Evolution of coal body damage and gas migration around borehole induced by cyclic dynamic disturbance: Theoretical model and numerical analysis

In deep coal mining, the accumulation of coal rock damage induced by dynamic disturbance coupling such as high ground stress, mining, blasting and fault activation significantly affects the effect of coal seam gas extraction, and even induces dynamic disasters. This study developed a coupled mathematical model for cumulative coal seam damage evolution and gas migration under cyclic loading based on an elastic damage model. Through multiphysics numerical simulations, we revealed the influence of pore-permeability changes in coal seams on gas extraction under dynamic disturbances. Research shows that: (1) Gas pressure exhibits distinct spatiotemporal variations with cyclic intensity. Near borehole areas, permeability increases during initial extraction phases, causing rapid gas pressure drops. In distant monitoring points, although permeability decreases with increasing cyclic intensity, gas pressure undergoes cross-reversal with earlier onset at greater distances from boreholes, persisting dynamically beyond 10 days. (2) Damage zones dominated by horizontal tensile stress significantly exceed vertical directions, showing nonlinear damage accumulation patterns that transition from rapid to gradual progression. (3) Gas pressure dynamics follow a three-stage evolution pattern (rapid decline-temporary rebound-steady decay), with phase transitions closely tied to permeability spikes induced by damage. (4) Effective extraction radius expands progressively with the increasing number of cycles, but expansion rate declines as damage zones stabilize. The study illustrates the coupling mechanism among dynamic disturbances, damage evolution, permeability changes, and gas migration. These findings provide scientific foundations for optimizing borehole layouts, parameter adjustments, and permeability enhancement measures in deep coal seam gas extraction under strong dynamic disturbances.