Simulation of Coupled Single-phase Flow and Geomechanics in Fractured Porous Media

Accurate predictions of the complex interactions between fluid-flow and mechanical deformation in fractured geologic formations is of interest in a wide range of reservoir engineering applications including subsurface CO2 sequestration, heavy-oil recovery, and wellbore stability. In this report, we describe a fully implicit method for coupled geomechanics and fluid flow in naturally fractured porous rocks. Specifically, we study single-phase fluid flow and poroelastic mechanical deformation for two-dimensional domains. A Discrete Fracture Model (DFM) with a static unstructured computational grid is employed, in which the fractures are represented explicitly as low-dimensional objects embedded in the matrix. The fracture segments lie at the interface between matrix elements (control volumes). The combination of unstructured grids and a DFM-based description allows for accurate representation of the complex geometry and the wide range of length-scales often observed for naturally fractured formations. We assume that the deformations are small, so that the computational grid remains as a function of time. We also assume that all the fractures are already present and represented explicitly, and that no failure (i.e., fracture propagation) will take place. A low-order finite-volume method is used to discretize the mass conservation equations of the matrix and the fractures. A finite-element method is used for the mechanics problem, in which a double-node numbering scheme is used for the fracture segments. The double nodes are enriched with additional degrees of freedom that are chosen to enforce the equilibrium conditions at the two fracture surfaces represented by the segment. The appropriate equilibrium conditions at discrete fracture segments and how to enforce them in the numerical model are discussed in detail. Fully implicit coupling of the flow and mechanics problems is performed by updating the