United States
                   Environmental Protection
                   Agency
 Robert S. Kerr Environmental
 Research Laboratory
 Ada, OK 74820
                   Research and Development
EPA/600/S2-90/022 Sept. 1990
c/EPA         Project Summary
                   A Feasibility Study of the
                   Effectiveness  of  Drilling
                   Mud  as a Plugging  Agent  in
                   Abandoned  Wells
                   Marvin D. Smith, Randolf L. Perry, Gary F. Stewart, William A. Holloway, and
                   Fred R. Jones
                    The Hazardous  and Solid Waste
                  Amendment of 1984 requires  the
                  Environmental  Protection  Agency to
                  assess environmental suitability of
                  liquid-waste injection into  subsurface
                  rock.  Accordingly,  the reaction
                  among injected wastes,  reservoirs,
                  and original formation fluids is under
                  evaluation.
                    The main  objective  of the
                  feasibility study described here was
                  to test the hypothesis that properly
                  plugged wells  are  effectively sealed
                  by drilling mud. While achieving such
                  an objective, knowledge  of the  dy-
                  namics of building mud cake on  the
                  wellbore-face is obtained,  as well as
                  comprehension of  changes that  oc-
                  cur in  drilling mud  from the time it is
                  placed in  a well  until  it  reaches
                  equilibrium.
                    A system   was developed  to
                  simulate (a) building mud cake in a
                  borehole, (b) plugging the well, and
                  (c) injecting salt water in a nearby
                  well, with  concomitant migration of
                  salt water into  the  plugged well. The
                  system "duplicates"  reservoir pres-
                  sures,  mud pressures, and reservoir-
                  formation characteristics  that devel-
                  op while mud  cake  is built,  as in
                  drilling a well.  Salt-water injection is
                  simulated, to  monitor  any fluid
                  migration through the reservoir.
                    A 2100-ft. well and ancillary equip-
                  ment permit controlled variation of
                  simulated  depth, porosity and per-
                  meability  of reservoir rock, fluid
composition, fluid pressure, injection
pressure, and mud properties. Data
can be  recorded continuously  by
computer.
  The synthetic-sandstone reservoir
is cylindrical, 3 ft. in diameter and 2
ft. thick. It has porosity and perme-
ability similar to those of several
natural reservoirs.
  Pressures  commensurate  with
those  in 5000-ft.-deep wells were to
be measured;  associated differential
pressures were required.  A system
developed to  measure differential
mud pressures  includes undim-
inished pressure-transmittal by dia-
phragm-interface.
  Also,  a  high-pressure,  low-flow-
rate,  high-accuracy  flow meter
system was developed to monitor the
slightest amount of fluid movement.
Flow meters  were developed  to
measure (a) fluid from  the  reservoir,
(b) mud-column flow from above the
reservoir,  and  salt water being
injected.
  An in-place system  provides for
extensive  testing  of the many
variables that  influence effective
plugging of boreholes.
  This  Project Summary  was
developed by EPA's  Robert S.  Kerr
Environmental  Research Laboratory,
Ada, OK,  to announce key findings of
the  research project  that is  fully
documented in a  separate  report of
the  same title (see Project Report
ordering information at back).

          {ZA) Printed on Recycled Paper

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Introduction

   The Environmental Protection Agency
is  required by the Hazardous and Solid
Waste Amendment of 1984 to assess the
environmental suitability  of injection of
liquid wastes into subsurface formations.
The  Agency's approach to this matter is
composed of three general activities: (1)
to  evaluate the construction  of injection
wells and the capability  for monitoring
them, in order to  detect  failures;  (2) to
assess the relationship among  the rock-
stratigraphic units, the fluids injected,  and
the integrity  of the  bounding  confining
beds; and  (3) to evaluate the reaction
among the injected waste, the formation,
and the  formation fluids.
   The primary objective of the research
described here is to  test this hypothesis:
Drilling  mud  in  abandoned,  properly
plugged  wells  effectively  seals  the
borehole. Therefore,  if fluids injected  into
reservoirs at  depth were  to  migrate up
the boreholes of properly  plugged wells,
filter cake  nevertheless  would  prevent
passage  of  these  fluids into  other
reservoirs. The alternate  hypotheses
need no elaboration
   A 2100-ft.  well and ancillary facilities
compose  a system  that  permits
controlled variation  of simulated  down-
hole conditions, including depth, porosity
and  permeability of reservoirs,  composi-
tions of  fluids,  pressures  of  fluids,
injection pressures,  and  properties of
plugging  agents.  Instrumentation  was
designed  and assembled, or manufac-
tured, in order to  test the feasibility of
monitoring variation  in  pressures and
rates of flow of  fluids,  under several
regimes of injection. Computer software
was  written for continuous reception  and
recording of  data.  Methods  were
developed for construction of an artificial
sandstone reservoir; porosity and perme-
ability of this reservoir and  some actual
reservoirs are similar.

