.
Six to eight steps are preferred for accurate interpre-
tation. The plot of pressure versus injection rate produces two
lines (Figure 8). The point of intersection of the lines defines
the formation fracture pressure. Step tests progressively alter
the formation fluid pressure. Therefore, the fracture pressure
derived from the plot will not be the pre-test fracture pressure.
Deviations in cement bonding can result in interpretation
problems. Cement bond problems are distinguished from step test
results by observing the slope of the second line. In the event
of poor cement bonding, the second line will drop below the
fracture pressure as injection rates decrease (Guerard, no date).
The fracture pressure gradient (F) can be obtained by dividing
the bottom hole fracture pressure by the depth. If the depth is
D, the specific gravity of the injected fluid G , and the
i
surface fracture pressure P , the fracture gradient is:
s
43
-------
1600
1400
of 1200
LLJ
cr
1900
at
FRACTURE PRESSURE
1000 psi AT SURFACE
-200 -400 -600 -800 -1000 -1200
INJECTION RATE. STB/D
Figure 8. Step-rate injectivity data plot (from
Guerard, Publication M13).
44
-------
F = 0.433 G D + P in psi
i s
D ft
where 0.433 is the hydrostatic pressure of one foot of water.
Steady state tests entail pumping fluid into the formation
with a constant pump rate. During the test, the pressure (at the
well head and/or at the bottom of the hole) is monitored. After
equalization of pressure, the pumping is terminated and the
pressure is allowed to return to static condition. Pressure
monitoring continues during this period.
The pressure fall-off test is a type of steady-state
injection test which provides data on reservoir permeability,
compressibility and rate of injection. The tests assume that the
reservoir is completely saturated and that the injected and in-
situ fluids have the same mobility. These assumptions are true
when the reservoir and injected fluid are compatible and when the
injected fluid front has a radius greater than the radius of the
area of investigation.
45
-------
FRACTURE GRADIENTS
The fracture gradient is the change in pressure with depth
needed to fracture the formation. It is expressed in psi/ft.
The fracture gradient depends on a number of factors,
namely: the pressure in the pore system, Young modulus, the over-
burden and surface pressures. Studies conducted by Calhoun,
et al., have shown that the pressure gradient tends to decrease
with increasing depth. Knowledge of the fracture gradient is
essential in planning cement for operations, matrix acidizing,
hydraulic fracturing and, secondary and tertiary recovery
methods.
Two basic methods exist for determining fracture gradient.
The first is by calculations involving the fracture surface
pressure and the weight of the overburden. The second is based
on measurements made in wells by increasing the injection
pressure in a well until the formation is fractured.
The second method, by far, is the most common for deter-
mining the fracture pressure of a formation. The technique
involves pumping water into the well (after completion of the
well) at a series of incrementally-increasing pumping rates.
Generally, six to eight .rates are used to bracket the estimated
fracture pressure. The injection pressure is monitored conti-
uously through out the test at either the wellhead or bottom of
hole with Hewlett-Packard 2811B quartz pressure gauge (Wycon
Chemical Company, 1983). During the early parts of the test,
when the pressure is below the fracture pressure, an increase in
injection rate will be proportional to each increase in
pressure. After the formation has fractured, increases in
injection rate will result in lower increases in injection
pressure. A plot of injection rate versus injection pressure
will typically yield two straight line segments. The inflection
point of these lines is the fracture pressure at the surface or
well head. The fracture pressure indicates that the injection
pressure equals or exceeds the formation fluid pressure.
The injected fluid pressure gradient is determined by
monitoring the well head pressure during the step test. The
depth may be the well depth or any depth of interest. The
following procedure should be used to determine the bottom hole
fracture gradient:
1. Proceed with step test as described.
2. Plot well head pressure (psi) versus injection rate
(STB/D) (Figure 8).
3. Determine the fracture pressure at the surface using the
plot from Step 2.
46
-------
4. Plug variables into Equation 1 to obtain the fracture
gradient.
An artificially induced fracture will occur if a fluid is
injected at a rate and pressure which exceed the least stress of
the formation. Normally, if the injection pressure is equal to
or exceeds the overburden pressure (approximately 1 psi/ft) the
resulting fracture will be horizontal. If the injection pressure
induces a fracture even though this pressure is less than the
overburden, then a vertical fracture will occur.
Falloff tests are also used to determine fracture gradients.
Appendix A gives detailed recommendations for aquifer pump tests.
47
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GEOPHYSICAL LOGGING
INTRODUCTION
Geophysical logging techniques are methods by which
formation characteristics can be determined by physical borehole
measurements. These measurements can be made before and/or after
the completion of the well. A variety of logging techniques are
commonly used to obtain different types of data on the subsurface
lithologies. Geophysical logging requires running a tool with
the appropriate instrumentation down the borehole. During its
descent or ascent, measurements are made according to the
particular type of log being run, and a continuous signal is sent
to surface recorders via a cable attached to the tool.
In general, geophysical techniques are used to measure
specific physical characteristics of the rocks surrounding the
borehole. These measurements are interpreted or translated into
indirect measurements of particular features of the rocks (e.g.,
lithology, porosity,< permeability, hydrocarbon presence). For
injection well testing, the most important parameters are permea-
bility and porosity, since these factors affect the degree to
which the formation can accept an injected fluid. The logs that
can measure formation properties include:
o resistivity log;
o sonic or acoustic velocity log;
o gamma-gamma, density log;
o neutron and pulse neutron log;
o nuclear magnetic; and
o spontaneous potential.
Most of the above logs can be used to determine porosity.
However, the primary logs used for this purpose are the acoustic,
gamma-gamma or density log, and neutron logs. In addition, a log
that is used to estimate qualitatively the formation permeability
is the nuclear magnetic log. This log will be described in more
detail below.
Other borehole geophysical methods which are used for
testing the mechanical integrity of injection wells can also be
used to test formation characteristics. For example, Pulse-
Neutron logs can be used to determine formation porosity, water
velocity and saturation, and noise logs can be used to determine
fluid production from wells. These applications will be discussed
in more detail in future technical assistance documents being
developed by EPA.
RESISTIVITY LOGS
There are two basic types of resistivity logs to record
formation resistivity or conductivity: a) induction logs and
d) latero logs.
48
-------
a) Induction Logs
An induction log is the basic resistivity log used in
formations displaying medium to high porosity. In conjunction
with an SP log, it aids in defining beds and determining
formation water resistivity.
More specifically, the induction log determines true
formation resistivity, defines formation boundaries, can be used
as a basis to establish lithologic picture of the subsurface, and
can supply enough data to compute water saturation.
Dual iDduction Log
The dual induction log is a sophisticated resistivity log
which can be used in medium to low porosity formations. This log
possesses three resistivity curves which cover different depths
of investigation. The first curve provides the resistivity of
the flushed zone. The second curve measures the resistivity of
the flushed and invaded zones and the third curve measures the
true resistivity of the formation.
Latero Log
The latero log is used primarily in conductive muds (salt
muds). It can measure the resistivity of a deeply invaded zone
and the true formation resistivity.
With this log, an analyst can do the -following: define
formation boundaries of even thin beds, provide a true reading of
the formation resistivity, and provide data to compute the water
saturation level.
ACOUSTICAL TRANSMISSION LOGS
Acoustical logs measure the speed of sound waves that are
propagated through the formation adjacent to the borehole by the
logging tool. A transmitter at one end of the tool emits high
frequency pulses of acoustic energy and several receivers spaced
at varying distances in the tool receive that energy. The travel
time of the sound waves through the formations is measured.
The velocity of a sound wave through a formation depends on
properties of formation fluids and the rock matrix. The sonic
wave is affected by formation characteristics. If data are
available on the nature of the formation fluids and the lithology
of the rocks, then formation porosity can be determined readily
from sonic velocity logs.
The sonic-amplitude log, another type of acoustical log,
measures the attenuation of sonic waves travelling through a
formation. It has been shown that sonic waves are attenuated by
certain formation features related to porosity, such as
49
-------
fracturing and the presence of vugs,
useful in identifying such features.
GAMMA-GAMMA LOGS - DENSITY LOG
and, therefore, can be
Gamma-Gamma logs measure the apparent density of the
formations surrounding the borehole, providing data which can be
used to determine the porosity and lithology of the formation.
To obtain gamma-gamma logs, a gamma ray is emitted from the
logging tool into the formation. The Gamma rays that collide
with electrons in the formation are back-scattered and detected
by the receiving part of the tool. The back-scattering is
proportional to the electron density of the formation. The gamma
device actually measures the bulk density of a formation based on
the relative amount and density of the geologic matrix and the
fluids contained within the matrix.
NEUTRON LOG
The neutron log is a porosity log that measures the induced
radiation from a formation when it is bombarded by rapidly moving
neutrons. The neutrbns emitted are captured by hydrogen atoms
existing in the formation. This phenomenon produces secondary
gamma ray emissions. In hard dense rocks, such as dolomites and
sandstones, high radiation levels are recorded. Low levels of
radiation are recorded opposite shale beds. Thus, this type of
log permits the determination of lithology and porosity of the
formation, allows the calculation of water saturation in the
pores, and distinguishes between gas zones and oil water zones.
PULSE NEUTRON LOG
of the
offers
14
N by nuclear
movement of the
velocity of the fluid
a lot of promise for
Oxygen atoms in water are converted to
bombardment. A sensor determines the rate of
isotopes and provides an estimate
behind the casing. This method
quantifying flow behind the casing.
NUCLEAR MAGNETIC LOGS
Nuclear magnetic logs are used in determining permeability
and porosity of subsurface formations. The logging tool exerts a
magnetic field and induces polarization of hydrogen nuclei in the
pore fluids in the formation. This method distinguishes hydrogen
atoms bound in water molecules from other types of hydrogen
atoms. Quantification of water content in a formation provides
information on formation porosity and permeability. This tech-
nique is also useful for distinguishing between hydrocarbons and
water in formations.
50
-------
SPINNER LOGS
Spinner surveys (also called flow meter surveys) determine
each injection zone's ability to receive fluid and can be used to
detect large leaks in the injection well casing. The tool used
is an impeller type flow meter (see diagram) which is hung on a
wireline cable to measure injected fluid flow. The rotational
rate of the impeller (rpm) is proportional to the flow velocity
of the injected fluid. The flow meter spin rate is continuously
recorded on surface equipment and plotted versus the depth of the
tool. The flow meter can be used in tubing as small as 1-5/8
inches with pressures as high as 15,000 psi.
