Repetitive Mining Stress and Pore Pressure Effects on Permeability and Pore Pressure Sensitivity of Bituminous Coal

Coalbed methane is a lucrative energy source, but its development relies heavily on the fracturing of coal seam and underground extraction during mining. This study of mining stress paths, pore pressure, and permeability of a coal body was aimed to overcome the constraints of low coal permeability under the stress applied to a coal seam group. Variation in the applied stress changes the effective stress in coal and its permeability (which also depends on pore pressure). The effects of different mining intensities, repetitive cyclic loading and unloading, and pore pressure on gas permeability in a coal mine are revealed and discussed. The permeability evolution curve can be described by quadratic polynomial equations and split into three (decreasing, stable, and increasing) stages. When the stress environment with low axial deviatoric stress changes to that with high axial deviatoric stress and when the stability stage becomes longer, the goodness of quadratic polynomial data fitting is reduced. Cyclic loading and unloading with different axial stress paths have different effects on coal permeability. Increased axial and confining pressures decrease the permeability and pore pressure sensitivity of permeability. Because the obtained permeability-pore pressure curves contain a stable stage, this sensitivity is more pronounce in the low and high pore pressure stages.