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Connate water saturation evaluation
In certain situations, it is necessary to obtain a reliable measurement for connate water saturation (Swc) in an oil reservoir. The single well chemical tracer (SWCT) method has been used successfully for this purpose.
Applications
The SWCT method has been used successfully for this purpose in six reservoirs.[1] The SWCT test for Swc usually is carried out on wells that are essentially 100% oil producers. The procedure is analogous to the SWCT method for Sor, taking into account that oil is the mobile phase and water is stationary in the pore space.
Because oil is the mobile phase, it is used to carry the chemical tracers into and back out of the formation. The ester dissolved in oil is injected and then pushed away from the well. The first bank carries the ester plus material-balance tracer, as in the Sor test. The push bank contains only the material-balance tracer.
During the flow into the formation, the ester partitions between the oil and water phases. This partitioning slows the ester velocity only slightly because the ester is more soluble in the oil phase. After the push step, the ester bank is located 10 to 20 ft from the wellbore.
During the 2- to 10-day shut-in period, a portion of the ester hydrolyzes (reacts with the connate water) to form the alcohol product tracer. The product alcohol is much more soluble in the connate water than in the oil phase.
The well then is placed on production. Samples of the produced oil are taken frequently and analyzed for tracer content. The unreacted ester comes back first because it travels at nearly the same velocity as the carrier oil. The product alcohol is produced later because it travels more slowly than the oil. Because of its preferential solubility in the stationary water phase, the alcohol has a large β value compared to that of ester. The difference in arrival volumes between the ester and the delayed alcohol tracer thus is a function of Swc.
As in the SWCT test for Sor, β can be calculated directly from the concentration vs. produced volume profiles. Using the partition coefficients measured in the laboratory, Swc can then be calculated. However, the best-fit Swc result actually is determined by matching the field tracer profiles using our multilayer simulator program for the connate water test.
Case study
An example of a production profile from a SWCT test for connate water is shown in Fig. 1.[1] The target formation was in the Prudhoe Bay Field on the Alaska North Slope, US. The reactive ester injected was isopropyl acetate, which was dissolved in light diesel oil. The product tracer in this case was isopropyl alchohol (IPA).
Fig. 1 – Field data and simulated tracer profiles for an SWCT test for Swc.
The best-fit simulation model for this case used three layers, to account for the slightly nonreversing flow behavior. The injected ester-weighted average result was Swc = 0.15 ± 2%. This is in close agreement with the average Swc determined from oil-based cores on the same well.[1]
References
- ↑ 1.0 1.1 1.2 Deans, H.A. and Mut, A.D. 1997. Chemical Tracer Studies To Determine Water Saturation at Prudhoe Bay. SPE Res Eng 12 (1): 52-57. SPE-28591-PA. http://dx.doi.org/10.2118/28591-PA.
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See also
Single well chemical tracer test
Residual oil evaluation using single well chemical tracer test