Impact of Geomechanics Model Simplifications on Assessing Flow and Transport in 3D Fracture Networks

Changes in subsurface stress conditions induce fracture aperture changes, altering the hydraulic properties of fractured rocks. Due to the high computational cost of full 3D geomechanical modeling, simplified models are usually adopted, using two main simplifications, namely (i) calculating local stresses on a fracture by projecting far‐field stresses onto individual fracture planes without considering nearby fractures, and (ii) assuming a linear elastic fracture mechanics‐based constitutive law for fracture shearing. In this study, we investigate the consequences of using these geomechanical model simplifications on assessing flow and transport in 3D fractured media by comparing the simplified model against a full geomechanical model that integrates local stress heterogeneity and a Coulomb‐type shear behavior. We explore varying stress conditions to determine when the simplified model closely aligns with the full model, and when and why it starts to deviate. Our results indicate that, for an assumed typical friction coefficient of 0.6 and shear stiffness of 10 GPa/m, under stress ratios of 1 to 3, shear deformation is in the elastic stage, and local stress variability does not result in significant differences between simplified and full models. However, at a high stress ratio (e.g., 4), plastic shear slip prevails, and a significant difference between the two models emerges. The full model accommodates more intense shear displacements, resulting in increased aperture heterogeneity, enhanced flow channeling, and earlier solute breakthrough, while the simplified model underestimates these effects. Our results suggest that great caution is needed when applying simplified models in practice, especially when the stress ratio is high.