Experimental research on the permeability of high-rank coal under a varying stress and its influencing factors

In order to investigate the permeability behavior of high rank coal during early depletion of CBM and its influencing factors, we collected 14 coal samples from Ordos Basin in northwest China and determined their air permeability under a varying effective stress of 2.5-20 MPa in laboratory. We used effective confining pressure to simulate effective stress. It turns out that high rank coal permeability is susceptible to effective stress. Permeability of coal samples declines exponentially with the rise of effective stress on the whole. When effective confining pressure variation ranges from 2.5 MPa to 10 MPa, permeability varies dramatically; when effective confining pressure rises to above 10 MPa, the curve of permeability decline gets gentle and the stress sensitivity becomes relatively weak. Coal permeability is also affected by coal moisture, maceral, fracturing and metamorphism degree. Permeability rises with the increase of vitrinite content and fracturing degree and decreases with the increase of moisture content. Permeability of wet and fractured coal samples is more sensitive to effective stress than dry intact ones and the irreversible permeability loss rate of fractured coal cores is significant, over 80%. Comparison of tests results of medium and low rank coal samples and high rank ones shows that, the stress sensitivity coefficient decreases with the increase of the maximum reflectance of vitrinite; if experiencing the same load-relief process, the irreversible permeability loss rate of low rank coal is less than that of medium and high rank coals.