Study on Seepage Characteristics of Different Rank Coals After CO2 Adsorption Saturation and Its Implications for Coal Seam Sequestration

CO2 geological storage in deep coal seams is a key carbon emission reduction approach, where coal permeability dominates sequestration efficiency. Current studies mainly concentrate on pre-CO2-adsorption permeability of various coal ranks, while studies on permeability evolution after CO2 adsorption saturation remain limited, which is vital for optimizing sequestration schemes. This work performed permeability experiments on different-rank coal samples saturated with CO2 under diverse axial confining pressures and temperatures. The sequestration rate, capacity, and engineering safety were analyzed to propose optimal sequestration conditions. Results reveal that coal permeability decreases significantly after CO2 saturation. With axial confining pressure increasing from 10 to 30 MPa, the permeability of low-, medium- and high-rank coals declines continuously. Within the temperature range of 20 °C to 60 °C, the permeability of low-rank long-flame coal decreases first then increases, whereas medium-rank coking coal and high-rank anthracite show the opposite trend. At 800 m burial depth, CO2 injection efficiency first declines then rises. Distinct permeability evolution exists in different coal ranks, so differentiated injection strategies should be adopted to balance sequestration efficiency and safety. By mathematically fitting the permeability variation curves of anthracite saturated with CO2 under varying axial and confining pressures, combined with the surrounding rock stress formula around the borehole, when CO2 at approximately 4.7 MPa is injected into anthracite seams, CO2 can permeate to a distance of 1.35 times the borehole radius from the borehole center around the borehole. The penetration radius ratio of 1.35 is only a qualitative indicator, not a fixed design parameter.