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An Unstructured Dual-Grid Model For Flow In Fractured And Heterogeneous Porous Media
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, ECMOR XVI - 16th European Conference on the Mathematics of Oil Recovery, Sep 2018, Volume 2018, p.1 - 9
Abstract
Discrete representations of highly heterogeneous porous media require high-resolution models, which are computationally expensive to simulate. Model reduction through upscaling is an effective way to accelerate flow simulations. Although single-grid upscaling techniques can provide accurate results for the pressure field, they may fail to capture the details of the saturation distribution when highly coarsened models are used.
One approach to address this issue is to use the coarse grid only for the pressure solution, and the original fine grid for transport solutions. Such procedures, commonly referred to as multiscale methods, have been extensively investigated in the reservoir simulation community. In this work we present a new dual-grid model that shares many similarities with existing finite-volume-based multiscale methods. Our dual-grid approach is, however, formulated as an extension of our previously developed aggregation-based upscaling procedure.
First, a coarse model is constructed for the pressure solution. The main flow parameters for this model are the transmissibilities between adjacent coarse (aggregated) cells. These are obtained using a flow-based upscaling procedure that (typically) requires two or three global fine-grid pressure solutions. The pressure fields constructed for transmissibility upscaling are used not only to evaluate the coarse transmissibility, but also to extract a fine-grid flux profile for each coarse (aggregated) interface. In the second step, fine-grid fluxes are calculated for the transport equation.
This is done locally within each coarse aggregate by solving a pressure equation with flux boundary conditions. These fluxes are determined by scaling the profile for each interface to match the coarse rate provided by the pressure solution. The overall procedure is implemented for unstructured fine and coarse grids. Examples involving two-phase flow in heterogeneous and fractured two-dimensional models are presented. Numerical results demonstrate the capabilities and flexibility of the overall methodology.