Description of Field Tests To Determine Residual Oil Saturation by Single-Well Tracer Method

Abstract

Two field tests were conducted using the single-well tracer test to measure residual oil saturation. A description of the tests is presented. A dilution effect or production from zones that did not take tracer injection occurred and had to be accounted for in the data analysis. Introduction Residual oil saturation measurements are important in the evaluation of tertiary recovery projects. Reliable values are necessary to evaluate a pilot performance and to determine the potential for commercial tertiary recovery projects. projects. The single-well tracer method to measure residual oil saturation in watered-out formations has been described by Tomich et al. This method has the attractive feature of measuring the residual oil saturation in a relatively large pore volume and involves the following steps:Injecting a bank of water containing a primary tracer, either ethyl acetate or n-propyl formate.Shutting in a well to allow time for some of the primary tracer to react, forming a secondary tracer of primary tracer to react, forming a secondary tracer of ethyl alcohol or n-propyl alcohol.Producing a well and analyzing water samples to determine concentration profiles of the primary and secondary tracers. The separation of the arrival times of the two tracers indicates the residual oil saturation. This separation is caused by different partitioning of the primary and secondary tracers between the oil and water. primary and secondary tracers between the oil and water. The greater the separation between the tracers, the higher the residual oil saturation. Continental Oil Co. has been licensed to conduct Exxon Production Research Co.'s single-well tracer method to determine residual oil saturation since 1973. Residual oil saturations were run at three locations in five different wells. A service company tested two wells at one location, while Conoco Production Research tested at the other sites. Logistic problems of scheduling the tests on short notice and economic considerations caused Conoco to conduct its own tests. Two wells were tested at the Maier lease in the Illinois Basin and one well was tested at the Big Muddy field in Wyoming. Both locations had low rates that limited test size. The Big Muddy is a fresh water reservoir, while salinity of the Maier lease is about 10,000 ppm. There was no appreciable gas production with either test. This paper coversthe details of the field tests conducted solely by Conoco,the determination of the partition coefficients, andthe fitting of the produced partition coefficients, and (3) the fitting of the produced tracer concentrations using Exxon's single-well tracer computer models. Several approaches were made to fit the data from the Big Muddy test because results were complicated by drift and the tracers were diluted by water from zones that did not accept them. Test Design The following items were considered when designing the test.A large test (up to 90 bbl of injection per foot of pay) was desirable. Injection of 50 bbl per foot of pay was pay) was desirable. Injection of 50 bbl per foot of pay was a good design size.The reservoir temperature dictated the selection of the primary tracer. Ethyl acetate was used for the higher temperatures and n-propyl formate for the cooler reservoirs. JPT P. 194