Module 15
Pressure Interference Testing
Pressure Interference Testing
Bob A. Hardage
Design Objectives
This module has a two-fold objective: to demonstrate how fluvially deposited
reservoirs tend to be segmented into compartments and to illustrate how
pressure interference tests can confirm that compartmentalization exists within a
reservoir interval.
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Knowledge Base
A map of the study area is shown in Figure 1. The solid dots show production
wells, many drilled at 40-acre spacing. The circled dots locate wells where
geologic and engineering control was acquired in the form of modern logs, cores,
or pressure tests. The 3-D seismic area is approximately 7.6-mi2. VSP control
data were recorded in two closely spaced wells, 175 and 202, inside the triangle
near the center of the 3-D seismic grid. The pressure interference test data that
will be analyzed were recorded between these same two wells.
Figure 2 is a horizon slice through the 3-D seismic data volume that cuts through
a targeted thin-bed reservoir system. Figure 3 is an enlarged view of the
reflection amplitude behavior on this horizon slice in an area of four critical
control wells, 75, 175, 197, and 202. The problem that is to be addressed is to
determine if these four wells penetrate the same reservoir compartment, or are in
different compartments and have no fluid-flow connection between them.
Static reservoir pressures measured during a 1-month period in the four wells are
shown in Figure 4. The bottom-hole pressure in well 75 was 300 psi at
abandonment. The different reservoir pressure at these four well locations
suggests that each well may be in a different reservoir compartment.
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A stratigraphic cross section of the thin-bed reservoir along the 4-well profile that
shows the depositional environments interpreted from log shapes and the initial
bottom-hole pressure (BHP) observed in each well is illustrated in Figure 5. The
date when each well was drilled is labeled above the BHP value. The log depths
are measured from the well’s kelly bushing (KB), and the curves are shifted to
align on a stratigraphic datum. The log curves infer that the reservoir in each well
was deposited in a channel environment that shows some evidence of splay
deposition. These log data, by themselves, do not provide enough information to
define where compartment boundaries may exist.
The seismic reflection amplitude behavior on a stratal time surface through the
reservoir interval penetrated by the four wells is helpful in developing a reservoir
compartmentalization model. The seismic image (Fig. 3) indicates some possible
compartment boundaries. For example, a likely cause of the compartment
boundary that separates well 197 from the other wells is the depositional
variation that creates the red/blue (positive/negative) amplitude changes that
trend north-south between crossline coordinates 130 and 140. Similarly, a
probable seismic indication of the compartment boundary that segregates well 75
from the other wells is the positive-to-negative (red-to-blue) amplitude variations
trending north-south between crossline coordinates 110 and 120. However, this
same logic of looking for inter-well reflection amplitude changes does not explain
why there is a compartment boundary between wells 175 and 202, which are
only 200 ft (61 m) apart. There is no appreciable change in the reflection phase
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Pressure Interference Testing
between these two wells (i.e., there is no color change in the reflection amplitude
plot between wells 175 and 202).
Our objective is to do a pressure interference test between wells 175 and 202 to
determine if they are connected in a fluid flow sense, or if there is a compartment
boundary in the inter-well space that prevents horizontal fluid flow.
Concept of Pressure Interference Test
A diagram of a simple pressure interference test is shown in Figure 6. Well 2 is
the pressure input well. Wells 1 and 3 are pressure observation wells. The idea is
to inject a known pressure pulse into the perforated zone of the input well and
then record the pressure reaction in the observation wells.
In this example, the pulse should propagate to well 1, but the impermeable
barrier between wells 2 and 3 should prevent the pulse from being transferred to
well 3. By interpreting the pressure response (or lack of response), reservoir
engineers can conclude that wells 1 and 2 penetrate a common compartment,
but well 3 is in a separate compartment.
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Exercise 1
A pressure interference test will be simulated using the 6-well pattern in Figure 7
(the figure can be clicked on, but only the well locations should be shown
(no responses). The objective is to determine how the reservoir system
spanned by the wells is divided into compartments. All six wells produce from the
same stratigraphic interval.
Well 4 will be the input well for the pressure test. Click on well 4 to define the
pressure pulse that is applied to the reservoir at that well’s coordinates. (show
figure 7 minus the responses or arrows here. When “well 4” is “clicked”,
the pressure response graph and arrow with text should appear in the
lower left-hand corner of well 4).
Click on wells 1, 2, 3, 5, and 6 in succession to see the response that is
measured in each well. (each time a well is clicked, it’s corresponding
response graph and arrow is shown and left on the figure).
Using the tools provided, draw reservoir system compartment boundaries. Once
you have finished, interpret the geologic facies for the system by using the fill
tools for either sand channel facies or shale facies. For further help, select “Ask
the Expert.” When you are through with your interpretation, select “Finished” to
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Pressure Interference Testing
compare your results with the expert’s version. (Expert’s answer = Figure 8).
Need to give the users a solid line tool to use for “compartment
boundaries,” a long-dashed line for “partial flow barriers,” and a short
dashed line for “severe flow barrier.” Need fill tools for: sand channel
facies (yellow fill in the expert’s version), and shale facies (brown fill in the
expert’s version)
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Ask the Expert (1)
Reservoir Compartment: What does the term mean?
