Effects of surface contact mode on fluid flow through three-dimensional rough-walled rock fractures with contact areas

As one of the controlling factors of the three-dimensional (3D) fracture flow, the effect of surface contact on fluid flow behavior is usually characterized by the parameter of contact ratio. However, the difference in the shape of surface contact induces different fluid flow pathways, thereby resulting in a more complex flow process, while the influence mechanism has not been fully revealed. This study focuses on the reexamination of the effect of surface contact characteristics on complex fluid flow behaviors. For that, the origin of hydraulic tortuosity in rough fractures with contact areas was identified by theoretical derivation, and a modified permeability estimation model was established on the basis of the triple-effect model. To further clarify the control mechanism of contact shape on permeability, a novel algorithm for the generation of 3D rough-walled fractures with different contact modes (i.e., isotropic and anisotropic contacts) was developed, and a series of lattice Boltzmann simulations of the fluid flow through the generated fractures were conducted. Numerical results indicate that it is inadequate to accurately characterize the effect of contact area only by contact ratio, and the contribution degree of contact ratio to hydraulic tortuosity depends on contact mode that represented by anisotropy factor of contact area. On this basis, as an important source of the hydraulic tortuosity, the effect of surface contact was identified to be characterized by two parameters, namely, contact ratio and anisotropy factor. Finally, the validity of the modified permeability model was validated, and its generalizability was also discussed.