The impact of fluid invasion on failure, deformation, and wellbore stability of deep coal seams

Coal-rock contains abundant cleats and fractures, drilling-fluid invasion can severely reduce strength and trigger wellbore instability. This study aims to quantitatively evaluate the weakening effect of fluid invasion on deep coal-rock and to develop a collapse-pressure model for coal-bearing formations. Triaxial compression tests are conducted on coal cores from the northern Kuqa region under simulated formation conditions. Dry specimens and specimens saturated with fresh water, original drilling fluid, and new high-sealing drilling fluid are tested, the resulting mechanical parameters are used to establish a collapse-pressure model. The results show that fluid invasion eliminates the yield point on the stress-strain curve, smooths the deformation response, and significantly reduces compressive strength. The weakening effect is governed primarily by a marked decrease in the internal friction angle of cleat surfaces, whereas the elastic modulus, Poisson’s ratio, and cohesion are less sensitive. The strength reduction follows the order fresh water > original drilling fluid > new high-sealing drilling fluid, indicating that increasing drilling-fluid density alone is insufficient to prevent wellbore collapse in coal-rock. The novelty of this work lies in the experimental quantification of fluid-invasion-induced mechanical degradation in both coal matrix and cleat/fracture system, and in integrating these degradation parameters into a mechanistic collapse-pressure model for deep coal seams.