Multi-scale assessment of mechanical properties of organic-rich shales: A coupled nanoindentation, deconvolution analysis, and homogenization method

Abstract

The boom of unconventional shale plays has brought significant attention during the last decade to shale rocks. Elastic properties of shale are one of the most important parameters to be known for hydraulic fracturing success which is necessary for production from organic-rich unconventional reservoirs. In this study, we combined experimental methods including, XRD, Rock-Eval, and nanoindentation on eight Bakken Shale samples with various thermal maturity, taken from different wells and depths in the Williston Basin, ND, to examine their mineralogy, and geochemistry with the emphasis on variations on the elastic properties. Expectation Maximization (EM) based statistical deconvolution was applied to the nanoindentation modulus data to distinguish individual mechanical phases. Then homogenization method was used to upscale the elastic properties to macro-scale, and the data were compared with the values reported in the literature. The results showed that the elastic properties of individual mechanical phases could be distinguished and evaluated through a statistical deconvolution approach. Three main mechanical phases were recognized for each shale sample. It was found that homogenized samples have Young's modulus values in the range of 18.2 GPa–38.8 GPa, which were consistent with the range of mechanical property values of the Bakken Shales reported in the literature. Correlative relationship analysis between mineralogy and thermal maturity with elastic properties showed that Young's modulus decreases with increasing TOC content and increases with the increasing maturity.