An Analysis of Grid Orientation Effect on Steamflood Simulations

Grid orientation effect (GOE) is the appearance of preferential flow along grid coordinate directions in numerical reservoir simulation. GOE is most evident in simulations with strong adverse mobility ratios, such as immiscible gas injection and steamfloods. Motivated by previous work, an eleven-point finite difference formulation for multiphase flow is investigated and found to reduce errors for steamfloods using structured grids. The eleven-point formulation is implemented in a parallel, fully-implicit reservoir simulator with thermal, black-oil and compositional formulations, and the implementation supports both local grid refinement (LGR) and dual-porosity, dual-permeability (DPDK) modeling. Systematic tests are performed for compositional steamflood cases with different grid resolutions and grid coordinate angles between wells. A comparison of seven and eleven-point formulation results, using different grid scales and hybrid unstructured grids, demonstrate that the eleven-point scheme is effective in mitigating GOE and can leverage the benefits of structured LGR and DPDK options. Using grid-refinement as a means of reducing GOE is case dependent and is not always successful. Additional results suggest that using grid refinement with local application of the eleven-point scheme around only the injector does help mitigate GOE with increased computational efficiency, but GOE is not reduced as well as when the eleven-point scheme is used in the entire grid-system.