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CASE HISTORIES
CASE 3 - INTERWELL TRACERS ADD LOW-COST
RESERVES
An understanding of reservoir heterogenic properties is
critical in the design and operation of all secondary and tertiary
flood projects. During injection operations, the injected fluid will
flow preferentially into the high-permeability intervals and
fractures. This type of fluid movement in the reservoir reduces
sweep efficiency, increases lifting costs, and reduces oil recovery.
The use of interwell tracers provides the only direct means of
tracking the movement of the injected fluid in the reservoir,
thereby allowing the identification of these reservoir
heterogeneities. Once these heterogeneities are known, steps can be
taken to minimize their negative effect on both operating costs and
oil recovery.
The following example illustrates how
interwell tracers were used to identify water channeling and to
assist in the design of gel polymer treatments, which resulted in
the addition of new reserves for a field under
waterflood.
In mid-1992 a West Texas
operator implemented a waterflood on a 280-acre lease producing from
the San Andres formation. The San Andres has a gross thickness of
approximately 350 feet. Core analysis indicates it is composed of
dolomitized grainstones and packstones separated by laterally
continuous shale intervals. The Upper San Andres is characterized by
its intergranular porosity and eight producing intervals, whereas
the Lower San Andres exhibits moldic porosity, with moderate natural
fracturing, and only three producing intervals. Because of this
difference in lithology, water injection into the San Andres was
managed with the use of isolation packers and flow regulators as
shown in Figure 1.
Within 6 months of initial
water injection, lease production increased from an average rate of
225 BOPD to a rate of 645 BOPD. By the end of the first year,
problems began to appear. Several wells began showing significant
increases in water production, resulting in high fluid levels that
could not be pumped down. In addition, the high H2S content in the
untreated injection water began deteriorating the flow regulators,
resulting in an adverse change in the vertical distribution of the
injected water.
Therefore, a two-phase
interwell tracer program was developed to assist in diagnosing these
problems. The objective of the Phase I program was to identify which
injection and producing wells were in communication. The Phase II
program was then designed to determine which of the 11 San Andres
intervals under waterflood were responsible for the communication.
Two pattern areas on the subject lease were selected for the
interwell tracer program.
In
February 1993 the Phase I interwell tracer program was implemented.
The program consisted of injecting tritiated water (HT0) into WIW
Nos. 33 and 40, and ammonium thiocyanate (CNS-) into WIW Nos. 38 and
39. The results of the Phase I program are shown in Figure 2. Tracer breakthrough occurred in
seven producing wells with breakthrough times ranging from less than
1 day to 15 days. These results were used in the design of the Phase
II interwell tracer program.
To
prepare for Phase II, the original injection intervals were again
isolated in WIW Nos. 33, 38, and 40. This allowed for the injection
of a different interwell tracer into each of the three Injection
Intervals in the San Andres formation. During Phase II, ethanol and
ammonium thiocyanate were injected into the Upper Injection
Interval, acetone and amino-g were injected into the Middle
Injection Interval, and tritiated water and ammonium nitrate ( N03-)
were injected into the Lower Injection Interval. The placement of
these interwell tracers are shown in Figure 3. Note that no tracers were
injected into WIW No. 39 during the Phase II program.
The results of the Phase II program are
illustrated in Figure 4. Rapid
breakthrough occurred in the Lower Injection Interval of the San
Andres formation where the tritiated water and ammonium nitrate were
used. The tracers injected into the Upper Injection Interval did not
show up at any of the producing wells in the pattern area. Tracer
breakthrough occurred in the Middle Injection Interval in only one
well in the two pattern areas.
The two interwell tracers
programs indicated not only that channeling occurred between
injectors and producers in both pattern areas investigated, but that
the majority of the channeling occurred in the Lower Injection
Interval. This information allowed the design of polymer gel
treatments for the injection wells to improve areal sweep efficiency
in the Lower Injection Interval and to provide improved vertical
conformance over all 11 productive intervals in the San Andres
formation.
In each of the four
injection wells in the two pattern areas, the Lower Injection
Interval was isolated and an aqueous acrylamide polymer solution was
pumped along with a chromic triacetate crosslinker. The total pumped
volumes ranged from 5,200 bbl to 8,100 bbl of the polymer solution,
with the polymer concentration increasing during the treatments from
3,000 ppm to 8,500 ppm.
Ten of the offset producing
wells began to display increases in oil production within a month of
the pumping of the injection well gel polymer treatments. The peak
response from the offset producing wells was an incremental 125 BOPD
with no increase in water production. The sustained response over a
period of 18 months was an incremental 75 BOPD. Incremental
reserves, resulting from the interwell tracer program and polymer
gel treatments, were calculated at 200 MBO for a cost of
approximately $3.00 per STBO.
In summary, the use of
interwell tracers identified and confirmed that communication
existed between many of the injectors and producers in the
waterflood area. In addition, the second phase of the interwell
tracer program allowed the identification of the vertical interval
responsible for the majority of the communication. This information
was critical in the development of the resulting polymer gel
treatments. Without the Phase II program results, the gel treatments
would have been designed based on the entire San Andres interval
thickness of 350 feet. Instead, the gel treatment designs used the
Lower Injection Interval thickness of approximately 50 feet,
resulting in a significant savings in the cost of the gel
treatments. The combination of using interwell tracers and gel
treatments not only increased production on the lease, but also
allowed the addition of approximately 200 MBO of reserves at a cost
just over $3.00 per STBO.
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