Experimental study on stress-dependent permeability of coal samples with different salinities of water in coal

The stress sensitivity and water sensitivity of coals are key factors restricting the productivity of coalbed methane (CBM) wells. In this study, stress sensitivity experiments with different salinities and X-ray diffraction (XRD) tests were conducted with coal samples from the Zhaozhuang (ZZ), Sihe (SH), and Xishan coalmine (XS) in the Qinshui Basin to analyse the correlation between permeability and the coupling of salinity and effective stress. To reveal the revolution of coal permeability under the coupling of salinity and stress, the permeability damage rate (PDR), the stress sensitivity coefficient (alpha k), and the water sensitivity index (IW) were introduced to evaluate the stress sensitivity and water sensitivity. The results showed that the permeability varied with the effective stress following a negative exponential function and was positively correlated with salinity within a certain range. When the salinity increased from 0 ppm to 5000 ppm, the stress sensitivity decreased; when the salinity increased from 5000 ppm to 10,000 ppm, the stress sensitivity remained constant, and the water sensitivity increased with increasing effective stress. Finally, a mathematical model for predicting coal permeability that considered the impacts of stress and salinity was established, and the control mechanism of stress and salinity on permeability was revealed.