Managing Experimental-Data Shortfalls for Fair Screening at Concept Selection: Case Study To Estimate How Acid-Gas Injection Affects Asphaltene-Precipitation Behavior

In a carbonate field under high pressure and high temperature (HP/HT), 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, including raw-gas injection 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. Fluid 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 precipitation. Because of high hydrogen sulfide (H₂S) concentrations of AGI fluid under HP/HT in-situ reservoir conditions, it is difficult to take an experimental approach for evaluating gas-mixed asphaltene-flow assurance at a normal laboratory. Hence, at the concept-selection stage, this paper focuses on an alternative approach for numerical-modeling analysis of the AGI scenario, and presents the way in which AGI impacts asphaltene-precipitation behavior. On the basis of the asphaltene model established by applying a cubic-plus-association equation of state, which was calibrated with the experimentally measured asphaltene-onset pressure, a new binary-interaction-parameter correlation between H₂S and hydrocarbons was incorporated to evaluate variation of the asphaltene-precipitation envelope (APE) with periodic compositional change observed from the AGI-flow simulation. In the AGI scenario, injection gas was assumed to be 90 mol% H₂S and 10 mol% carbon dioxide (CO₂). The original reservoir fluid contains 15 mol% H₂S concentration. During the 3D reservoir-simulation study for the AGI scenario, H₂S concentration in produced fluid was observed to increase up to 76 mol% at a well located near acid-gas injectors. In the APE sensitivity analysis that was conducted independently for each composition of H₂S and CO₂, the asphaltene model revealed that the base APE decreased as the H₂S concentration increased and expanded as the CO₂ concentration increased. As a result, for the mixed compositions, the opposing effects on the APE offset each other, and the acid-gas addition produced a subsequent decrease of the APE. In summary, this work supported a relative merit of AGI from a thermodynamic asphaltene-flow-assurance point of view, while verification is needed with experimental data in the next defined/detailed engineering stages.

Read or download the full SPE paper 170585-PA.