Generalized prism grid: a pillar-based unstructured grid for simulation of reservoirs with complicated geological geometries

Grid generation is critical to numerical reservoir simulations. High-quality grids guarantee the fidelity of a reservoir model and keep the flow calculations simple. In this study, we propose a 3D unstructured grid, the generalized prism grid (GPG), to model reservoirs with complicated geological geometries, including horizons, pinch-outs, faults, fractures, and bore holes. GPG is a layered, pillar-based grid. The location of a face node is specified by its elevation, and the pillar to which it is attached. Compared with the hexahedral corner point grid (CPG), GPG is a polygon prism and therefore more flexible; whereas, compared with the 2.5D perpendicular bisection (PEBI) grid, GPG allows polygons morphing through the stratum. We built a gridding algorithm to fulfil the features of GPG. The algorithm first constructs a 2D triangular mesh for one layer by setting up control points and grid densities for geological objects, such as fractures, faults, and wells, distributing triangular grid points with the "advancing front method," and performing Delaunay optimization to the points. The polygon mesh is the dual grid of the triangular mesh. Taking the polygon mesh as a reference, the mesh for each layer of the strata is a morphing of it, with edges being stretched and points being assigned with heights. We also designed a compact file format to store GPG data and implemented the flux calculation method for GPG in a reservoir simulator. The attractive features of GPG are demonstrated through four examples. The conciseness and flexibility of GPG make it a potential new standard grid format replacing CPG.