Mechanical Properties and Brittleness of Shale with Different Degrees of Fracturing-Fluid Saturation

The mechanical characteristics of Longmaxi Formation shale with different degrees of fracturing-fluid saturation were characterized by applying triaxial compression tests at a confining pressure of 50 MPa. The test samples were collected from fresh outcrop shale in Dayou, Chongqing, China and the shale brittleness was evaluated based on brittleness-drop coefficient, stress decrease coefficient and softening modulus. The weakening of related rock parameters of shale specimens being immersed in fracturing-fluid for different time periods was studied and discussed. The degree of deterioration of the peak strengths, elastic and softening moduli and brittleness were significant and varied exponentially when the samples were soaked in fracturing-fluid. The samples were found to fail by shear on the whole accompanied by varying degrees of bedding plane cracking. With increase of sample immersion time, the number of shear failure surfaces changes from one to two and finally to more than three. The length and number of cracks parallel to bedding planes increased gradually, however, no cracks were formed perpendicular to the bedding plane even when the shale was soaked for a long time. The weakening of the brittleness and mechanical parameters with sample fracturing-fluid saturation are mainly related to change of stress state at the crack tips caused by hydration swelling, the dissolution caused by alkaline fracturing-fluid and the formation of liquid film on the surface of shale particles, all of which are the results of mechanical–physical–chemical coupling.