Fast computation of multiphase flow in porous media by implicit discontinuous Galerkin schemes with optimal ordering of elements

We present a family of implicit discontinuous Galerkin schemes for purely advective multiphase flow in porous media in the absence of gravity and capillary forces. To advance the solution one time step, one must solve a discrete system of nonlinear equations. By reordering the grid cells, the nonlinear system can be shown to have a lower triangular block structure, where each block corresponds to the degrees-of-freedom in a single or a small number of cells. To reorder the system, we view the grid cells and the fluxes over cell interfaces as vertices and edges in a directed graph and use a standard topological sorting algorithm. Then the global system can be computed by processing the blocks sequentially using a standard Newton–Raphson algorithm for the degrees-of-freedom in each block. Decoupling the system offers greater control over the nonlinear solution procedure and reduces the computational costs, memory requirements, and complexity of the scheme significantly. In particular, the first-order version of the method may be at least as efficient as modern streamline methods when accuracy requirements or the dynamics of the flow allow for large implicit time steps.