Vertex-centred Discretization of Multiphase Compositional Darcy Flows on General Meshes

This paper introduces a vertex centred discretization on general 3D meshes of multiphase Darcy flows in heterogeneous anisotropic porous media. The model accounts for the coupling of the mass balance of each component with the pore volume conservation and the thermodynamical equilibrium. The conservative spatial discretization of the Darcy fluxes is based on the Vertex Approximate Gradient scheme (VAG) which is unconditionally coercive for arbitrary meshes and permeability tensors. The stencil of this vertex-centred scheme typically comprises 27 points on topologically Cartesian meshes. On tetrahedral meshes, the number of unknowns is considerably reduced, by typically a factor five, compared with usual cell-centred MultiPoint Fluxes Approximations, which is a key asset for multiphase flow simulations on unstructured meshes. An adaptive choice of the pore volume at the vertices ensures the accuracy of the discretization even for coarse meshes on highly heterogeneous media. This approach can easily be implemented on existing reservoir simulators using a graph of transmissibilities for the computation of the fluxes. The efficiency of our approach is exhibited on several two phase and three phase Darcy flow examples. In particular it includes the nearwell injection of miscible CO2 in a saline aquifer taking into account the precipitation of salt.