The Slipping Characteristics of Bedding Interface Under Shearing Stress in Unconventional Reservoirs

ABSTRACT: The bedding interfaces with different cohesions usually exist in unconventional reservoirs. During reservoir stimulation, the slips induced by hydraulic fractures are related to the fracturing effects. In this paper, the rock specimens with cemented and uncemented bedding interfaces were selected to conduct direct shearing experiments, and the slipping behaviors were analyzed using the digital image correlation (DIC) method and acoustic emission (AE) technique. The results show that the slip weakening mode exists in cemented bedding interface during slipping, and the corresponding variation of AE b-value is small. The entire uniform slippage will appear in the uncemented bedding interface during slipping, accompanied by the larger variation of AE b-value. The effects of the interlayer cohesion on the slip weakening zone length, AE magnitude, and the difference between static and kinetic friction were discussed. It is found that the opening feature is not significant during slip weakening, but obvious during cohesionless slipping. And a relationship model was established to describe the opening displacement affected by slipping. This study can help to understand the slip mechanisms during fracturing in unconventional reservoirs. 1. INTRODUCTION Unconventional reservoirs, such as shale, have been the key areas in oil and gas production (Gao et al., 2022). Unlike conventional reservoir rocks, unconventional reservoirs are usually characterized by bedding development. Therefore, investigating the slip of bedding plane in unconventional reservoirs during fracturing is one of the focuses in rock fracture mechanics (Zhao & Gray, 2021). Microseismic monitoring results of in-situ fracturing demonstrate that hydraulic fractures can induce interlayer slip (Rutledge et al., 2016; Tan & Engelder, 2016a). The enhancement of hydraulic conductivity by slip (Zhao et al., 2013; Ye & Ghassemi, 2018) is a crucial factor for promoting shale oil and gas production (Urbancic & Baig, 2016; Wang et al., 2019; Zoback et al., 2012). In field hydraulic fracturing, the slip fractures are usually monitored by microseismic moment tensor analysis (Tan & Engelder, 2016b) and micro-resistivity scanning imaging technology (Zoback & Lund, 2019). Laboratory experiments have also demonstrated that hydraulic fractures can induce interlayer slip at the bedding plane (Chen et al., 2023). Some researchers think that the shear slips of interlaminations during fracturing are part of complex fracture networks (Bunger et al., 2013; Huang et al., 2013; Min et al., 2010; Ye & Ghassemi, 2018).