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

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.