Efficient Fluid Behavior Computations in a Sequential Compositional Reservoir Simulator

Subramanian, G., Trangenstein, J.A., Mochizuki, S., Shen, E.I-C., Exxon Production Research Co.
Presented at SPE Symposium on Reservoir Optimization, San Antonio, Texas, February 1987.


A compositional reservoir simulator using a sequential formulation of the fluid flow and phase equilibrium equations requires that the phase equilibrium and associated derivatives of the dependent phase equilibrium variables with respect to the pressure and moles of individual components be computed every time step for all the grid blocks in the system. The phase equilibrium calculations are computation intensive, since they solve a nonlinear system of equations and are thus iterative in nature. This paper describes techniques for implementing the phase equilibrium and the associated derivative calculations in a sequential compositional reservoir simulator designed for large reservoir models. These techniques use strip mining and optimal strategies for treating gridblocks at similar mathematical convergence or physical states and have been very successful in reducing computation times and memory requirements for iterative computations in the simulator, both on the CRAY and IBM computers. The derivative calculations are unified in that a single matrix can be used to generate derivatives of phase equilibrium variables with respect to any of the dependent variables used in the flow/conservation steps used in the simulator. Results are presented to illustrate various cases.