Procedure
   The  facility is designed  for testing
under conditions that simulate  a well
plugged for abandonment. In actual wells,
the wellbore  above  the  protected zone
(Figure 1) is to be filled with drilling mud
and  capped  with a  cement plug. The
specific  requirements are in regulations
set out by the states. A zone in the upper
region  of  the simulated  well  is  an
underground  source  of  drinking  water
(protected  zone,  Figure  1),  and  the
intention is to not contaminate it. Below
the fresh-water-bearing formation  is  a
formation  used for  injection  (Figure 1),
pressurized by disposal of salt water  into
a  nearby well.  Pressure  is  translated
through the  injection  zone  to  the
abandoned well.  Therefore, a potential
exists for salt water  to  migrate  up the
wellbore and  invade  the  underground
source of drinking water. The purpose of
the testing design  is  to determine the
array of conditions  that  could  allow
invasion  of the  zone  of fresh water to
occur.
   The testing facility  is divided into four
basic areas,  which are  associated  with
zones in a plugged and abandoned well,
shown  diagrammatically in  Figure 1.
These areas are dedicated to study of the
wellbore  above the  reservoir   being
protected (Figure  1,  Region  1),  the
protected reservoir and wellbore (Region
2),  the  wellbore below the  protected
reservoir, and the salt-water disposal
reservoir (Region 3), and the overall part
of the  facility  that simulates drilling the
well and building mud cake on  the wall of
the wellbore.  Regions  1  and 2 shown in
Figure  1  are simulated  by facilities
located  above ground  level,  whereas
Region 3 is an actual well, 2100 ft. deep.
The  part of  the  facility that  simulates
building of  mud  cake  is  also  above
ground.
   Figure 2 is a  plan view of the  facility.
Individual systems are required to obtain
quantitative data  on results of injecting
salt water into a  reservoir and the effects
of invasion on a shallow,  fresh-water-
bearing  formation in a nearby abandoned
well  The Instrumentation Building  houses
the computer  used for data acquisition.
About 15 ft. east of the building,  at the
site labeled  "Artificial  Reservoir"  (Figure
2) is  the Instrumentation Console, the
main source of test data. The Assembly
Stand  (Location  A,  Figure  2)  is  the
mounting stand for the reservoir housing,
used  when  the  artificial  reservoir is
poured  (Figure 3) and for  determining
porosity and permeability of the reservoir.
The  salt-water tank,   lines  and  pump,
effluent  tank  and connecting lines, mud
tank, mud mixer, mud pump, controls and
pipe  network  are  clustered   in  the
northeastern part of the facility  (Figure 2).
Casing and tubing are stored on the pipe
rack and are  moved to  Location B
through the v-door on the northern part of
the pipe rack.
   Figure 4  is a functional schematic
drawing  of  the  system. It shows the
general  configuration of the components,
their  interconnections, controls,  and
instrumentation.
   To simulate a drilling stage and then a
plugging stage, the reservoir first  is filled
with water under pressure commensurate
with the depth being simulated. Then the
drilling  operation  is  simulated  by
circulating mud from  bottom to  top  past
the porous medium, which is maintained
at reservoir pressure.  Mud in the column
is maintained at the pressure appropriate
for depth of the well  and density of the
mud. This process is continued until  mud
cake is fully developed - when there is
no more  flow of filtrate into the reservoir.
   Because  communication from  an
injection well through  a subsurface
injection  zone  has the potential of mixing
salt water  with  drilling  mud   and
considerably  raising the pressure in  the
mud  column,  it  is  not  sufficient  to
simulate  the only direct effect that depth
and borehole volume  have  on  the
process.  Thus it was determined to make
possible  a range  of  depths from about
200 ft. to 2000 ft. This was accomplished
by drilling the 2100-ft. well, cementing it
from  bottom  to top,  and  placing a  full
open head on the top casing joint with 5
1/2-in.  slips.  Casing  can  be run  in  the
hole to the  desired depth and  hung on
the casing-head slips. Rather than drilling
an  adjacent well and  injecting salt water
in it, hoping that  some  of the salt water
would  get to  the test  wellbore,  the
simulation is done by running  a  string of
1  1/4-in.  tubing on the  outside  of the 5
1/2-in.  casing  and supplying  salt water
directly into the  casing at  the  injection
point. Injection pressure for the  saltwater
is supplied by  an  accumulator  with
nitrogen  in the bladder and the column
head of  salt water going to the  injection
point.
   In  large reservoirs, at places distant
from the borehole, reservoir pressure is
maintained until a large amount of fluid is
injected  into  the  reservoir. Because a
virgin fresh-water  reservoir is simulated in
the case at hand,  and  because  this
reservoir pressure  would influence the full
development  of mud cake,  a  constant
reservoir pressure must be maintained.
Pressure is developed by a nitrogen-filled
accumulator bladder.
   The largest feasible  artificial  reservoir
was desired.  Expense and  handling-
operations were the limiting factors.  The
resulting  dimensions of the reservoir
housing  are 2 ft.  in height and  3  ft.  in
diameter. The  synthetic-sandstone reser-
voir (Figure 3) is composed of quartzose
sand and resin. Mixing and pouring were
designed  to  simulate  porosity  and
permeability of actual reservoirs. Fluid is
injected through the core of the reservoir.
Associated with this reservoir housing is
a  hose system (Figure 5),  to provide a
path for fluid forced out of the reservoir at
its periphery to be directed to the effluent
tank (Figure 3).

