A numerical simulator has been used in the study of a hydraulic fracture with two nearly parallel branches. The pressure required to propagate a branched fracture is found to be significantly higher than the pressure needed to extend a single planar fracture. Interactions with natural fractures have also been studied. The results show that hydraulic fractures may develop offsets at crosscutting natural fractures. Slip occurring on weakness planes after the hydraulic fracture crosses them has been modeled, demonstrating a reduction in fracture width at the site of the offset. This reduction in width may lead to proppant bridging at that location. An elastic calculation for the size of zones within which sliding may occur on preexisting weakness planes around a pressurized fracture indicates that the extent of these zones is proportional to the length of the pressurized fracture. Slip in such zones is controlled by the far-field stress, the pressure in the fracture, and the orientation and frictional properties of the weakness planes.

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