Low-Tension Waterflood at Salem Unit: Postpilot Evaluation (1979)

Previously published preliminary evaluations of a low-tension waterflood Previously published preliminary evaluations of a low-tension waterflood pilot reported less-than-expected oil recovery, partly the result of flow pilot reported less-than-expected oil recovery, partly the result of flow path distortion. Based on data from postflood tracer tests, tertiary path distortion. Based on data from postflood tracer tests, tertiary recovery was 37 to 43 % of the oil in the affected reservoir volume. Causes of the distortion and production shortfall are discussed. Complete reservoir definition when designing tertiary floods is necessary. Introduction In 1974, a joint Texaco Inc.-Mobil Research and Development Corp. test to evaluate a low-tension waterflood (LTWF) process was initiated in the Benoist sand at Salem Field, Marion County, IL. The process used a sequence of freshwater preflush, process used a sequence of freshwater preflush, chemical pretreatment, surfactant solution, a mobility-control (polymer) solution, and field brine. Descriptions of the process, its implementation, and preliminary evaluations are found in earlier studies. preliminary evaluations are found in earlier studies. Tertiary oil recovery was less than expected from the original flood plan, and various explanations of the flood performance were proposed. To provide additional information, further testing began in 1976. This paper describes the results of those tests and presents revised evaluations of the Salem LTWF pilot performance. pilot performance. Review To summarize pilot test performance briefly, the Salem LTWF program was designed to provide balanced injection with symmetrical fluid flow distribution in a 5-acre (20 x 10(3) m2), normal five-spot pattern. Four existing backup injection wells on 20-acre (81 x 10(3) m2) spacing (Fig. 1) were used to help confine the injected chemicals to the pilot area and to increase effectiveness of chemicals in the pilot. Studies indicated that 50 % of the injected chemicals should displace tertiary oil to the producing well, with the balance exiting the pattern.Oil recovery was substantially less than expected, only about 25 to 30 % of the recovery predicted using the concept of symmetrical displacement in all pattern quadrants, with recovery factors based on pattern quadrants, with recovery factors based on earlier laboratory investigations. Various explanations for the production shortfall were considered, including lower-than-expected oil displacement efficiency for the chemical system or a smaller pattern reservoir volume that actually was flooded and contributed to oil production. If the tertiary oil originated from a smaller swept reservoir volume, then the recovery efficiency assigned to the process obviously would be larger than that process obviously would be larger than that associated with a larger swept volume. Postflood Tests Postflood Tests To resolve the questions raised in preliminary evaluations, and to define better the amount of tertiary chemicals captured and the reservoir volume contributing to oil recovery, further testing began in 1976. A major goal of the postflood testing program was to ascertain whether flow path distribution was symmetrical (as intended) or distorted. Accordingly, for 1 month different tracers were injected into the respective injectors of the north and south quadrants. Response was measured using producing well samples. JPT P. 1185