Effects of pulse wave on the variation of coal pore structure in pulsating hydraulic fracturing process of coal seam

Coal pore structure is not only closely related to gas adsorption, but also affect the dynamic characteristics of gas desorption and diffusion. In order to understand the influence of pulsating hydraulic fracturing on the variation of coal pore structure, in this paper, the mercury intrusion method and liquid nitrogen adsorption method were used to quantitatively analyze the changes of coal porosity and pore size distribution under the action of pulse wave. And combined with scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) test to study the variation of minerals on the surface of coal samples. The results show that the influence of pulsating load on coal porosity is greater than that of static pressure load. The cumulative volume of micropores and their proportion decrease with the increase of pulsating frequency and pressure; the cumulative volume and proportion of macropores show an increasing trend; the cumulative volume of mesopores increases with increasing frequency, but the proportion increases in the low frequency phase, and decreases in the high frequency phase. There are two types of pores in the coal sample affected by the pulse waves: one is the expansion of the original pores, and the other is the new pores because the mineral crystals embedded in the coal body are washed out. The results have an important theoretical basis for the analysis of gas microscopic dynamics during pulsating hydraulic fracturing.