Compositional Reservoir Simulation: A Review

Compositional reservoir models have many uses, including petroleum reservoir simulation, the simulation of flow and remediation of nonaqueous petroleum liquids, and the study of various carbon- capture technologies. The present work describes these models in a general way but concentrates on isothermal compositional models based on an equation of state (EOS). These models are used to study petroleum reservoirs containing gas condensate or volatile oil fluids and various gas injection processes like CO2 injection. The technology is documented from the earliest rudimentary models which first appeared more than 50 years ago to more recent work. The system of differential- algebraic equations is described along with the physical nature of the parameters. Model formulations are compared with respect to choices of variables and other computational features. Descriptions of efficient and reliable computational procedures are given. In addition to basic algorithms, we detail important side issues of (1) testing for phase appearance, (2) temporal stability, (3) material balance, and (4) other efforts to speed up the calculations and/or reduce the calculation requirements. Information in the literature is supplemented with some work which has not been presented previously. Correct rendering of the physics and chemistry is emphasized which is often intertwined with the numerics. Although many improvements have been made, the work is not finished. We conclude with a discussion of areas needing additional work.