Plastic Deformation and Resulting Enhancement in Caprock Failure Limit

Loss of caprock integrity is a well-known complication of CO2 geo-sequestration projects. Existing studies on caprock integrity analysis often apply a yield criterion on elastic solutions for the caprock geomechanics problem and, consequently, would make an underestimation of the permissible change in total volume of fluid injection. This work considers plastic deformation after yielding but before caprock failure through an analytical solution for a spherically symmetric proxy model of reservoir and surrounding caprock. The part of solution pertaining to pore fluid flow and geomechanics of reservoir is time-dependent and based on the fully coupled theory of poroelasticity while the elastoplastic deformation of caprock is treated by use of Drucker-Prager plastic flow models with strain hardening. The latter problem (plastic caprock) is formulated using Lagrangian formulation of elastoplastic deformations while the time-dependency of the former problem (poroelastic reservoir) is treated using the Laplace integral transform method. Continuity of displacement, stress and pore fluid flow at the boundary between the caprock and reservoir connects the two solutions. Findings indicate that consideration of a plastic strain allowance for the caprock indeed increases the injectable volume of CO2 in the reservoir, substantially. This volume is larger for the undrained caprock if elastoplastic strain allowance is relatively large while reverse is true for smaller elastoplastic strain allowances. In either case, strong dependence of injectable fluid volume on the rock strain hardening parameter is observed. This result is rationalized by studying the time-dependent evolution of the elastoplastic stress paths corresponding to the drained and undrained caprock solutions.