A Coupled X-Ray Imaging and Experimental Permeability Study of Propped Hydraulically Induced Fractures

The recovery of unconventional natural gas is a potential solution to the increasing worldwide energy demand and therefore plays a crucial role in the global energy landscape. However, the rapid reduction of production often experienced during the initial recovery phase of unconventional natural gas resources is a major concern. Therefore, an advanced understanding of realistic fracture characteristics and flow behaviour in hydraulically fractured shale and siltstone formations has become a subject of interest. This paper reports on the adoption of 3-D synchrotron imaging, 3-D fracture surface scanning, and statistical analysis to study the fracture morphology and characteristics of hydraulically induced fractures. The fractures generated in siltstone specimens exhibited fracture surfaces with high surface roughness (Joint Roughness Coefficients (JRCs) of 9-12) compared to shale formations (JRCs of 1.3-1.7). In addition, whatever the type of formation, hydraulic fracturing generates fractures with varying fracture profiles and fracture complexities. 3-D synchrotron imaging was utilized to measure the tortuosity of fractured specimens. Flow behaviour in proppant-filled fractures was studied to evaluate the influence of different proppant concentrations, fracture tortuosities, and confining stresses. Importantly, for low-proppant concentrations and high confining stress levels, the severity of fracture tortuosity determines the overall flow behaviour in fractures. The results reveal that fracture permeability increases by a magnitude of 20.62 and 28.4 darcy, when fracture tortuosity varied from 8.4 to 3.8 and 7.4 to 3.2, respectively. Furthermore, an increase of proppant concentration from 40 to 80% proppant coverage increased fracture permeability by a magnitude of 40.81 darcy, whatever the tortuosity of the fracture. 3-D synchrotron imaging of fractured permeability-tested specimens detected the greater crushing of proppants in low proppant-concentrated fractures than higher proppant-concentrated fractures.