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

Numerical simulation of gas injection processes requires coupling of phase equilibrium equations with complex multiphase flow equations. Standard EOS-based phase equilibrium modeling in reservoir simulators involves computationally intensive and time-consuming calculations for stability analysis and flash calculations. Therefore, speeding up stability analysis and flash calculations is an active research area. Recently, the tie-simplex based phase behavior modeling in reservoir simulators has been applied and investigated as a potential method for improving the computational speed of EOS-based reservoir simulators. The most promising of the tie-simplex based approaches in terms of practicality and computational efficiency is Compositional Space Adaptive Tabulation (CSAT). We implemented CSAT in UTCOMP, The University of Texas's in-house IMPEC compositional reservoir simulator to investigate its computational efficiency compared to the phase behavior algorithm in the original UTCOMP. Several example miscible and immiscible case studies are presented. The results show that using CSAT to generate initial estimates for flash calculations is inefficient compared to the original UTCOMP's phase behavior algorithm. Applying CSAT to only skip stability analysis does improve the computational time, but only when a significant portion of the grid blocks are in the single-phase region and no other technique for avoiding stability analysis is used. CSAT may skip as much as 99% of the stability analyses performed in the base case original UTCOMP, which may amount to about 30% improvement in total computational time in such cases. However, in most cases there is little or no computational advantage to use CSAT when the simple option in UTCOMP is used where stability analysis is skipped for blocks surrounded by single-phase regions. Thus, we see no reason to implement CSAT into an IMPEC type simulator over other simpler schemes to avoid stability analyses. 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 number of tie lines generated depends on tolerances specified for tie-line detection and for acceptance of a phase stability skip. The results shed light on applicability of CSAT in the IMPEC type compositional reservoir simulators and show that the advantages of CSAT are not as great as initially thought.