Approach to determination of residual oil saturation in Bach ho sandstone reservoir by single well tracer test (SWTT)

In the tertiary oil recovery, Residual oil Saturation (Sor) means the oil percentage remaining in formation may be recovered by improved method. Thus, Sor is very important information in making tertiary recovery project feasible. The Single Well Tracer Test is reliable method amongst other methods for determination of Sor. In SWTT tracer in form of ester of alcohol and fat acid, which is able to participate in oil and water is injected with water into water flooded production well to create an annulus surrounding well bore. After a sufficient well shutin time, a part of ester will be hydrolyzed into secondary tracer, alcohol, which is able to participate in water phase only. When re-open of the well both tracers ester and alcohol will move backward to the well with different velocity due to chromatographic retardation effect. The retardation depends on oil/water partitioning coefficiency of the ester and certainly on the percentage of oil in the rock pore which contacts with moving water (Sor). Therefore, Sor is determined basing on the retardation of ester in re-produced water in comparison with alcohol in tracer concentration curves by means of water analysis and partitioning coefficiency determined in the Lab test. The Project implemented in two year 2003-2004 aims at development of the method in the laboratory in regard with Vietnam reservoir condition, firstly in Sandstone which is popular, rather familiar in application. The results obtained shall be the base to further application in field. The results were establishment of methodology of SWTT, study of hydrolysis, partition of 10 distinguished esters in order to select 4 tracer candidates such as Ethyl Acetate, Ethyl Propionate, Ethyl Lactate and Ethyl 2 Hydroxy Isobutyrate which have low thermal sensitivity and can work well in high temperature reservoir as well as other technical requirement. The result of Core Tests in Bach ho and Brea Sandstone rock gave good agreement with referenced Sor. And lastly the technical procedure was designed for the further application in the field. (author)