The fracture toughness of rock mode I is of great significance for quantitatively evaluating the fracturing ability of shale reservoirs. Based on the cracked chevron notched Brazilian disc test (CCNBD), the mode I (opening-mode crack) fracture toughness of the Longmaxi Formation shale specimen in southern Sichuan is determined. Based on the experimental data, the relationships among the fracture toughness of mode I, the density, the acoustic time and the shale content are analyzed, and the prediction model of shale fracture toughness of mode I is established by the geophysical logging data. The results indicated that the fracture toughness of predication model had a good correlation with the measured data from CCNBD. The model can provide continuous data of fracture toughness mode I along the well bore for field hydraulic fracturing operation, and has certain guiding significance in the exploration and development of oil and gas reservoirs.

1 Introduction

In China's energy sector, the exploration and development of shale gas has become a research hotspot in recent years (Ding et al., 2011), and the Longmaxi Formation shale in southern Sichuan has great exploration potential for shale gas resources (Li et al., 2016, Ma & Xie, 2018). Shale gas reservoirs have the characteristics of ultra-low permeability and natural fracture development(Guo et al, 2015, Shan et al., 2017), so large-scale fracturing is required to form industrial productivity(Chen et al, 2013, Wang et al., 2014). At present, the main drilling method for shale gas exploitation is horizontal wells, and the hydraulic fracture initiation and expansion of horizontal well sections involve mode I, mode II and mode I-II composite fractures(Yuan et al., 2012). Fracture toughness is an important mechanical parameter reflecting the ability of rock to resist crack propagation. When the stress intensity factor at the crack tip exceeds the rock's fracture toughness, the rock will crack(Xiong et al., 2019, Yasin et al., 2018). However, the rock samples in the deep part of the stratum can only be obtained by a small number of drilling cores, so it is impractical to determine the fracture toughness of the whole well by conventional methods. On the other hand, the deep formation rocks are subjected to high temperature and pressure, simulating the formation conditions by conventional methods are difficult. Therefore, it is necessary to develop effective methods for testing and predicting fracture toughness of deep formation rocks.

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