Testing the Key Fracture Mechanical Parameters of Shale-Sand Bedding Interface by Digital Image Method

Artificial specimens are created with similar materials. The fracture mechanical parameters are studied by three-point bending test and digital image method. The shale-sand interface's tensile stiffness (Ks) and fracture toughness (KIC) were tested. The research findings indicate that (1) The tensile strength and stiffness of the bedding plane decrease as the specimen size increases and the fracture toughness of the bedding plane is not significantly impacted by the size, and the average fracture toughness is 0.16 MPa/m1/2; (2) There is a positive linear relationship between the tensile stiffness and tensile strength of the bedding plane; (3) The tensile stiffness and tensile strength value of the bedding interface affect the hydraulic fracture propagation path at shale-sand bedding interface. Smaller tensile stiffness and tensile strength values make it easier for hydraulic fractures to open the bedding interface and form T-shaped hydraulic fractures when they extend to the interface; (4) The propagation path of hydraulic fractures at the interface of shale and sandstone is influenced by the tensile stiffness and tensile strength of the interface. The size effect of interfacial tensile stiffness and tensile strength suggest that specimen sizes should be focused when conducting rock core parameter testing. 1. INTRODUCTION The success of hydraulic fracturing in shale-sand reservoirs is closely linked to the propagation of hydraulic fractures in the height direction (Zhu et al., 2015; Khanna et al., 2016). However, due to the impact of the mechanical properties of bedding planes, the propagation of hydraulic fractures in shale-sand reservoirs can be quite complex (Gu et al., 2008). Therefore, understanding the fracture mechanical parameters of bedding planes is crucial to studying the propagation law of hydraulic fractures. Several scholars have carried out theoretical calculations to determine the hydraulic fracture penetration height, using fracture toughness of reservoir rocks and stress intensity factor of fracture tips (Chen et al., 1997; Zhao et al., 2009; Liu et al., 2017). In order to test the fracture toughness of shale, three-point bending experiments were conducted, and fracture propagation morphology was analyzed (Heng et al., 2019). A mathematical and physical model has been established to calculate the geometric parameters of hydraulic fractures, taking into account the impact of interlayer interface effects (Peirce et al., 2009; Gu et al., 2008). However, Peirce's and Gu's study only considers shear slip that occurs at the interface and interfacial shear stiffness while ignoring the interfacial tensile stiffness. These studies conflict with the interface tensile failure proposed by (Zhao et al. 2010; Zhong et al. 2009) used interface tensile stiffness to describe the interface and analyze the interface failure behavior. At the same time, many scholars have agreed that different ground stress conditions also significantly affect the penetration of hydraulic fractures (Wu et al., 2017; Li et al., 2014).