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High capillary pressure has a significant effect on the phase behavior of fluid mixtures. The capillary pressure is high in unconventional reservoirs due to the small pores in the rock, so including the effect of capillary pressure on phase behavior is necessary for reliable modeling of unconventional shale gas and tight oil reservoirs. We show that the tangent plane distance method cannot be used to determine phase stability and present a rigorous thermodynamic analysis to determine phase stability with capillary pressure. We then demonstrate that there is a maximum capillary pressure (Pcmax) where capillary equilibrium is possible and derive the necessary equations to obtain this maximum capillary pressure. We also discuss the implementation of the capillary equilibrium in a general purpose compositional reservoir simulator and the numerical challenges involved with its application to unconventional reservoirs. Three simulation case studies for gas condensate and tight oil reservoirs were performed to illustrate the influence of capillary pressure on production behavior. These results clarify the influence of capillary pressure on production behavior in low-permeability reservoirs. We show that the choice of the capillary pressure function and parameters significantly affects the results.

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.

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