Interpretation of the Alkaline-Surfactant-Polymer Pilot in West Salym Using Tracers

Abstract

An Alkaline-Surfactant-Polymer (ASP) pilot was executed in the West Salym oil field in the Russian West-Siberian oil province. To successfully interpret the project outcome an extensive surveillance plan was put in place. A tracer program formed an important, stand-alone part of the plan. Tracers injection was designed and executed to evaluate the incremental oil production due to ASP injection by using A) change in volume swept between the pilot wells and B) change in oil saturations due to ASP flooding. This paper focusses on the practical aspects of the tracer program execution and how the tracer program results were used for the interpretation of the pilot. The West Salym reservoir is a sandstone formation with 83°C temperature, 2 cP crude oil viscosity, permeabilities ranging from 10 to 250 mD and porosity ranging from 18 to 22%. The field is operated as a mature waterflood, with oil production having peaked in 2011. To increase the recovery factor, a tertiary oil recovery technique (ASP) was selected. A confined five spot well pattern was chosen for conducting the ASP field trial. Due to low remaining oil saturation after the waterflood (executed also as a pre-flush for the ASP flood) the production watercut reverse due to the ASP injection changed only from 98% to 88-90%. Hence, it was important to evaluate the efficiency of ASP flooding using several independent approaches. In addition to field injection/production data, analytical and modelling techniques, the tracer data interpretation became a valuable source of information. Four tracer injection stages were conducted during West Salym ASP pilot. Passive and partitioning tracer injection/production data were analyzed using Shook's analytical method and supported by the reservoir modelling. Analytical analysis of field data was complicated by the production and injection upsets, as well as the changes in injected viscosities. Even though the requirement for steady state conditions were not fully met, the passive tracer recovery data provided an important input to the history matching of pilot dynamic model helping to determine the sweep increase due to injection of viscous chemical solutions. The partitioning tracer recovery data in the water post-flush were used to confirm the low residual oil saturation after ASP flooding.