Permeability characterization tests of continuous graded coal bodies under effective stresses

The stress change of coal body around the gas drainage borehole is one of the important factors affecting its permeability. In order to study the permeability with different degrees of fragmentation around the gas drainage boreholes, some coal particle composite samples with different range of particle diameter were prepared by the method of continuous grading, and the triaxial permeability tests with different Talbol power exponent n were carried out to analyze the influence of the effective stress on the internal structure and permeability characteristics of the fragmented coal samples. The results show that during the loading process, the axial pressure affects the permeability by changing the porosity of the broken coal, which decreases exponentially with the increase of the axial pressure. At the early stage of loading, the increase of axial pressure makes the volume of coal sample compressed, and the internal voids are closed, which causes the decrease of porosity and the weakening of permeability, resulting in the decrease of permeability. When the axial pressure is loaded with the porosity tends to 0.1, the internal compact structure of the coal sample makes a stable permeability. In the process of increasing the Talbol power exponent n of broken coal from 0.1 to 1.0, the permeability k shows three growth periods that are slow, linear and exponential stages, which are affected by the low-grade size particle, ratio of particle size and high-grade size particle respectively. The velocity of permeate is affected by the Talbol power exponent, pressure gradient, confining and axial pressure, however, the effect of pressure gradient is more significant. The larger the Talbol power exponent and pressure gradient with the smaller the confining pressure and axial pressure are, the larger the velocity of permeate is. There are the Talbol power exponent promotion effect of permeate velocity and the axial pressure and confining pressure inhibition effect of permeate velocity. The effective stress can characterize the inhibition effect of permeate velocity caused by the axial and confining pressure very well. The permeability of broken coal with different Talbol power exponents generally decays exponentially with the increase of effective stress, but the promotion effect of the Talbol power exponent on permeability gradually weakens, while the inhibition and amplitude effect of the effective stress on permeability gradually strengthen. The parameter change of the equation between the effective stress and permeability has three stages corresponding to the permeability curve. With the increase of the Talbol power exponent n, the parameter a of the equation decreases gradually, and the inhibition effect of the effective stress increases, while the increase of b, and the amplitude effect is gradually strengthened.