Pore size characteristics of tectonic coal and its influence on gas bearing properties

The coal pores have an important effect on the gas bearing property of the reservoir, especially in the tectonic coal reservoir. The low temperature liquid nitrogen experiments were carried out in the coal samples with 4 kinds of coal structure, the coal samples were collected from the No. 13 coal seam of Panyi coal mine, Huainan coalfield. The characteristics of pore structure of coal reservoir at the nanoscale (1.7-20 nm) and its relationship with fractal dimension were systematically analyzed based on the least square method principle and the FHH fractal model. The results show that the increase of coal destruction increases the BJH pore volume and BET surface area, and the content of transitional pores and the micropore in mylonite. The capillary condensation in tectonic coal began to occur at 2-3 nm, gradually increased with the increase of relative pressure. The pores with size of 5 nm are the main contribution to gas adsorption, the better development of which leads to the best gas content of mylonitized coal. Besides the primary tectonic coal, the fractal dimension of nanoscale pores of the other coal reservoir in the study area is more than 2.6, and the average pore size is negatively correlated with the fractal dimension and the coefficient of correlation is above 0.9, which shows that the pore has obvious fractal characteristics and the complexity of pore structure is high. The comprehensive pore characteristics indicate that the more complex pore structure in tectonic coal and the higher the adsorption pore content with near 5 nm, the stronger the gas content.