The Impact of Bedding Plane Density on the Hydraulic Fracture Vertical Propagation and the Near-Wellbore Fracture Patterns in Shale Oil Reservoirs

Abstract

ABSTRACT: The Shahejie shale oil reservoir of the Jiyang Depression exhibits lithological homogeneity with finely developed bedding planes (BPs). BPs are prone to open and leak off during hydraulic fracturing, inhibiting vertical fracture propagation. Understanding the characteristics of BP development on fracture propagation is crucial for optimizing hydraulic fracturing strategies. This study conducts hydraulic fracturing experiments using shale cores with varying BP characteristics, employing a triaxial hydraulic fracturing simulation system combined with CT scan analysis. The results indicate that the density of BPs near the initiation point significantly affects the effectiveness of hydraulic fracture penetration, with higher breakdown pressures and greater fracture heights observed in specimens with lower BP density. High-density BPs near the initiation point hinder the vertical propagation of hydraulic fractures penetrating layers. Overall, hydraulic fractures exhibit a significant trend of upward propagation, resulting in wider fracture width and penetrating more BPs. BP widths typically remain smaller than those of primary hydraulic fractures at the same depth, after the hydraulic fracture penetrates BPs, the width of the fracture narrows, limiting proppant transport effectiveness in activated BPs. This study provides new insights and optimization criteria for hydraulic fracturing in laminated shale reservoirs. 1. INTRODUCTION Organic-rich laminated rock facies favor the in-situ enrichment of oil and gas and are preferred targets for shale oil exploration (Guo X. et al., 2023). Numerous scholars have conducted extensive research on the morphology of hydraulic fractures in the multi-layered formation, the extent of fracture propagation, and the interaction between fractures and bedding interfaces (AlTammar et al., 2019; Anderson, 1981; Athavale & Miskimins, 2008; Barree et al., 2010; Daneshy, 1978; Fisher & Warpinski, 2012; T. Guo et al., 2014; Y. Guo et al., 2020; Huang & Liu, 2017; Tan et al., 2020; Teufel & Clark, 1984; Warpinski et al., 1998; Yan et al., 2016; R. Zhang et al., 2019; Y. Zhang et al., 2019; Zhao et al., 2018; Zou et al., 2018). Through true triaxial fracturing experiments, it has been found that under significant horizontal stress differentials, bedding planes can still be opened or induce shear failure to connect natural fractures (NFs). However, vertical propagation is often constrained, leading to overall uncertainties in fracture morphology (Barree et al., 2010; Daneshy, 1978; Fisher & Warpinski, 2012; Warpinski et al., 1998; Zou et al., 2018).In reservoirs with developed laminations, hydraulic fractures interact with BPs in three modes: (i) penetration: hydraulic fractures directly penetrate the BPs; (ii) capture: hydraulic fracturing fractures propagate along the BPs; (iii) deflection: hydraulic fractures propagate along the BP for a distance before penetrating and extending beyond the BP. These scholars have also investigated bedding planes' angle, strength, and thickness. Bedding planes with lower strength and thinner thickness are prone to forming complex fracture networks, while thicker and stronger bedding planes result in higher breakdown pressures (Athavale & Miskimins, 2008; Heng et al., 2015; Zhao et al., 2016). Fluctuations in pressure curves during experiments can indicate that hydraulic fracturing fractures continually encounter and penetrate bedding planes during propagation (Yan et al., 2016). In general, controlled by differences in interlayer mechanical properties, interlayer stress differentials, and interface characteristics, simple fractures tend to form in vertically homogeneous formations, while complex fractures tend to form in formations with developed laminations (AlTammar et al., 2019; Anderson, 1981; Teufel & Clark, 1984). laminated shale typically exhibits significant mechanical anisotropy. It is highly prone to fracturing under high-pressure fluids or induced stresses, with laminations (or bedding planes) significantly influencing fracture height and overall fracture morphology (Yushi et al., 2016, 2017).