Single-Well Tracer Tests for Evaluating Chemical Enhanced Oil Recovery Processes

Abstract

Summary A method is presented for testing enhanced oil recovery (EOR) processes in a single well. The method uses a multitracer version of the single-well tracer test (SWTT) for determining residual oil saturation to measure the oil displaced by an EOR slug process. A test was conducted to demonstrate the validity of the method. This test and its results are described. Introduction Piloting an EOR process is both time-consuming and expensive. Conventional pilot tests can take 3 years or more to conduct and cost several million dollars. A single-well test for evaluating tertiary slug processes has been developed that is both faster and less expensive than conventional piloting by at least one order of magnitude. The basis for this method of evaluating tertiary slug processes is the SWTT for measuring residual oil saturation developed by Exxon Production Research Co. The large depth of investigation from the wellbore of the SWTT and the ability to control this investigation depth are unique properties that make the single-well test for evaluating EOR slug processes possible. This single-well test consists of the following steps.Conduct an SWTT to determine the residual oil saturation after waterflooding.Inject a small slug containing an oil recovery chemical to establish an oil saturation profile within the SWTT pore volume (PV) of investigation.Inject a mobility control bank if needed.Inject a brine bank to displace the mobile banked oil from the test area, and recover the inaccessible PV caused by the mobility control bank if one was used.Conduct another SWTT with several different esters, each having different partition coefficients, so that average oil saturations can be measured for different distances from the wellbore. Steps 1 and 5 can be analyzed to determine oil saturation curves as a function of contacted PV and percent oil recovery vs. chemical slug size. These data are very useful in scale-up to larger projects and are not directly available from multiwell pilot tests. Another application of this testing method could be to conduct several tests with different chemical formulations in a given area. This would aid in selecting the best system for a particular field. Conoco Inc. has conducted a 1-acre (4047-m2) pilot test of a surfactant process at the Big Muddy field in Wyoming. Success of that pilot led to expansion of the surfactant process to 90 acres (364 km 2). In April 1979, Conoco ran a single-well surfactant test in conjunction with the surfactant expansion project. This single-well test was conducted to measure residual oil saturation after waterflooding and to determine the potential for evaluating tertiary slug processes by use of the single-well method. The same surfactant formulation used in the pilot test was used in the single-well test to provide a basis for checking the single-well test. This paper discusses theory, design, operation, results, and interpretation of the single-well surfactant test conducted in the Big Muddy field. Theoretical Development The theoretical basis and the general technique for measuring residual oil saturation by the single-well tracer method are well documented in the literature. Briefly, the method relies on determination of the chromatographic retardation factor of a tracer, which partitions between the oil and water phases in the reservoir. In practice, this determination is made by injecting a slug of primary tracer, such as an organic ester (having solubility in both water and oil), followed by a relatively large volume of water. A shut-in period then follows, during which the ester partially hydrolyzes to form an alcohol (having essentially no solubility in oil); this is the secondary tracer. JPT P. 1887^