Most spinner surveys are performed by pulling the tool
opposite to the direction of flow, but an additional run in the
flow direction can enhance accuracy. A loss of fluid from the
tubing well results in a lower velocity. In an open hole, a
decrease in fluid velocity indicates a more transmissive rock
formation. Areas of tubing which show a decrease in flow
velocity usually are losing fluid through a leak, whether due to
a casing leak or a more transmissive rock formation.
SPONTANEOUS POTENTIAL
The following discussion of spontaneous potential (SP)
logging has been reproduced from a previous EPA document (EPA,
1982).
The spontaneous potential log, also known as self potential
or SP log, is used to correlate stratigraphy, provide a qualitat-
ive indication of the amount of clay or shale present, identify
permeable beds, and indicate formation-water resistivity. The SP
log measures the natural electric potential established between
the borehole fluid and the formation fluid with relationship to a
fixed potential electrode located at the surface. The relative
potentials observed are dependent on formation lithology, and
borehole and formation-fluid characteristics.
51
-------
CABLE
CASING
INSTRUMENTATION
SECTION
MAGNETIC _
TRANSDUCER
IMPELLER-
-------
REFERENCES
-------
REFERENCES
ATLAS WIRELINE SERVICES, 198?. Oxygen Activation Logging
Technical Manual. Western Atlas International, September 1987.
BENTALL, R., et al. 1963. Shortcuts and Special Problems in
Aquifer Tests, U. S. Geological Survey Water Supply Paper 1545-C,
117 PP.
BLACK, J. H. and KIP, K. L.1977. Observation Well Response Time
and Its Effect Upon Aquifer Test Results, Journal of Hydrology
34. (297-306).
BREDEHOLFT, J. D., et. al. 1983. Regional Flow in the Dakota
Aquifer: Study of the Role of Confining Layers, U. S. Geological
Survey Water Supply Paper 2237.
CAMPBELL, M. D. and LEHR, J. H. 1972. Water Well Technology,
McGraw-Hill Book Company, New York 10020, 681 pp.
CLARK, W. E., 1967. 'Computing the Barometric Efficiency of a
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EDWARDS, A. G. and WINN, W. H., 1974. A summary of Modern Tools
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ENVIRONMENTAL PROTECTION AGENCY (EPA). Injection Well Construc-
tion Practices and Technology. EPA #68-01-5971, 1982. 203 PP.
FERRIS, J. G., et. al., 1962. Theory of Aquifer Test, U. S.
Geological Survey Water Supply Paper 1536-E, pp 69-174.
FETTER, C. W. Jr. Applied Hydrogeology. Charles E. Merill,
Columbus, Ohio, pp. 253-303, 1980.
CAREER, M. S. and KOOPMAN, F. C., 1968. Methods of Measuring
Water Levels in Deep Wells, Techniques of Water Resource Inves-
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HAMLIN, S. N., 1983. Injection of Treated Wastewater for Ground
Water Recharge in the Palo Alto Baylands, California, Hydraulic
and Chemical Intervention, U.S. Geological Survey Water Resources
Investigation 82-4121.
JOHNSON, E. E., 1966. Ground Water and Wells, Edward E. Johnson,
Inc., St. Paul, Minn. 440 pp.
KING, H. W., 1982. Handbook of Hydraulics, McGraw-Hill.
-------
KRAUSEMAN, G. P. and DeRIDDER, N. A., 1979. Analysis and
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200 pp.
LEUPALD and STEVENS. Stevens Water Resources Data Book.
Beaverten, Washington.
MORAN, J. H. and FINKLEA, E. E. Theoretical Analysis of Pressure
Phenomenon Associated with the Wireline Formation Tester.
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MURPHY, W. C. The Interpretation and Calculation of Formation
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SCHULTZ, A. L., BELL,. W. T., and URBANOSKY J. J., 1975. Advance-
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Technology of Subsurface Wastewater Injection, U.S. Environmental
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-------
U. S. ENVIRONMENTAL PROTECTION AGENCY. 1982. Handbook for
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600/4-82-029.
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APPENDIX A
AQUIFER TESTS
-------
DESIGN OF PUMPING FACILITY
Reliable Power
Continuously available power for the duration of the test is
crucial to a successful test. Should power be interrupted, it
would be necessary to allow the well to fully recover and then to
start the test all over.
fump
During an aquifer test, all necessary measures should be
taken to assure the pump will be continuously operable during the
test. Should a pump fail, the time, effort, and expense of
conducting the test could be wasted. Generally, electrically
powered pumps are recommended for aquifer tests because they
produce the most constant discharge. Gas-powered pumps can
produce a large variation of discharge over a 24-hour period and
require constant monitoring of the discharge during the test.
It is necessary to have a check valve installed at the base
of the pump column pipe to prevent the back flow of water into
the well when the test is terminated. Any back flow into the
well will interfere with or obliterate the water level recovery
of the aquifer, making any aquifer analysis based on recovery
data questionable.
Discharge=Control Eguip.rnent
The wellbore and discharge lines must be accessible for
installing discharge control and monitoring equipment. When
considering an existing well for use as a discharging well, the
well must either already be equipped with discharge measuring and
regulating equipment or the well must have been constructed so
that the necessary equipment can be added.
Control of the pumping rate during the test requires an
accurate means of measurement and a convenient means of maintain-
ing a constant pump rate. Common methods of measuring well
discharge include the use of an orifice plate and manometer, an
inline flow meter, an inline calibrated Pitot tube, a calibrated
weir or flume, or, for low discharge rates, observance of the
length of time taken for the discharging water to fill a
container of known volume. A valve installed in the discharge
line is the best method of controlling the discharge rate while
conducting a pump/aquifer test using an electric powered pump. A
rheostat control on the electric pump will also allow accurate
control of the discharge rate. The use of a gas-powered pump
requires fine throttle adjustments such as a screw adjustment to
control discharge rate.
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Water Disposal
Discharging water immediately adjacent to the pumping well
can cause problems with the test, especially in tests of shallow
unconfined aquifers. The water becomes a source of recharge
which will affect the results of the test. The produced water
must be transported away from the control well so that it cannot
return to the aquifer during the test. If the produced water is
contaminated, proper disposal is mandatory. If such disposal is
required, the viability of the test from an economic standpoint
is severely constrained.
Water Level Measurement Access
Means must be provided to measure the water level in the
pumping well before, during, and after pumping. The quickest and
often the most accurate means for measuring the water levels in
the pumped well during an aquifer test is to use a pressure
transducer system. This system can be costly and may be
difficult to install in an existing well. Appendix B presents
information on two commonly used pressure transducers. Page B-8
gives a summary of chemical resistance for the SE-1000 pressure
transducer. Pages B-10 through B-13 present pertinent informa-
tion on the transducer model 570 series. This information is
presented as a summary of operating specifications which can be
applied to a number of systems. If the well is not accessible to
a sounder or a calibrated steel tape, an airline can be used with
somewhat less accuracy than electric sounder. Steel tapes cannot
be used accurately or quickly in a pumping well, except in
modified wells with a small depth to water (less than 200 feet)
where the pump test crew has a fair amount of experience. A 3/4
inch pipe is typically installed in the well as access for the
measuring device. The pipe should be capped at the bottom with
numerous 1/16 to 1/8 inch holes drilled in the pipe and cap.
This will dampen the surging caused by the pump and will
eliminate the problems caused by cascading water. Generally, the
use of a steel tape should be confined to the later stages of the
pump tests when rapid changes in water levels are not occurring.
Well Screen
. The diameter, depth and position of all intervals open to
the aquifer in the control well must be known. The diameter must
be large enough to accommodate a test pump and allow for water
level measurements. All openings to the aquifer(s) must be known
and only those openings located in the aquifer to be tested
should be open to the well during the testing. If the pumping
well has to be drilled, the type, size and number of perforations
should be established using data from existing well logs and from
information obtained from drilling the new well. The screen or
perforated interval should be designed to have sufficient open
area to minimize well losses caused by entry of the ground water
into the well (Campbell, et al., 1972 and Johnson, 1966). The
A-2
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well should be completed with a gravel pack between the well
screen and the earth material. The sizing of the gravel pack
will depend on the nature of the aquifer material and the well
screen design. The gravel pack should extend at least one foot
above the top of the well screen; a seal of bentonite pellets
should be placed on top of the gravel. At a minimum, three feet
of pellets should be used.
Well DeyeloBment
Information on how the well was drilled and completed should
be collected during the initial file review. It may be necessary
to interview the driller. If the well was not properly completed,
the data collected may not be representative of the aquifer. The
efficiency of the well, for instance, may be affected, causing
increased drawdown in the pumping well. If no information is
available on the well or there is suspicion that the well was
poorly constructed, the well should be stressed with a surge
block or a pump for up to eight hours. Sometimes, running a step
drawdown test to determine well efficiency after the well has
been surged can provide important prepump test data (Johnson,
1966 and Rorabough, 1953).
Aayifer Data Needs
Depth to, thickness of, and areal limits of the aquifer to
be tested should be known.
Nearby aquifer discontinuities caused by changes in litho-
logy or by incised streams and lakes should be mapped. All known
and suspected boundaries should be mapped so that observation
wells can be located where the well(s) will provide the best
opportunity to yield valuable information about the responsive-
ness of the aquifer.
Estimates of all pertinent hydraulic properties of the
aquifer and adjacent rocks must be made by any means feasible.
Estimates of transmissivity and storage coefficient should be
made, and if leaky confining beds . are suspected, leakage co-
efficients should be estimated. Trial calculations of well
impact using these values should be made to finalize the design,
location, and operation of test and observation wells, (Campbell
and Lehr, 1972 and Rorabough, 1953). If local perched aquifers
impact the pump test, this impact should be estimated where
possible.
Design of Observation Welles}
Verification of Well Response
The responses of all wells to changing water stages should
be tested by injecting a known volume of water into each well and
measuring the subsequent decline of the water level. A file
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search should be made to collect any available construction data
for existing wells which might be used as observation wells.
