Sequential implicit approach for compositional reservoir simulation in conjunction with unstructured grids

Abstract

Various formulations are used to solve numerically complex systems of partial differential equations that describe important physical phenomena. The approach for solving the resulting equations from the modeling of the fluid flow has a great effect on the simulator’s performance making its use possibly prohibitive in practice from computational point of view. In compositional reservoir simulation, the numerical formulations are usually divided into two broad classes: volume balance and mass balance. The former formulation uses a volume balance equation to determine the pressure for each gridblock. In this work, a semi-implicit volume balance formulation was implemented into The University of Texas Compositional Simulator (UTCOMP) for unstructured grids in conjunction with the finite-volume method. The semi-implicit formulation implemented is implicit in pressure and phase saturations and explicit in moles (IMPSAT). This formulation is compared with the original IMPEC (implicit pressure, explicit compositions) formulation of UTCOMP, which was previously validated with analytical solutions and commercial simulators, implemented to unstructured grids. The results obtained suggest that for coarse grids, the IMPES formulation presents the lowest computational time, however as the grids are refined the semi-implicit approach improves its performance in terms of computational time when compared to the IMPES formulation.