Coal permeability enhancement via cyclic percussion using low-temperature liquid CO₂ and high-temperature water vapor and simulations of resultant gas drainage performance

Objective In the application of the coal seam permeability enhancement technique based on liquid CO₂, pore water in coal seams freezes due to low temperatures, blocking pathways for gas migration. Hence, this study developed a technique for coal permeability enhancement through cyclic percussion using low-temperature liquid CO₂ and high-temperature water vapor (also referred to as the cyclic cold and hot percussion).

Methods This study delved into the permeability evolutionary patterns of coals under cyclic cold and hot percussion using a low-field nuclear magnetic resonance (NMR) spectrometer, an automatic coal permeability tester, a contact angle tester, and 3D CT scanner.

Results and Conclusions  Key findings are as follows: (1) The area of the NMR T₂ curves of coals subjected to cyclic cold and hot percussion increased with the number of the cold and hot percussion cycles. (2) The incremental coal permeability was exponentially related to the number of cold and hot percussion cycles. The coal permeability and wettability increased with this number. (3) After 12 cold and hot percussion cycles, the pore volume ratio of seepage increased by 23.57%, the permeability increased by 0.009 3×10⁻³μm², and the contact angle decreased by 39.45°. (4) The coal permeability was correlated positively with the pore volume ratio of seepage and exponentially related to the contact angle. Based on the experimental results, this study conducted numerical simulations of the field engineering application of the cyclic cold and hot percussion of coal seams using the COMSOL software. The experimental results reveal an effective temperature influence radius of 0.7198 m as the coal seam temperature was restored to room temperature (25 ℃). This effective temperature influence radius was used to simulate gas drainage from coal seams in a coal mine, revealing that the coal seam temperature was directly proportional to the gas extraction efficiency. The experimental and simulation results reveal the on-site borehole arrangement, as well as gas drainage and transport patterns, in the application of the technique for coal permeability enhancement through cyclic cold and hot percussion, providing a reference for subsequent field application of this technique.