Atomic Force Microscopy (AFM) vs. Nanoindentation: What is Their Difference on Measuring Mechanical Properties? A Case Study on Organic-Rich Shale

Abstract:

Obtaining the geomechanical properties of unconventional shales rocks through conventional experiments is challenging due to the difficulties of getting good quality core plugs and the strong heterogeneous of shale samples. Atomic Force Microscopy (AFM) and Nanoindentation are two main powerful techniques used to characterize the properties at nano-scale. In this study, we investigate the application of these two methods in same samples, characterize the geomechanical properties of Bakken shale and compare the results to demonstrate the advantages and shortcomings of two methods. The nanoindentation curves tested on the Bakken shale were first analyzed to identify the elastic-plastic essence. Multi-cluster deconvolution method was adopted in the statistical analysis of the nanoindentation results, demonstrating the 3 clusters found due to the complex composite material of shale. Next, AFM technique, integrating scanning electron microscope (SEM) images, and energy dispersive spectroscopy (EDS) chemical analysis, were used to characterize the mechanical properties of Bakken shale in 2D maps, distinguishing three mineral phases and the corresponding modulus. In the last, the result obtained by two techniques were compared, illustrating the better detection accuracy of the organic matter in shale by AFM method than by nanoindentation. It is recommended that each experiment technique should be applied depending on the research purpose and scenario.