Pore structure characteristics of middle and low rank coals and their influence on gas desorption characteristics

The pore structure of coal is an important factor affecting gas storage and migration. In order to further study the gas desorption characteristics of low and medium metamorphic coals, 6 sets of coal samples from the Fukang mining area were selected for mercury intrusion and gas desorption experiment. Mercury intrusion and extrusion curves of coal samples were drawed, the pore structure characteristics of each pore segment were counted, the gas desorption curve was drawed, and the pore morphology, pore volume, pore specific surface area and volume fractal dimension of coal and their influence on the gas desorption capacity and desorption rate were analyzed. The results show that there are different proportions of open pores in coal samples. Macropores are mainly composed of open pores, the middle and small pores are mainly composed of semi-closed pores, and the micropores are mainly composed of closed pores. In addiction, the total pore volume are mainly contributed by micropores and macropores, and micropores contributes the most to the total specific surface area. By drawing the gas desorption curve and the desorption rate scatter diagram, it is found that the gas desorption amount increases rapidly with the desorption time and then tends to a stable value, and then the fitting finds that the gas desorption curve of middle and low rank coal can be expressed by 1/Q=m/t^0.75+n, the degree of fitting is above 0.995, where m is a parameter related to the gas desorption rate, n is a parameter related to the desorption volume constant,and the coefficient of 0.75 may be related to the degree of coal metamorphism, and the desorption curves with different degrees of metamorphism can be analyzed in the later stage. And the desorption rate decreases exponentially with the desorption time. Fitting the pore structure characteristics of different pore sizes with the desorption characteristic parameters, it is found that the gas in the macropores is preferentially desorbed in the initial stage of desorption. As the pore size decreases, the priority rate gradually decreases. The fractal dimension of mesopores and macropores is between 2.879 1−2.991 5, which has obvious fractal characteristics. It is proposed that the initial desorption velocity and fractal dimension show a significant positive correlation. The relationship between n value and fractal dimension is not obvious.