The failure mode and fracture morphology of shale under dry and water fracturing situation

Hydraulic fracturing is an essential means to achieve economic benefits for shale oil and gas, with the main goal of forming complex fracture networks. Water-based fracturing fluid is a commonly used fracturing fluid. Gas bearing shale has special rock physical properties, such as microcracks development and high clay minerals, which will inevitably lead to physical and chemical interaction between shale and water-based fracturing fluid, and affect the failure mode and fracture morphology. In this paper, Lujiaping Formation shale in Chongqing area, southwest of China is selected for the compression experiments under dry and water fracturing condition, respectively. Through the analysis of mechanical parameters, deformation characteristics, the influence of water on the failure mode and fracture morphology is systematically studied. The experimental results indicate that both dry shale and water fracturing shale have shear localization bands, with dry shale one shear band and water shale several shear bands (main fracture), which is different from previous study that water saturated shale forms a dispersed fracture network without a main fracture. In fact, the marine shale of the Lujiaping Formation has a strong spontaneous imbibition ability. When shale is saturated with water, the water absorbed into shale increases pore pressure (reducing normal stress), which leads to further propagation of microcracks and increases the initial microcrack volume. The increase in initial crack volume will enhance the interaction between cracks. In addition, the water entering the cracks further reduces the friction resistance between crack surfaces, making the cracks prone to shear sliding, dilatation and forming several shear bands. The widely developed initial microcracks connect various shear bands, forming a more complex fracture network that increase the overall deformation width of the sample compared to dry sample.