CO2 phase fluctuation-induced topological damage enhances shale strength

Abstract

Pressure drops and low-temperature fracture zones readily induce CO₂ phase-state transitions, significantly affecting methane recovery and storage safety. This study conducted a series of CO₂ phase-transition erosion experiments coupled with triaxial cyclic loading-unloading tests under temperature-confining pressure (T-P_c) conditions. Key findings include: (1) Shale structural discontinuity is exacerbated by cyclic CO₂ phase transitions under loading, which cause physicochemical damage. As a result, the macroscopic mechanical characteristics and damage variables (quantified by dissipated energy and stiffness degradation) become disconnected. We show that, contrary to the conventional assumption that damage always weakens materials, the controlled damage distribution can counterintuitively improve mechanical performance. (2) To appropriately define damage evolution, we created a multiscale spatial-structure complexity evolution model (D_T) based on the inherent nature of damage: the geometric complexity evolution of structural defects. The dynamics of damage progression are accurately quantified by this model. The system produces enough microdefects to induce distributed damage when D_T exceeds 0.67, which enhances fatigue resistance. (3) A higher phase-transition frequency concurrently decreases permeability (P₂ < 0.0469 μm²) and increases porosity (P₁ > 6.65 %). This reduces pore connection and promotes the spread of distinct micro-defects within the spatial structure of shale. Paradoxically, macroscopic mechanical characteristics are significantly improved by such intensified damage. (4) A fundamental shift in the mechanism of shale damage is induced by cyclic loading with CO₂ phase transitions (15 h/cycle). Our results reveal that static damage parameters are insufficient for predicting macroscopic mechanical responses. Importantly, the spatial configuration of internal damage patterns is the major factor driving macroscopic strength evolution in rocks.