Abandoned wells tend to become totally or partially plugged, and
consequently the response test is one of the most important
prepumping examinations to be made if such wells are to be used
for observation (Stallman, 1971, and Black, et al. 1977). Any
wells which appear to have poor response should either be
reworked or replaced.
Total Depth
Generally, the observation well should penetrate the tested
aquifer to a point where the bottom of the well is at an
elevation which is the midpoint of the well screen or perforated
interval of the pumping well. This assumes that the observation
well is used for monitoring the response in the same aquifer from
which the discharging well is pumping. Actual screen design will
depend on aquifer geometry and site specific lithology. If the
aquifer test is designed to detect for hydraulic interconnection
between aquifers, the observation well should be terminated in
the stratum in which hydraulic interconnection is suspected.
Observation wells can be located above or below the aquifer
tapped by the pumping well (in addition to those completed in the
pumped aquifer). Depending on how much information is required,
additional wells in other strata may be necessary (Black, et al,
1977 and Bredeholft, et al, 1983).
Well Diameter
Observation wells should be cased with tubing having a
diameter just large enough to allow for accurate, rapid water
level measurements. A two inch well casing is adequate, under
some conditions, for use as an observation well in shallow
aquifers (depth to water of less than 50 feet). Care must be
taken in determining as accurately as possible these well water
levels. A special effort must be made to eliminate fluctuations
in the water level caused by gravel packing in the vicinity of
the borehole. However, if a water depth recorder is going to be
used, a four or six-inch diameter well casing may be required
depending on the type of recording equipment to be used. In
addition, the difficulties in drilling a straight hole usually
dictate that a well over two hundred feet deep be at least four
inches in diameter.
Well Screen
Ideally, the observation well(s) should have five to twenty
feet of perforated casing (or well screen) on the bottom of the
well. The final open interval will depend on the nature of
geologic conditions at the site and the parameters to be
estimated. The open interval and gravel pack of a given well
should not be open to more than one aquifer. The annular space
between the casing and the open hole should be gravel packed
A-4
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outside the perforated interval(s) to be tested. The size of the
gravel should be based on the grain size distribution of the
aquifer and the size of the well screen. The space above the
gravel should be sealed with a sufficient amount of bentonite or
other grout to seal off the gravel pack from vertical flow from
above. It will be necessary to put a cement seal on top of the
bentonite prior to backfilling the remaining annular space.
After installation is completed, the observation well should be
developed by surging for several hours with a surge block and/or
submersible pump (Campbell, et al, 1972, and Johnson, 1966).
Radial Distance from the Pumped. Well
If only one observation well is to be used, it is usually
located 50 to 300 feet from the pumped well. However, each test
situation should be evaluated individually, since hydraulic
conditions may warrant the use of closer or more distant
observation wells. The wells are generally placed in a straight
line which includes the pumping well, unless multiple boundary
effects are observed or suspected. In the case of multiple
boundaries, the observation wells need to be located in a manner
which will identify the location and effect of the boundaries.
If aquifer anisotropy is expected, the wells should be located
at right angles to each other (Ferris, 1962 and Hamlin, 1983).
NECESSARY EQUIPMENT FOR DATA COLLECTION
Besides the equipment to b.e installed in the well(s),
equipment is needed to measure water levels, to measure and
regulate well discharges, to record the time from the start of
the test, and to record all the data accumulated during the
testing procedure.
Water Levels
The equipment for water level measurement can be electric
sounders, airlines and pressure gauges, calibrated steel tapes,
or pressure transducers (Bentall, 1963).
Calibrated Electric Sounders
Accurately calibrated electric sounders are recommended for
use in the pumping well because they will allow for rapid,
multiple water level readings as required during the early stages
of pump/aquifer/recovery tests.
One sounder should be assigned to each observation well used
throughout the duration of the test. This is particularly
important in ground-water quality studies to prevent cross
contamination.
Each sounder to be used during the test should be calibrated
prior to the commencement of testing to assure accurate readings.
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Airlioes and. Pressure Ganges.
Airlines are only recommended for use when electric
sounders, steel tapes, or pressure transducers cannot be used to
obtain water level measurements in the wells. Their usefulness
is limited by the accuracy of the gauge used and by difficulties
in eliminating leakage from the airline. A gauge capable of
being read to 0.01 psi will be needed to obtain necessary level
of accuracy for determining water level change. A continuous
copper line of known length should be carefully strapped to the
column pipe when the pump is installed. This will minimize the
potential for leaks.
When airlines are used, each well should be assigned a
pressure gauge and the same gauge used throughout the test. Each
pressure gauge to be used should be recently calibrated to assure
accurate readings.
Calibrated Steel Tapes
Calibrated steel tapes are recommended for use in observa-
tion wells when possible, unless rapid water level drawdown or
buildup is anticipated. If rapid drawdown, cascading water, or
high frequency oscillation are anticipated, electric sounders,
float actuated recorders (which use a steel tape or calibrated
wire line) or pressure transducers are preferred (Garber and
Koopman, 1968) .
If tapes are used, the well must be equipped with a means of
dampening fluctuating water levels and preventing problems with
cascading water.
Pressure Transducers
Pressure transducers are often used in situations where
access to the well is restricted such as a well where packers are
used to isolate a certain zone. They may also be applicable in
large scale tests using a computerized system. The most common
installation uses downhole transducers with the recording of the
result taking place on the surface.
Transducers should be calibrated prior to installation.
Transducers being used should be capable of accurately detecting
changes of less than 0.005 psi. Transducer systems which will
accurately record water level changes of 0.001 feet are
available.
After installation, the transducers and recording equipment
should be calibrated by comparing pressure readings to actual
water level measurements. Measurements should be taken with a
steel tape, if possible.
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The effect of barometric changes on the transducers should
be determined prior to and during the test. This will require
continuous monitoring of the barometric pressure as well as
periodic comparisons of water level and transducer readings
(Clark, 1967).
Discharge Measur.emgnt
The equipment commonly used for measuring discharge in the
pumping well includes the orifice plate, an in-line water meter,
a Parshall flume and recorder, a V-notch weir or, for low
discharge rates, a container of known volume and a stop watch.
The choice of method will depend upon a combination of factors.
Some of the major influences are i) accuracy needed, ii) planned
discharge rate, iii) facility layout, and iv) point of discharge.
If, for instance, it is necessary to discharge the water a half
mile from the pump, a flume or weir will probably not be used.
The distance between the point of discharge control and the point
of discharge would make logistics too difficult. An in-line flow
meter or a pitot tube would be the most easily calibrated device
(Leupald, Stevens; U..S. Bureau of Reclamation; and King, H.W.).
Orifice Plate
Orifice plates and manometers are an inexpensive and
accurate means of obtaining discharge measurement-s during
testing. The thin plate orifice would be the best for a typical
pump test situation. This is a plate which has an opening
smaller than that of the discharge pipe. The manometer is
installed directly into, and onto the end of the discharge pipe.
The diameter of the borehole must be small enough to ensure that
the discharge pipe behind the plate is full at the chosen rate of
discharge. The difference shown on the manometer represents the
difference between the upstream and downstream heads.
Assuming the devices are manufactured accurately and the
equipment is installed correctly, the orifice plate will provide
an accuracy of measurement between two and five percent.
The accuracy should be calibrated prior to testing by
pumping into a container of known volume over a given time. The
test should be repeated for several rates.
In=Line Flowmeter
In-line flowmeters can give accurate readings of the flow if
they are installed properly and calibrated properly. For a
discussion of flowmeter design and calibration, see "The
Application and Calibration of Pressure Instruments, Flowmeters,
and Flow Control Devices" as applied to the Underground Injection
Control Program (UIC-QA Workgroup and SMC Martin Inc., 1987).
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Using meters it is a relatively easy way to monitor the
discharge rate by recording the volume of flow through the meter
at one minute intervals and subtracting the two readings. Some
meters register both instantaneous rate of flow and total flow
volume.
The meter should be calibrated after installation and prior
to the test.
Flumes and. Weir
A Flume is a gorge through which flow occurs. A Weir is an
enclosure placed in a flowing stream for catching floating
objects. To improve the measurements, flumes and weirs are
installed in appropriate locations in proximity to the pump.
There are numerous accurate flumes and weirs on the market.
The choice will depend mainly on the approximate discharge, the
location of the discharge point and the nature of the facility.
The cost of installation will preclude use at many nonpermanent
facilities.
The discharge canal should have sufficient upstream channel
so as to not affect the accuracy of the chosen weir or flume.
Pitot lube
The pitot tube is a velocity meter which is installed in the
discharge pipe to establish the velocity profile in the pipe.
Commercially available devices consist of a combined piezometer
and a total head meter. The tube must be installed at a point
such that the upstream section is free of valves, tees, elbows,
etc. for a minimum distance equal to 15 to 20 pipe diameters.
Since the pitot tube becomes inaccurate at low velocities,
the diameter of the pipe should be small enough to maintain
reasonably high velocities.
Contain.er of Known Volume and Stop Watch
The use of a container of known volume and a stop watch is a
simple way to measure the discharge rate of a low volume,
discharging well. The discharge rate is estimated by simply
recording the length of time taken for the discharging water to
fill a container of known volume. This method can be used only
where it is possible to precisely measure the time interval
required for a known volume to be collected. If rates are
sufficiently high so as to cause splashing in the container, or
to prohibit development of a relatively smooth surface on the
water in the container, this method is likely to be inaccurate.
The restriction of this method to flows that are less than 10 gpm
is a conservative rule of thumb.
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Discharge RegulatiQD
The size of the discharge line and the gate valve should be
such that the valve will be from one-half to three-fourths open
when pumping at the desired rate (during the initial phase of the
test) with a full pipe.
The valve should be placed a minimum of five pipe diameters
downstream from a flow meter to ensure that the pipe is full and
flow is not disturbed by excessive turbulence.
Time
A stop watch should be used during a pump/aquifer/recovery
test. Time should be recorded to the nearest second when draw-
down is rapid, and to the nearest minute as the time period
between measurement is increased, beyond 15 minutes.
If more than one stop watch is to be used during the
testing, all watches should be synchronized to ensure there is no
error caused by the. imprecise measurements of elapsed time.
Accuracy of time is critical during the early part of a pump or
aquifer test. Later in the test the time recorded to the nearest
minute becomes less critical.
A master clock should be kept on site for tests longer than
one day. This will also provide a backup in case of stop watch
problems.
