An in situ study on coalbed methane seepage related to multiple physical fields: A case study from the Qinnan-East Block in the Qinshui Basin, China

Coalbed methane (CBM) seepage is a significant geological factor affecting the capacity of CBM wells. Using the example of the Qinnan-East Block of the Qinshui Basin, in this work, the CBM seepage law under different in situ stresses, coal reservoir pressures, and temperatures is systematically analyzed using on-site well test data. A permeability calculation model that includes the coal reservoir stress, pressure, and temperature is established, and the coal permeability and its control mechanism under the action of multiple fields are revealed. The results demonstrate that the in situ stress, pressure, and temperature in the study area increase linearly with increasing depth. The coal reservoir geothermal gradient is 0.72-2.11 degrees C/100 m, and the mean value is 1.67 degrees C/100 m. The coal reservoir geothermal gradient rises with an increasing depth as a logarithmic function. The permeability of the coal reservoir is 0.01-0.20 x 10(-3) mu m(2), the mean value is 0.05 x 10(-3) mu m(2), and the CBM seepage under the action of multiple physical fields is relatively low. With increasing effective stress and temperature, the permeability continuously decreases following a negative exponential function. The CBM seepage is tightly bound to the fracture aperture and is controlled by the change in the in situ stress, the pore fluid pressure, and the temperature stress.