Optimizing nano-dynamic mechanical analysis for high- resolution, elastic modulus mapping in organic-rich shales

An elastic modulus mapping technique based on spatially continuous dynamic nanoindentation is applied to map microscale variations in a fine-grained, kerogenrich shale consisting of inorganic minerals with an interpenetrating network of microscale pores filled with organic matter. Advantages and limitations of the application of this technique to shales are explored through varying sample preparation and scanning procedures. Filtering techniques are developed to remove data that are negatively impacted by topography and other issues inherent to the mapping technique. As a result, spatial variations of elastic modulus in kerogen-rich regions are seen at substantially higher resolution than has previously been reported. Spatial resolution and continuous mapping across high stiffness-contrast material boundaries are further improved with stringent sample preparation and the use of a sharp tip. Typical modulus values measured by this technique include approximately 10 GPa for kerogen, 15–45 GPa for clay depending on the morphology and orientation, and 50–70 GPa for quartz. Introduction Organic-rich shales are heterogeneous composite sedimentary rocks that form in sedimentary basins where abundant masses of living organisms are deposited along with silicic and carbonate minerals. The organic material, kerogen, intertwines throughout the matrix of the shale [1– 3]. As the shales are buried and exposed to high temperature and pressure, the kerogen matures to produce hydrocarbons that are stored in the mature organic-rich shales as well as in adjacent formations. Economic hydrocarbon production is only feasible through high-conductivity conduits generated by the process of hydraulic fracturing. The mechanical properties of kerogen and the physical arrangement of the material within shale reservoirs are of interest for building upscaled rock models to predict fracture propagation for hydraulic fracturing design and to interpret exploration seismic data. Mechanical properties of shales have conventionally been assessed at the cm scale and above, using uniaxial and triaxial compression tests [4], but a growing interest in determining the properties on the microscale has emerged. Serial sectioning techniques, acoustic wave technology, and computed tomography have been used to understand the arrangement of kerogen and its properties at mm to lm scales [5, 6]. Elastic modulus variations in clay minerals, shales, and natural cements have been mapped using nanoindentation, in which a diamond probe of known geometry is pressed 10–100 s of nanometers into a material while simultaneously measuring force to extract local mechanical properties [1, 7–10]. Arrays of quasistatic nanoindentations have revealed spatial variations in properties over areas of *150 9 150 lm [1, 11]. When these pointwise modulus maps are compared to scanning electron or optical micrographs of the same area, good correspondence is generally seen in locations of T. M. Wilkinson C. E. Packard (&) Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401, USA e-mail: [email protected] T. M. Wilkinson e-mail: [email protected] S. Zargari M. Prasad Department of Petroleum Engineering, Colorado School of Mines, Golden, CO, USA 123 J Mater Sci (2015) 50:1041–1049 DOI 10.1007/s10853-014-8682-5