Experimental study on CO2 sealing capacity of salt-gypsum caprock

• Permeability decays exponentially with stress; 40% irreversible loss after unloading in salt-gypsum caprock. • CO 2 breakthrough pressure highest for supercritical phase, increases linearly with depth to 15.64 MPa at 1000 m. • Plastic domain at > 2600 m depth with 5.15 MPa tensile strength inhibits fractures for long-term sealing. CO 2 geological storage is a pivotal technology for climate mitigation, and the sealing capacity of the caprock is paramount for its long-term safety. Salt-gypsum caprocks, known for their ultra-low permeability and ductility, are ideal candidates, yet their sealing mechanisms under multiphase CO 2 conditions are not fully understood. This study systematically investigates the sealing capacity of salt-gypsum caprock from the Wolonghe Gas Field through integrated experiments, including transient pulse decay permeability tests, phase-controlled breakthrough pressure tests, and triaxial compression tests. The results demonstrate that the permeability decreases exponentially with effective stress, with a 40 % irreversible loss post-unloading. The breakthrough pressure, governed by CO 2 phase state (sc-CO 2 > liquid > gaseous), increases linearly with burial depth, reaching 15.64 MPa at 1000 m. The caprock transitions into the plastic domain beyond 2600 m, exhibiting a tensile strength of 5.15 MPa sufficient to inhibit micro-fracture propagation. This research validates the long-term containment potential of salt-gypsum caprocks and identifies a synergistic “low permeability, high breakthrough pressure, self-healing” triple-sealing mechanism, providing a theoretical basis for site selection in CO 2 geological storage projects.