Research on evolution law of coal micropore structure based on acid-heat coupling

In order to reveal the mechanism of acid-heat coupling in improving the permeability of coal seams, combined with the advantages of the low-temperature nitrogen adsorption method and mercury intrusion method under different pore size tests, X-ray diffraction（ＸＲD） phase analysis was used as an auxiliary method to analyze the microscopic and multi-dimensional angle analysis of coal pore structure. The research results show that the coupling conditions of acid and heat have a significant effect on the pore structure of the coal body, and it shows a trend of first strengthening and then weakening with the increase of temperature. At 50 ℃, the 5% HF+5% HCl mixed acid solution has the most significant effect on pore reformation. Specifically, in the micro-pore stage（measured by the low-temperature nitrogen adsorption method）, the pore volume increased by 22.3%, the specific surface area increased by 23.9%, and the average pore size increased by 57.1%. The pore volume increased by 110.2% in the visible crack stage（measured by the mercury intrusion method）.