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, S_or 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 S_or 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 S_or 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
• Hydrocarbon-miscible

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.

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Case study

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:

1. An SWCT test for S_wc (see Connate water saturation evaluation)
2. A multiple tracer test to determine oil/water fractional flow vs. saturation
3. A waterflood to reduce the near-wellbore oil saturation to near-residual conditions
4. An SWCT test for S_or
5. Injection of a bank of hydrocarbon-miscible solvent
6. A waterflood to displace the IOR bank away from the test well
7. A second SWCT test for S_or, 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 S_or measured in step 4 is 32 ± 3%.

• 

  Fig. 1 – One-spot pilot evaluation of hydrocarbon-miscible IOR before IOR bank.

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 S_or of 6% to produce the best fit, the late layer appears to have an S_or 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.

• 

  Fig. 2 – One-spot pilot evaluation of hydrocarbon-miscible IOR after IOR bank.

Noteworthy papers in OnePetro

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External links

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See also

Single well chemical tracer test

Well to well tracer tests

Miscible flooding

Enriched hydrocarbon miscible flooding case studies

PEH:The_Single-Well_Chemical_Tracer_Test_-_A_Method_For_Measuring_Reservoir_Fluid_Saturations_In_Situ