The granular and polymer composite nature of kerogen-rich shale

Abstract

In the past decade, mechanical, physical, and chemical characterization of reservoir shale rocks, such as the Woodford shale, which is kerogen-rich shale (KRS), has moved toward micro- and nanoscale testing and analyses. Nanoindentation equipment is now widely used in many industrial and university laboratories to measure shale anisotropic Young’s moduli, kerogen stiffness, plastic yield parameters, and other isotropic and anisotropic poromechanical and viscoelastic properties. However, to date, failure analyses of KRS and the effects of organic components on the tensile strength have not been observed or measured at the micro- or nanoscales. In this study, preserved kerogen-rich Woodford shale samples manufactured in micro-beam and micro-pillar geometries were mechanically tested and brought to failure in tension and compression, respectively. These tests were conducted in situ using a nanoindenter inside a scanning electron microscope (SEM). The load versus displacement curves of prismatic micro-cantilever beams were analyzed in light of high-resolution images collected during tensile fracture initiation, propagation, and ultimately sample failure. The micro-pillar geometries were subjected to a uniaxial compressive load and were also brought to failure while capturing measurements of stress and strain. It was found that, within just a few hundred microns of the KRS micro-cantilever beams, both brittle and ductile failure modes were observed. In the ductile plastic domain, strain-softening and strain-hardening behaviors were identified and characterized. These were not due to confining stress variations, but due to the volume of the organic matter and the way it is interlaced with the shale minerals in and around the failure planes. The tensile strength characteristics and the large modulus of toughness of kerogen, which is a cross-linked polymer, definitely weigh heavily in our engineering field applications, such as hydraulic fracking, which is a Mode I tensile fracture opening and propagation phenomenon. This practice demands that, due to the complex composite nature of KRS, mechanical characterization be not only for unconfined compressive strength but also for unconfined tensile strength and moduli of ruptures. At the end of this study, the need for nanometer scale mechanical characterization of KRS will become apparent. These nano- and micro-scale shale failure tests reinforce our previous understanding of the heterogeneous composite nature of Woodford KRS and its complex behavior, as well as other source shale reservoir formations.

Appendix

In Fig. 24, the micro-beam cantilever test, T1, is herein detailed for the load–displacement curve. The linear elastic early performance followed by the various slopes in the strain-softening regimes extended the micro-cantilever beam rupture to a very large displacement compared to the 600 nm for the early pure linear elastic deformation. From real-time in the SEM visualization, the dashed line represents the first major fracture observed at the fixed support at a load of 809 μN. In other words, the kerogen after that point was supporting most of the load, thus preventing the beam from reaching its rupture strength. The rebound slope at the bottom after stage 3 shows a linear elastic rebound in the figure. This is the proof that the kerogen cross-linked elastomer nature did not reach its rupture strength, but rather that mass of kerogen extended the initial shale granular deformation and failure by almost 10 times to 4500 nm and still reserving an elastic rebound. Not to overload the paper, micro-beam in Test 2, T2, in strain-hardening (KRS micro-beam) could also be easily explained in a similar way in the loading–unloading and when brought eventually to failure.

Fig. 24

Linear elastic load and rebound curves, in addition to the step-wise linear strain-softening behavior plotted in details