Experimental investigation on different effects of fracturing fluids on mechanical properties and failure mechanism of continental shale.

Continental shale is abundant with unconventional oil and gas and has considerable potential for exploitation. However, due to differences in the deposition environment, continental shale presents unique characteristics in terms of mineralogy and mechanical properties compared to marine shale. Furthermore, the mechanical influence of fracturing fluid immersion must be considered because of the dual effects on the productive shale system caused by fracturing fluids remaining in the formation. In this study, tri-axial compression tests were performed on Jurassic continental shale specimens collected from an exploratory well (2778–2816 m deep) in the Sichuan Basin of China before and after exposure to fracturing fluids. Meanwhile, two quantitative indicators, the roughness R , which characterizes the widely distributed yield deformation, and the complexity F , which evaluates the complex multi-fractures, were used to distinguish the different brittle-plastic characteristics of specimens before and after fracturing fluid immersion. The results showed that the immersion of fracturing fluids weakened the mechanical properties of continental shale. Two types of fracturing fluid immersion effects were distinguished by the statistics of the failure characteristics of the different layers: lubrication-dominated and softening-dominated. Unlike conventional smooth tri-axial stress-strain curves, many of the post-peak curves were in a 'step-like' shape, corresponding to two sources: one part came from the formation of multi-fractures (brittle characteristics), and the other part came from the softening deformation of main failure surface (plastic characteristics). A new brittleness index, BI new , was proposed considering failure characteristics with different brittleness effects, which could effectively distinguish the brittleness difference before and after exposure to fracturing fluids. • Mechanical variations of continental shale along longitudinal profile were studied. • Different effects of fracturing fluids were distinguished by failure characteristic. • Different sources of additional energy dissipation were recognized. • New brittleness index was proposed incorporating different failure characteristics. [ABSTRACT FROM AUTHOR]