Shale microstructure governs mechanical properties. The long-time productivity of a well in unconventional hydrocarbon formations such as shale gas plays is strongly influenced by the relationships between the shale microstructure and the stress induced by hydraulic stimulation, which govern for example the stress- induced fracture-healing rate of artificially generated cracks (Xxxxxx and Mecklenburgh, 2017). The fracture-healing rate of those rocks depends on different factors like microstructure, porosity and mineralogy, differential stress, pressure and temperature. Measured stress-strain curves of tested samples are shown in Figure 1, revealing that Posidonia and Alum shales, as well as one porous and TOC-rich Upper Xxxxxxx shale, are relatively weak with low Triaxial Compressive Strength (TCS) and pronounced inelastic deformation. In comparison, the Xxxxxxx shale is much stronger and more brittle, almost independently of the relative carbonate and quartz/feldspar/pyrite (QFP) content. In general, shales with a low fraction of ‘weak’ phases reveal high TCS (Figure 1a), in particular if the carbonate content (and not the QFP fraction) is also high. The static Young’s moduli increase with increasing strength as observed in other shales (Xxxxxxx et al., 2015). Similar to TCS, high E-values were obtained for samples with low fraction of weak phases and particularly with high carbonate content (Figure 1b). Our results suggest that the triaxial compressive strength and static Young’s modulus of shales depend on whole rock composition, where dense rocks with a low amount of TOC and phyllosilicates, but relatively high carbonate fraction, display the highest values and preferentially brittle deformation behaviour. Compared to other European shales, the Xxxxxxx shale formation represents a strong and brittle shale type that may show a good ‘frackability’ and low fracture-healing rate, but contains also a low TOC content. In summary, using traditional characterisation techniques, the relative values of total organic content (TOC), porosity, petrophysical and phase fraction quantities, mechanical properties, and permeability were measured for selected shale samples. The obtained experimental values, and the correlations between them both increase our understanding of the fundamental mechanisms responsible of fracture formation and propagation in shale rocks by hydraulic stimulation, and they also provide the critical inputs for models of this behaviour. Via improved understanding and modelling, the community will ...
