Mass transfer, phase transition processes and mechanisms of transcritical-CO2 in rapid leakage from offshore saline aquifer storage: A visualization study

Geological storage of CO 2 in saline aquifers is a critical strategy for major global economies to achieve carbon neutrality. With the transitions from pilot-scale demonstrations to large-scale implementation, ensuring its safety and effectiveness has become a primary focus. The phase transition and thermodynamic characteristics of CO 2 leakage are the theoretical basis for leakage prevention and control. Herein, we used visual PVT-cell to record the entire process of CO 2 from stable storage to complete leakage in brine. Subsequently, we developed a multiphysics coupled numerical model, and integrated with experimental observations to elucidate the coupled mechanisms of convection, phase transition, interfacial mass transfer, and heat transfer during leakage. The results reveal that rapid depressurization at the moment of leakage triggers a transcritical-phase-transition of CO 2 , forming a mist-like layer through heterogeneous nucleation at the interface, with structural trapping dominating the initial leakage. When the pressure drops to about 9.0 MPa, the CO 2 phase undergoes a rapid density reconstruction, accompanied by waterfall-flow due to the critical opalescence principle. Meanwhile, jellyfish-flow emerges in the saline, indicating the destabilization of solubility trapping. In the later stage of leakage, due to the loss of temperature induced by phase transition, extensive CO 2 hydrates generate, which temporarily hinders the leakage of CO 2 but lead to concentration accumulation. Hydrates decompose driven by geothermal heat, leading to rapid leakage dominated by CO 2 in solubility trapping. Therefore, the timing of hydrate formation is the bottom line for storage safety monitoring. This paper fills the gap in visual research on the entire process of CO 2 leakage and provides a reliable research framework for multiphysics coupled solubility trapping.