Stability evolution and failure mechanism of coalbed methane extraction process in Chiyu Coal Mine: Implications for development

Abstract

The dynamic evolution of in-situ stress during coalbed methane drainage governs coal seam stability, and instability-induced damage critically impacts well productivity. Triaxial deformation and methane adsorption-induced expansion experiments were conducted to investigate the stability evolution mechanisms and controlling factors in mid-deep intact and fractured seams. Results show that permeability surges at the peak stress, then declines sharply and eventually stabilizes. Adsorption pressure correlates positively with adsorption-induced strain, with the maximum volumetric strain of 1.756%. Fractured zones are more prone to instability failure than intact coal seams. Instability failure occurs under normal faulting stress regimes with moderate-to-strong desorption capacity and strike-slip stress regimes with strong desorption capacity. Seams under reverse faulting regimes remain stable. Stress difference, critical desorption pressure, elastic modulus, internal friction angle, cohesion, and fault friction coefficient are inversely related to coal seam stability. In late-stage production, failure of faulted zones in the #2 and #4 coal seams occurred at pore pressures of 1.17 and 1.70 MPa, respectively. These failures induced five abrupt bottom-hole pressure drops, each lasting 26–54 min, severely compromising well productivity. Pore pressure inversion yields fracture-wellbore distances of 15–111 m.

Keywords

In-situ stress evolution / Reservoir stability / Instability-induced failure / Coalbed methane drainage

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