Enhancing coalbed methane recovery with liquid CO2 fracturing in underground coal mine: From experiment to field application

CO2 is both a primary greenhouse gas and an available energy source. In this study, a new type of underground coal permeability enhancement technique by liquid CO2 (L-CO2) fracturing is proposed. The key parameters that determine the feasibility of the technique are identified: L-CO2 injection pressure and flow rate. Based on the principle of stress superposition and the experiments for L-CO2 fracturing under true triaxial stress conditions, the calculation models of the initiation pressure and flow were determined. The mathematical model of the initiation pressure of the L-CO2 fracturing of coal is verified, and the quantitative relationship between the injection flow rates of L-CO2 and hydraulic fracturing is derived. Accordingly, a complete set of underground high-pressure (30 MPa) L-CO2 fracturing equipment for coal permeability enhancement is developed. A field test of the coal permeability enhancement by fracturing in an upward cross-borehole was performed. The results confirm that after the fracturing, the gas drainage mixed flow and pure flow of the observation borehole that was located 25 m distant from the fracturing borehole increased by 3.2 and 3.3 times, respectively, and the maximal CO2 concentration in the observation borehole was 40%. The influence radius of the L-CO2 fracturing was determined to be approximately 25 m. Compared with areas with no coal permeability enhancement measures, the average single-borehole gas drainage concentration increased by approximately 1.57 times, and the pure flow increased by approximately 1.82 times within a radius of 25 m after L-CO2 fracturing.