Effects of Mechanical Properties on Gas–Water Flow Characteristics around Boreholes: Implementation of Damage-Based Coupling Models

Abstract

Neglecting the coal damage effect around a borehole could result in low accuracy of gas extraction seepage analysis. A fluid–solid coupling model incorporating coal stress and damage, gas diffusion, and seepage was established. Reliability of the proposed model was validated using field data. Variation characteristics of gas–water phase parameters in the borehole damage zone during gas drainage were analyzed. Meanwhile, effects of equivalent plastic strain, lateral pressure coefficient, internal friction angle, cohesion, Young’s modulus, and Poisson’s ratio on the damage state and spatiotemporal change properties of gas extraction flow were investigated. Results indicate that due to coal damage, permeability shows a three-zone distribution around the borehole, among which the fracture zone has the highest permeability, approximately 40 times of the original value. Permeability in the plastic zone decreases rapidly, while permeability is the smallest in the elastic zone. Coal permeability within the damage zone increases with continuous gas extraction. A smooth and low-value zone occurs for both fracture and matrix gas pressures. With the increase in equivalent plastic strain, the damage zone decreases, while peak permeability in the damage zone rises, and gas pressure in the smooth low-pressure zone continues to drop. The damage zone becomes smaller with an increasing lateral pressure coefficient, while those plastic and elastic zones become larger. The damage zone area corresponding to the lateral pressure coefficient of 0.89 is 82.3% smaller compared with that of 0.56. As internal friction angle and cohesion rise, the damage zone gradually decreases and shifts from a butterfly shape to elliptical shape. When Young’s modulus is heterogeneously distributed, except for concentrated shear damage zones around the borehole, punctate microdamage zones are also found at positions far from the borehole. Those damage zones gradually become smaller as shape parameters of the Weibull distribution get larger. The above findings are expected to offer theoretical support and practical guidance for borehole drilling and efficient extraction of clean methane resources.