Experimental insights into the borehole plugging mechanism of shale and the coupling effects of hydraulic pressure and creep-induced argillization

Severe deformation-induced borehole plugging poses a critical challenge in deep shale gas extraction. This study investigates the rheological behavior of Longmaxi shale through hydro-mechanical coupled creep tests on hollow cylinder specimens. Results reveal a pronounced physicochemical fluidization process, where the brittle solid matrix progressively degrades into a viscous, clay-rich state. This degradation produces distinct morphological zonation around the borehole: an outer intact region, an intermediate fractured zone with micro-cracks, and an inner argillized zone. Progressive failure is governed by the interplay between hydraulic wedging and confining stress—low confinement induces tensile splitting, while high confinement drives radial extrusion of fluidized material. A generalized Burgers damage model accurately captures the nonlinear transition from steady-state creep to viscoplastic extrusion. These findings provide a theoretical basis for predicting long-term borehole stability in water-sensitive shales.