Brittleness and Rock Strength of the Bakken Formation, Williston Basin, North Dakota

摘要

The geomechanical characterization is essential for the development of hydraulic fractures in unconventional finegrained reservoirs. Rock brittleness is used to determine the best hydraulic fracturing candidate. The formation with higher brittleness is considered to be more likely to develop fracture networks. We investigate the mineralogy-based and elastic property-based brittleness index of the Bakken Formation. The results exhibit excellent correlation between two methods. It also indicates that the black organic-rich Upper and Lower Bakken Shale typically appears to be more ductile compared to the Middle Bakken and Pronghorn Members. However, certain quartz-rich shale successions, indicated by high brittleness index, could be as brittle as Middle Bakken/Pronghorn Members. The core description of these brittle zones also shows the presence of vertical natural fractures. Additionally, Leeb hardness tests were conducted to better understand the rock strength. Previous geomechanical studies on Eagle Ford Shale and Niobrara Formation have suggested a good correlation between rock hardness and unconfined compressive strength. In contrast to the traditional triaxial and acoustic tests, hand-held rebound hammer provides a fine-scale, cost-saving and non-destructive method to estimate rock strength. This study demonstrates a relatively low correlation between unconfined compressive strength and Leeb hardness. However, this is very likely due to measurement errors and the poor core conditions.