Mechanism of pressure pulsation in borehole during the process of liquid CO2 phase change displacement for coal seam CH4
MI Wansheng
WEN Hu
FAN Shixing
CHENG Xiaojiao
LI Rijun
LIU Bocong
JIN Feiyang
Abstract：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 ana-lyzes 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 injec-tion 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 expres-sion 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 conductiv-ity varies as an exponential function with respect to the coal sample radius and surface temperature decrease.It was found that the effect-ive thermal conductivity shows a monotonically increasing function with the decrease in surface temperature of the coal sample.By com-bining 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 mechan-ism of pressure fluctuation changes during CO2 phase change displacement of coal seam CH4.
Keywords：liquid CO2liquid-gas phase transitiondisplacementcoal seam CH4borehole pressurepulsatile mechanism
Publication Date：2025-08-31
Online Publishing Date：2025-09-12(First online date of this platform, not the publication date of the document)
Pages：11( 281-291 )