The numerical simulation using extended finite element method (XFEM) and cohesive zone model (GZM) was conducted to find out the effect of layer spacing on layered shale's fracture propagation. Five numerical models with different layer spacing were created to conduct the uniaxial compression tests simulation. The final fracture morphology, energy characteristics, load-displacement curves of the models were analyzed. The obstruction effect of bedding on fracture propagation was observed, fracture initiated in the area away from bedding plane in a sloping way, then turned into the vertical direction, finally spanned through the bedding and was connected by other inclined fracture. In the same time, the distribution law of shear cracks and tensile cracks was also found, shear cracks initiated away from the bedding, and became tensile when turning vertical. As the spacing of the bedding decreased, the compressive strength of the model increased, and the cumulative AE count has a trend of decreasing. The results are conductive to understand the law of fracture propagation in the semi-brittle layered rock like shale, and predict the fracture propagation path.

1. Introduction

It is vital to find out the law of crack propagation in shale reservoirs to improve the efficiency of oil and gas collection (Hou and Diao, 2017). The bedding structure in shale is common, which has an obvious effect on the fracture's propagation in the shale (Hou and Chen,2014). But the spacing of the bedding planes in different shale is varied (Hou and Zhang,2018). In order to explore the influence of bedding on fracture propagation, previous scholars have done a lot of research.

M. Bahaaddini and G. Sharrock (2103) researched the effect of geometric parameters of joints on the rock mass failure mechanism, unconfined compressive strength and deformation modulus, and found that the failure mode is determined principally by joint orientation and step angle. Baotang Shcn and Ovc Stcphansson (1995) performed a series of uniaxial compression tests on gypsum specimens with preexisting fractures to study the failure mechanism of fractures and rock bridges in fractured rock masses, found that coalescence of fractures with surface contact and friction requires loads as much as 35% higher than that for coalescence of fractures without contact and friction. Yanlin Zhao and Lianyang Zhang (2016) investigated the cracking and stress-strain behavior, especially the local strain concentration near the flaw tips, of rock-like material containing two flaws, and found the stress-strain curves of the specimens containing two flaws are closely related to the crack development and coalescence process. Hou bing and Chang Zhi (2019) conducted true triaxial hydraulic fracturing tests to study the initiation and propagation of hydraulic fractures, and revealed that guar and slick water each play a different role in hydraulic fracturing.

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