The concept of a reservoir compartment is controversial, and researchers have
different opinions about how to define the terms compartment and
compartment boundary and about what reservoir properties should be used to
support those definitions. A popular definition is that two points (A and B) within
a reservoir system are in different compartments if oil or gas cannot flow between
points A and B in a time period that satisfies the economic requirements for
production. This definiton is popular with people who judge reservoir
connectivitiy from the economic view of how efficiently fluid moves within the
reservoir system. Other people think that reservoir compartrments should be
defined on the basis of some fundamental variation in the petrophysical
porperties of the reservoir facies, such as a significant change in porosity or
permeability. A third community of people prefers that observed differences in
engineering-related parameters, such as reservoir pressure, production rate, or
fluid chemistry, be used to define the internal compartmented architecture of a
reservoir system. In this module, we will infer the presence or absense of
compartment boundaries by observing effects on pressure pulses that travel
through the pore spaces that connect perforation zones in our well with
perforation zones in another well.
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Exercise 2
The following pressure interference test was done in one of the fluvially
deposited Frio reservoirs in Stratton field, South Texas.
Wells 175 and 202 are separated only 200 ft, which is a short distance (Fig 5). It
is remarkable that the two wellbores appear to be on opposite sides of a
compartment boundary (Fig. 4).
A pressure interference test will be done to determine if wells 175 and 202 are or
are not in pressure communication. The test has to check the following
possibilities:
1. Is the perf zone in well 175 connected to both perf zones in well 202 (Fig.
5)?
2. Is the perf zone in well 175 connected to only one perf zone in well 202
(Fig. 5)?
3. Is the perf zone in well 175 not connected to either perf zone in well 202
(Fig. 5)?
To conduct the pressure interference test, perform the following actions in
sequential order: (still Fig. 5 – but need to zoom in on just wells 202 and 175,
with perf Zone 1 and 2 labeled)
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Pressure Interference Testing
1. Click on perf zone 1 in well 202 to set isolation packers around that perf
interval. (need to interactively draw a packer symbol above and below
zone 1)
2. Click on the perf zone in well 175 to create the pressure pulse that was
introduced into the reservoir. (when they click on the zone, figure 9
pops up with the pressure pulse line bolded and drawn animated
from left to right)
3. Click on perf zone 1 in well 202 to retrieve the pressure response that was
observed. (when they click on the zone, figure 9 is shown again, this
time with the line from step 2 drawn, and then the straight, dashed
response from this zone bolded and drawn animated from left to
right)
4. Click on perf zone 2 in well 202 to set isolation packers around that perf
interval. (same animation as for #1, just for zone 2 instead)
5. Click on the perf zone in well 175 to create the pressure pulse that was
introduced into the reservoir. (same as #2)
6. Click on perf zone 2 in well 202 to retrieve the pressure response that was
observed. (same as #3)
Question: The perf zone in well 175 is connected to which perf zone in well 202?
A.
Perf zone 1
B.
Perf zone 2
C.
Both zones 1 and 2
D.
Neither zone 1 nor zone 2.
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Question: Is there a compartment boundary between wells 175 and 202?
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Ask the Expert (2)
A compartment boundary can shut off lateral fluid-flow and act as an absolute
barrier to pressure variations that occur on one side of the boundary. Conversely
it can “almost” shut off lateral fluid-flow and allow an attenuated version of a
pressure pulse to propagate between wells that are on opposite sides of the
boundary. Both types of compartment boundaries are depicted in this problem.
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Ask the Expert (3)
Note the length of time required for a good pressure interference test, which is
approximately 10 days in this real-life example. Note also the length of time
required to build a good pressure cycle; typically one to two days.
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References
Hardage, B. A., Levey, R. A., Pendleton, V., Simmons, J., and Edson, R.,
1994, A 3-D seismic case history evaluating fluvially deposited thin-bed
reservoirs in a gas-producing property: Geophysics, v. 59, p. 1650-1665.
Figure Captions
Fig. 1. Map of the study area. The solid dots show production wells, many
drilled at 40-acre spacing. The circled dots locate wells where additional
geologic and engineering control was acquired in the form of modern logs,
cores, or pressure tests. The 3-D seismic area is approximately 7.6-mi2. VSP
control data were recorded in two closely spaced wells, 175 and 202, inside
the triangle near the center of the 3-D seismic grid. The pressure interference
test was done between these same two wells.
Fig. 2. Static reservoir pressures measured during a 1-month period in the
four wells shown in Figure 5. The bottom-hole pressure in well 75 was 300 psi
at abandonment. The different reservoir pressure at these four well locations
suggests that each well may be in a different reservoir compartment.
Fig. 3. Stratigraphic cross section of the reservoir along the 4-well profile
showing the depositional environments interpreted from log shapes and the
initial bottom-hole pressure (BHP) observed in each well. The date when
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each well was drilled is shown above the BHP value. The log depths are
measured from KB and the curves are shifted to align on a stratigraphic
datum. The log curves infer that the reservoir in each well was deposited in a
channel environment that shows some evidence of splay deposition. These
log data, by themselves, do not provide enough information to define where
compartment boundaries may exist.
Fig. 4. Seismic reflection amplitude behavior on a stratal time surface through
the reservoir interval penetrated by the four wells. The seismic image
indicates some possible compartment boundaries. For example, a likely
cause of the compartment boundary that separates well 197 from the other
wells is the depositional variation that creates the red/blue (positive/negative)
amplitude changes that trend north-south between crossline coordinates 130
and 140. Similarly, a probable seismic indication of the compartment
boundary that segregates well 75 from the other wells is the positive-tonegative (red-to-blue) amplitude variations trending north-south between
crossline coordinates 110 and 120. However, this same logic of looking for
inter-well reflection amplitude changes does not explain why there is a
compartment boundary between wells 175 and 202, which are only 200 ft (61
m) apart. There is no appreciable change in the reflection phase between
these two wells (i.e., there is no color change in the reflection amplitude plot
between wells 175 and 202).
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