The nano-mechanical morphology of shale

Abstract

Shale, the sealing formations in most hydrocarbon reservoirs, is made of highly compacted clay particles of sub-micrometer size, nanometric porosity and different mineralogy. In this paper, we propose and validate a technique to identify the nano-mechanical morphology of such a nanocomposite material. In particular, by means of a massive nanoindentation campaign at two different scales on a large range of shale materials, we show that the highly compacted plate- or sheet-like clay particles have a distinct nano-mechanical morphology with no privileged orientation of the particle-to-particle contact surface, as evidenced by a mechanical percolation threshold of η 0 ⩾ 0.5. Furthermore, the nanoindentation results provide strong evidence that the nano-mechanical elementary building block of shales is transversely isotropic in stiffness, and isotropic and frictionless in strength. These observations lead to a sphere-like mechanical morphology for visibly plate- or sheet-like clay particles. The contact forces between the sphere-like particles activate the intrinsicly anisotropic elastic properties within the clay particles and the cohesive bonds between the clay particles. The mechanical stiffness and strength properties of porous clay scale with the clay packing density toward a unique set of shale-invariant material properties. The determination of mechanical microstructure and invariant material properties are of great importance for the development of predictive microporomechanical models of the stiffness and strength properties of shale. The approach presented here also applies to other chemically and mechanically complex materials exhibiting nanogranular behavior.