The nanogranular nature of shale

Abstract

Despite their ubiquitous presence as sealing formations in hydrocarbon bearing reservoirs affecting many fields of exploitation, the source of anisotropy of this earth material is still an enigma that has deceived many decoding attempts from experimental and theoretical sides. Sedimentary rocks, such as shales, are made of highly compacted clay particles of submicrometer size, nanometric porosity and different mineralogy. In this paper, we present, for the first time, results from a new experimental technique that allows one to rationally assess the elasticity content of the highly heterogeneous clay fabric of shales from nanoand microindentation. Based on the statistical analysis of massive nanoindentation tests, we find (1) that the in-situ elasticity content of the clayfabric at a scale of a few hundred to thousands nanometers is almost an order of magnitude smaller than reported clay stiffness values of clay minerals, and (2) that the elasticity and the anisotropy scale linearly with the clay packing density beyond a percolation threshold of roughly 50%. Furthermore, we show that the elasticity content sensed by nano-and microindentation tests is equal to the one that is sensed by (small strain) velocity measurements. From those observations, we conclude that shales are nanogranular composite materials, whose mechanical properties are governed by particle-toparticle contact and by characteristic packing densities, and that the much stiffer mineral properties play a secondary role.

Digital carbonate rock physics

“…However, if nanoindentation technique is a well-established technique in material sciences, which deals with homogeneous, purely elastic materials, this is, as of today, not the case for rocks, which are heterogeneous materials with both elastic and nonelastic behaviour (creep). Though nanoindentation tests provide significant insights into elastic properties of heterogeneous rocks such as carbonates (Lebedev et al, 2014;Vialle and Lebedev, 2015) or shale (Ulm and Abousleiman, 2006;Abousleiman et al, 2007), there are still some points to be looked at before using the derived values of Young's (or shear and bulk) moduli in a quantitative way for DRP: the value of Poisson's ratio to be used, effect of surface roughness, local mechanical damage induced on the sample's surface by polishing techniques, etc. Nonetheless, the histograms of the indentation moduli of both samples show a broad distribution of moduli values ranging from very low values (a few GPa, where the indenter tip measures stiffness of an area mostly made of a pore) to values consistent with calcite.…”

Multi-scale approach for modeling the transversely isotropic elastic properties of shale considering multi-inclusions and interfacial transition zone

“…According to [2], the transversely isotropic properties of shale rock are mainly due to the transversely isotropic properties of the clay particles at nanoscale. For the transversely isotropic matrix or inclusions, the solutions for the micromechanical equations, like Eq.…”

“…Shale rock is especially critical for success of many fields of petroleum engineering and may also be important for the development of sustainable nuclear waste storage solutions [1,2].…”

A simplified multiscale damage model for the transversely isotropic shale rocks under tensile loading

“…Shale rocks may also be important for the development of sustainable nuclear waste storage solutions (Thomsen, 2001; Ulm and Abousleiman, 2006).…”