Single-factor physical simulation on matrix contraction effect of coal rocks with different ranks

In order to analyze the dynamic change characteristics of permeability of coal rocks with different ranks under the single factor of matrix contraction effect, the LFLab-Ι triaxial stress and strain(matrix contraction expansion) test system developed by Langfang Branch was used to carry out the single factor physical simulation of the matrix contraction effect on the coals of different ranks with constant effective stress methods, and analyze the effect of matrix contraction on coals permeability.By controlling a single variable, the effect of coal rank and constant effective stress on the potential of coal rocks “desorption induction” was analyzed.The experimental results show that① under the influence of the matrix contraction effect, the normalized permeability of the four sets of parallel experiments of Baiyinhua coal samples under the constant effective stress of 1.5-7.5 MPa rebounded by 1.57-3.68 times, with an average of 2.23 times; the normalized permeability of Xingxian coal samples rebounded by 1.64-2.39 times, with an average of 1.94 times; the normalized permeability of Yangcheng coal samples rebounded by 1.92-3.60 times, with an average of 2.45 times.② As the gas equilibrium pressure decreased, the permeability of low, medium and high coal rank coal samples increased as an exponential function, basically with the gas equilibrium pressure of 3 MPa as the critical point, and then the permeability increased rapidly.③ Under the conditions of different constant effective stresses in the same coal rank, as the constant effective stress increases with the coal samples of low, medium and high ranks, the permeability rebound effect became worse.Under the same constant effective stress and different coal rank conditions, when the constant effective stress was 1.5 MPa and 3.5 MPa, the permeability rebound effect of low coal rank coal samples was better; under the constant effective stress of 5.5 MPa and 7.5 MPa, the permeability rebound effect reversed.In addition, compared with the effect of coal rank, the constant effective stress played a decisive role in the dynamic permeability rebound effect.