Research on permeability model of coal reservoir along producing wellbores

Permeability is an important parameter to characterize gas flow. In order to ensure the safe and efficient gas drainage, it is necessary to explore the evolution characteristics of coal reservoir gas seepage at different positions from the producing wellbores. However, the effective stress, the gas adsorption/desorption capacity of coal matrix and the temperature of coal reservoir are constantly changing with the gas drainage, and even the drainage damage may occur, which makes the gas migration behavior in coal seam extremely complicated. In order to explore the seepage characteristics of coal reservoir gas during gas drainage, the space-time evolution function of pore pressure affected by temperature was constructed in the cylindrical coordinate system with consideration to the influence of pressure and temperature fields on the permeability of coal reservoir, and the permeability model of coal reservoir under the action of stress and temperature was established accordingly. The results show that the established model could reasonably describe the evolution law of pore pressure along the producing wellbores and the migration characteristics of gas, that is, the pore pressure at different positions decreases firstly and then changes gently with the increase of drainage time under the condition of constant external stress, while the permeability of coal reservoir decreases at first and then increases. In addition, the calculated permeability of coal reservoir considering the temperature is lower than that at the same position without considering temperature. It is found through discussion that the gas drainage could be enhanced with the increasing of drainage time by setting a reasonable negative pressure drainage mode according to the different effects of fracture compression and matrix contraction on permeability evolution.