The Effects of Proppant on Hydraulic Permeability of Granite Fractures Under Shear Stimulation

Abstract

Hot dry rock (HDR) was commonly fractured rock mass with high temperature and lack of water, which has high potential as a geothermal reservoir. The geothermal exploitation in HDR needs a high permeability of the fractures in a reservoir. Proppants have been widely used in expanding the fracture aperture in petroleum engineering, which increases the fracture permeability effectively. However, there lacks of understanding in applying the proppannt in a HDR reservoir to the enhancement of hydraulic permeability.
This study investigates the evolution of hydraulic permeability and roughness features of propped granite fractures under shear stimulation. The roughness of the fracture surface was characterized by 3D morphological scanning and the fracture surface roughness coefficient (JRC), the slope root mean square (Z2) and structure function (SF) were considered. Shear tests were implemented on raw fracture and propped with 0.1 mm, 0.50 mm and 1.0 mm particle size. The results show both proppant placement and shear dilation expands fracture aperture.
The fracture dilation is sensitively affected by the particle size of proppant. The larger the particle size of proppant is, the larger the change value of normal displacement is determined. The shearing dilation could increase the hydraulic conductivity by around 1800 times. The fracture hydraulic conductivity can increase up by 3700 times by combination of both shear and propped dilation. Furthermore, the 3D morphological analysis indicates the proppant has protected the shear damage of asperity effectively by comparing the raw factures with propped fractures. To examine the evolution of fracture aperture under shearing process, the fluid flow with varying shearing displacement were tested. In order to observe the flow behavior, a transparent fracture was modeled. The effect of coupled shear dilation and proppant on the hydraulic permeability of rough fractures was analyzed.