Evaluating Residual Oil Saturation using the Single-Well Chemical Tracer Test Method (SWCTT)

By: Wei Tian and Alex Darnley

Introduction:
The single well chemical tracer test (SWCTT) is a technique which utilizes chemical tracers to determine the residual oil saturation (Sor) within key sections of the reservoir. Residual oil saturation is defined as the fraction of pore volume occupied by the oil after displacement. Determining residual oil saturation is a critical step before, during, after any IOR/EOR project to improve reservoir characterization, reduce risk, and forecast recoveries in the field. The single well chemical tracer test is a simple, cost-effective, and well-scale method of calculating residual oil within approximately 20’ from the wellbore. This basic procedure for the SWCTT method involves injecting chemical tracers into the target wellbore, shutting in the well, and flowing back the well for approximately 1-2 weeks.

A partitioning tracer, typically an ester, is required for precise measurement of oil saturation during a SWCTT. Partitioning tracers are chemical compounds that are soluble in both oil and water by reaching an equilibrium concentration in both phases. The degree of partitioning between oil and water phases is controlled by the partition coefficient. Each specific partitioning tracer has a unique partition coefficient, and the partition coefficient is dependent on the physical and chemical features of the target zone or reservoir. The injected ester will react with water to produce a mixture of acid and alcohol, which acts as the conservative water tracer during the test.

Ester+H_2 O=Acid+Alcohol

Advantages of the SWCTT method:

• Determines residual oil at a much larger reservoir scale (500-1000 bbl) versus traditional core flood laboratory measurements which determine residual oil at core scale (0.001 bbl)
• Test duration of 2-3 weeks, which is significantly faster than core analysis
• Cheaper than traditional core flood experiments

The ResMetrics Advantage

Conventional SWCTT is restricted to evaluate immobile oil saturation near the wellbore. The interpretation of saturation data from a SWCTT is not accurate when mobile oil is present within the target zone or reservoir. ResMetrics has a fine-tuned injection methodology of modified SWCTT to analyze residual oil saturation regardless of its mobility. The physical and chemical properties of the partitioning tracer are carefully examined to ensure its performance in the reservoir.

Tracer Injection Strategy

For target intervals with significant mobile oil, water injected during the conventional SWCTT will displace nearby oil further into the formation. This results in the reservoir volume contacted by the tracer to contain an average oil saturation which is different from the initial oil saturation. Therefore, it is more difficult to assess the initial oil saturation via the conventional SWCTT method.

To maintain saturation values throughout the duration of the tracer test, ResMetrics has modified the procedure to improve tracer test applicability. Mixture injection is the key difference from the conventional SWCTT and the modified SWCTT provided by ResMetrics. The following is the modified procedure:

1. Obtain production data from the operator and determine producing water-oil ratios for the target well.
   Inject the partitioning tracer (ester) along with a mixture of oil and water at the ratio obtained in the first step. It is preferred to use well-specific reservoir fluids as the injected carrier for the partitioning tracer.
2. Flush the injected tracer slug away from the wellbore with the same mixture of oil/water as in step 2. The goal of this step is to push the partitioning tracer away from the wellbore to the radius of investigation. The radius of investigation is typically 5-20 ft away from the wellbore.
3. Shut in the well to give sufficient time for the hydrolysis reaction of the partitioning tracer to take place.
4. Produce the well, collect oil and water samples, and record the concentration of both the ester and alcohol in each sample.

Tracer Data & Interpretation

A typical tracer response curve is shown in Figure 1 below. The partitioning tracer (ester) and conservative tracer (alcohol) will arrive at the wellbore at different times. This is because the conservative tracer travels at the same velocity as the water, whereas the velocity of the partitioning tracer is affected by the mobility of the residual oil. Such difference in the arrival time is the key to determine the remaining oil saturation near wellbore. The new injection and interpretation technique offered by ResMetrics will greatly improve the accuracy of the single well chemical tracer test (SWCTT) in determining residual oil saturation.