Insights of microstructures and lithofacies on fracture process zone of organic-rich shale

The fracture process zone (FPZ) plays a critical role in determining the mechanical properties of shale,as a quasi-brittle material. In this study, the effects of microstructure on fracture process zones of Carbonate Dominated (CD) shale and Quartz-Feldspar Dominated (QFD) shale are revealed by semi-circular bend tests combined with digital image correlation. Results indicate that the FPZ shape of CD shale is a fat ellipse and the FPZ shape of QFD shale is a narrow and long ellipse. The average FPZ width and length of CD shales are 4.70 mm and 2.38 mm, while the average FPZ width and length of QFD shales are 3.84 mm and 2.93 mm. The main reason for this phenomenon is the difference of mechanical properties between bedding and rock matrix. The Young's modulus, stiffness, and fracture toughness of the (QFD) shale bedding planes are 42%, 70%, and 50% greater than rock matrix. While these differences are 25%, 35% and 29% respectively in CD shale. The interfacial mechanical contrast between bedding planes and the rock matrix affects the energy distribution during fracture, ultimately governing the elliptical morphology development of fracture process zones (FPZ). Energy dissipation during fracture propagation occurs more readily along the sides of the pre-crack tip in CD shale, leading to a wider and shorter elliptical FPZ. A numerical simulation model based on the combined finite-discrete element method (FDEM) was conducted under conditions consistent with the experimental setup. The simulation results confirm that a high fracture energy has a significant impact on the post-peak performance of the element, making its plastic stage longer and thus generating a larger FPZ. This study provides beneficial insights into the anisotropic behaviors of oil shale.