Recording Forms
All forms for recording the test data should be prepared
prior to the start of the test and should be attached to a clip
board for ease of use in the field.
The forms should allow for the following data to be recorded
on every page:
o date
o temperature
o time
o weather
o well location
o well number
o owner of the well
o type of test (drawdown or recovery)
o description of measuring point
o type of measuring equipment
o radial distance from center of pumped well to the
center of the observation well (r).
o static depth to water
o person recording the data
o page number of total pages
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In addition, the forms should include columns for recording
of the following data:
o the elapsed time since pumping started shown as the
value (t)
o the elapsed time since pumping stopped shown as (tx)
o the period of observation before the start of test (t )
o
o the depth in feet to the water level
o drawdown shown as the value(s)
o the time since pumping started divided by the elapsed
time since pumping stopped shown as (t/t')
o the recovery of the water level shown as (s')
o the discharge rate shown as the value (Q)
•
o a column for comments to note any problems encountered,
weather changes (i.e., barometric changes, precipitat-
ion), natural disasters, or other pertinent data.
FIELD PROCEDURES
It is necessary to establish a baseline trend in the water
levels in the tested wells prior to 'beginning the test.
Changes in depth to water level observed during the test may
be due to several variables such as recharge, barometric
response, "noise" resulting from operation of nearby wells, or
loading of the aquifer in response to surface loading, such as
trucks, trains, etc. (King, 1982). It is important to identify
major activities which may impact the test data.
As a general rule, the period of observation before the
start of the test (t ), should be at least twice the length of
o
the pumping test.
During the baseline trend observation period, it is desir-
able to monitor and record the barometric pressure to a sensiti-
vity of plus or minus 0.01 inches of mercury. The monitoring
should continue for at least one day to a week after the
completion of the recovery measurement period. This data when
combined with the water level trends measured during the baseline
period can be used to correct changes in the water depths
measured during the test as the barometric pressure changes.
During the baseline measurements, the on-off times of nearby
wells should be recorded. The discharge rates of the wells
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should be noted as should any observed changes in the proposed
test pumping and observation wells. Significant effects due to
nearby wells can often be removed from the test data with
accuracy if the on-off times of the wells are monitored before
and during the test. Interference effects may not be observable,
but nearby pumping wells will make analysis more difficult. If
possible, the cooperation of nearby well owners should be
obtained.
When feasible, all pumping wells in the vicinity of the
tested wells should be shut off at least 24 hours prior to the
start of the test. The absence of pumping will allow the water
levels in the test well and observation well(s) to recover to
near static conditions before testing.
TEST PROCEDURES
Immediately before pumping is to begin, static water levels
in all test wells should be recorded. Measurements of drawdown
in the pumping well can be simplified by taping a calibrated
steel tape to the electric sounds wire. The zero point of the
tape may be fixed at the point representing static water level.
This will enable the drawdown to be measured directly rather than
by depth to water.
If drawdown is expected in the observation well(s) soon
after testing begins and continuous water level recorders are not
installed, then an observer should be stationed at each observa-
tion well to record water levels during the first two to three
hours of testing. Later a single observer can usually record
water levels in all wells because it is not necessary to have
simultaneous measurements.
Table A.I gives the recommended minimum time intervals to
use for recording water levels in the pumped well.
Measurements in the observation well(s) should occur often
enough and soon enough after testing begins that the initial
drawdowns will not be missed. Actual timing will depend on the
aquifer/well conditions, which vary from test area to test area.
Estimates for timing should be made during the planning stages of
aquifer testing using estimated aquifer parameters.
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TABLE A.1
MINIMUM RECOMMENDED TIME INTERVALS FOR PUMP TEST
WATER LEVEL MEASUREMENTS
0 to 15 minutes every minute
•
15 to 50 minutes every 5 minutes
50 to 100 minutes every 10 minutes
100 minutes to 5 hours every 30 minutes
5 hours to 48 hours every hour
48 hours to 6 days every 8 hours
6 days to shutdown every 23 hours
NOTE: The above are only the minimum recommended time intervals.
More frequent measurements can be taken should the test
conditions warrant.
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Discharge of fluids in the pumping well should be monitored
and recorded at the same time as drawdown measurements.
Ideally, the pretest discharge must equal zero. If it is not,
the discharge should be measured for the first time within
a minute or two after the pump is started. How frequently the
discharge needs to be measured and adjusted for a test depends on
the pump, well aquifer, and power characteristics. Output from
electrically driven equipment requires less frequent adjustments
than from all other pumping equipment. The discharge rate should
never be allowed to vary more than plus or minus 10 percent
(Fenes, personal communication). The lower the discharge rate,
the more important it is to hold the variation to less than 10
percent.
Recovery measurements should be made in the same manner as
the drawdown measurements. After pumping is terminated, recovery
measurements should be taken at the same frequency as the
drawdown measurements as listed in Table A.1.
The amount of time the aquifer should be pumped depends on
the type of aquifer, location of suspected boundaries, the degree
of accuracy needed to establish the storage, coefficient and
transmissivity, and the rate of pumping. Pumping however, should
continue until the water levels in the well(s) stabilize or the
influence of boundaries is observed. These conditions may occur
a few hours after pumping starts or may happen days or weeks
later. Some aquifer tests never achieve equilibrium or exhibit
boundary effects. Although it is not absolutely necessary for
the .pumping to continue until equilibrium is reached, it is
recommended that pumping be continued for as long as possible and
at least 24 hours. Recovery measurements should be made for a
similar period or until pre-pumping water levels have been
attained. Recovery measurements should be taken with the same
frequency of readings as the pump test. The cost of running the
pump a few extra hours is low compared with the total costs of
the test, and the difference in data should be the difference
between a conclusive or an inconclusive aquifer test.
If the water being pumped is contaminated, it must be
disposed of properly. Lined on-site water storage ponds can be
used. Pond design must take the planned length of the test and
the planned discharge rate into account.
An accurate recording of the measurements and comments
during the test will prove valuable and necessary during the data
analysis stage following the test.
During the test, a plot of drawdown versus time on semi-log
paper can be useful. This plot will reveal the effects of
boundaries or other disturbing influences if they are encountered
during the test, and it will also give some indication when
enough data for'a solution have been recorded.
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Pr.eeaytio.ns
Care should be taken that all observers use the same
measuring points for each well. If it is necessary to change the
measuring point during the test, the time at which the point was
changed should be noted and the new measuring point described in
detail, including the elevation of the new point. If a water
level should be missed at a prescribed time during the test, the
actual time the measurement was obtained should be recorded. It
is important to remember to start all stop watches at the time
pumping is started or stopped if performing a recovery test.
Comments can be valuable in analyzing the data. It is important
to note any problems, or situations which may alter the test data
or the accuracy with which the observer is working.
DATA ANALYSIS
Data analysis involves transforming the raw field data into
calculated values of transmissibility and storage coefficient.
If the design and field-observation phases of the aquifer test
are conducted successfully, data analysis should be routine and
successful. The method(s) of analysis utilized will depend on
the type of aquifer conditions, on whether the wells are
partially or totally penetrating, and on the discharge used
(constant rate or constant pressure drawdown). In most cases, it
is good practice to analyze the test data using more than one
method, especially for semi-confined or totally unconfined
aquifers (U.S. Environmental Protection Agency, 1982).
The solution of the unsteady state radial flow partial
differential equation subject to the boundary and initial
conditions of the aquifer under consideration will result in a
time-drawdown type curve analogous to the time-drawdown field
data curve.
The matching of these two curves by the method of Theis and
Jacob will give the coefficients of transmissibility and storage
of the aquifer. The plots should also be used to identify
changes in pumping trends which may represent boundaries or
leakage. After the test the data is analyzed to determine if
data corrections are needed for such things as barometric
effects, any needed corrections should be made. The final
analysis should include a repeat of the preliminary analyses
using the corrected data (unless the original effort was
corrected).
GUIDELINES FOR SUBMITTED AQUIFER TEST DATA AND RESULTS
Tabular Data
All raw data in tabular form should be used to determine
test results along with the analysis and computations. The data
should clearly indicate the well location(s), and date and type
of test.
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Graphs
All graphs or plots should be so drafted that the individual
points which reflect the measured data can be retrieved. Semi-
logarithmic and logarithmic (log-log) data plots should be on
paper scaled at three inches per cycle. All X-Y coordinates
shall be carefully labeled on each plot. All plots must include
the well location, date of test, and an explanation of any points
plotted, or symbols used.
Calculations
All calculations and data analyses shall accompany aquifer
test data submitted to the individual analyzing the results. All
calculations should clearly show the data used for input, the
equations used and the achieved results.
Results
The results of an aquifer/pump/recovery test should be in
narrative format. The narrative report should include the raw
data in tabular form, the plots of the data, the complete
calculations and a summary of the results of the test. The
assumptions made in utilizing a particular method of analysis
should be included.
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AQUIFER (RESERVOIR) TESTS
The most effective method of obtaining the aquifer parameters that
are necessary for input into analytical equations or numerical models
for pressure buildup calculations is by performance of a field pumping
or injection test.
The equations given by Warner, Koederitz et al (1979) are the basis
for interpretation of such tests and the assumptions employed in the
development of those equations apply.
During field testing, the following data are obtained:
q = pumping or injection rate
t = time since initiation of pumping or injection
Ah = change in fluid level in the observation well
or Ap = change in aquifer or reservoir pressure in the
observation well
From these data and employing the appropriate method of analysis,
the following aquifer (reservoir) parameters are obtained:
•
k = permeability
c = compressibility
Additionally, such things as well skin factor (s), distance to
boundaries, presence and nature of fractures, etc., can be analyzed.
