Advantages of Using Vertical Stair Step Faults in Reservoir Grids for Flow Simulation

摘要

Conventional reservoir modeling relies on the construction of 3D grids which conform to stratigraphy and whose columns are aligned with faults. When modeling non-vertical faults, traditional "pillar technology" yields large volume distortions between cells leading to non-orthogonality problems. It is difficult or impossible to construct conventional grids in structurally complex environments: reverse, dying faults, X-, Y-, lambda-faults. Modelers can spend days simplifying their model by removing or verticalizing faults. Deciding on which data to alter is cumbersome and results in inaccurate models. This affects connectivity and compartmentalization, impacting significantly reservoir management decisions. Therefore, it is essential to construct grids that honor the integrity of the complete reservoir structure regarless of its complexity, and yet that are compatible with commercial flow simulators. A solution, in terms of structured grids, is to consider vertical stair-step faults. They have many advantages: all types of faulting patterns can be accurately represented; columns can be (sub-) vertical; cells are less likely to be distorted and orthogonality can be ensured. These grids are ideal for reservoir flow simulation purposes. However, since it has been difficult to construct them in the past, few engineers have experience with them. In this paper, a methodology to construct such grids that preserve the integrity of the structure and stratigraphy of reservoirs is presented. Approaches to robust reservoir property modeling and upscaling are also discussed. Test cases are presented, comparing results from conventional pillar grids with grids in which faults are defined as vertical stair-steps. The integration of this type of grid with a high-performance reservoir simulator is also highlighted.