Influencing Factors Discussion on Gas Pressure Sensibility of Coal Reservoir Stratum Permeability

To explore the dynamic evaluation laws of coal reservoir stratum permeability in the process of gas drainage and extraction, the experimental study on evolution laws that coal permeability decreases with gas pressure is carried out, which is made from K₂ coal seam in Songzao Mine under different stress and temperature. The results show that the permeability of coal has a tendency that declining first and then rising again during the gas pressure unloading. And the gas pressure corresponding to the minimum permeability is the critical value of gas pressure, which divided the curve of coal permeability into two stages: initial stage and second stage. We define the variation of coal permeability which is caused by reducing unit gas pressure as the sensitive coefficient of gas pressure C_p, the relationship between C_p and gas pressure can be fitted by logarithmic function. Due to two aspects of effect including effective stress and matrix shrinkage effect, the response degree of permeability to gas pressure at initial stage and the critical gas pressure both show an increase trend with the increase of confining pressure or temperature. Then based on the C_p, the functional relationship between permeability of coal and gas pressure is derived, which can provide a basis for the reasonable gas drainage scheme.