Potential evaluation and favorable zone optimization of CO₂ geological sequestration in deep coal reservoirs

CO₂ geological sequestration in deep, unmineable coal seams is a key technology for carbon capture, utilization, and storage (CCUS). This study focuses on the No. 3 major high-rank coal reservoir in the southern Qinshui Basin, utilizing high-pressure and high-temperature CO₂ isothermal adsorption experiments coupled with multiple adsorption models (Langmuir, BET, D-R) to establish a sequestration capacity calculation model. The study investigates CO₂ sequestration mechanisms and evaluates potential and favorable zones for sequestration. The results show that CO₂ sequestration mechanisms vary with temperature and pressure conditions. Specifically, adsorption dominates in the middle-deep subcritical zones but decreases with depth, while free-phase sequestration increases in the deep supercritical zone. Regarding model applicability, the BET model best fits supercritical CO₂ adsorption, effectively capturing the sharp adsorption increase near the critical point. Quantitative assessments indicate that the optimal sequestration depth is 800-1100 m, with a total sequestration potential of 575.5 Mt, 65.4% of which is in the deep supercritical zone. The supercritical zone's sequestration abundance reaches 956.1 × 10³ t/km², representing a 53.5% increase over the subcritical zone. Furthermore, adsorption and free-phase sequestration account for over 99% of the total potential, while dissolution and mineralization are negligible. Based on these evaluations, two deep coal reservoir units in the northern region are identified as optimal sequestration zones, coinciding with areas of high CBM potential, which could facilitate integrated CBM development and CO₂ sequestration. This study provides a theoretical and methodological framework for evaluating CO₂ sequestration potential and identifying favorable zones in deep coal reservoirs.