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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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