Abstract
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.
Highlights
Gas explosions are a critical factor limiting the safety of coal mines.1,2 Gas extraction is effective in preventing gas disasters and reducing air pollution and a prerequisite and a guarantee for safe and efficient production in mines with high amounts of gas and in mines with coal and gas outbursts
This increases the permeability of coal and the connectivity within the coal matrix, thereby demonstrating that the acidification technology can promote the permeability of the coal seam, which is beneficial for gas extraction
The effect of acidification treatment on coals has been studied in four main respects: the mineral content in coal; the microscopic morphology of the coal surface; the permeability and porosity of coal before and after acidification; the effect of confining pressure and temperature on the permeability; and the porosity of coal with the synergistic effect of acidification
Summary
Gas explosions are a critical factor limiting the safety of coal mines. Gas extraction is effective in preventing gas disasters and reducing air pollution and a prerequisite and a guarantee for safe and efficient production in mines with high amounts of gas and in mines with coal and gas outbursts. The pores and fractures in coal reservoirs are filled with several types of minerals, which makes gas extraction difficult and substantially affects gas extraction efficiency. At present, treatments such as hydraulic fracturing, hydraulic slitting, and hydraulic punching can increase the permeability of the coal seam and are widely used. To improve the permeability of coal for the process of gas extraction, an acid solution containing hydrochloric acid, hydrofluoric acid, and acetic acid can be injected into coal to dissolve minerals, thereby increasing the number of gas migration channels and the connectivity of pores and fractures.. The effect of acidification on coal seams has been studied in four respects: the mineral-filling condition of coal; the microscopic morphology of the surface of coal; the permeability and porosity of coal before and after acidification; and the effect of confining pressure and temperature on the permeability and porosity of coal with the synergistic effect of acidification
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