Turning Abandoned Mines into Carbon Sinks: Laboratory Demonstration of CO2 Hydrate Sequestration in Cold-Region Goafs under Permafrost Conditions

Carbon capture, utilization, and storage (CCUS) is a key technology for carbon neutrality, but conventional CO2 geological sequestration suffers from high injection pressure, limited stability, and restricted storage space. Hydrate-based CO2 sequestration offers high capacity, low cost, and intrinsic safety. This study proposes a novel in situ CO2 hydrate formation and sequestration method in coal mine goafs containing permafrost layers in cold regions. The natural low-temperature permafrost environment provides favorable conditions for hydrate formation, while goafs offer large storage space. To avoid hydrate plugging and improve engineering feasibility, a liquid-first CO2 constant-pressure supply and predissolution mode was adopted. We simulated goaf conditions at depths of 110–130 m and ∼1 MPa. Experiments tested three injection temperatures (293.15, 298.15, and 303.15 K) and five goaf temperatures (269.15–277.15 K) using tetrahydrofuran and tryptophan as promoters. Results proved efficient sequestration at a low pressure of 1 MPa. Under optimal conditions (injection temperature of 298.15 K, goaf temperature of 271.15 K), capacity reached 60.9 mmol of CO2/mol of water (mmol/mol) with an efficiency of 135.36 (kg·m–3)/MPa. The 26.7 h stability experiments under depressurization showed 4.21–16.09 mmol/mol dissociated initially, but 66.38%–90.77% CO2 remained sequestered. Further, one-month tests under the pressure-release condition revealed hydrate dissociation almost stopped after day 1, yielding a 65.32% permanent sequestration ratio. Crucially, under the pressure-maintained condition, the system continued to form CO2 hydrates over the 30-day period, resulting in an additional 31.04% CO2 storage. These results validate the feasibility and long-term stability of this novel approach.