Investigation of coupled seepage - heat transfer - mass transfer mechanisms in rough rock fractures based on lattice Boltzmann method

To investigate the coupled mechanisms of seepage, heat transfer, and mass transfer in a rough rock fracture, a three-dimensional numerical model is proposed based on the lattice Boltzmann method (LBM). The model employs three distribution functions to capture the evolution of the velocity, velocity temperature, and concentration fields, respectively accounting for the dependence of fluid kinematic viscosity, thermal diffusivity, and solute diffusivity on temperature. Model accuracy is verified against two benchmark cases. The model is subsequently applied to investigate the effects of rock temperature, pressure gradient, and fracture roughness on solute transport within fractures. A comprehensive sensitivity evaluation analysis is performed to evaluate the overall sensitivity of five key influencing factors: parameters: fracture aperture, pressure gradient, rock temperature, fracture roughness, and initial solute diffusion coefficient. The results reveal that increasing rock temperature reduces the kinematic viscosity of the fluid while enhancing the solute diffusion coefficient, thereby significantly accelerating solute transport in the rock fracture. Specifically, when rock temperature rises from 20℃ to 70℃, kinematic viscosity decreases to 48% of its initial value, while the solute diffusion coefficient increases to 5.17 times its initial value. Higher seepage velocity further promotes solute transport through the coupling effect between flow velocity and temperature. However, increased flow velocity partially offsets the temperature-induced acceleration. In the near-wall regions of fractures, solute transport exhibits higher sensitivity to temperature variations, with temperature exerting a more pronounced control than the pressure gradient. Increasing fracture roughness leads to reduced fluid velocity and elevated temperature within the fracture. The deceleration of flow exerts a substantially greater inhibitory effect on solute transport than the enhancement of diffusion arising from temperature elevation. Consequently, greater fracture roughness results in slower solute transport. Among all investigated factors, fracture aperture exhibits the highest sensitivity, whereas the initial solute diffusion coefficient shows the lowest sensitivity.