Experimental Investigation of Pore Pressure Effect on Coal Sample Permeability Under Different Temperatures

When extracting deep coalbed methane (CBM), coal reservoir permeability would change with variation in temperature and pore pressure. It is essential for CBM development to reduce the sensitivity of coal reservoir permeability to stress and temperature. To explore the permeability evolution law during CBM production under different temperatures, CH4 permeability tests were conducted on raw coal samples under varying pore pressures of 0.2-3.0 MPa under five different temperatures (25-65 degrees C) in the laboratory. The coal permeability evolution laws with temperature and pore pressure were obtained. Permeability damage rate, stress sensitivity coefficient and temperature sensitivity coefficient were used for quantitative evaluation of the impact of effective stress and temperature on coal reservoir permeability. Moreover, a coupling coal permeability function that considers impacts of temperature and pore pressure was established. It turns out that coal sample permeability declines in a negative exponential manner with decreasing pore pressure or increasing temperature, and coal samples exhibit obvious sensitivity to stress and temperature. Coal sample stress sensitivity decreases with decline of pore pressure and temperature. Additionally, coal sample temperature sensitivity fluctuates when temperature increases and shows a downward trend when pore pressure decreases. With simultaneous rise in pore pressure and temperature, coal sample permeability shows a complex nonlinear downward trend. The coal permeability coupling model achieved good fitting results. Deep coal reservoir permeability continually change during CBM production, which significantly relates to the effects of stress sensitivity, matrix shrinkage as well as gas slippage. With increase in temperature, the downward trend of permeability profile slows down, and the permeability recovery degree increases in low-pressure stage. The results have theoretical and practical significance to guide reasonably drainage of CBM wells in different geothermal fields.

Automated summary

The permeability of deep coal reservoirs in coalbed methane extraction is affected by temperature and pore pressure. This study explores the permeability evolution during CBM production under different temperatures and establishes a coupling coal permeability function. The results show the sensitivity of coal reservoir permeability to stress and temperature, and provide guidance for reasonable drainage of CBM wells in different geothermal fields.