Why Fracking Works and How to Optimize It
  Although spectacular advances in hydraulic fracturing, aka fracking, have taken place and many aspects are well understood by now, the topology, geometry and evolution of the crack system hydraulically produced in the shale still remains an enigma. Expert opinions differ widely and fracture mechanicians must wonder why fracking works. Fracture mechanics of individual pressurized cracks has recently been clarified but the vital problem of stability of interacting hydraulic cracks escaped attention. Progress in this regard would likely allow optimization of fracking and reduction of environmental footprint. The present article first focuses on the classical solutions of the critical states of localization instability of a system of cooling or shrinkage cracks and shows that these solutions can be transferred to the system of hydraulic cracks. It is concluded that if the profile of hydraulic pressure along the cracks can be made almost uniform, with a steep pressure drop at the front, the localization instability can be avoided. To achieve this kind of profile the pumping rate (corrected for the leak rate) must not be too high. Subsequently, numerical solutions are presented to show that an idealized system of circular equidistant vertical cracks propagating from a horizontal borehole behaves similarly. It is pointed out that one important role of proppants, as well as acids that promote creation debris in the new cracks, is that they partially help to limit crack closings and thus localization. Based on the extremely low permeability of gas shale, one must imagine a hierarchical progressively refined crack systems in which the finest cracks have spacing in the sub-centimeter range. The overall conclusion is that what makes fracking work is the suppression or mitigation of localization instabilities of crack systems, which requires achieving uniform pressure profiles along the cracks. 
Z.P. Bazant, M. Salviato, V.T. Chau
physics.geo-ph