Abstract
Fault modeling has become an integral element of reservoir simulation for structurally complex reservoirs. Modeling of faults in general has major implications for the simulation grid. In turn, the grid quality control is very important in order to attain accurate simulation results. We investigate the dynamic effects of using stair-step grid (SSG) and corner-point grid (CPG) approaches for fault modeling from the perspective of dynamic reservoir performance forecasting. We have performed a number of grid convergence and grid-type sensitivity studies for a variety of simple, yet intuitive faulted flow simulation problems with gradually increasing complexity. We have also explored the added value of the multipoint flux approximation (MPFA) method over the conventional two-point flux approximation (TPFA) to increase the accuracy of reservoir simulation results obtained on CPGs. Effects of fault seal modeling on grid-resolution convergence and grid-type sensitivity have also been briefly examined. For simple geometries, both SSG and CPG can be used for fault modeling with similar accuracy in conjunction with the pillar-grid approach. This is evidenced by the fact that simulation results from SSG and CPG converge to identical solutions. SSG and CPG yield different results for more complex geometries. Simulation results approach to a converged solution for relatively fine SSGs. However, a SSG only provides an approximation to the fault geometry and reservoir volumes when the grid is coarse. On the other hand, non-orthogonality errors are increasingly evident in relatively more complex faulted models on CPGs and such errors cannot be addressed by grid refinement. It has been observed that MPFA partially addresses the discretization errors on non-orthogonal grids but only from the total flux accuracy perspective. However, transport related errors are still evident. Grid convergence behaviors and grid effects are quite similar with or without fault seal modeling (i.e., dedicated fault-zone modeling by use of scaled-up seal factors) for simple geometries. However, in more complex test cases, we have observed that it is more difficult to achieve converged results in conjunction with fault seal modeling due to increased heterogeneity of the underlying problem.
Highlights
Faults are associated with reservoirs of oil and gas in many basins of the world
More stringent criteria and focusing on the earlier part of the production time profiles are required. For such studies local grid refinement can be considered, which is kept outside the scope of our investigations. Both the stair-step grid (SSG) and corner-point grid (CPG) approaches can be used for fault modeling with similar accuracy in conjunction with the pillar-grid approach
This is evidenced by the fact that simulation results from SSG and CPG approaches converge to identical solutions
Summary
Faults are associated with reservoirs of oil and gas in many basins of the world. A fault is a break or planar surface in brittle rock across which there is observable displacement. Non-orthogonal grids lead to large discretization errors in the solution of flow equations by reservoir simulators that utilize the TPFA (e.g., Wu and Parashkevov 2009). The CPG approach leads to modeled faults that more accurately follow the interpreted surfaces but results in a non-orthogonal grid. The gridlines used for constructing a SSG are recommended to be parallel to the line connecting the injector and the producer, not diagonal, which means the resulting grids provide more accurate solutions when aligned with the main flow directions. Another disadvantage of SSG is its inflexibility in the representation of well locations.
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