Modeling and experimental analysis on the permeability evolution of coal containing methane

The change of coal permeability related to gas pressure is important to coal mining and CBM exploitation. The permeability evolution of various kinds of coal is different and changes with stress conditions. To investigate the permeability evolution of coal samples changed in gas pressure,the strain and permeability of coal samples collected from the No. 9 coal seam in Zhaogezhuang mine,Kailuan,China were measured during gas pressure reduction under constant temperature,axial and confining stress by using Triaxial servo-controlled seepage rig. The experimental results showed that coal shrinkage strain increased with the decrease of gas pressure. And the permeability of coal cores may increase gradually or decrease firstly and then increase ( The gas pressure cor-responding to permeability rebound is less than 1. 0 MPa). When the gas pressure reduced to 0. 3 MPa,the perme-ability was 1. 9-2. 9 times of the initial condition (gas pressure was 2. 0 MPa). Considering that coal axial and radial strains are approximately equal,a theo- retical model was established to describe the permeability evolution of the three-dimensional deformation coal. The model also reflected the effects of gas pressure and gas desorption on coal permeability. The analysis showed that in the process of reducing gas pressure,the permeability of coal will change from decreasing to increasing at a certain gas pressure (rebound gas pressure pr ). The equation of rebound gas pressure pr was derived. It indicated that pr was de- termined by coal volume modulus,K,and Langmuir coefficients related to the adsorption,εp and pL . The greater volume modulus K and the adsorption deformation coefficient εp are,the higher pr is. And pr was independent of the external stress and the internal gas pressure. According to the derived permeability model,the permeability curves and the re- bound gas pressure pr of coal samples under various stresses and gas pressures were calculated by referring to the ex- perimental data of this paper and other published articles. The numerical results were close to the experimental data with a maximum relative error less than 8. 9% . The trend of coal permeability changing with gas pressure in the exper- iment depends on the rebound gas pressure pr and the experimental gas pressure. When pr ≥pmax( the maximum gas pressure in the experiment),coal permeability decreases with gas pressure increasing. When pr≤ pmin (the minimum gas pressure value in the experiment),coal permeability increases with gas pressure. When pmin