TEST AND ANALYSIS METHODS
I. Test Methods
A. Pumping tests
1. Constant rate-single well
2. Variable rate-single well
3. Constant or variable rate with
observation wells (interference
testing)
B. Injection tests
1. Constant rate-single well
2. Variable rate-single well
3. Constant or variable rate with
observation wells (interference
testing)
II. Analysis Methods
A. Analysis of transient data
1. Curve matching (Theis or Ramey)
2. Semilog Methods (Theis, Jacob, Horner
and Miller-Dyes-Hutchinson)
3. History Matching
B. Analysis of Steady-State Data
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INJECTION WELL TESTING
INTRODUCTION
Injection well testing is similar to production well testing except
that a pressure buildup occurs rather than a pressure decline. An
injectivity test measures pressures while injection occurs, or is similar
to a drawdown test for producers. A pressure falloff test occurs when
injection ceases and pressures at the wellbore decrease; this is anala-
gous to a buildup test after a production cycle. For injection well tests,
when the injection string is- filled with liquid, surface pressures may
readily be corrected to bottom-hole as long as the density of the injected
fluid is known; this eliminates the need of running a bottom-hole pressure
sensor. The bottom-hole pressure can be calculated from
P = P f + — (1)
H Hsurf 144 ^ '
where
p = bottom-hole pressure, psi
p ,. = surface pressure, psi
3
p = fluid density, lb./ft
H = height of fluid column, ft.
which for pure water (p = 62.4 lb./ft ) becomes
P = Psurf + 0.433H
and for brine, a common approximation is
P = Psurf + 0.466H
Injection wells may be tested to determine formation permeability,
formation compressibility and to check for formation damage due to
A-17
-------
plugging, clay swelling, precipitates, etc. Such formation damage is
referred to as a "skin effect" and is measured by a "skin factor."
PRESSURE FALLOFF ANALYSIS
A falloff test consists of injecting a fluid (usually water) at a
constant rate, shutting-in the well and recording pressure readings during
the shut-in period. If a skin problem exists, it can be corrected, or if
a gradual skin effect is occurring, it can be diagnosed and prevented. A
knowledge of the reservoir permeability and compressibility will allow
future estimations of rate of injection pressure buildup.
Liquid Fillup, Unit Mobility Ratio
The basic falloff test assumes that the reservoir is liquid filled
and that a unit mobility ratio exists.
H = ^
This situation obviously exists when the fluid properties are such that
H=l; however, it also exists when the injected fluid bank has a radius
greater than the radius of investigation during the falloff test.
Assuming radial flow, the radius of the injected fluid is
(3)
rif =
5.615 W.Bl1
irh
where
r.f = radius of injected fluid, ft.
W. = cumulative injected fluid, STB
B = injected fluid formation volume factor, RVB/STC
h = net formation thickness, ft.
4> = porosity, fraction
A-18
-------
and the radius of investigation is
- 0.029
k At H
~:i<-
(4)
where
r. = radius of investigation, ft.
k = permeability, md.
t = test time, hrs.
<|> = injected fluid viscosity, cp.
c. = total system compressibility, psO
Semilog Plot Analysis
A Horner plot is constructed as shown in Fig. 1 and the slope of the
straight line section is determined.
PWS'PSI
100
10
t+At
At
Fig. 1 Horner'-Plot - Falloff Test
A-19
-------
The pressure change is defined as Ap = p f - p . The
effective permeability is then determined from
. 162.6 qyB /rx
K " mh UJ
where
k = permeability, md.
q = injection rate prior to test, STB/day
n = viscosity, cp.
B = fluid formation volume factor, RVB/STB
m = slope of the straight line section from the Horner plot, psi
h = net formation thickness, ft.
and the skin factor can be calculated using
S.MS, J!wL_fihn.log_i_T+3.23J (6)
where
p . = pressure at start of test, psi
plhr = pressure after one hour of test time obtained from the
straight line extension, psi
= porosity,-fraction
c. = system compressibility, psi"
r = well bore radius, ft.
w
Type curve analysis
The Ramey type-curves can be used to analyze falloff data.
An overlay plot is constructed to fit Fig. C.6,
and a match is obtained. Permeability is calculated from
_ 141.2 QUB PD match
h Ap match
A-20
-------
APPENDIX B
DETERMINATION OF TRANSMISSIBILITY BY
USING SLUG TEST METHOD
CHEMICAL AND PERFORMANCE DATA ON
PRESSURE TRANSDUCERS
-------
DETERMINATION OF TRANSMISSIBILITY
BY USING WATER SLUG METHOD
Given: Recovery data for a well at Speedway, Indiana, after a
39-gallon slug of water was added to well.
To Find: Use modified formula shown on following page to deter-
mine the transmissivity (T) of the aquifer.
The recovery data shown in the following tables was plotted
on an arithmetic graph of residual head (ft) vs. 1/t. There was
some scatter of points because of the difficulty in obtaining
good measurements. A straight line was then drawn through best
fit with one point on the line being the origin.
The data used in the formula was then obtained off the
graph.
•
114.6 V(1/tm)
T =
3
V =39 gallons
s = 0.261
1/tm = 0.800
2
114.F6 x 39 x 0.80 13.750 gpd/ft T artesian
T = x aquifer
0.26
B-l
-------
SLUG FORMULA
Terminology:
Symbol
Description
Unit in System Noted.
ft/day" ~ gal/ft/day
s
t
tm
V
T
Residual head above static
water level
Time since injection of slug
Time since injection of slug
Volume of slug injected
Coefficient of transmissibility
feet
days
—
cu.ft.
2
ft /day
feet
—
minutes
gallons
gpd/ft
Equations:
ft^day. system:
V (1/t)
T =
4iis
gilZft/day system:
114.6 V (1/tm)
s
T =
REFERENCE: Ground-Water Notes Series, No. 26.
B-2
-------
RECOVERY OF WATER LEVEL IN A 6-INCH TEST WELL
FOLLOWING THE INSTANTANEOUS INJECTION OF A 39-GALLON
SLUG WATER
Depth to Water
Time
(minutes)
-20
-15
-10
0
1.25
1.33
1 .50
1.92
2.17
2.30
2.37
2.42
2.67
2.72
2.77
2.92
3.00
3.22
3.28
3.33
Below Measuring
Point
(feet)
42.39
42.40
42.40
introduced
39 gallons
42.14
42.15
42.20
42.23
42.24
42.25
42.26
42.26
42.28
42.28
42.28
42.29
42.29
42. 30
42. 30
42. 30
Residual
Head
(feet)
___
0.26
0.25
0.20
0.17
0.16
0.15
0.14
0.14
0.12
0.12
0.12
0.11
0.11
0.10
0.10
0.10
1/t
( 1/minutes)
0.800
0.750
0.667
0.521
0.461
0.435
0.422
0.413
0.375
0. 368
0. 361
0.3^2
0.333
0 . 31 1
0.305
0. 300
B-3
-------
RECOVERY OF WATER LEVEL IN A 6-INCH TEST WELL
FOLLOWING THE INSTANTANEOUS INJECTION OF A 39-GALLON
SLUG OF WATER
(continued)
Depth to Water
Time
(minutes)
3. MO
3.^7
3.55
3-67
3-77
3.87
4.10
4.33
4.52
4.58
4.72
5.17
5.28
5.45
6.10
6.40
6.83
7.17
7.75
8.58
Below Measuring
Point
(feet)
42.31
42.31
42.31
42.31
.42 . 31
42.32
42.32
42.32
42.33
42.33
42.33
42. 34
42.34
42.34
42.35
42.35
42.35
42.36
42.36
42.36
Residual
Head
(feet)
0.09
0.09
0.09
0.09
0.09
0.08
0.08
0.08
0.07
0.07
0.07
0.06
0.06
0.06
0.05
0.05
0.05
0.04
0.04
0.04
1/t
( 1/minutes)
0.294
0.288
0.282
0.272
0.265
0.258
0.244
0.231
0.221
0.218
0.212
0.193
0.189
0.183
0.164
0.156
0.146
0.139
0.129
0.117
B-4
-------
RECOVERY OF WATER LEVEL IN A 6-INCH TEST WELL
FOLLOWING THE INSTANTANEOUS INJECTION OF A 39-GALLON
SLUG OF WATER
(continued)
Time
(minutes)
Depth to Water
Below Measuring
Point
(feet)
Residual
Head
(feet)
1/t
(1/minutes)
9.37
10.12
11.00
12.50
1 3.00
42.37
42.37
42.37
42.37
42.37
0.03
0.03
0.03
0.03
0.03
0.107
0.099
0.091
0.080
0.077
B-5
-------
CP
6
sf>
6
Ejtp
a
f
:rr
oJ
Q
UT peaq
B-6
-------
CHEMICAL RESISTANCE OF THE SE-1000 PRESSURE TRANSDUCER
(In-Situ Inc., Laramie, Wyoming)
-------
CHEMICAL RESISTANCE OF THE SE-1000 PRESSURE TRANSDUCER
(In-Situ Inc., Laramie, Wyoming)
ACIDS
Acetic, 5% G
Formic, 20% P
Hydrochloric, 10% F
Oleic F-G
Sulfuric 20% F
ALCOHOLS
Ethanol P
Isopropanol F-P
Isopropanol, 50% F-P
Methanol P
ALKALI
Sodium hydroxide, 20%
ORGANICS
Acetone P
ASTM Fuel A G
ASTM Fuel B F
ASTM Fuel C F-P
ASTM Oil #1 G
ASTM Oil #2 G
ASTM Oil #3 G-F
Benzene P
Brake fluid, Type A P
Brake fluid, (H.D.) F-G
Butane G
Carbon tetrachloride P
Cyclohexanone D
Dimethyl formamid D
Dimethyl sulfoxide D
1, 4-Dioxane D
Dioctyl Phthalate
Ethyl ether
Ethylene glycol
Ethylene glycol 50% H2
Gasoline, 100 octane
Hexane
Kerosene
Methylene chloride
Metheyl ethyl ketone
N-Methyl-2-Pyrrolidene
Oil, Texas crude
Oil, detergent 20W
Oil, nondetergent 20W
Oil, Skydrol type B
Oil, Skydrol type 500A
Oil, Skydrol type 500B
Oil, transmission type A
Perchlorethylene
Pyridine
Tetrahydrofuran
Toluene
Trichloroethylene
Turpentine
MISCELLANEOUS
Chlorox (5%)
Calcium chloride
saturated solution
Freon 113
Freon 11B
Freon 12
Hydrogen disulfide (5%)
Sodium chloride
saturated solution
Synthetic perspiration
Tide (1%)
Water
D
F-G
G
v
F
F-G
G
P
P
)
F-G
G
G
D
F-P
F-P
G
P
D
D
P
P
G
P
P
G
E
G
G
G
G
Legend: E - excellent, little or no change
G - good, slight loss in properties, slight swell
F - fair, swelling and some loss in properties
P - poor, significant loss of properties and significant
swelling
D - dissolves
CHEMICAL COMPATABILITY TABLE (room temperature)
TRANSDUCER CABLE JACKET AND POTTING COMPOUND
B-7
-------
PERFORMANCE INFORMATION FOR THE MODEL 570 TRANSDUCER
(AMETEK INC., Feasterville, Pennsylvania)
-------
CONTINUOUS FLUID LEVEL
MEASUREMENT SYSTEM
"»e«I»-C
'"*•"<.,
i^L%!^?
