Abstract
Rock brittleness is an indicator to characterize the intensity of failure, widely used in shale hydraulic fracturing for reservoir stimulation and tunnel construction. Representative measurement approaches of brittleness are reviewed. A brittleness determination method comprised of rock physics techniques and mechanical tests is proposed for dry samples of shale, sand, limestone and concrete. The samples are tested with ultrasonic pulse under hydrostatic pressure. Triaxial tests of samples are carried out under various confining pressure with constant temperature, recording stress-strain relations and AE (acoustic emission) signals originated from macro rock damage. The SEM (scanning electron microscope) image of failure shale samples show cracks of brittle failure in rock matrix under test load. The method for brittleness determination is presented with comprehensive consideration of pre-peak damage and post-peak strength release rate based on stress-strain relationship. It appears that the method, which revealed intrinsic brittleness of intact samples and failure under artificial load, appropriately characterizes brittle behavior of rock with matrix damage. The brittleness is independent of mineral composition alone. Shale samples are far more brittle than other samples used for tests. The shale and sand samples in various confining pressure show more brittle behavior in moderate confining pressure.
1. INTRODUCTION
Rock brittleness is an indicator to characterize the intensity of failure, widely used in unconventional reservoir hydraulic fracturing for stimulation consideration. However, even up to now the definition of brittleness varies from authors to authors and areas to areas [1-8]. At present, it’s widely accepted that the pursuit for one universal brittleness determination method is laborious and fruitless, as specific assessment methods are required to serve in sorts of practice.
