Pressure Coupling for Geomechanical Multi-Phase Flow Simulation Using Vertex Centered Flow Elements and Unstructured Grids

Abstract

Abstract Significant work has been done for generating unstructured grids. Coupled geomechanics simulation and hydraulic fracture flow for gas shale simulation have given a new impulse for unstructured gridding. The objective of the paper is to couple flow and geomechanics using unstructured grid models, to demonstrate the ability to apply a more efficient pressure coupling using discretization at vertices on both sides (mechanical and flow equations). Coupled equations are discretized and solved on an unstructured flow grid and a geomechanical finite element grid which are composed of various types of elements such as tetrahedrons and hexahedrons. On the flow side, a recent multi-point flux method, the Vertex Approximate Gradient (VAG) (Eymard 2012) is investigated for solving the reservoir equations on such unstructured grid. SPE paper 173309 (Samier, Masson 2015) presented the implementation of VAG scheme inside a next generation reservoir simulator designed for handling unstructured grids. This paper proposes an iterative coupling scheme with full pressure coupling at vertices. The geo-mechanics equations fully coupled to a single phase flow are solved using global pressure. Then the resulting deformations are iteratively coupled to the multi-phase flow simulator. Since most geo-mechanic simulators propose fully coupled single phase flow features, the main advantage of this method is the ability to use a full pressure coupling method with industrial simulators. The convergence of this new scheme is discussed and results are presented for two cases described below. The two cases are validation cases used by other SPE papers. Faults are modeled using specific cohesive elements. Results are compared with standard loose iterative coupling method using TPFA cell centered elements. SPE paper 173232 (RSS 2015, Doster et al) presented a full pressure coupling method using cell centered elements. The aim of the paper is to extend the method to unstructured grids and to vertex centered elements which are more adequate for a coupled problem since the multi-phase flow pressures, component masses and displacements unknowns are now discretized at the same location.