Frictional properties of proppant-filled fractures/faults under a shear-flow condition and the implication for fracture/fault stability during geoenergy extraction

With the global expansion of geoenergy extraction, the fault stability associated with the industry exploitation have drawn extensive attention. During the hydraulic fracturing process, proppants are usually injected into the fractures together with the fracturing fluid to maintain the permeability of the reservoir. However, the mechanism by which proppants affect the shear behavior of fractures during the injection process has not been clearly explored, and this may change the shear motion state of faults. In this study, granite was used as the experimental material to simulate fractures, and ceramic sand (<106 μm) were used as the fracture filling medium. The evolution of the frictional strength and frictional stability of fractures containing proppants under double direct shear conditions was investigated, and the effects of changes in normal stress, fluid flow rate of water, and shear velocity were studied. The results show that as the normal stress increases, the friction coefficient continuously decreases. When the normal stress is relatively high, the friction coefficient shows a decreasing trend with the increase in the fluid flow rate of water. At a lower fluid flow rate, the value of frictional stability (a – b) also increases with the increase in normal stress. The value of (a – b) is only negative when the normal stress is 1 MPa, and when the normal stress is increased to 2 MPa, all values of (a – b) are positive. The transient changes in frictional strength follow the same trend as the step changes in shear rate. Under the same normal stress, The fluid flow rate has a non-linear effect on the value of (a – b). These findings enable a deeper understanding of the possible impacts of granular proppant on fault frictional stability during the geoenergy extraction.