Mixed hybrid finite-element formulation for general purpose reservoir simulation

We present a mixed hybrid finite-element (FE) formulation for modeling subsurface flow and transport for general-purpose compositional reservoir simulation. The formulation is fully implicit in time and employs a hybrid FE method for the spatial discretization of the conservation equations. The hybrid FE formulation is implemented in the Automatic Differentiation General Purpose Research Simulator (ADGPRS); consequently, the new FE-based methodology inherits all the physics' capabilities of ADGPRS, including compositional EOR models. The high-order mixed hybrid FE discretization scheme works for many types of finite elements and can handle highly anisotropic material properties. The formulation is locally conservative. The momentum and mass balance equations are solved simultaneously, including Lagrange multipliers on element interfaces. The fully implicit scheme uses the automatic differentiation capability to construct the Jacobian matrix. The hybrid FE approach accommodates unstructured grids, which are needed for honouring the complex geometry of the subsurface, in a straightforward manner. We present compositional test cases with full permeability tensors, and we discuss the accuracy and computational efficiency of the formulation. We also compare the performance of the hybrid FE-based scheme with finite-volume based Multi-Point Flux Approximation (MPFA) methods.