Time-dependent physico-mechanical degradation of Longmaxi shale subjected to acidic–alkaline fluid immersion

Chemo-mechanical interaction between shale and chemically active fluids critically governs wellbore stability and fracture conductivity. Time-dependent degradation of Silurian Longmaxi shale under acidic (pH = 2, 4), neutral (pH = 7), and alkaline (pH = 12) conditions is investigated over 21 days. Multi-scale experiments (mass/porosity analysis, triaxial testing, SEM/XRD) reveal a pH-dependent degradation hierarchy: Acidic ≫ Neutral > Alkaline. Strong acid immersion induces catastrophic deterioration, with 67.4 % strength loss and a transition from brittle splitting to strain-softening ductile failure. Quantitative analysis identifies two distinct chemo-mechanical pathways: (1) Dissolution-Dominated (Acidic)—proton attack dissolves carbonate cements, creating extensive honeycomb voids; and (2) Debonding-Dominated (Alkaline)—hydroxyl attack targets grain boundaries via silicate etching. Notably, alkaline environments exhibit higher “Damage Sensitivity” (strength loss per unit porosity), indicating interfacial micro-cracks compromise structural integrity more efficiently than volumetric dissolution voids. A unified framework linking fluid chemistry to macroscopic instability is established, offering vital insights for fluid optimization in complex geochemical settings.