Effect of CO2–Water–Coal Coupling on the Mechanical Stability of Deep Coal Seams

Abstract CO2 geological storage in deep coal seams provides dual benefits of carbon storage and enhanced coalbed methane recovery. However, the CO2–water–coal interactions during storage significantly deteriorate the coal’s mechanical properties through swelling and softening mechanisms, seriously threatening reservoir stability and long-term storage integrity. Therefore, the complex effects of multi-fluid interactions on the mechanical properties of the reservoir still require thorough investigation. Consequently, at a constant temperature of 50 °C, we conducted a series of experiments under vacuum, 5 MPa CO2, 10 MPa CO2, 10 MPa deionized water, and 10 MPa CO2–water coupled conditions. The key findings are as follows: (1) water plays a dominant role in the deterioration of coal strength, with high-pressure deionized water causing the most pronounced mechanical degradation, primarily because the scale of fractures induced by clay mineral swelling upon water absorption is far larger than that caused by CO2 adsorption; (2) under CO2 saturation, the mechanical parameters of coal samples degrade in an approximately linear manner over time. Supercritical CO2, owing to its stronger adsorption affinity and higher penetration capacity, inflicts significantly greater damage than subcritical CO2; (3) the coupled action of CO2 and water exhibits distinctive damage characteristics: CO2 can penetrate smaller microfractures and, in synergy with water, form a denser internal microcrack network, resulting in the most pronounced reductions in wave velocity and increases in porosity in the coupled group. However, the competitive adsorption between water and CO2 constrains the superposition of their respective damage effects, resulting in a smaller reduction in mechanical strength under coupled conditions than in the water-only saturated group. These findings provide a comprehensive theoretical foundation for assessing the long-term stability of CO2 storage projects in deep coal seams under complex geological conditions.