Non-iterative phase behavior model with application to surfactant flooding and limited compositional simulation

Hand's method is typically used to empirically calculate the equilibrium compositions for ternary systems between two liquid phases. Oil field application of Hand's method is generally limited to surfactant phase behavior with oil and brine, primarily because the excess oil and brine phases are nearly immiscible. Hand's method is not accurate to represent liquid-vapor equilibrium, especially as oil and gas become miscible. It also requires iterations, which means there is no guarantee of convergence. In this paper, we present a new empirical phase behavior model to replace Hand's method. The new method is faster and more accurate, and applicable for both surfactant phase behavior and liquid-vapor equilibrium. The new approach is non-iterative and always finds a tie line or its extension even for the limiting tie line at the critical point. Our approach transforms tie lines to a new compositional space, where all tie lines become parallel. Equilibrium compositions are then easily determined in the transformed space. Besides improved accuracy and robustness, the flash calculations for ternary systems show that the new method is up to 100 times faster than conventional calculations using a cubic equations-of-state (EOS), and up to seven times faster than Hand's method. When incorporated in a compositional simulator, the new method reduces flash calculation time to nearly zero compared to the solution of the pressure/compositional equations. Thus, speedup is proportional to the fraction of time occupied by flash calculations within the simulator. For example, if flash calculations are 50% of total simulation time, speedup is nearly a factor of two using the new approach. This approach is ideally suited for fast recovery estimations for miscible gas floods, and fills the gap between standard or modified black-oil models and fully compositional simulations.