Feasibility on exploitation of coalbed methane by SC-CO₂ shock fracturing

The efficient exploitation of coalbed methane (CBM) bears the safety responsibility of ensuring the efficient production of coal mines and reducing the gas accidents. At the same time, it undertakes the important mission of supplying the clean energy and contributing to the realization of the dual carbon goal. A novel idea of using supercritical carbon dioxide (SC-CO₂) shock fracturing to stimulate CBM was proposed. This technique utilizes the high-pressure SC-CO₂ to dynamically fracture the coal body, and cooperates with the energy shock and tube-casing injection to realize the multiple and continuous injection, so that the coal reservoir creates complex fracture networks with a certain scale that are not controlled by in-situ stress. This technique has some advantages such as water-free, environmental protection, safety and high efficiency. A self-developed true triaxial SC-CO₂ shock fracturing experimental system was used to conduct the laboratory fracturing experiment of high-rank and low-rank coal rocks, and the borehole pressure characteristics and fracture patterns were compared with the conventional water fracturing and the SC-CO₂ fracturing. The results show that the tortuosity and complexity of coal fractures in the SC-CO₂ shock fracturing are higher than that of the conventional water fracturing and the SC-CO₂ fracturing. Consequently, the fracture networks in which main fractures, micro-fractures and bedding fractures connected with each other are created. With the increase of shock pressure, the fracture becomes branched. There is a reasonable range of shock pressure for specific coal to ensure the generation of fracture networks and reduce the pulverized coal. The SC-CO₂ shock fracturing is suitable for primary structured coal. In the field applications, it requires less professional equipment and has strong controllability. The conventional CO₂ fracturing equipment and shock tools can meet the fracturing requirements. The SC-CO₂ shock fracturing has the advantages of sufficient fracturing energy, little damage to the wellbore, and low economic cost. It can be used as a new reserve technique for revitalizing old CBM areas and reforming new CBM areas. It is suggested that the combination of the SC-CO₂ shock fracturing and the water fracturing should be adopted in the field application to ensure the generation of fracture networks, the expansion of the scope of stimulation and the long-term conductivity. This research verifies the feasibility of the SC-CO₂ shock fracturing to exploit CBM, which is expected to provide a new idea for the integration of efficient CBM exploitation and CO₂ geological storage.