CO2 seepage & breakthrough process and multifactorial influence mechanisms on it in unsaturated low-permeability sandstones

• CO 2 breakthrough pressure in sandstone with varying CO 2 phases is studied. • Relationship between CO 2 phase and CO 2 breakthrough pressure is revealed. • Influence of experimental pressure on CO 2 breakthrough pressure is investigated. • Function of CO 2 effective permeability and breakthrough pressure is proposed. CO 2 breakthrough pressure is a key indicator for studying CO 2 percolation behavior and assessing the sealing capacity of caprocks, which is critical for ensuring the safety and effectiveness of CO 2 geo-sequestration. However, existing research on how CO 2 phase, initial pressure difference, and confining pressure influence CO 2 breakthrough pressure is limited, with inconsistent and unreliable conclusions. To address this gap, this study investigated the mechanisms behind CO 2 breakthrough pressure using natural low-permeability unsaturated sandstone as the experimental material. A total of 27 CO 2 breakthrough pressure experiments were conducted under various CO 2 phases, initial pressure difference, and confining pressures using a step-by-step methodology. The results showed that there was no fixed or predictable relationship between breakthrough pressure and CO 2 phase. However, in the presence of a single CO 2 phase, the breakthrough pressure declined exponentially as the pressure increases. The influence of pressure on breakthrough pressure was attributed to the combined effect of CO 2 -H 2 O interfacial tension (IFT), contact angle of CO 2 -H 2 O-rock system, and two-phase viscosity ratio. The study clarifies interactions influencing CO 2 breakthrough pressure under varying conditions and elucidates the role of CO 2 phase behavior. These findings provide experimental data to improve assessments of CO 2 sequestration safety and caprock integrity, enabling more accurate evaluations of geological storage potential.