In the past decade, chemical, physical, and mechanical characterization of source-rock reservoirs has moved toward micro- and nanoscale testing and analyses. Nanoindentation is now widely used in many industrial and university laboratories to measure stiffness and strength as well as other mechanical properties of shales. However, to date, tensile failures of shales have not been studied at the micro- or nanoscale.
In this work, a scanning electron microscope (SEM) coupled with a focused ion beam (FIB) and a special nanoindenter (NI) testing configuration (SEM-FIB-NI) is used to bring organic-rich shale samples (preserved Woodford shale from a wellsite in Ada, Oklahoma, USA) to failure in tension. Microcantilever beam geometries were milled and loaded to failure in tension while monitoring in situ with SEM. The force-displacement curves were generated while videos recording in-situ real-time displacements and failures were collected simultaneously.
The microcantilever beam tests of this composite natural material demonstrate linear elastic behavior followed by elastic/plastic yield before complete failure. This behavior was clearly observed to correlate with the amount of organic matter (OM) at the fractured surface of the microcantilever beam supports. Energy-dispersive X-ray spectroscopy (EDS) analyses were conducted along the prepared microbeam samples before loading. In addition, post-failure EDS analysis was performed on the resulting fractured faces of the failed microbeams, and the correlation between tensile behavior and shale OM content was shown. Large tensile moduli of rupture, or moduli of toughness, were associated with high OM, or kerogen, present at the failed supports of the kerogen-rich-shale (KRS) microcantilever beams. The moduli of toughness as a measure of work or energy needed to bring these samples into tensile failure were ten times less when OM was missing or barely present at the support, in terms of shale microbeam volume.