Acid–Shale Interaction-Induced Pore–Fracture Reconstruction and Damage-Dependent Creep Evolution

Acid–shale interaction can substantially alter shale microstructure and time-dependent mechanical behavior, yet the link between mineral-scale reactions, pore–fracture reconstruction, and damage-dependent creep remains insufficiently quantified. Shale specimens were exposed to acidic solutions with initial pH values of 5, 3, and 1, followed by solution chemistry analysis, X-ray diffraction (XRD), nuclear magnetic resonance (NMR) T2 relaxation analysis, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDS), micro-computed tomography (micro-CT), and stepwise triaxial creep testing. A comprehensive damage variable Dc integrating mass loss, P-wave velocity degradation, and porosity increase was introduced to describe the post-reaction damage state independently of the mechanical parameters subsequently used in constitutive modeling. Preferential dissolution of calcite and dolomite weakened carbonate-bearing cement and promoted pore–fracture reconstruction. The total NMR porosity increased from 3.82% to 8.62%, while the long-T2 (>100 ms) signal fraction increased from 5.5% to 18.0%. Correspondingly, the final cumulative axial strain increased from 0.687% to 2.480%, and the steady-state creep rate at 30 MPa increased from 1.55 × 10−4 to 10.40 × 10−4 h−1. The estimated long-term strength threshold exceeded 30 MPa for the control shale but decreased to approximately 22 MPa after strong acid exposure. Incorporating Dc into a history-dependent viscoelastic–viscoplastic formulation enabled the progressive degradation of elastic stiffness, viscous resistance, and the nonlinear creep threshold to be described within a unified framework. Specimen-level cross-validation showed improved predictive performance relative to conventional Burgers and Nishihara models. The results therefore provide a quantitative state-to-response framework linking independently characterized acid-induced physicochemical damage to subsequent creep evolution.