The observations from recent core-through experiments in Unconventional shales show existence of hundreds sub-parallel fractures. While core analysis suggests that many of these fractures were created during or after hydraulic fracturing process, their genesis and structure remain unclear. A hypothesis that the observed fractures are connected to the main hydraulic fractures and result from complex fracture propagation process has been suggested earlier.

In this presentation we describe a very different mechanism for generating multiple extension fractures adjacent, sub-parallel but isolated from the main hydraulic fracture by the rock matrix. A leak-off into heterogeneous layers with sufficiently high hydraulic diffusivity results in the area of tensile effective stresses adjacent to the propagating hydraulic fractures. A simple solution is presented. The effect of the key subsurface parameters on the extent of the potential failure zone is discussed and further research directions are suggested.