Invited Lecture – Carbon Dioxide Injection into Deep Aquifers: a Geomechanical Perspective
CO2 storage in deep aquifers is considered a potential technology to reduce the greenhouse effects of CO2. Practically, a large-volume (>1 Mt/year) of CO2 could be injected into a system that consists of a highly porous host aquifer covered by a low-permeability sealing caprock. High-rate injection could result in the abrupt build-up of fluid pressures, deforming the aquifer and compromising the integrity of the caprock. The interaction between the fluid pressure and mechanical reaction of the host aquifer results in a complex coupled system. The understanding of these hydromechanical processes is crucial to secure the injection. We investigate numerically the hydromechanical effects induced by CO2 injection on aquifer stability and the related interactions with the caprock. The proposed simulation incorporates the physical properties of supercritical CO2 such as the density, viscosity and fugacity. A conceptual deep aquifer is modelled to investigate the state of the stress and strain during the injection of CO2. The responses during the simulation show that significant geomechanical variations occur during the early period of injection in which fluid pressures increase sharply. This overpressure decreases the effective stress, which induces a volumetric expansion around the injection well. Because of the increase in porosity and permeability through hydromechanical coupling, the fluid flows more easily. As the injection continues, the stress path moves away from the failure line, and the material returns to its elastic state. In this study, the safety of carbon dioxide injection is assessed primarily from a geomechanical point of view. The most significant physical processes and mechanical behaviours are highlighted within this study, which are suggested to be accounted for in future risk management of CO2 storage projects.