Pilot test on pressure pulsation and pumping efficiency of liquid CO₂ phase change displacement for CH₄ drilling in coal seams

Methane in coal seams (CH₄) significantly impacts coal mine safety, contributes to clean energy supply, and affects the greenhouse effect in the atmosphere. Injecting liquid CO₂ is not only a powerful measure to reduce coal seam gas pressure, lower the risk of coal and gas outburst accidents, and minimize the impact of CH₄ emissions on the atmospheric environment, but also an effective way to improve the quality and efficiency of coal seam CH₄ extraction. To investigate the enhancement effect of the pulsation of the borehole pressure on coalbed CH₄ extraction during liquid CO₂ injection, CO₂’s effective influence radius, CH₄ extraction concentration, borehole extraction mixture, and pure CH₄ extraction from boreholes were used as indicators. A series of in-situ comparative experiments were conducted at the Shuanglong Coal Mine in Huangling, Shaanxi, using continuous injection methods at pressures of 2.0 MPa, 3.4 MPa, and 5.0 MPa. Results showed that injection pressure exhibited a “∏-shaped” trend over time. At 3.4 MPa, borehole pressure fluctuated with an amplitude and period of 0.52 MPa and 10.7 minutes, respectively. The effective influence radius of CO₂ in the coal seam reached 25 meters, consistent with the 5.0 MPa pressure case and higher than the 2.0 MPa case. Additionally, compared to pressures of 2.0 MPa and 5.0 MPa, CH₄ extraction concentration, borehole extraction mixture, and pure CH₄ extraction efficiency increased by 29.9% and 7.2%, 43.6% and 27.3%, and 84.0% and 36.3%, respectively, with CO₂ injection. The phase change displacement of CO₂ in the coal seam effectively promotes CH₄ extraction efficiency.