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                             Salt Water Injection
                                                                                      Ground
                                                                                       Level
Cased Salt Water
Disposal Well
                                                                                                     Region #2
                                                                                                     Region #3
          Figure 1. Representative injection and abandoned we/I.
   Information  from  this  reservoir  is
acquired with differential-pressure trans-
ducers,  pressure transducers, temper-
ature  sensors  and a  flow meter. These
data,  in  conjunction  with  the  axial
differential pressure, will  provide the
reservoir radial pressure gradient.  This
gradient  will be used for  permeability
calculations  and  for  correlating  the
potential  invasion  flow-rate across the
mud cake.
           In  order  to  determine  the  mud
         characteristics and dynamic behavior  of
         the  mud  column in  the injection area, a
         sequence of differential-pressure transdu-
         cers was placed on the 5 1/2-in. casing
         and  run  down-hole.  Signals  are  trans-
         mitted to the  computer by a multiplexer,
         which requires only one cable from the
         surface. Multiple sensors can be attached
         to the casing from a series of locations
         below the  multiplexer.  The multiplexer
can  serially select  a given sensor  and
send that part of the signal up-hole, cycle
to the next and repeat the operation until
all sensors are sampled.

Conclusions  and
Recommendations
1. Feasibility of designing and equipping
  a shallow well for the purpose of the
  experiment has been demonstrated.

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                                                     OSU Petroleum Technology
                                                           EPA Protect

                                                      Wellsite Location Layout
        Pipe Rack for
        5 12" Casing
                                                                 20' 0'
      Figure 2.  Plan-view schematic drawing of test facility.
2.  A technique  and  hardware  were
   developed  to measure down-hole
   pressure gradients accurately.
3.  A multiplexer to transfer  data from
   down-hole  to  the  surface was de-
   signed and  built, as were a computer
   board and software, to accept, process
   and store data.
4.  Other equipment designed,  built, and
   developed  included  a diaphragm-seal
   housing  assembly, a  temperature-
sensor circuit, a flow-meter and  flow-
control system (for uncommonly low
rates of flow  at high  pressure), and a
mud-maintenance,  mud-flow  network
and control system.
An  artificial   reservoir  with  lithic
properties,  porosity, and  permeability
similar to actual  injection-formations
was  constructed,  complete  with
housing and attendant instrumentation.
After  initial guidance by  Halliburton
Company, techniques were developed
for composing, mixing, emplacing and
consolidating  reservoir  material, to
obtain porosity and permeability within
specified limits.  Moreover,  methods
were developed  to isolate   and
measure radial flow through the  large
artificial  reservoir.
A cased-well system,  designed  and
constructed,  allows  simulation of
conditions below the artificial reservoir

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  Figure 3. Artificial reservoir, nearly completed. Several "lifts" of fine-grained material emplaced and compacted within the hardened,
           coarse-grained outer shell of reservoir.

of depths  as great as  2000 ft.  and
controlled injection of fluids at depths
of 100 to 2000 ft. The  facility  could,
and  should,  be  used to  define the
entire array  of  critical  conditions  of
mud-plugging.  Also, it  should  be
employed for experimentation  and
development of  new products  and
techniques for protecting  fresh-water
aquifers.

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Figure 5.  Artificial-reservoir housing assembly.
                                                                                •&U.S. GOVERNMENT PRINTING OFFICE: 1993 - 7M-07I/MMHI

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   Marvin D. Smith, Randolf L Perry, Gary F. Stewart, William A. Holloway, and Fred
        R. Jones are with Oklahoma State University, Stiltwater,  Oklahoma 74078
   Don C. Draper is the EPA Project Officer (see below).
   The complete report, entitled "A Feasibility Study of the Effectiveness of Drilling
        Mud as a Plugging Agent in Abandoned Wells,"  (Order No.  PB 90-227
        232/AS; Cost: $31.00, subject to change) will be available only from:
            National Technical Information Service
            5285 Port Royal Road
            Springfield, VA22161
            Telephone: 703-487-4650
   The EPA Project Officer can be contacted at:
            Robert S.  Kerr Environmental Research Laboratory
            U.S. Environmental Protection Agency
            Ada, OK 74820
United States                   Center for Environmental Research
Environmental Protection         Information
Agency                         Cincinnati OH 45268
Official Business
Penalty for Private Use $300

EPA/600/S2-90/022

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