Accurate coarse reservoir modeling using unstructured grids, flow-based upscaling and streamline simulation

This work addresses the generation of accurate coarse models for reservoir simulation. The coarse representations are obtained from detailed geological models which are too large to be used directly in flow simulations. The coarse model must be able to represent complex reservoir geometries and to preserve the basic flow responses of the detailed model. The methodology proposed entails the use of flexible unstructured grids, flow based upscaling and streamline based simulations to accurately capture the reservoir geometry, account for the subscale permeability heterogeneity, and assess the quality of the models constructed. The model accuracy is achieved via grid adaptation, using information extracted from single phase fine scale flow simulations. Several new capabilities are developed in order to implement this methodology. These include a streamline technique suitable for use with a multipoint unstructured finite volume scheme, a transmissibility upscaling methodology applicable for unstructured grids, and grid construction techniques.; The key to accurate streamline simulation on unstructured grids is the determination of an appropriate velocity interpolant. The technique presented entails a local postprocessing of the numerically computed fluxes, leading to a flux-continuous and flux-consistent piecewise constant velocity defined on a subgrid. The transmissibility upscaling technique enforces local flow equivalence between the discretized fine and coarse simulation models. Single phase flow problems are solved on regions extracted from the fine scale model that contain a volume comprised of the union of flow-connected coarse cells. This upscaling method implies a two-point approximation of the fluxes on the coarse grid. This approach is compared to a generalized permeability tensor upscaling technique (also developed in this work) appropriate for a multipoint flux treatment. For the types of models considered here, the transmissibility upscaling appears to be the superior technique. Unstructured grids are adapted to specific global flow problems. Complex geometrical constraints and fine scale flow information are incorporated via target grid resolution maps.; The methods described are implemented in a Gocad software platform that allows for the construction of problem specific adapted coarse grids. The accuracy of the methods is demonstrated and illustrations of the methodology on synthetic large reservoirs are provided.