Modeling of caprock discontinuous fracturing during CO2 injection into a deep brine aquifer

Abstract

In this paper, we present a discontinuous method for simulating coupled multiphase fluid flow and hydraulic fracturing, and apply this method to deep underground CO2 injection. In this modeling approach, the individual physical processes involved in hydraulic fracturing are identified and addressed as separate modules: (1) a rock-discontinuous-cellular-automaton method for continuous-discontinuous geomechanical fracturing analysis, and (2) an integrated finite volume method for representing nonisothermal, multiphase fluid-flow processes. With this approach, the real fracture propagation path (straight or curved) induced by CO2 injection can be modeled without the need for remeshing. We verify the method and the numerical model against analytical solutions for fracture opening and propagation. We then simulate CO2 injection-induced fracturing within a brine aquifer, demonstrating the capability and applicability of our coupled numerical method for simulating fracturing processes driven by multiphase fluid flow. Our study focuses on the role of initial caprock damage in geologic carbon sequestration, and how natural fractures could impact caprock sealing integrity. We find that, given initial damage or fracturing in the lower part of the caprock, injection-induced pressure can diffuse into the fracture and potentially propagate upwards across the caprock, creating a new flow path by which CO2 could migrate out of the intended storage aquifer. However, our modeling also shows that an injection pressure limited by minimum principal compressive stress (which might be estimated from leak-off or mini-frac tests) would be appropriate for safe injection with respect to maintaining caprock sealing integrity. Finally, our modeling also highlights the importance and usefulness of pressure and deformation monitoring—potentially effective techniques for early detection of deep fracture propagation breaking through a caprock layer.