Stress sensitivity and difference of porosity and permeability in coal reservoirs in Hangcheng CBM block

In order to discuss the relationships between the physical properties of coal reservoir and the stress, the stress sensitivity experiments of the porosity and permeability of the coal samples from No.3, No.5 and No.11 coal seams in Hancheng coalbed methane (CBM) block were carried out under various pressure and the corresponding mathematical models were established. The results showed that the porosity and permeability of the samples decrease as the confining stress increases and the stress damage for permeability is much larger than that for porosity. However, the stress damage is different for different coal seams. When the confining stress increased from 4.14 MPa (600 psi) to 12.42 MPa (1800 psi), the stress damage of the sample from No.11 coal seam was highest in the samples from three coal seams, with the porosity damage of 76.5% and permeability damage of 93.3%, that from No.3 coal seam was lowest with the porosity damage of 38.5% and permeability damage of 77.9%, and that from No.5 coal seam is relative high with the porosity damage of about 45% and permeability damage of 83.9%. It was suggested that the occurrence of the fractures in coal is resulted mainly from the stress damage difference among the coal seams. Additionally, the power function is more accurate than exponential function for fitting the measured value of porosity and permeability in the range of test stress.