**2.1 Hydraulic fracturing**

Hydraulic fracturing is the artificial initiation and proliferation of fractures by high-pressured injection of fluid into the rock. The fracturing fluid is pumped into the rock at a pressure that surpasses the rock failure stress [1]. The operation is aimed at generating new fractures, reopening/expanding and extending the reach of existing fractures and increasing their connectivity. The concept of this well stimulation technique was foremostly introduced by Hubbert and Willis [2] and has since been developed into an effective and widely used method of increasing oil and gas reservoir productivity. Hydraulic fracturing has been successfully applied in various conventional (for enhanced oil recovery/gas recovery (EOR/EGR)) and unconventional reservoirs/source rocks, such as oil shales and tight rocks (i.e. tight oil/gas sandstones, limestones, shale, etc.) (e.g. [1, 3, 4]). Various designs of hydraulic fracturing operations are currently implemented in practice; an example is multicluster-multistage horizontal well fracturing (e.g. [5, 6]).
