Creep Deformation in Shale at Submicron Scale

Stanford University, Stanford CA

Investigators

Abstract

Shale is a fine-grained sedimentary rock consisting of a mixture of clay, quartz, feldspar, pyrite, carbonate, and organic materials forming a highly heterogeneous nanocomposite. As a seal rock, shale has been used to contain toxic, long-lived chemical and radioactive waste products in the ground. In fact, crystalline rocks and thick shale sequences have long been considered as prime storage sites because of the shale's ability to resist fracture. The mechanical properties of shale depend on the properties of its basic constituents, including those of clay particles and organic inclusions, as well as the porosity of the mixture. Due in large part to its clay and organic content, shale exhibits significant creep deformation that can create subsidence issues on a regional scale, as well as alter its effectiveness as a seal for toxic waste products that must be contained for thousands of years. This award supports fundamental research to investigate the creep deformation behavior of shale at the nanometer scale. Investigation into the mechanical properties of this heterogeneous material provides insight into the fundamental processes governing creep at a scale critical for its function as a seal, as well as allows interpretation of the creep phenomena at larger scales. The research involves several disciplines including materials science, biomechanics, and other scientific disciplines concerned with the studies of nanoporous materials. It supports a female doctoral student who will help broaden the participation of underrepresented groups and undergraduate students. The project investigates the creep properties of shale at nanometer scale using a combination of laboratory testing and numerical modeling. Experiments include nanoindentation testing to determine basic mechanical properties of the shale constituents, and imaging of the pore structure with nanometer-scale resolution using full-field Transmission X-Ray Microscopy (TXM), a scarce resource available at the Stanford Synchrotron Radiation Lightsource within the SLAC National Accelerator Laboratory. Numerical modeling includes a novel formulation of mixture theory for solid-solid mixture of softer matter and harder matter, reflecting a realistic physical representation of the heterogeneous structure of shale at submicron scale. Interaction between these two matters, which possess starkly different stiffness and creep properties, could potentially shed light onto the nature by which micro-cracks form and propagate through a heterogeneous nanocomposite. From a modeling standpoint, mixture theory is used for the first time to delineate a solid-solid mixture of softer matter and harder matter at the nanometer scale.