How Acid Gas Affects Asphaltene Precipitation Behavior

Abstract

AbstractIn a carbonate field, a gas injection scheme has been assessed to improve oil recovery through pressure maintenance and miscible displacement. The potential study assumed sequential application of several gas injection concepts: Raw Gas Injection (RGI) and Acid Gas Injection (AGI). Flow simulation studies of these concepts revealed a variety of compositional changes to the in-situ fluid depending on the injection scheme and composition of the injected gases. Compositional change is a common trigger of asphaltene instability; therefore, to ensure a robust gas injection development, it is important to evaluate the risk of asphaltene deposition. Due to high H2S concentrations in potential developments, it is difficult to take an experimental approach for evaluating gas-mixed asphaltene flow assurance. Hence, this paper will focus on one AGI scenario, and present how AGI impacts asphaltene precipitation behavior through numerical modeling analysis. Based on the asphaltene model established by applying Cubic Plus Association (CPA) equation of state (EoS), which was calibrated with the experimental measured asphaltene onset pressure (AOP), a new Binary Interaction Parameters (BIP) correlation between H2S and hydrocarbons was incorporated to evaluate variation of asphaltene precipitation envelope (APE) with periodical compositional change observed from the AGI flow simulation. Acid Gas (AG) was assumed to be 90mol% H2S and 10mol% CO2. The produced fluid H2S concentration used in this study was assumed to be ~15mol%. During this study, H2S concentration was observed to increase up to 76mol% at a well located near AG injectors after long term flow simulation. In the APE sensitivity analysis that was independently conducted for each composition of H2S and CO2, the asphaltene model revealed the base APE shrunk as the H2S concentration increased while it expanded as the CO2 concentration increased. As a result for the mixed compositions, the opposing effects on the APE offset each other; the AG addition produced a subsequent shrinking of the APE. In summary, this work supported acid gas injection from a thermodynamically asphaltene flow assurance point of view.