Pilot test of high-pressure water jet slotting with liquid CO2 injection to displace CH4 and improve coal seam permeability

Abstract

High-gas and low-permeability coal seams commonly exist in China; however, there is still a gap between gas drainage volume and planned production. Hydraulicization and CO2 injection displacement of CH4 are technologies that unilaterally improve the safety and efficiency of coal seam gas extraction. This study combines the high-pressure water jet slotting technology in boreholes with the liquid CO2 injection technology to displace coal seam CH4. Field experiments were conducted on the 401,102 working face of the Mengcun Coal Mine and the experimental results were compared with the original boreholes. The results indicate that, based on the time and cumulative volume of slag return from single blade slotting in boreholes, the optimal slotting pressure and slotting time are 90 MPa and 18 min, respectively. The average coal seam CO2 content in test area I, which adopted the combined technology, was 1.8–3.37 times higher than that in test area II, which adopted common drilling and CO2 injection; the influence radius of CO2 is 1.44–1.6 times higher. The average concentration of CH4 in boreholes in test area I increased by 1.06–4.76 times in relation to that in test area II, the pure extraction volume of CH4 in boreholes increased by 0.6–12 times, and the impact period of liquid CO2 injection was extended by 36.33 %. Therefore, this pilot experiment provides theoretical support and reference basis for the application and promotion of combined technologies.