Compositional modeling of multicomponent gas injection into saline aquifers with the MUFITS simulator

Abstract Reservoir simulations of the co-injection of natural gas, carbon dioxide, and other non-condensable gases into saline aquifers are in demand in many areas of subsurface utilization, ranging from excess energy and greenhouse gas storage to the substitution of cushion gas with CO2. We aim at extending our reservoir simulator MUFITS for modeling flows in porous media related to these applications. For the accurate prediction of phase equilibria in aqueous systems, we implement a modified method of Soreide and Whitson in the compositional module of the software. The modified equation of state (EoS) involves the most recent literature data on the binary interaction coefficients and several new correlations for volume shifts to improve the accuracy of the EoS for predicting the partial volume of dissolved gases. All the aggregated and developed correlations are built into the simulator to facilitate its usage by the scientific community. The extended software is validated against a benchmark study of pure CO2 injection into a saline aquifer. Furthermore, two more complicated studies concerning the co-injection of several gases are conducted to demonstrate the potential application areas of MUFITS. First, we simulate the injection of impure carbon dioxide into an aquifer and briefly discuss the influence of N2 impurity on the propagation of the gas plume. We show that the plume is significantly wider in the case of impure CO2 injection. Second, we simulate the injection of supercritical CO2 into a synthetic natural gas storage reservoir to substitute cushion gas. We show that the mechanical dispersion can cause a rapid CO2 spread into the wells used for natural gas production.