Experimental and Numerical Simulation Study of the Main Controlling Factors of Borehole Stability with Different Coal Physical Properties

Abstract To resolve the low efficiency of gas drainage in soft coal seams caused by borehole breakout during coalbed gas drainage, it is essential to understand the mechanisms of borehole deformation and the main controlling factors of borehole stability with different coal physical properties. This paper investigates the geometric differences of borehole breakout in different preloaded coal samples under various factors using the Particle Flow Code (PFC). First, the tensile and compressive strengths were measured through Brazilian splitting and uniaxial compression tests. By integrating multiple parameters such as stress–strain curves and macroscopic fracture morphology, precise calibration of DEM macro-mesoscopic parameters was achieved. Subsequently, a series of batch simulations were conducted to investigate the critical conditions and failure modes of coal borehole instability under the influence of different factors. The results show that: (1) Through the trial-and-error calibration method using both Brazilian splitting and uniaxial compression tests, the error in key macro-mesoscopic mechanical parameters can be controlled within 3.13%, revealing the existence of stability-failure-collapse states during the borehole instability process. (2) By integrating key mechanical parameters and taking the microscopic parameter of coal physical cohesion (Cohesion, Coh) in PFC as the main controlling factor affecting borehole stability, it is found that hard coal (Coh = 23.5 MPa and 17 MPa) remains stable under stress difference (Stress Difference Ratio, N) ≤ 3.0 and borehole diameter (Diameter, D) ≥ 120 mm. Medium-soft coal (Coh = 10.5 MPa) exhibits a critical stress difference N = 2.5, a critical failure borehole diameter D = 90 mm, and a collapse borehole diameter D = 100 mm. Soft coal (Coh = 4 MPa) has an extremely low critical value, making it prone to collapse. Based on these findings, the critical conditions for stability-failure-collapse states are established. (3) Beyond the critical stress difference or borehole diameter, the V-shaped breach deepens and widens, with its normalized depth and fracture span angle increasing nonlinearly as the critical value for failure increases. The critical conditions for gas drainage borehole instability are universal and can provide theoretical support for the stability of drainage boreholes.