Correlation between permeability and strain of coal samples under loading

In order to study the correlation between coal permeability and deformation under loading, the radial strains, axial strains and permeability of coal samples from No.9 coal seam of Zhaogezhuang Coal Mine in Kailuan mining area were measured with the triaxial servo-controlled seepage device.The experiments were carried out in two different conditions.One condition was to reduce the inlet gas pressure while the confining stress was constant.The other condition was to reduce the gas pressure and confining stress synchronously.The experimental results showed that the strains of coal samples containing non-adsorbing gas (helium) were linear with gas pressures.Compared with coal sample containing helium, the swelling of coal samples containing carbon dioxide gas was remarkable under the same experimental conditions.The swelling was resulted from the adsorption of carbon dioxide gas in coal samples.The experimental results showed that under the same experimental conditions of gas pressure, the strain of coal sample adsorbed by constant confining pressure and decreasing gas pressure was larger than that by decreasing confining pressure and gas pressure synchronously, and the difference between them was less than 5.4%.The decrease of external stress in this test had little effect on the adsorption strain of coal sample.With the decrease of gas pressure, the experimental results showed that the macroscopic volume of coal sample shrank, but the permeability increased.The pores and fractures of coal sample were not narrowed with the shrinkage of coal samples.The reason was that the permeability of coal sample was affected by the deformation of coal caused by effective stress and the deformation of coal matrix caused by gas desorption.The change characteristics of permeability were not only related to the macroscopic deformation measured on the surface of coal sample, but also related to the change of pore and fracture structure in coal sample caused by the deformation of coal matrix closely.The strain change measured on the surface of coal samples was not enough to explain the relationship between coal permeability and strain.Considering that coal permeability was influenced by both the effective stress and gas adsorption, the strain of coal samples containing carbon dioxide gas was divided into two parts:the strain caused by the effective stress and the strain caused by gas adsorption.Based on the S& D permeability model, the exponential correlation equation was derived to describe the correlation between coal permeability and the above two kinds of strain variations.The presented equation was used to match the experimental data, and the fitting coefficients were more than 0.9.