Direct Energy Minimization Algorithm for Numerical Simulation of Carbon Dioxide Injection

We discuss numerical simulation of carbon dioxide injection considered by oil and gas companies. Complex behavior of multicomponent reservoir fluids mixed with carbon dioxide may cause the occurrence of vapor-liquid-liquid equilibria (VLLE), salt precipitation in aquifers, pore-clogging, etc. We propose a simple algorithm for phase equilibria calculations based on the minimization of the multicomponent system free energy. This algorithm can be used to calculate phase separations and component partitioning between the phases under various conditions (critical region, two- and three-phase equilibria, etc.). We demonstrate the applicability of the proposed algorithm in a series of calculations. We consider binary and ternary mixtures that include carbon dioxide and hydrocarbons. We examine the algorithm in two- and three-phase equilibrium calculations and compare its performance with the popular iterative fugacity equilibration technique. We show that both calculation techniques give near-identical results for the considered mixtures. Thus, we show that the free energy minimization algorithm can be used interchangeably with the fugacity equilibration technique for calculating phase equilibria. This algorithm is applicable for VLLE calculations, which is important when considering multicomponent reservoir fluids that include carbon dioxide.