Model and Effects of Controlling the Pore Water Pressure Distribution Around Coal and Rock Masses on Crack Initiation

Abstract

As mining depth increases, vertical fracturing cracks become more prevalent, complicating crack initiation and extension through controlled holes. Current mechanical models inadequately consider initiation angles, and the mechanisms governing the directional propagation of hydraulic fractures are not well understood. In this paper, a pore water pressure distribution equation for coal and rock around a control hole is established, along with a mechanical model for hydraulic fracture initiation. The water pressure field around the control borehole is coupled with the global stress field model to analyze pore pressure and effective stress distribution, highlighting how water injection pressure affects initiation pressure and angle and revealing the mechanisms of hydraulic fracture initiation influenced by pore water pressure. Results show that: (1) Crack initiation pressure negatively correlates with control pore pressure, with cracks forming in the controlled direction on one side and the maximum principal stress direction on the other. (2) Pore pressure decreases along the line connecting the holes, while effective stress increases sharply. The maximum principal stress is aligned with this line, and the gradient pore pressure inversely correlates with the distance from the control hole. (3) The pore water pressure in the control hole reduces the fracture initiation pressure. Increased water injection pressure aligns the fracture initiation angle more closely with the direction between the two holes. Comparing these results with relevant tests confirms the reliability of the theoretical model. In field applications, control hole parameters should be arranged according to the desired induced direction, guiding the implementation of controlled fracturing technology.