Impact of coal creep on permeability evolution under different temperatures and stresses

The permeability of coal reservoir is the key factor determining the effectiveness of coal-bed methane (CBM) exploitation. In CBM exploitation, the deep coal undergoes time-dependent deformation by the coupling of long-term temperature and stress, resulting in seepage channels evolution and reducing the reservoir permeability. In this paper, the creep and permeability tests of coal at different temperatures and stresses were conducted to study the creep of coal and its impact on permeability. The results shown that during the multi-stage creep, the permeability of coal undergoes instantaneous strain, volume-compressive creep and volume-expansive creep, decreasing gradually and then increasing. The increasing temperature contributes to the occurrence of coal creep. At high temperatures, the creep of coal develops more rapidly, and the impact of coal creep on permeability reduction increases. At the temperature of 150 °C, the permeability reduction is up to 40.35%. Based on the temperature-dependent elastic modulus and viscosity coefficient, a creep model of coal in tri-axial stress and a permeability model considering creep and sorption deformation were established. The creep model and permeability model can well describe the time-dependent deformation and the permeability evolution of coal very well. The results are of great significance for the efficient exploitation of CBM.