Effects of geological pressure and temperature on permeability behaviors of middle-low volatile bituminous coals in eastern Ordos Basin, China

Coal permeability is of great importance for coalbed methane (CBM) development from coal reservoirs. The abundant CBM resource in middle-low volatile bituminous coals in the eastern margin of Ordos Basin has necessitated study on seepage features of these coal reservoirs. In order to better understand the control mechanism of geological pressure and temperature on coal permeability, this paper systematically addressed the permeability behaviors of different middle-low volatile bituminous coals from eastern Ordos Basin with different gases (helium and nitrogen) under various pressures and temperatures. The results show that the permeability decreases exponentially over effective stress. Coals with a great proportion of macro- and mesopores have a higher permeability level and a faster permeability decreasing rate to effective stress than coals with micro- and transition pores in majority. Under a constrained state, the permeability decreases over temperature because of the interaction between thermal stress and interior stress inside coal body. When the temperature is quite high, the permeability sensitivity to effective stress or temperature weakens correspondingly. Due to distinct adsorption abilities, the permeability tested by nitrogen is smaller than that tested by helium and their difference decrease with the rise of temperature. Furthermore, the permeability changing rate caused by effective stress, matrix deformation and gas slippage were calculated to visualize the pressure and temperature effects on permeability behaviors. The gas slippage-induced permeability improvement, affected by the pore fluid pressure, temperature, coal permeability and gas properties, was deeply discussed.