Numerical modeling of the tensile fracture reactivation under the effects of rock geomechanical properties and heterogeneity during CO2 storage

Abstract

The injection of CO2 into aquifers increases fluid pressure and induces a change of the geomechanical state. This is expected to be a cause of fracture reactivation, which is an important issue for the safety of CO2 storage. Using the geomechanics module, fracture reactivation, which was induced by tensile failure, was considered for the investigation of a potential CO2 leakage. Numerical simulations were conducted to analyze the effects of the rock geomechanical properties on the fracture reactivation in a two-dimensional system, and then the properties of fracture propagation were extended to the different rock types, hard shale and weak shale, in a three-dimensional system. Changes of the stress state, rock deformation, and the pressure build-up induced by CO2 injection were examined. When a rock has a high Young's modulus, effective stress decreases rapidly, leading to an earlier fracture reactivation. The total reduction of effective stress in the rock with a low Poisson's ratio was less than that of the rock with a high Poisson's ratio. The magnitude of pressure build-up is lower in the high-permeability rock, so that the fracture propagates more slowly. Hard shale's caprock is more favorable than that of weak shale for CO2 storage because fractures are opened later, retarding the CO2 leakage. The injected CO2 flows through a preferable region with reactivated fractures in the heterogeneous caprock. This study broadens our perception of the mechanical behavior that is induced by CO2 injection and it could be helpful for the precise safety assessment of CO2 storage projects.