Abstract: Coal is a porous medium structure. During the hydraulic fracturing process, fluids may escape along pre-existing pores and fractures, known as filtration effect, which reduces the effectiveness of hydraulic fracturing operations. Additionally, there is no established system for the relationship between pressure changes during coal seam hydraulic fracturing and fracture propagation, making it impossible to accurately assess the damage caused by hydraulic fracturing to coal. To address this issue, the Wudong Coal Mine is chosen as the research background, and theoretical analysis, laboratory experiments, and on-site tests are conducted. Firstly, the initiation mechanism of hydraulic fracturing under loss conditions is analyzed based on the stress analysis theory of surrounding rock in rock mechanics circular hole problems, coal seepage theory, and the theory of tensile fracture criterion induced by stress concentration on hole walls. Secondly, an experiment of hydraulic fracturing for steeply inclined top coal is conducted to determine the relationship between the pressure curve of steeply inclined top coal and the fracture of coal rock mass according to the crack propagation situation of the experimental coal sample. Thirdly, an on-site hydraulic fracturing test is carried out to further determine the relationship between the on-site pressure curve and the fracture of top coal. Finally, four types of pressure curve evolution patterns are summarized as a basis for judging the effectiveness of hydraulic fracturing in coal rock mass. The research results show that when the internal water injection pressure in the borehole reaches the tensile strength equal to that of the coal rock, the first crack appears at the borehole wall. During the hydraulic fracturing process, the water injection flow rate must be greater than the leakage rate, and the higher the water injection rate, the shorter the pressurization process duration. The pressure curves can be categorized into four types: (1) single initiation type, (2) fracture connection type, (3) fluid loss type, and (4) secondary initiation type. A smaller proportion of fluid loss type during the fracturing process indicates a better fracturing effect. These findings provide a basis for judging the damage effect of hydraulic fracturing on coal bodies on-site.

Key words: hydraulic fracturing, steeply inclined coal seams, injection pressure, primary pores and fractures, filtration effect