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While the hydraulic fracturing technology, aka fracking (or fraccing, frac),has become highly developed and astonishingly successful, a consistentformulation of the associated fracture mechanics that would not conflict withsome observations is still unavailable. It is attempted here. Classicalfracture mechanics, as well as the current commercial softwares, predictvertical cracks to propagate without branching from the perforations of thehorizontal well casing, which are typically spaced at 10 m or more. However, toexplain the gas production rate at the wellhead, the crack spacing would haveto be only about 0.1 m, which would increase the overall gas permeability ofshale mass about 10,000$\times$. This permeability increase has generally beenattributed to a preexisting system of orthogonal natural cracks, whose spacingis about 0.1 m. But their average age is about 100 million years, and a recentanalysis indicated that these cracks must have been completely closed bysecondary creep of shale in less than a million years. Here it is consideredthat the tectonic events that produced the natural cracks in shale must havealso created weak layers with nano- or micro-cracking damage. It is numericallydemonstrated that a greatly enhanced permeability along the weak layers, with agreatly increased transverse Biot coefficient, must cause the fracking toengender lateral branching and the opening of hydraulic cracks along the weaklayers, even if these cracks are initially almost closed. A finite elementcrack band model, based on recently developed anisotropic spherocylindricalmicroplane constitutive law, demonstrates these findings.

Branching of Hydraulic Cracks in Gas or Oil Shale with Closed Natural Fractures: How to Master Permeability