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The production of hydrocarbons from unconventional reservoirs, like tight shale plays, increased tremendously over the past decade. Hydraulic fracturing is a commonly applied method to increase the productivity of a well drilled in these reservoirs. Unfortunately, the production rate decreases over time presumably due to fracture closure. The fracture closure rate induced by proppant crushing and embedment depends on mechanical properties of shales and proppants that are influenced by confining pressure (pc), temperature (T), and shale composition. We performed constant strain rate deformation tests at ambient and in situ conditions of a typical shale reservoir (pc ≤ 100 MPa, T ≤ 125 °C) using European shale samples exhibiting variable mineralogy, porosity and maturity. We focused on a comparison of Posidonia shale with Bowland shale, which is believed to be the most prospective shale formation in the United Kingdom. Compression tests were performed perpendicular to bedding orientation. Stress–strain curves show that Bowland shales are relatively strong and brittle compared to Posidonia shale which display semibrittle deformation behavior. Brittleness estimated from elastic properties is in good agreement with the recorded stress–strain behavior but shows no clear relation to composition. Compressive strengths (σUCS = uniaxial compressive strength, σTCS = triaxial compressive strength) and static Young’s moduli, E, reveal a strong confining pressure and mineralogy dependence, whereas temperature and strain rate only have a minor influence on σTCS and E. The coefficient of internal friction for both shales is  ≈ 0.42 ± 0.03. With increasing amount of weak minerals (e.g., clay, mica) σUCS, σTCS and E strongly decrease. This may be related to a shift from deformation supported by a load-bearing framework of hard minerals to deformation of interconnected weak minerals at about 25–30 vol% of weak phases. At the applied conditions, the triaxial compressive strength and Young’s moduli of most shales deformed normal to bedding are close to the Reuss bound. To our knowledge, this is the first study, which presents results of experimental investigations carried out to characterize the mechanical behavior of Bowland shale. The observed results are helpful to estimate the potential of the Bowland reservoir with respect to the economical extraction of hydrocarbons.

Deformation Experiments on Bowland and Posidonia Shale—Part I: Strength and Young’s Modulus at Ambient and In Situ pc–T Conditions