Shear-thinning rheology reshapes hydrodynamic dispersion in heterogeneous fractures

Many subsurface fluids, such as polymer solutions, drilling fluids, and geothermal brines, exhibit shear-thinning behavior, meaning that their viscosity decreases as the flow becomes faster. In fractured rocks, where fluid flow and solute transport mainly occur through narrow and irregular pathways, the combined effects of fracture heterogeneity and shear-thinning rheology remain poorly understood. In this study, we develop a pore-scale lattice Boltzmann model to simulate fluid flow and solute transport in heterogeneous fractures with different aperture correlation lengths and injection velocities. The results show that fracture heterogeneity controls the overall shape of the solute front, leading to either diffusive broadening or channelized transport. At the same time, shear-thinning rheology enhances geometrical dispersion but suppresses Taylor dispersion by flattening the velocity profile across the fracture aperture. We also find that shear-thinning fluids promote earlier eddy formation in stagnant zones, which increases local mixing beyond what would be expected from molecular diffusion alone. These findings improve our understanding of how non-Newtonian fluids move through fractured rocks and help guide applications such as groundwater remediation, hydrocarbon recovery, and geothermal energy production.