The deformation of an initially intact caprock, due to the depletion of an underlying reservoir during oil extraction, is geomechanically modeled in three dimensions using a fully coupled poro-elastic model, incorporating plastic damage and fracture models in the caprock. Fractures are represented explicitly in an arbitrary tetrahedral mesh. An isotropic damage model accounts for micro-fracture growth in response to the accumulation of plastic strain. Stress intensity factors, computed using the reduced virtual integration technique, assess potential fracture growth by measuring energy concentrations around the tips. The objectives are to evaluate the locations in the caprock, where damage and fracture propagation are most likely to occur. As a result of poro-elastic deformation during the onset of reservoir depletion, shear driven damage in fractures above and around the well are observed. Extension driven growth is observed in fractures located away from the well. Damage and fracture growth both potentially decrease the caprock integrity, and locally enhance the permeability.
CO2 sequestration at industrial scales is a potentially a viable long-term pathway to reduce atmospheric greenhouse gas concentrations. Such a solution relies on the ability to effectively trap CO2 within saline aquifers or oil reservoirs over hundreds or thousands of years, without leakage. The reservoir caprock is an important trapping mechanism which should help ensure that the greenhouse gas remains underground. Caprocks are low permeability rocks, typically shale or anhydrites, which lie on top of the reservoir and mechanically prevent fluids from migrating towards the surface. Caprocks are geological features that have formed over millions of years, and therefore contain a variety of heterogeneities including natural fractures and faults. If reactivated, these fractures and faults could potentially become conduits for fluid to flow to the surface, jeopardizing the effective integrity of the caprock. However, for a caprock to be eligible for CO2 injection, it must have kept its integrity during oil extraction.