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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