">V-«^-
ViZi*'*'-****"
V^*^!^l
:<~- (-i-taflt A«_f .fctl^r
SMte^
•^yififlafltr.
*':$t&''
FEATURING THE MODEL 570 SERIES
PRESSURE TRANSDUCER
coNtRoLs biVi^loN
-------
A POTENTIALLY DANGEROUS
SITUATION OBSERVED
Lack of rainfall, combined with the
needs of our ever-increasing popula-
tion, are depleting the water supply
in many parts of our country.
The danger, of course, is that
once a well is pumped down to a
point where the underlying water
table (or aquifer) cannot replenish
it, the well will be drawn dry. Once
this happens, that water supply can
usually be written off for good.
Period.
IT'S NEEDED NOW
The good news is that a well that is
allowed to recharge itself can be
operated almost indefinitely. And
obviously the key to effective well
management is continuous, accurate
waler level monitoring—at costs ac-
ceptable for widespread industrial
and municipal requirements.
Until recently, water level moni-
toring was accomplished by using a
complicated compressed air method.
This system required an elaborate
hook-up consisting of air-compres-
sors, air lines, galvanized pipe and
periodic visits by personnel to read
well levels. Clearly, there was a need
for a simpler, cost-effective method.
Twin Transducers Mounted on Pump
Casing.
B-9
THE MODEL 570 SERIES
TRANSDUCER. IT'S REALLY
A "SIMPLE MACHINE"
The transducer is less than 5" long
and weighs less than 13 dunces.
The Model 570 Series Transducer
is simply suspended in the well by a
shielded electronic cable. Con-
tinuous level data is sent to our
above ground digital meter con-
tinuously—making it possible to
monitor the water level.
Installation is simple and inexpen-
sive. And the system is virtually
maintenance-free. And because of
its small size, the Model 570 Series
can be retrofitted into many existing
systems—keeping conversion costs
to a minimum.
THE MODEL 570 SERIES. THEY
ARE ACCURATE, RELIABLE
AND BUILT TO LAST
The system includes a transducer
(sensor), cable and electronic read-
out unit.
The heart of the sensing unit is a
diffused "sensing chip" that reacts
to external pressures to send up a
continuous fluid level reading. The
transducer body is 300 series stain-
less, and the electrical connection is
a 4 wire, 20 gauge shielded cable.
There are no moving parts to jam,
clog or corrode.
-------
Fluid level measurement doesn't lust mean water
And although the Model 570 Series
was designed as a water well unit, its
construction can make it suitable
for a wide range of environments.
It's available in eight ranges: 0-5,
0-15, 0-30, 0-60, 0-100, 0-150, 0-200
and 0-300 psig. Applications other
than water well monitoring might
include process fluid containers,
storage tanks, canals, dams—you
name it. The Model 570 Series can
be designed to meet almost any in-
TYPICAL APPLICATIONS
dustrial fluid level measurement re-
quirement.
RELIABLE PERFORMANCE
BEGINS HERE.
Primarily because our own rigid
quality control standards are ap-
plied throughout the entire manu-
facturing process—from the sensing
chip to the final assembly of the
transducer. All critical assembly is
done in a semi-clean room—a pre-
caution which is crucial during the
manufacture of diffused semi-con-
ductors—to provide stable trans-
ducer performance.
This is just one example of how
AMETEK quality control standards
are applied—and why our custo-
mers in industrial applications and
municipal agencies can depend on
the Model 570 Series Transducer
system for continuous, accurate and
reliable monitoring of our most
precious resource.
SPECIFICATIONS
TRANSDUCER
Ranges:
PSI: 5, 15, 30, 60, 100, 150, 200, 300
FT. H:O: 12, 35, 69, 138, 230, 346, 460, 700
Media compatibility: Ranges 30,60,100,150,200, 300;
Potable well water, gas or liquid not affecting stain-
less, all welded construction.
Ranges 13,15, 30; Potable well water, any media not
affecting silicon or epoxy based adhesives.
Operating temperature limits: +33°F to 150 °F
Compensated operating temperature: +40°F to 80 °F
Overall system accuracy: ±3%
DIGITAL READOUT
Readout in Engineering units
14.2 mm high LED display
Single or dual set point control
Control and alarm capability
.01% maximum error
Contact closure output (5 AMP)
Analog output available
Power requirements—110VAC ±10%—3 watts
B-1Q
-------
SUBMERSIBLE 5,15, 30 PSI RANGE
29.4
too
12.7
r
1000
23.4
*
THREADED
•M2 UNC-2 B i
(2-HOLES)
DEEP
BED I *|INPUT
REOSHEATM BLACK |-I INPUT
SILVER GROUND
WHITE I -I OUTPUT
GREEN SHEATH BLACK NOT USED
SILVER GROUND
GREEN ( + > OUTPUT
BLUE SHEATH BLACK NOT USED
SILVER GROUND
REDUCING BUSHING • -L51- HEX
2C.8
-
LE)
15,7
- -
275
M.«
]
1.M
{~2
1
42.2
(APPHOX)
U NPT (FEMALE)
REDUCING BUSHING • -~- HEX
312
< 1M
1
42.2
(APPROX)
SUBMERSIBLE DIAPHRAGM SEAL 60, 100, 150, 200, 300 PSI RANGE
THREADED
•t-32 UNC-2 B i -~- D€EP
(J-HOLES)
RED(f) INPUT
RED SHEATH BLACK {-) INPUT
SILVER GROUND
WHITE (-) OUTPUT
GREEN SHEATH BLACK NOT USED
SILVER GROUND
GREEN (»| OUTPUT
BLUE SHEATH BLACK NOT USED
SILVER GROUND
REDUCING BUSHING • 1°? HEX
20,9
2M
U NPT (MALE)
31.7
J1.7
LENGTH OF WIRE
AS REQUIRED
4. BY CUSTOMER-
REDUCING BUSHING
1—
1
U NPT (FEMALE)
t (
J12
1
SYSTEM ORDERING INFORMATION
CABLE
OPTION
METER DIMENSIONS
i. « . «i L, «• j
p(1B8)*] ^(24) p (3781 ^
""
4 0
17)
1
.
=
==
S
^
W
s
=
~
=
=
r
1 ^_f-
. 4SO .
135 4
(3
\
^
13)
x_ SLIDE
RETAINER
— CASE
oxmoxm,
Hi ' ifP
i
1
1
141 5
(5.57)
.1
)\ SLIDE
RETAINER
/ ' T"
570
PANEL CUTOUT
NOTES: DIMENSIONS IN MILLIMETERS ,23 MM
AND IN (INCHES) 1.01 IN.
EXAMPLE- Moo*/ 570, 0-5 ptl.112 II. ol wtttr, 200 U. ol ctbl» with dull iff point
mtttr 57CHX5-200-D-2.
RANGE PSI
12 FT.
35 FT.
69 FT.
138 FT.
230 FT.
346 FT.
460 FT.
890 FT.
WATER
WATER
WATER
WATER
WATER
WATER
WATER
WATER
005
015
030
060
100
150
200
300
WRITE IN t OF FEET
STO—3 FT.
METER OPTIONS
N = NO SET PT.
S = SINGLE SET PT.
D - DUAL SET PT
READOUT DISPLAY
1 • %
2 • FT. OF H,O
3 - METERS OF H.O
«o:«
"»»:SJ)
1 1 1 06) R
4 PLCS
PANEL
, THICKNESS
64(24| MAX
0>(03)MIN
REAR VIEW
(TERMINAL BLOCK COVER AND BE2EL
NOT SHOWN FOR CLARITY)
CLAMP RINGS ROTATED AND SLIDE RETAINERS
REMOVED AS SHOWN FOR INSTALLATION
1C < 81
\METEK
CONTROLS DIVISION • 860 PENNSYLVANIA BLVD., FEASTERVILLE, PA 19047
TELEPHONE: (215) 355-6900 TELEX: 83-4799
-------
APPENDIX - C
Example of Evaluation of Constants
-------
This appendix is written for the purpose of showing the
reader how constants appear in some formulas. As an example, we
shall use relation (2) which appears on page 41 of the text.
This formula is -
162.6 Q u B
M = -— (1)
K h
As indicated above, the point is to demonstrate how the
number 162.6 was arrived at.
First let us begin by saying that if M, which has the
dimension of pressure is expressed in dyne per square centimeter
and if Q is expressed in cubic centimeter for second and u the
2
viscosity in dyne sec/cm and if K and h are respectively
expressed in square centimeter and centimeter, the constant 162.6
in the above formula will not be needed. This is shown in this
manner:
3
(Q) cm (ju) dyne sec (B)
x
2
(M) dyne sec cm
= (2)
2 2
cm (K)cm x cm(b)
Making suitable simplifications in the second member of the
above relation, one would easily show that:
M dyne = Q p B dyne
(3)
cm2 k h cm2
As one can easily see if all the variables are expressed in
the C.G.S. system, there will not be any need to have a constant
in the above formula. However, if practical units are to be used
the following conversions must be made:
2 5
M in psig = M in dyne/cm x 10 71.4504 (4)
3
Q in bbl/day = Q in cm /sec x 1.8401 (5)
2 -3
ju in cps = ju in dyne-sec/cm x 1.8245x10 (6)
C-l
-------
2
K in mds = K in cm /1.018x10 (7)
h in feet = h in cm x 30.48 (8)
B is a dimensionless factor.
