Mechanism of pressure pulsation in borehole during the process of liquid CO₂ phase change displacement for coal seam CH₄

Injecting liquid CO₂ into coal seam boreholes is an effective measure to improve CH₄ extraction efficiency. We established a physical model for CO₂ phase change displacement of coal seam CH₄ based on the actual injection process in boreholes. The model analyzes the composition of borehole pressure, using basic theories of heat transfer such as the ideal gas law, heat calculation formulas, and Fourier's law of heat conduction. It explains the essential causes and influencing factors of pressure fluctuations in boreholes, and provides mathematical expressions for these fluctuations. It is found that during constant pressure injection, borehole pressure variations depend on the phase change pressure of liquid CO₂. This phase change pressure is jointly regulated by the effective thermal conductivity of the injection process and the real-time temperature inside the borehole. Therefore, based on the principle of radial heat flow, an experiment was conducted to determine the variation of effective thermal conductivity with the surface temperature of coal samples of varying diameters. The theoretical expected value of the phase change pressure of liquid CO₂ during injection was derived. Ultimately, a mathematical expression for the pressure fluctuation in boreholes during the CO₂ phase change displacement of coal seam CH₄ was provided. The study results show that the maximum injection pressure of liquid CO₂ in boreholes with different diameters is 2.0 MPa. The temperature variation trends of the coal sample surface measured by infrared thermography and temperature sensors are consistent, and the effective thermal conductivity varies as an exponential function with respect to the coal sample radius and surface temperature decrease. It was found that the effective thermal conductivity shows a monotonically increasing function with the decrease in surface temperature of the coal sample. By combining the quantitative relationship between CO₂ phase change pressure and effective thermal conductivity, mathematical expressions for the phase change pressure of liquid CO₂ and real-time pressure fluctuations in boreholes were obtained, revealing the formation mechanism of pressure fluctuation changes during CO₂ phase change displacement of coal seam CH₄.