A locally conservative multiscale method for stochastic highly heterogeneous flow

In this paper, we propose a local model reduction approach for subsurface flow problems in stochastic and highly heterogeneous media. Generally, we apply the mixed generalized multiscale finite element method (MGMsFEM) to solve the single-phase flow equation. As for the two-phase flow, we use mixed finite element method (MFEM) with piecewise constant bases on a fine mesh to deal with the transport equation and a standard Implicit Pressure Explicit Saturation (IMPES) scheme is utilized in the time-marching process. To guarantee the mass conservation, we consider the mixed formulation of the flow problem and aim to solve the problem in a coarse grid to reduce the complexity of a large-scale system. We decompose the entire problem into a training and a testing stage, namely the offline coarse-grid multiscale basis generation stage and online simulation stage with different parameters. In the training stage, a parameter-independent and small-dimensional multiscale basis function space is constructed, which includes the media, source and boundary information. The key part of the basis generation stage is to solve some special local problems. With the parameter-independent basis space, one can efficiently solve the concerned problems corresponding to different samples of permeability field in a coarse grid without repeatedly constructing a multiscale space for each new sample. A rigorous analysis on convergence of the proposed method is presented. In particular, we consider a generalization error, where bases constructed with one source will be used to a different source. In the numerical experiments, we apply the proposed method for both single-phase and two-phase flow problems. Simulation results for both 2D and 3D representative models demonstrate the high accuracy and efficiency of the proposed model reduction techniques.(c) 2023 Elsevier B.V. All rights reserved.