Combining relations (4), (5), (6), (7) and (8) with
relations (3), results in
162.6 Q (Bbl) x u(cp) x B
day
M(psig) =
K(mds) h(ft)
Therefore 162.6 is nothing but a conversion factor resulting
from the use of practical engineering units.
c-2
-------
APPENDIX D
INTERPRETATION OF FORMATION TEST PRESSURE CHARTS
(Excerpted from Murphy, undated material)
-------
INTRODUCTION
A number of papers have been written on the interpretation
of formation test charts. These papers have been very helpful in
guiding geologists, engineers and testers in a correct interpre-
tation and subsequent evaluation of the test. Other papers of a
more technical nature have been written concerning theoretical
methods of calculating formation characteristics from formation
test data. The purpose of this paper is to expand on the inter-
pretation of current practices of formation testing and present
proved and accepted formulas with practical examples for
calculation of formation characteristics.
Testing for formation productivity was first accomplished by
allowing the fluids to move to the wellbore and measured by
bailing the open hole or swabbing the casing. With the advent of
rotary drilling, a method of testing through the drill-stem was
introduced by Halliburton in 1926. The method became known as
"Drill Stem Testing."' However, current trends are to call it the
"Formation Testing." This method proposed for temporarily
relieving the hydrostatic head of drilling fluid from the
formation by the use of downhole tools without removing such
fluids from the hole. This was done by setting a cone packer on
the shoulder of a reduced hole which had been drilled ahead into
a potential oil bearing formation. A simple gear-operated valve
provided the opening and closing operations of the test. A
pressure recording device was introduced in the string to verify
the operation of the tools. Later the need for more reliable
formation pressures became apparent; therefore, one or more
gauges of greater accuracy were used to record these pressure
changes. The present day combination of tools provides for very
accurate flowing and build-up pressures recorded in relation to
time. This information recorded on a chart offers an important
aid in evaluating a formation. The first, and the most important
step in this evaluation, is a correct interpretation of the
pressure charts.
1. INTERPRETATIONS
A number of preliminary checks should be made to compare
pressures with known data. Hydrostatic pressures should be
checked to determine if they are consistent with reported mud
weight and depth. This is the first indication that the gauges
are recording correctly. The flowing pressures are indicative of
drill pipe fill-up; therefore, the final flow pressure should
equal the hydro-static head of recovery in the drill pipe. This
D-l
-------
will not be true if the bottom-hole or surface choke has caused a
back pressure during fluid or gas entry. It is very important to
accurately measure each type fluid recovered, and its density, so
that the recovery may be confirmed or to calculate net fluid
fill-up. The initial flow pressure on the chart should reflect
the use or the absence of a water cushion. Horizontal lines,
while going into the hole, indicate a delay and may indicate a
water cushion was being introduced at that time. This is
especially true during the early part of the operation.
Horizontal lines near the end of this operation may indicate a
single was being picked up.
The base line is the basis for all pressure measurements on a
formation test chart. The pressure lines on the chart should
zero in and out of the hole. In other words, the stylus should
coincide with the base line before going into and after coming
out of the hole and any vibrations of the gauge should be noted
as fluctuations of the stylus at right angles above and below the
base line. If the base line and the initial pressure recordings
do not coincide, the difference in these two readings is the
amount of error incurred by the incorrect drawing of the base
line. This error may be minimized by reading all pressures from
an imaginary baseline drawn through the initial and final
fluctuations. Fig. 1 shows the base line inconsistent with the
initial pressures of the test.
Sharp reciprocating movements co-incident with each stand
pulled or added may be an indication of swabbing or a tight hole.
Fig. 2a indicates that considerable difficulty was encountered in
attempting to reach bottom. Under normal conditions this action
will have no bearing on the test itself, unless the test tool
opens during prolonged spudding through a bridge. This indicates
poor hole conditions and could contribute to sticking a tool.
At points of delay, while going into the hole or a total
depth, a decrease in recorded pressure is indicative of a loss in
the hydrostatic head as shown by Fig. 2b. There are two
possibilities:
NOTE: All chart illustrations are sections of the actual chart
and have been photographically enlarged or reduced to
show the greatest detail.
D-2
-------
FIG. 1- Base line drawn incorrectly,
FIG. 2 - (a) Tight hole indicated,
(b) Leaking drill pipe,
D-3
-------
FIG. 3 - A Stair-stepping gauge,
FIG. 4 - Clock stopped,
D-4
-------
FIG. 5- Clock "running away",
FIG. 6 - Leaking dual closed-in-pressure valve,
D-5
-------
1. The hole may be taking fluid.
2. The drill pipe may be leaking.
If the fluid recovery contains an abnormally large quantity
of mud, a leak in the pipe may be assumed. This deems the flow
period not usable: therefore, production calculations cannot be
made.
A "stair-stepping" appearance in a build-up curve, as shown
in Fig. 3 is caused by guage malfunction; therefore, this curve
should not be considered for calculation purposes. This
mechanical problem is caused by an intermittent escapement of the
chart drum and may be attributed to any of the three known
causes.
1. The Chart Drum Lugs and/or Inner Case Runners may be
dirty.
2. The Lead Screw may not be straight.
3. The Inner Case or Cover may be crooked or rough.
The clock is that part of the gauge assembly which measures
the time of each of the operations of a formation test. It
operates as an escapement mechanism for the chart drum. Although
time is independent of pressure recordings, ,it is important for
interpretation of results. Proper selection of clock speeds will
aid in accurate interpretation and reading. Excessive rough
treatment mechanical difficulty, or lack of power may cause the
clock to stop running. This problem is characterized by time
discontinuance, as shown in Fig. 4, while the pressure continues
to increase and or decrease in the same vertical line. For a
clock to stop and to start running again before the test is
complete makes a very confusing chart; yet, it will become quite
clear when compared with its companion chart. A clutch spring
malfunction may cause the clock to "run away" as shown in Fig. 5.
This occurrence is usually caused, by excessive rough treatment
but does not damage the clock.
Closed-in pressure equipment that leaks will cause erratic
build-up curves that should not be used to extrapolate static
reservoir pressure. However, if this leak occurs after sufficient
closure, an extrapolation may be made on that portion of the
uninterrupted curve. Leakage is characterized by a decrease in
the build-up pressure and normally a subsequent rise,, as shown in
Fig. 6, but not necessarily so. In the event that the tool
washes out, the build-up would tend to equalize with the flowing
pressure.
D-6
-------
Build-up curves in formation testing are essentially made up
of three components:
1. Initial inflection
2. Curved section
3. Straight section
The initial inflection of the curve is developed immediately
after the starting of the closed-in period and is influenced by
conditions near the wellbore; however, the use of this part of
the curve is difficult to establish such conditions. In
formations with extremely low permeability this portion of the
build-up curve may not seem to exist. The curved section of the
build-up curve is an indication of the rate of the build up of
the bottom-hole pressure in that area drained by the previous
flow period. Its rate of build-up is influenced by damage near
the wellbore, formation permeability and reservoir pressure. The
straight section is the portion of the build-up curve that is
tangent to the curved section and has very little change in rate
of build-up. It is this section of the curve that is the most
conclusive evidence of its slope. The remaining section of a
build-up curve is that portion which has little or no change in
rate. Because of the long shut-in time required, this portion of
the curve is not usually attained during a formation test.
However, this section reflects the most accurate measurement of
the static reservoir pressure. The rate of build-up of this
portion of the curve is influenced primarily by the drainage at
the extremeties of the radius of investigation during the test.
A common point of great concern in a dual closed-in
operation of formation testing is the frequent differences in the
initial and final build-up pressures. For the same time duration,
it is quite normal for the initial build-up curve to be slightly
greater in pressure, than the final build-up curve. This may be
attributed to a shorter first flow period (less disturbance)
resulting in a greater rate of build-up. This condition will not
affect the ultimate static conditions of the curve.
Supercharge is the most common condition resulting in an
abnormally high initial build-up of pressure. During drilling or
completion a formation may have an invasion of pressure due to
excessive hydrostatic head or pressure surges. This pressure
invasion is called supercharge. This abnormal pressure is
suddenly alleviated when the first flow period is commenced. The
time required for this pressure to equalize with reservoir
pressure depends upon the magnitude of pressure invasion,
permeability, and formation damage. If a closed-in pressure is
attempted prior to the dissipation of this supercharge, the
resulting build-up curve will exhibit excessive pressures, as
shown in Fig. ?a.
D-7
-------
FIG. 7 A
D-8
-------
An unreliable build-up curve is sometimes difficult to
distinguish from a good curve. This distinction can only be made
by comparison; therefore, it is almost imperative that a dual
closed-in pressure be taken, in order that a visual comparison
between the initial and final closed-in pressure curves can be
made. This, however, is not conclusive within itself. A further
comparison should be made by extrapolating these pressures. Both
closed-in pressure curves should extrapolate to the same value.
If the initial extrapolates noticeably higher than the final (see
Fig. 7b), one must determine why this occured before a valid
evaluation of the formation may be made. It therefore becomes
necessary to have a first flow long enough to dissipate the
pressure that is not inherent to the formation. A "rule of
thumb" cannot be made for this operation, but the action in the
bubble bucket is the best indicator. A very slow bubble rate is
an indication of low productivity, therefore, a longer first flow
period is desirable. A very permeable formation will sometimes
require as little as 5 minutes flow time; however, most wells
require as much as 20 minutes to completely dissipate super-
charge. If the permeability is exceptionally low, as much as 30
minutes or more may be required.
Some objection to this practice is based on the belief that
the first pound of reservoir pressure that moves after closing
the tool for a closed-in pressure should contribute to the
initial build-up curve. This results in the shortest duration of
build-up time with no reservoir drawdown prior to shut-in. This
is ideal and theoretically correct; however, an operator is at a
loss to know how he may do this. In attempting to dissipate the
supercharge, a drawdown of the reservoir is often noted. This is
of no consequence since the subsequent build-up curve will
extrapolate to the static reservoir pressure. If this drawdown
does cause a decrease in static pressure, then it may be assumed
that the reservoir pressure will decrease rapidly when the well
is placed on production. This condition is due to a limited
reservoir.
A depleting reservoir will also exhibit a high initial
build-up curve; however, it will differ from the supercharged
condition in that a good first flow was observed. Furthermore,
the flow period will indicate a decreasing rate of flow. The
difference in the extrapolated static reservoir pressures of the
initial and final build-up curves indicates the loss in reservoir
pressure during the second flow period. These conditions are
D-9-
-------
FALSE BUILD-UP
Ps
o>
LU
CC
LLI
DC
a.
