Abstract: In this study, the effects of shear displacement and the number of cycles on the nonlinear hydraulic characteristics of three-dimensional rough fracture during cyclic shear are numerically studied. First, a three-dimensional fracture surface is generated based on the elevation data of the natural rough fracture surface, and the normal displacement during the process of cyclic shear is determined through previous studies. Sixteen fracture aperture fields are established so as to obtain the contact area and aperture distribution under different shear displacements and cycles times. It is found that when the number of cycles is constant, the upper and lower surfaces of the original mesh will stagger with the increase of shear displacement, the average mechanical aperture and anisotropy increase, and the contact area decreases; when the shear displacement is constant, with the increase of the number of cycles, the upper and lower surface undulations are smoothed under the action of shear stress, the contact area increases, and the average mechanical aperture decreases. Finally, the model is imported into the numerical simulation software to calculate the flow rate data. The results show that the variation of hydraulic gradient with flow rate conforms to Forchheimer’s law. The linear Fochheimer coefficient decreases by 88.3% and the nonlinear Fochheimer coefficient decreases by 95.2% as the shear displacement increases from 4 mm to 10 mm under 1 cycle; the linear Fochheimer coefficient decreases by 95.4%, the nonlinear Fochheimer coefficient decreases by 99.7% as the shear displacement increases from 4 mm to 10 mm under 2 cycles, and the reduction range of the two coefficients increases with the increase of the number of cycles. Further analysis shows that the increase of shear displacement boosts the fracture permeability, while the increase in number of cycles inhibits the permeability. In addition, the variation of normalized transmissivity coefficient with Reynolds number conforms to the research results of Zimmerman et al., and the numerical simulation results of this study are fitted with the normalized transmissivity coefficient prediction formula proposed by Zimmerman et al. The fitting coefficient approaches to 1, where Fochheimer coefficient β decreases with the increase of shear displacement and increases with the increase of cycle times.

Key words: cyclic shear, shear fluid flow, Reynolds number, normalized transmissivity coefficient