Abstract
Corner point grids and pillar based unstructured grids do not provide an effective workflow for translating earth models into simulation models. It requires grids that allow an accurate representation of the near well flow, that preserve geological accuracy and offer flexible resolution control. Hence a 3D unstructured approach is required.
Significant work has been done for generating unstructured grids and modeling hydraulic fracture flow for gas shale simulation has given a new impulse for unstructured gridding. Recent methods such as VAG or more mature ones such as MPFA provide a numerical scheme based on multipoint stencil more physical than two point flux approximation methods.
This paper presents the implementation of VAG and MPFA schemes inside a next generation reservoir simulator starting from a source code calculating multi-point flux non neighbor connections for any polygonal shaped control volume.
The unstructured scheme approach has been developed as an in-house extension to a next generation multi-company collaborative reservoir simulator (which is designed for handling unstructured grids).
Main issues addressed are the introduction of vertices unknowns among the usual cell centers variables, the assignment of vertices properties in the reservoir simulator model, the parallel solver conditioning and the link with the well model. Incidentally the definition of an exchange format to describe the unstructured geometry (vertices, edges, faces, control volumes) on a large reservoir simulation model is proposed.
Three simulation examples are presented and compare results, accuracy and performance of multipoint scheme methods such as VAG and MPFA on unstructured grids. Results are compared with TPFA methods on refined structured CPG grids and TPFA methods on unstructured PEBI grids. The two first test cases are academic models and the third one is a field model.
