Numerical simulations of the effects of bedding planes on hydraulic fracture propagation law in oil shale

Hydraulic fracturing serves as a major technical means for the exploitation of oil shale reservoirs presently, and the hydraulic fracture morphology in oil shales is primarily affected by differences in the characteristics of bedding planes. However, current studies mostly focus on the influence of the developmental degree of bedding planes on fracture propagation, overlooking the effects of the bedding plane thickness itself on hydraulic fracture propagation. This study delves into the oil shale in the Xunyi area, Ordos Basin. Based on the theory of linear elastic fracture mechanics, this study developed a stress-damage-seepage model of hydraulic fracture propagation. Using the global numerical simulation method termed finite element method-cohesive zone method (FEM-CZM), this study analyzed the effects of the in-situ stress field and bedding planes’ thickness and spacing on hydraulic fracture propagation, followed by comparing fractures’ failure type and length and the area connected by bedding planes under different influencing factors. Key findings of this study are as follows: (1) The thickness of bedding planes affects their capacity to intercept hydraulic fractures. A large thickness will lead to a strong tendency of fracture propagation along laminae planes, thus inducing tensile failure. Accordingly, long fractures and a large area connected by bedding planes will be formed. (2) The bedding plane spacing affects the time for hydraulic fractures to reach bedding planes. In the case of a small spacing, hydraulic fractures will directly penetrate bedding planes. The spacing will increase the resistance to fracture propagation. A larger bedding plane spacing is associated with longer fractures and a larger area connected by bedding planes due to tensile failure. (3) the in-situ stress field determines the direction of the hydraulic fracture propagation. In the case of a large vertical in-situ stress difference, the vertical stress will compact bedding planes. As a result, hydraulic fractures are more prone to penetrate bedding planes. When the vertical in-situ stress difference is small, hydraulic fractures will bend and branch in the process of propagation on bedding planes, leading to increased fracture length and area connected by bedding planes. Based on these findings, it is recommended that hydraulic fracturing should be performed in areas with large thicknesses and spacings of bedding planes and a small vertical stress field, which are more favorable for the formation of efficient seepage and heat transfer channels. This study can provide guidance for the hydraulic fracturing construction for oil shale in the Xunyi area.