Experimental study of influence of hydraulic punching on microscopic pores and structural components of coal

Coal mine gas disasters seriously restricted the development of China’s coal industry,and most of which occurred in the lowpermeability coal seams with low permeability and poor extraction efficiency. At present, hydraulic punching technology is a relatively effective method to control low permeability coal seams.In order to study the influence of hydraulic punching technology on the microscopic pore and structural components of coal, the microscopic pore, methane adsorption properties and structural components of coal before and after hydraulic punching were studied by means of mercury intrusion, liquid nitrogen, methane isotherm adsorption, Fourier infrared spectroscopy and Raman spectroscopy. The results showed that the hydraulic punching processhas a certain reforming effect on the microscopic pores in the coal body, which was mainly concentrated in the transition pore (especially below 50 nm) and microporous section. The methane adsorption properties of coal were improved significantly at the different temperatures after hydraulic punching, and the increase range could reach 1~2 times. After punching, the intensities of each peak of coal sample in Fourier infrared spectrum decreased, and the areas of D peak and G peak in Raman spectrum increased.That is to say, the hydraulic punching process also affected the structural components of coal, which mainly included the change of relative content of functional groups, the development of structural defects and the enrichment of aromatic carbon. Hydraulic punching effect can not only greatly improve the pore channels inside the coal body, so as to enhance the desorption, diffusion and seepage capacity of methane; but also could generate gas with small molecule structure, which can be stored in the rich pore structure inside the coal body. These changes in microscopic pore and structural components after hydraulic punching could well explain the increase in gas drainage efficiency of the coal body after hydraulic punching at the production site, the smaller gas flow attenuation coefficient and the stronger continuous extraction ability.The research results of this paper were not only beneficial supplement to the pressure relief and anti-reflection mechanism of hydraulic punching, but also could effectively guide the actual production site.