How does geomechanics model simplification impact assessment of solute transport in 3D fracture networks

ABSTRACT: Geomechanics models used to study stress-induced aperture changes in fractured rocks capture the complex interactions between stress concentration, rock deformation, and fracture displacement. However, their complexity and high computational demands often lead to the use of simplified approaches. These simplified models reduce complexity by, for example, decoupling fracture-fracture mechanical interactions and neglecting stress-redistribution effects. This study investigates how a particular geomechanics model simplification impacts solute transport predictions in 3D fracture networks under varying stress conditions. We explore under which stress conditions solute transport predictions by the simplified model are close to the full-complexity model, and when they start to deviate and why. Our results indicate that, for an assumed typical friction coefficient of 0.6, under low stress ratios (e.g. 1 to 3), both simplified and full models predict similar transport behavior, as reflected in their breakthrough curves. However, at higher stress ratio (e.g. 4), significant difference emerges, with the full model showing much earlier solute breakthrough than the simplified model. This can be attributed to the fact that the full model accounts for local stress variations which can promote fracture dilation and therefore the formation of preferential flow pathways. We show that while the simplified model may be sufficient for low-stress ratio conditions, it fails to capture key transport dynamics under high-stress conditions, for which great caution is needed when applying them in practice.