Study on temporal and spatial evolution of stress around borehole in hydraulic fracturing of soft coal

In order to reveal the cracking characteristics around borehole of soft coal, the fracturing process was divided into 6 stages according to the time-step equal division method. And spatial distribution of the stress around the borehole, the variation characteristics with time and the evolution of the difference coefficient of the horizontal stress were simulated by PFC^2D discrete element method. The results show that the stress around borehole increases greatly in the direction of the maximum principal stress in the first to fourth stages of hydraulic fracturing simulation. The stress increases rapidly in the direction of minimum principal stress in the 5 and 6 stages. The stress concentration around the borehole is obvious, and the stress level near the borehole is high. In the influence range of stress concentration, the trend of stress path around borehole in x direction and y direction is the same, and the difference coefficient of horizontal stress is small, but the stress paths and horizontal stress difference coefficient outside the influence range of stress concentration is just the opposite. The stress difference coefficient can reflect the uniform degree of crack development, and the degree of stress concentration affects the development and expansion range of crack. Based on this, the behavior characteristics of crack induced by borehole enlargement and stress shifts outward around hydraulic borehole of soft coal is revealed.