CO₂ Penetration Time Effect in the Sand-Mudstone Combination Caprocks within the Bending Subsidence Belt in the Closed Goaf

CO2 storage in the closed goaf is an essential negative carbon technical reserve to solve the carbon emissions in the coal industry, and ensuring the safety of carbon storage is a prerequisite for realizing this technology. The CO2 breakthrough pressure of rocks and penetration time of strata in the bending subsidence belt are critical reference indices for evaluating the safety of CO2 storage in a closed goaf. This study conducted CO2 permeability and breakthrough pressure tests of rocks from a bending subsidence belt using the coal/rock solid–gas coupled comprehensive test system with the hydrogeological conditions of a real closed goaf. The effects of the axial pressure and water saturation on the rock CO2 permeability and breakthrough pressure were analyzed. Based on the theory of gas–liquid two-phase flow in porous media, the CO2–H2O two-phase flow model of the fissure-matrix dual porous media was established, and a numerical simulation of CO2 penetration in sand-mudstone combination caprocks at the site scale was performed innovatively. The effects of water saturation and fissures on the CO2 penetration time and damage were analyzed. Finally, we provided an outlook on the CO2 sealing enhancement techniques for bending subsidence belts. The results indicated that CO2 breakthrough pressures in water-bearing sandstone range from 3.1 to 4.5 MPa in the mechanical environment of the bending subsidence belt. Internal fissures weakened the sealing ability of the combination caprocks to CO2, but the increase in water saturation enhanced the sealing ability of the combination caprocks to CO2. The penetration time of combination caprocks containing fissures decreased by 5.98–13.61% compared to those without fissures as the water saturation of the sandstone stratum increased from 25% to 100%. The higher the water saturation of the sandstone stratum, the greater the damage to the combination caprocks after CO2 penetration, and the average damage of the mudstone is larger than that of the sandstone stratum. The internal fissures in the combination caprocks weaken this damaging effect. The study provides a foundation for the safety evaluation of CO2 storage in a closed goaf.