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Evaluating IOR potential with single well chemical tracer test
The single well chemical tracer (SWCT) test can be used to evaluate an Improved oil recovery (IOR) process quickly and inexpensively. The one-spot procedure takes advantage of the nondestructive nature of the SWCT method.
Testing reduction in residual oil saturation
The single-well (one-spot) pilot is carried out in three steps. First, Sor for the target interval is measured (see Residual oil evaluation using single well chemical tracer test. Then an appropriate volume of the IOR fluid is injected into the test interval and pushed away from the well with water. Finally, the SWCT test is repeated within the treated region. Typically, the entire process is completed in a few weeks, as compared to the much longer duration of pilot programs, even small-patterned ones.
The reduction in Sor observed in the postflood SWCT test is a measure of the IOR flood performance. By using several esters with different partition coefficients, it is possible to measure Sor as a function of radial position from the test well. This procedure also can be used to evaluate the stability of the IOR process because tracer test volume is increased relative to the IOR displaced volume.
- The single-well IOR pilot concept has been applied in a number of different projects, in the course of which the following IOR processes have been evaluated:
- Surfactant, surfactant-polymer, and alkaline-surfactant-polymer (ASP)
- CO2 miscible
- Caustic and caustic-polymer
In every case, the process was evaluated successfully using the SWCT test. In most of these sequences, the IOR fluid injection and push steps were carried out by the SWCT test field group as a part of the field test program. A typical single-well pilot evaluation of an IOR process requires four to six weeks to complete.
In-situ evaluation of hydrocarbon-miscible IOR
A series of SWCT tests was performed in the Aurora Field on the Alaska North Slope, US, to provide an in-situ formation evaluation and to evaluate hydrocarbon-miscible IOR. The target well produced 100% oil.
The steps in the test sequence were:
- An SWCT test for Swc (see Connate water saturation evaluation)
- A multiple tracer test to determine oil/water fractional flow vs. saturation
- A waterflood to reduce the near-wellbore oil saturation to near-residual conditions
- An SWCT test for Sor
- Injection of a bank of hydrocarbon-miscible solvent
- A waterflood to displace the IOR bank away from the test well
- A second SWCT test for Sor, to determine the post-IOR oil saturation in the flooded region
Steps 4 through 7 constitute the "one-spot" pilot for the hydrocarbon-miscible IOR process.
The results of step 4 are shown in Fig. 1. The field data are plotted, along with the best-fit four-layer simulation model. Two nonideal effects are indicated by these data:
- Nonreversing flow is evidenced by the early hump and the extended tail on the tracer profiles.
- A radial gradient of oil saturation apparently was present in the formation because the waterflood of step 3 was not large enough to reduce the formation to true residual oil.
The four-layer model can account for both of these effects. The ester-weighted Sor measured in step 4 is 32 ± 3%.
The field data and best-fit simulation of the aftertest (step 7) are shown in Fig. 2. Again, two distinct nonideal effects are apparent:
- Nonreversing flow again is a factor. An early hump and a tail indicate that small layers are needed to improve the fit of the simulation model.
- In this case, the late layer, which produces the tail, also is anomalous regarding oil saturation. Whereas both of the other layers require an Sor of 6% to produce the best fit, the late layer appears to have an Sor of 26%. A likely explanation for this remarkable difference is gravity override by the miscible solvent bank. There appears to be a layer that was not contacted by the IOR bank.
- Sheely Jr., C.Q. and Baldwin Jr., D.E. 1982. Single-Well Tracer Tests for Evaluating Chemical Enhanced Oil Recovery Processes. J Pet Technol 34 (8): 1887-1896. SPE-8838-PA. http://dx.doi.org/10.2118/8838-PA
- Holland, K.M. and Porter, L.T. 1983. Single-Well Evaluation Program for Micellar/Polymer Recovery, Main and 99 West Pools, West Coyote Field, California. Presented at the SPE Annual Technical Conference and Exhibition, San Francisco, California, 5-8 October 1983. SPE-11990-MS. http://dx.doi.org/10.2118/11990-MS
- Edinga, K.J., McCaffery, F.G., and Wytrychowski, I.M. 1980. Cessford Basal Colorado A ReservoirCaustic Flood Evaluation. J Pet Technol 32 (12): 2103-2110. SPE-8199-PA. http://dx.doi.org/10.2118/8199-PA
- Cockin, A.P., Malcolm, L.T., McGuire, P.L. et al. 1998. Design, Implementation and Simulation Analysis of a Single-Well Chemical Tracer Test To Measure the Residual Oil Saturation to a Hydrocarbon Miscible Gas at Prudhoe Bay. Presented at the SPE Annual Technical Conference and Exhibition, New Orleans, Louisiana, 27–30 September. SPE-48951-MS. http://dx.doi.org/10.2118/48951-MS
- Carlisle, C.T. et al. 2002. Aurora Single Well Tracer Test (SWTT) Results. Poster presented at the SPE Western North American Region/Pacific Section AAPG Joint Meeting, Anchorage, 20–22 May.
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