Tie-line Based Parameterization for Thermal Compositional Reservoir Simulation

Thermodynamic equilibrium computations are usually the most time consuming component in compositional reservoir flow simulation. A Compositional Space Adaptive Tabulation (CSAT) approach has been developed in order to speed up the Equation of State (EoS) computations. The methodology adaptively parameterizes the compositional space using a small number of tie-lines, which are assembled into a table. The phase equilibrium information for a composition is interpolated from the tie-line table. In this work, we extend the CSAT approach to thermal problems. Given an overall composition at a fixed temperature, the boundary between suband super-critical pressures is represented by the critical tie-line and the corresponding minimal critical pressure. A small set of sub-critical tie-lines is computed and stored for a given temperature. This process is repeated for the pressure and temperature ranges of interest, and a regular tie-line table is constructed. Close to the critical boundary, a refined tie-line table is used for phase identification. A combination of regular and refined interpolation improves the robustness of the tie-line search procedure and the overall efficiency of the computations. Several challenging tests are used to demonstrate the robustness and efficiency of this general thermal-compositional tie-line based parameterization method. Our results indicate that CSAT provides very accurate treatment of the near-critical region with a small number of preconditioned EoS flash iterations. Moreover, the computational efficiency of the method is at least an order of magnitude better than that of standard EoS-based approaches. We also show that the efficiency gains relative to standard techniques increase with the number of gridblocks in the simulation model. iv