A Phase-Field Based Approach for Modeling the Cementation and Shear Slip of Fracture Networks

Abstract

Modeling the geomechanical deformations of fracture networks has become an integral part of designing enhanced geothermal systems and recovery mechanisms for unconventional reservoirs. Stress changes in the reservoir can cause large variations in the apertures of fractures resulting in drastic changes in their transmissivities. At the same time, sustained high injection pressures can induce shear slipping along existing fractures and faults and trigger seismic activity. In this work, a novel approach is introduced for the simulation of cementation and shear slip of fractures on very general semi-structured grids. Natural fracture networks are represented in large scale reservoirs using the phase-field approach. The fluid flow through fractures is simulated on spatially non-conforming grids using the enhanced velocity mixed finite element method. The geomechanics equations are discretized using the continuous Galerkin finite element method. The single-phase flow and mechanics equations are decoupled using the fixed stress iterative scheme. The model can predict shear slipping and opening/closure of fractures due to induced stresses and poromechanical effects. Two synthetic examples are presented to model the effects of injection/production processes on the cementation and shear slip of fractures. The impact of the fractures' orientation and their connectivity on the hydromechanical response of the reservoir is also considered. The examples illustrate the strong impact of the dynamic behavior of fractures and the accompanying poroelastic deformations on the safety and productivity of subsurface projects.