X
Log
t + 0
FIG. 7B - Extrapolation indicating a false build-up of pressure,
D-10
-------
indicated in Fig. 8 and Fig. 9 for a depleting liquid and gas
reservoir, respectively.
If a test exhibits these characteristics, then it would be
advantageous to go back into the hole and run an initial flow of
sufficient duration to definitely establish that all supercharge
has been removed. Closed-in pressures of such durations that
were dictated by the previous test should be run for comparison
of the reservoir pressure drop during this test. The remainder
of the time that can be safely spent in the hole can be used for
the second flow period. A minimum back pressure should be
maintained to create a maximum drawdown on the formation. With
the above information a more accurate determination of the well's
future behavior can be made.
A barrier may be detected in the build-up curve on a
formation test. If the barrier is indicated in only the final
build-up curve, the initial will have a higher pressure but it
should extrapolate to the same static pressure as the final. The
final build-up curve, as shown in Fig. 10, is peculiar in
appearance as compared to the initial. It has two radii: a
sharp radius after the initial inflection, and a long radius in
the remainder of the build-up. This unusual appearance indicates
a barrier. It is sometimes rather hard to see this disturbance
in the buildup curves, but it becomes quite evident when the
extrapolation plot of the build-up curve abruptly breaks and
doubles its slope, as in Fig. 11, a barrier is assumed to be
present. This peculiarity is caused by a reflection of build-up
pressure from an impermeable barrier. This action may be
compared to the bouncing of a ball off a wall. An extrapolation
plot of the build-up curves does not orient or define the nature
of the barrier; however, it may be assumed that it is within the
radius of investigation of the test. Static reservoir pressure
is derived from that portion of the plot with the greatest slope.
If "slipping the packer" is necessary to reach the bottom,
the initial flow should be lengthened to compensate for this
piston action or supercharge of the formation. This pressure
invasion must be relieved prior to taking the initial closed-in
pressure for a valid curve to be recorded. The initial flow rate
is not usable in many cases because of the abnormally• high
production rate resulting from supercharge. If slippage of the
packer prematurely opens the tester valve, an abnormal quantity
of mud may be recovered.
D-li
-------
FIG. 8
D-L2
-------
FIG. 9 - Depleting gas reservoir,
FIG. 10- Indicates a barrier within the Radius
of Investigation.
D-13
-------
BARRIER
Ps
o>
w
Q.
I
111
oc
D
HI
oc
Q.
Log
t + 0
FIG. 11
- Extrapolation of a build-up curve indicating a barrier,
D-14
-------
Many build-up curves are not usable for the purpose of
extrapolation of static reservoir pressure because of insuffi-
cient closure resulting from very low formation permeabilities.
Approximately 75% closure must be attained for an accurate
extrapolation plot of the build-up curve. This figure will vary
with well conditions; that is, if a single phase is produced,
less closure will be required while multiple phases usually
require more closure. Fig. 12 illustrates low permeability and
low reservoir pressure while Fig. 13 illustrates a similar per-
meability with a high reservoir pressure. These curves are
characterized by a very slow rate of increase in pressure, which
requires a longer closed-in time for sufficient closure. Many
times these build-up curves resemble a triangle, but if given
enough time, they will develop into a good closure. The previous
flow period must be used as a guide to determine the duration of
closed-in time. For example, if the surface blow is very weak,
the flow period should be extended for at least 30 minutes, or
more in some cases, ' to relieve all supercharge. The build-up
time should be at least 50? longer than a normal flow period.
Coupled with pressure, the recovery is the most important
piece of information derived from a formation test. The recovery
of oil or gas is significant, but not conclusive, unless a rate
is known. If the well flows, the production rate of fluid may be
measured in a stock tank or the gas rate measured with a Pitot
Tube, orifice Well Tester, or other measuring devices. If a
well does not flow, the drill pipe recoveries of different fluids
should be measured with great care and the densities of each
reported. The sum of the hydrostatic pressures of the recovery
should equal the final flow pressure. In the event that a
complete flow period is plugging, the production rate does not
reflect the capabilities of the formation to produce. If a
portion of the flow period, either before or after plugging, is
uniform, the rate can be calculated using a pressure change
D-15
-------
FIG. 12 - Low permeability formation with a low
reservoir pressure,
FIG. 13 - LOW permeability formation with a high
reservoir pressure,
D-16
-------
during that period. This condition is indicated in the latter
part of the final flow period, as shown in Fig. 14.
A build-up curve which suggests a damaged formation does not
affect the use or the extrapolation of such. In many cases this
curve resembles a supercharged formation, but can usually be
distinguished by a visual comparison with the other build-up
curve. This question will arise quite frequently when consider-
ing the initial closed-in pressure. A damaged zone for a liquid
system may be characterized by:
1. The very sharp rise after shut-in.
2. A short radius curve.
3. A reasonably flat slop.
4. A high differential pressure between closed-in and final
flow pressure'.
High capacity gas wells exhibit .the same characteristics due
to the back-pressure through the bottom-hole choke. However,
damage is also indicated on a gas system by the same characteris-
tics, but they may be a little more difficult to distinguish.
This type of build-up curve is created by a pressure drop across
a low permeability (damaged) zone near the wellbore; therefore,
only a small quantity of fluid or gas need be produced after the
tool is closed to reach static reservoir pressure. Damage may be
present in a high, or low productive formation, as shown in
Fig. 15 and Fig. 16, respectively. A dual closed-in pressure
type formation test many times offers the opportunity to
determine if a zone with damage will clean itself up during a
flow period. The initial build-up may no longer be present after
the final flow period, as shown in Fig. 17. This indicates that
the damage was removed during the flow period.
The "S" shaped build-up curve is very misleading. In its
early development, an increase in rate of build-up is noted with
closed-in time. This early part is even more confusing if it is
not developed to the normal part of the build-up curve. However,
if given enough time, it would reverse in its direction and close
as a conventional build-up of formation energy. This type build-
up curve may be caused by one of three known conditions:
D-17
-------
FIG. 14 - Plugging flow period with a uniform segment,
FIG. 15 - High productivity - High damage.
D-18
-------
FIG. 16- Low productivity - High damage
FIG. 17- Formation damage indicated on initial
build-up - No damage indicated on the
final build-up.
D-19
-------
1. Vertical permeability through the formation around the
packer transporting energy from the hydrostatic head in the
annulus. See Fig. 18.
2. Gas may come out of the solution on low permeability
wells during the flow period and go back into solution during an
increase of pressure in the build-up period. Therefore, this
after production of oil would cause an increase in the build-up
rate. See Fig. 19.
3. Large valume below the Dual Closed-In Pressure Valve or
Disc Sub (set high) compared to the flow capacity of the
formation. See Fig. 20.
It is very difficult, and many times impossible, to
distinquish between these three conditions. A relatively short
duration "S" curve may be caused by after production, while the
long duration "S" curve may be attributed to vertical perme-
ability or compressibility under a Dual Closed-In Pressure
Valve or Disc Sub set high. In any case, an unreasonable permea-
bility and reservoir pressure will deem this type of curve
unreliable.
The second flow period of a conventional formation test
should be of such duration as to relieve abnormal pressure
invasion and establish a uniform rate of flow. A flow period
longer than this will be rig time wasted and could be utilized to
a greater advantage. In some instances, a longer flow period can
spoil the build-up curve. If the well is allowed to equalize
before attempting a closed-in pressure, as illustrated in Fig.
21, a build-up pressure cannot be taken nor can the equalized
flow pressure be used. In the event a decrease in rate at the
bubble bucket is noted; a closed-in pressure should be taken
before the formation equalizes with the produced column of fluid
in the drill pipe.
Plugging is one of the most common technical problems
confronted in formation testing. It is usually characterized by
sharp pressure fluctuations, if the plugging alternately plugs
and frees itself, as in Fig. 1. However, when the plugging is
sustained momentarily, the action will as small build-up curves,
as shown in Fig. 22. If this occurs in the tool both charts will''
look alike. V
**
Some wells are swabbed during a formation test. This action
will cause a drawdown in the flow period outlined by a decreasing
sequence of small "inverted fishhooks," as shown in Fig. 23. A
subsequent flow and swabbing action may very often be noted.
D-20
-------
FIG. 18- "S" curve indicating possible vertical
permeability,
FIG. 19 - "S" curve indicating possible after-
production of bypassed gas,
D-21
-------
FIG. 20 - "S" curve developed by fluid compression
resulting from a Disc Sub set too high,
FIG. 21 - Equalized flow,
D-22
-------
FIG. 22- Plugging flow,
\
FIG. 23 - Swabbing,
D-23
-------
The flow pressures of a single phase flow are usually very
uniform; however, a combination of phases will result in an
irregular pattern. If gas is breaking through the fluid, this
pattern may be very "lumpy" in appearance. A uniform "rippling"
appearance, as illustrated in Fig. 24, occurs when a well is
flowing in heads. The prettiest and smoothest charts usually
have water recovery.
An abrupt change of rate in the flow period can usually be
attributed to a change in the I.D. of the pipe. Ordinarily, this
change to a slower rate will indicate when the fill-up has
cleared the drill collars (see Fig. 25).
A pressure chart should be evaluated by comparison in the
final analysis. Two gauges should be run on all tests: One in
the flow stream, and the other "blanked-off" from the flow
stream. By comparing the charts from the two gauges that have
been run under these conditions, a reasonable assumption of the
validity of the charts may be determined. A common problem in
formation testing is to evaluate a test as "dry" on the basis of
the top chart. When the test is evaluated on the basis of both
charts, it may reveal "plugging" flow perforations. This
condition is illustrated in Fig. 26 and 27. The top chart
indicates little or no change in flowing pressures, while the
"blanked-off" bottom chart indicates an increased pressure. Both
of the charts should be similar on a conventional single pack-off
test; other-wise, plugging of the flow perforations or some other
malfunction is evidenced. On a straddle test with equalizing
tube, the bottom "blanked-off" chart (below both packers) should
reflect the hydrostatic head. If an equalizing tube is not used,
this gauge will reflect a drawdown during the test due to the
seepage into the formation below the bottom packer.
D-24
-------
FIG. 24 - Flowing in heads,
FIG. 25 - Fill-up transition from drill collar
to drill pipe,
D-25
-------
FIG. 26 - Plugging in the flow perforations as
indicated by. both charts,
FIG. 27
D-26
------- |