From grain-scale dissolution to reactive fracture: A multiscale geomechanical study of chemo-mechanical couplings in reservoir rocks

Chemical weathering induced by reactive fluid circulation critically affects the mechanical properties of sedimentary rocks involved in subsurface energy applications such as geothermal systems and underground energy storage. This work investigates chemo-mechanical degradation in bonded granular geomaterials through a multiscale approach combining discrete and continuum modeling.

At the grain scale, Discrete Element Method (DEM) simulations are used to study dissolution-driven debonding under oedometric conditions. The evolution of the lateral earth pressure coefficient k0 used as a proxy for stress state, is analyzed as a function of cementation degree, confining pressure, initial stress anisotropy, and loading history. Progressive dissolution leads to convergence toward an attractor stress state, with k0​ stabilizing between 0.3 and 0.4 independently of initial conditions. This behavior results from the collapse of cement-stabilized force chains and chemical softening of grains.

At the continuum scale, a phase-field fracture model coupled with damage-enhanced reactive diffusion is developed, informed by micromechanically derived degradation laws from DEM simulations. The model reveals that higher initial cementation delays brittle fracture initiation, while increased acidity may induce a chemical ductilization effect that counter-intuitively postpones fracture due to localized softening ahead of crack tips. The competing effects of chemical softening and degradation of fracture toughness are quantitatively characterized.