Tie-simplex-based phase-behavior modeling in an IMPEC reservoir simulator

Recently, tie-simplex-based phase-behavior modeling in reservoir simulators has been applied and investigated as a potential method for improving the computational speed of equation-of-state (EOS) -based reservoir simulators. We implemented compositional-space adaptive tabulation (CSAT), the most promising tie-simplex-based method, in UTCOMP, the University of Texas' in-house IMPEC compositional reservoir simulator, to investigate its computational efficiency compared with the phase-behavior algorithm in UTCOMP. The results show that applying CSAT only to skip stability analysis does improve computational time, but only when a significant portion of the gridblocks are in the single-phase region and no other technique for avoiding stability analysis is used. However, in most cases, there is little or no computational advantage to use of CSAT when the simple option in UTCOMP is used where stability analysis is skipped for blocks surrounded by single-phase regions. We explore in detail the performance of CSAT, which depends significantly on the specific gas flood modeled, and the number of tie-lines generated during adaptive tabulation. The results shed light on applicability of CSAT in the IMPEC-type compositional reservoir simulators and show that the advantages of CSAT in this type of simulator are not as great as are reported in the literature for fully implicit or adaptive implicit formulations.