Shear and tension hydraulic fractures in low permeability rocks

Laboratory hydrofracture experiments were performed on triaxially stressed specimens of oil shale and low-permeability granite. The results show that either shear or tension fractures could develop depending on the level of differentials stress, even in specimens containing preexisting fractures. With 1 kb of confining pressure and differential stress greater than 2kb, hydraulic fluid diffusion into the specimens reduced the effective confining pressure until failure occurred by shear fracture. Below 2kb of differential stress, tension fractures occurred. These results suggest that hydraulic fracturing in regions of significant tectonic stress may produce shear rather than tension fractures. In this casein situ stress determinations based on presumed tension fractures would lead to erroneous results.