Achieving more accurate microindentation hardness measurements of shale rocks – Insight into the effect of sink-in and pile-up

Abstract

Shale rock has become a major source of oil and natural gas production. Hence, understanding the mechanical properties of shales is important for field applications, such as drilling and fracking. In this work, a workflow was developed for assessing the hardness of shale rocks, with the kerogen-rich and calcite-rich shales as the model systems, using Vickers microindentation coupled with confocal microscopy. The use of confocal microscopy facilitates the direct observation of indent impressions, overcoming the challenges posed by the complex contrasts of shales seen in traditional optical microscopy. Sink-in and substantial elastic recovery were observed in kerogen-rich shales due to the large amount of organic content. However, sink-in did not affect the hardness calculation. Apparent pile-up was observed in calcite-rich shales due to its granular nature, which could lead to overestimation of hardness if not accounted for. The hardness measurement in calcite-rich shale is further complicated by spallation. Therefore, only hardness ranges, instead of individual hardness values, can be obtained in calcite-rich shales. This research not only refines the methodology for determining the hardness of shales but also provides insight into indentation artifacts, such as sink-in and pile-up, to improve the accuracy in measuring and analyzing the microhardness of complex geological materials.