Hydraulic fracturing experiments have been performed in the laboratory in pre-fractured material under triaxial states of stress. Tests have been run on naturally fractured blocks of Devonian shale as well as blocks of hydrostone in which the angle of approach of the hydraulic fracture to the pre-fracture was varied in a systematic way. It was found that hydraulic fractures tend to cross pre-existing fractures only under high differential stresses and high angles of approach. In most cases the hydraulic fractures were either diverted or arrested by the pre-existing fractures.

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