New Insights on the Mechanical Characterization of Kerogen-Rich Shale, KRS

Abstract

Abstract In the past decade, chemical, physical and mechanical characterization of source rock reservoirs has moved towards micro- and nano-scale analyses, primarily driven by the fact that the representative elementary volume (REV) for characterizing shales is at the nanometer scale. Nanoindentation is now widely used in many industrial and university laboratories to measure both stiffness and strength and other mechanical properties of shales, such as anisotropic Young's Moduli and plastic yielding parameters. However, to date, tensile failures of shales have not been studied at the micro- or nanoscale. In this work, a nanoindenter is used to bring organic-rich shale (preserved Woodford shale from a well site in Ada, Oklahoma) to failure in tension. Micro-cantilever beam geometries (~25 microns in length and ~5 microns in width) were milled and loaded to failure while monitoring in-situ via scanning electron microscopy (SEM). The force-displacement curves were analyzed in light of the high resolution images collected during fracture initiation, propagation, and ultimate failure. Complementary studies of the mineralogical composition, particularly at the failure faces, as well as the organic content were also performed. Failure planes and tensile fracture initiation in the micro-beam were associated with the various phases of the mineral and organic cluster components. The micro-beam tests of this composite natural material demonstrate linear elastic behavior followed by plastic yielding before complete failure. This behavior was clearly observed to correlate with the amount of organic matter at the fractured surface. Energy dispersive X-ray spectroscopy (EDS) analyses were conducted at the post-failure stage on the resulting fracture faces and the relationship between mechanical behavior and composition was established. It was observed that when high mineral content was found at the faces, a brittle failure took place, while when the fixed support had high kerogen content the micro-beam failed in a ductile mode. These results reinforce our growing understanding of the heterogeneous nature of shale and the importance of nano- and micro-scale analyses to understand our reservoir source rocks.