Hydraulic fracturing operations are necessary to extract hydrocarbons and geothermal energy from tight reservoirs or hot dry rock. Particularly challenging is the design of hydraulic fracturing in anisotropic rock. A Devonian Slate was characterized for its rock mechanical properties in order to provide a sound database for numerical modeling of fracture initiation and propagation. Each parameter showed strong anisotropy. This paper summarizes anisotropic features of the most relevant rock mechanical parameters and demonstrates their effects with two examples from laboratory hydraulic fracturing tests.
By nature, most rock masses are not isotropic due to either intact rock properties and / or discontinuities. The texture and the petrographic content of slate form a highly anisotropic rock. Slate is a fine-grained metamorphic rock and is formed from shales by low-grade regional metamorphism. Because of this process, slate shows an expressed cleavage. This feature allows splitting slate into very slim plates when correctly executed. Slate was used as an electrical insulator, roof tiles or serves now as a possible host rock for gas.
There exists some information of rock mechanical data on slate, however, the focus was mainly placed on its anisotropic triaxial strength (Donath 1964, Ramamurthy 1993, Kwasniewski 1993). Alam et al. (2008) reported about elastic properties and fracture toughness of a slate. Haimson & Avasthi (1975) published results from hydro-frac laboratory experiments. One reason for the rather sparse database may be the difficulty in preparing lab specimen according to the ISRM suggested methods (Ulusay & Hudson 2007). The cleavage in the slate separates easily under drilling, sawing and finishing operations and samples disintegrate into slim discs not fit for testing. With the slate at hand, we observed approximately 60% of vain specimen preparation work. The use of non-waterbased drilling and sawing fluid was necessary to prepare proper specimen from large blocks obtained from a roof slate mine in Central Germany.
