A brittleness index evaluation method for weak-brittle rock by acoustic emission technique

Abstract

Brittleness is a significant indicator of a series of engineering problems in the oil and gas community. However, the majority of brittleness evaluation methods focus on brittle rock, but few pay much attention to weak-brittle rock. In this study, an attempt has been made to study the relationship between released acoustic emission (AE) energy and brittleness during the fracture process of rock samples. An accumulative acoustic energy-based brittleness evaluation method was proposed according to the energy accumulation and dissipation in the pre-peak and post-peak stages. Moreover, the suitability and reliability of this method was verified by tri-axial compression tests on shale, tight sandstone, and glutenite. The experimental results indicate that both the complete stress vs. strain brittleness method and the accumulative acoustic energy-based brittleness method can reflect the effect of confining pressure, but deformation and failure characteristics that transition from brittle to weak-brittle behaviors with increasing confining pressure make the post-peak feature difficult to distinguish. Thus, the conventional complete stress vs. strain brittleness method fails to determine brittleness. However, the accumulative acoustic energy-based brittleness evaluation method is more sensitive to rock failure, even if the rock is weak-brittle or ductile, while acoustic energy can be directly monitored rather than calculated by a mathematical model. Thus, the accuracy of brittleness evaluation can be improved. The good match between the fracture morphology and accumulative acoustic energy-based brittleness results demonstrate that this novel method is more suitable for brittleness evaluation in weak-brittle or even ductile rock; therefore, this brittleness method can be efficiently used in deep formations under high-confining pressure, benefitting various engineering applications.