Multifactor analysis and dominant factor identification for coal seam gas drainage borehole instability

Abstract

Abstract To reveal the main controlling factors of instability in gas drainage boreholes in coal seams, this study investigated the main controlling factors of instability in coal seam gas drainage boreholes through Thermo-hydro-mechanical coupled triaxial tests, acoustic emission monitoring, and FLAC 3D simulations. Experimental results demonstrate: (1) Increasing confining pressure significantly enhances coal sample parameters including peak strength (76.1% increase), peak strain (74.4% increase), and elastic modulus (28.1% improvement), while elevated gas pressure reduces peak strength by 22.6% and weakens elastic modulus by 32.0%. Temperature rise induces 37.1% and 42.4% reductions in peak strength and strain respectively. (2) Acoustic emission energy evolution exhibits a "slow-accelerated" two-stage characteristic, with initial energy mutation point stress increasing with confining pressure (36.8–52.8 MPa) but decreasing with gas pressure and temperature elevation. Numerical simulations indicate PP screen pipe support effectively restrains borehole surrounding rock deformation (84.5% vertical displacement reduction) and optimizes stress distribution (35.1% peak stress position reduction), with its synergistic bearing effect increasing coal mass peak stress by 27.19%. Response surface models confirm confining pressure exerts the most significant impact on stability (P < 0.0001), followed by gas pressure and temperature, providing theoretical and technical support for preventing instability in deep coal seam gas extraction boreholes.