Comparison of shale fracture toughness obtained from scratch test and nanoindentation test

Fracture toughness is an important parameter in the hydraulic fracturing design, which is the major tool in the development of unconventional resources. Laboratory techniques for fracture toughness measurements usually require intact core samples and time-consuming sample preparation. The objective of this study is to compare fracture toughness values obtained by two less conventional methods: nanoindentation test and scratch test, which could facilitate the evaluation of this important parameter on smaller samples and at different scales. A set of 5 Antrim shale samples characterized by different mineral compositions is used to test this approach. For the scratch test, the fracture toughness and hardness are linearly correlated and show the same changing trend along the scratch length on all tested samples. For the nanoindentation test, the fracture toughness also increases with the increasing hardness. Most importantly, the results show that the fracture toughness values derived from these two methods are very similar, despite the difference in the scale of the measurements. This study is the first to compare fracture toughness between scratch and nanoindentation tests. Our results suggest that these two methods can be used to quickly evaluate fracture toughness from the shale core intervals containing both intact and nonintact parts.