Onset of reactive brittle cracking in sandstones: DEM-informed phase-field modeling

Chemical degradation of bonded granular materials sustaining mechanical loads is a critical process governing the long-term stability of natural geological systems and the safety of subsurface energy engineering operations. To investigate the interplays controlling reactive cracking in geomaterials, this study develops a multi-scale model that couples the phase-field fracture mechanics with damage-enhanced reactive diffusion. The novelty lies in a micromechanically-derived degradation law embedded in the phase-field fracture model, where material properties are informed by Discrete Element Method (DEM) simulations of intergranular bond dissolution. Our results show that a higher initial cementation level substantially postpones the triggering of the brittle fracture, as more time is required for the accumulated mass removal to cause the critical amount of degradation in the matrix ahead of the crack tip. A chemical ductilization effect is identified upon the onset of fracturing, where a higher environmental acidity counter-intuitively results in a delay in the initiation of the brittle fracture. This phenomenon is attributed to the acidity-enhanced softening around the crack tip, leading to a less steep increase in the maximum tensile stress perpendicular to the crack growth direction, which reaches the peak at a lower value. Furthermore, a competition between the environmentally induced softening effect that delays the onset of fracturing and a direct degradation of the intrinsic fracture toughness that promotes it, is quantitatively illustrated. The time required for inducing the brittle fracturing in a chemically degrading carbonate-cemented geomaterial shows a linear dependence on the material's critical energy release rate, under the dynamic interplay between the two counteracting mechanisms. The findings are applicable to a broad category of subsurface engineering concerning geomechanics, providing a fundamental basis for assessing the long-term integrity and maintenance of geostructures subject to reactive environments. The findings could be calibrated through controlled laboratory fracturing experiments equipped with acoustic emission detection.