An experimental investigation of the effect of acid stimulation on gas extraction from coal

This study presents a novel method of acid stimulation to improve the permeability of coal seams and promote gas extraction. The mineral composition and pore structure characteristics of coal, before and after acidification, were studied using x-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The XRD images suggest that the main mineral present in coal before acidification was kaolinite. The minerals that fill the pores and fractures in coal decrease or even disappear after acidification. Furthermore, SEM images indicate that acidification increases the number and width of fracture and increases the connectivity of the fracture system. The mechanism of acid stimulation has been studied by investigating the permeability and porosity of coal. The results demonstrate that the permeability and porosity of coal show an overall increasing trend with an increase in the duration of acidification. To replicate the practical conditions in coal mines, the effects of confining pressure and temperature on permeability and porosity, with the synergistic effect of acidification, were studied for three different coal samples. The permeability of coal shows an overall decreasing trend with increasing confining pressures at constant temperatures, and the permeability first increases and then decreases with increasing temperatures at constant confining pressures. The trends in permeability and porosity are approximately consistent. The results are important for enhancing permeability and, therefore, gas extraction from coal.