Stability of Perforation During CO2 Injection

Abstract The injected CO2 in geological storage layers has lower downhole temperature compared to the storage formation. This induces cooling effects in the formation rocks surrounding the injection wells and their perforations. The stability of the perforations is affected by the cooling. Perforation stability is investigated for different reservoir pressures, bottom hole pressures and temperatures. The tendency for shear failure at the perforation wall is investigated through a series of customized finite element simulations, while the risk of tensile failure in the perforations is modelled using an analytical approach. The tensile failure analysis is done for two major time steps at the start and at the end of the CO2 injection. The tendency for shear failure is quantified by the plastic strain magnitude calculated around the perforation tunnel in the numerical simulations. The perforations oriented to the side of the well are more unstable compared to the perforations oriented to the top of the well for the selected normal-faulting stress regime. As expected from the theory of thermo-elasticity, the perforations are more stable with greater cooling. The plastic strain increases with higher drawdown, while less plastic strain is calculated in overbalance-injection scenarios. The cooling effect reduces the fracture initiation pressure, i.e. increases the risk of tensile failure. The fracture initiation pressure is compared against the planned highest bottom hole pressure for individual cases to ensure the stability of the perforations. Overall, injecting CO2 with a temperature lower than the formation temperature does not induce excessive compressional or shear stresses, and is unlikely to cause collapse of the perforation. The risk of shear failure tends to be higher during the early stages of injection when the reservoirs are fully depleted. The cooling effect from CO2 injection may induce tensile fractures in the perforation tunnels, which requires a case-by-case detailed investigation.