Experimental Study on the Failure Mechanisms in Brittle Shales

The brittle failure of Chengkou shale occurs throughout the exploration and development processes of hydrocarbons. To investigate the failure mechanisms of Chengkou shale and analyze the associated mechanical behavior such as crack initiation, propagation, and coalescence at different stress levels, a series of laboratory experiments were conducted on servo-controlled triaxial cells equipped with ultrasound monitoring. The experimental results show that key mechanical parameters such as peak stress σ_p, stress onset of dilation σ_ci, and strain at peak stress ε_p exhibit nearly linear relationships at various confining pressures. In rock bodies, the wave velocity evolution at different stress levels very consistently reproduces the shape of stress-strain curves, while shear wave velocity v _s is more sensitive to crack damage than compressional wave velocity v _p. Furthermore, the Hoek-Brown failure criterion has an advantage over the Mohr-Coulomb fracture criterion due to the former's higher correlation coefficient r ². The wing crack damage models with sandwiched multilayers help explain the mixed tensile and shear failure mechanisms of Chengkou shale. The experimental results provide significant guidance for optimizing the design of drilling and well completion jobs, especially hydraulic fracturing operations, both in Chengkou shale and in other brittle shales around the world.