Study on the effect of acid-heat coupling on the damage characteristics of coal pore-fissure structure.

[Display omitted] • The mixed solution of HF and HCl can effectively expand the pores and cracks inside the coal. • The effect of acidification of coal will change with temperature. • The best acidification temperature for this experiment is 50 °C. • The acid-heat coupling affects the element composition on the coal surface and the internal pore structure of the coal. To study the effect of acid-heat coupled fracturing on the damage of coal pores and fractures, scanning electron microscope-energy dispersive spectrometry (SEM-EDS), mercury intrusion porosimetry (MIP) were used to analyze. The results of the study show that the acid-heat coupling effect not only changes the microscopic morphology and element composition of the coal sample, but also promotes the formation of a pore-fracture network within the coal, and the connectivity is significantly increased, which is conducive to the diffusion and migration of coalbed methane. The optimum acidification temperature for the experiment is 50 °C. SEM observed that the acid-heat coupling changed the local stress sensitivity of the coal body and weakens the strength of the coal body to form a fracture network. EDS analysis shows that the element distribution on the coal surface changes regularly with the increase of acidification temperature, and the reduction rate of mineral content on the coal surface is highest at 50 °C. MIP and fitting analysis show that the pore parameters of coal samples will increase with the increase of acid-heat coupling temperature, reaching a peak at 50 °C, and the acidification effect will be relatively slow after increasing the temperature. This research is of great significance for exploring the optimal acidification conditions of coal mines, reducing acidizing fracturing costs, and ensuring coal mine safety production. [ABSTRACT FROM AUTHOR]