On uncertainty of elastic modulus measurements via nanoindentation mechanical testing and conventional triaxial testing

Geomechanical properties of rocks vary across different measurement scales, primarily due to heterogeneity. Micro-scale geomechanical tests, including micro-scale “scratch tests” and nano-scale nanoindentation tests, are attractive at different scales. Each method requires minimal sample volume, is low cost, and includes a relatively rapid measurement turnaround time. However, recent micro-scale test results – including scratch test results and nanoindentation results – exhibit tangible variance and uncertainty, suggesting a need to correlate mineral composition mapping to elastic modulus mapping to isolate the relative impact of specific minerals. Different research labs often utilize different interpretation methods, and it is clear that future micro-mechanical tests may benefit from standardized testing and interpretation procedures. The objectives of this study are to seek options for standardized testing and interpretation procedures, through two specific objectives: (1) Quantify chemical and physical controls on micro-mechanical properties and (2) Quantify the source of uncertainties associated with nanoindentation measurements. To reach these goals, we conducted mechanical tests on three different scales: triaxial compression tests, scratch tests, and nanoindentation tests. We found that mineral phase weight percentage is highly correlated with nanoindentation elastic modulus distribution. Finally, we conclude that nanoindentation testing is a mineralogy and microstructure-based method and generally yields significant uncertainty and overestimation. The uncertainty of the testing method is largely associated with not mapping pore space a priori. Lastly, the uncertainty can be reduced by combining phase mapping and modulus mapping with substantial and random data sampling.