Description: Structured and unstructured grid generation for reservoir simulation, must honor classical key constraints and be boundary aligned such that control-volume boundaries are aligned with geological features such as layers, shale barriers, fractures, faults, pinchouts and multilateral wells. In this work Delaunay grid generation and advancing front point placement in conjunction with reconnection algorithms are employed to develop methods of grid generation for both 2.5D and 3-D that are boundary aligned. The schemes used here are control-volume distributed (CVD) with flow variables and rock properties sharing the same control-volume location and are comprised of a family of multipoint flux approximation methods (CVD-MPFA). In standard simulation primal grid cells act as control-volumes (cell-centred), and grid generation is performed with primal grid cell boundaries aligned with key interior constraint boundaries. We note a tetrahedral mesh contains approximately between 5 and 6 times as many cells (tets) as there are vertices, making a cell-vertex approximation the optimal choice on unstructured grids. However, the cell-vertex approximation (dual) control-volumes must be constructed around primal grid vertices. The dual grid generation process requires (dual) control-volumes must be constrained to satisfy interior boundary alignment, which is an additional grid generation constraint compared to primal grid generation.

Application: Optimal boundary aligned grids for solid walls and geological features such as layers, shale barriers, fractures, faults, pinchouts and multilateral wells.

Results Observations: The quality of the grids generated are assessed via a comparative performance of cell-vertex versus cell-centred CVD-MPFA finite-volume formulations using equivalent degrees of freedom. The relative benefits of both types of approximation is made clear in terms of flow resolution and degrees of freedom required. The cell vertex method proves to be beneficial with respect to accuracy and efficiency.

Significance: A novel grid generation procedure is proposed that automates (dual) control-volume boundary alignment and yields an essentially voronoi based mesh in 2-D and/or in 3-D. The actual grid is generated such that dual boundaries are aligned with key internal constraint boundaries and cell-vertex approximation becomes the natural choice. CVD-MPFA schemes are developed for the grids presented providing a unique comparison and insight into performance.

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