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.
Knowledge Base
A map of the study area is shown in Figure 1. The solid dots show production wells, many drilled at 40acre 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 3D 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.
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 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.
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 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)
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 engineeringrelated 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.
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)
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
5.
6.
animation as for #1, just for zone 2 instead)
Click on the perf zone in well 175 to create the pressure pulse that was introduced into the
reservoir. (same as #2)
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.
B.
C.
D.
Perf zone 1
Perf zone 2
Both zones 1 and 2
Neither zone 1 nor zone 2.
Question: Is there a compartment boundary between wells 175 and 202?
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.
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.
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 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-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 between these two wells (i.e., there is no color change in the reflection amplitude
plot between wells 175 and 202).