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.