Three-phase EoS-based Reservoir Simulation with Salinity Dependent Phase-equilibrium Calculations

Light gases (CO2, H2S, …) are soluble in water at typical reservoir conditions, particularly when salinity is low; besides, at high enough temperatures water can significantly vaporize into the gas phase. Such gas-water mass exchanges can play an important role in the recovery mechanisms and need to be appropriately accounted for in reservoir simulation. In this context, the Søreide and Whitson equation of state (SW-EoS) is an attractive option to model those phase equilibria since it is reasonably predictive and incorporates salinity effects, while being simpler than more advanced EoS such as CPA or SAFT. Most publicly available reservoir simulators treating gas-water mass exchanges either use K-values or two-phase EoS models combined with Henry’s law, or the SW-EoS but limited to a two-phase (gas/water) environment. In this work, the SW-EoS has been implemented in our fully compositional research reservoir simulator. The model is first tested with constant salinity. Two-phase simulations of an Asian gas field with high CO2 content are presented, and results benchmarked against a commercial simulator. Three-phase simulations of a French CO2 sequestration pilot with significant gas-water exchanges are then discussed, matching the experimental production data. The model is further extended to account for salt transport and precipitation. To the best of our knowledge, this is the first time in the literature that the SW-EoS is coupled to dynamic salinity. The phase equilibrium algorithm is modified to reach quadratic convergence even when the attractive term of the cubic EoS depends on dynamic salinity. The algorithm is tested on 3D simulations of a tertiary gas injection process with dry-out and salt precipitation effects in the near-well region, and comparisons with simulations based on the Peng-Robinson EoS or considering static salinity with the SW-EoS are presented, showing significant impact on oil recovery.