An insight into shale's time-dependent brittleness based on the dynamic mechanical characteristics induced by fracturing fluid invasion

Traditionally, the shale's brittleness is defined as a constant, actually brittleness is a variable during the fracturing fluid invasion, which is crucial to decide the optimum fracturing time. In this paper, the brittleness evolution under fluid-shale interaction based on the shale's multi-scale mechanical behaviors were investigated. The results indicate that the stress-strain curve of the original samples shows obvious linear elastic characteristics, while the performance of the elastic stage and the unsteady fracture development stage of the samples are distinctive as interaction proceeding. The treated samples failures manifest as the radial tensile failure and shear slip failure along bedding plane compared with the blasting failure of original samples. Moreover, the energy and damage accumulations show a staged increase at pre-peak stage due to the duration of elastic growth stage of crack is shortened, while that of unstable development stage is extended. Finally, a time-dependent brittleness model with comprehensive multi mechanical factors is established through the analytic hierarchy process (AHP), which is verified by five independent brittleness indexes. The results show the new brittleness indicator can accurately describe the time-dependent characteristics of shale's brittleness. This investigation has proposed a new insight into evaluate the time-dependent brittleness induced by fluid-shale interaction, further, to determine the fracturing's optimum time points.