Fracture toughness of organic rich shale via nanoindentation: A comparison of energy-based methods

Fracture toughness is an important mechanical parameter for proper modeling of reservoir stimulation which is necessary for production from tight formations. To measure fracture toughness, several models have been proposed based on nanoindentation data, however, the comparison of these models in organic rich shale is still limited in the literature. To obtain a better understanding of the suitability of these models, we applied two distinct energy based methods, one which takes advantage of the whole force-displacement curve (KIC_whole) and the other one that considers the fracturing-induced pop-in events (KIC_pop). To estimate and compare the fracture toughness from either of these methods, nanoindentation data is acquired from a sample that was collected from the Wolfcamp Formation, USA. The results showed that for the soft mechanical phase (clay minerals), the KIC_whole varied from 0.020 to 0.313 MPa m0.5 while the KIC_pop from 0.02 to 0.330 MPa m0.5. In the hard mechanical phase (carbonate and quartz), the KIC_whole was found between 0.380 and 0.84 MPa M0.5, and the KIC_pop was estimated from 0.38 to 0.780 MPa m0.5. These values for the intermediate phase was, 0.220-0.588 MPa m0.5 and 0.215-0.500 MPa m0.5, respectively for each method. Statistical analysis of the results based on the t-tests verified that the fracture toughness of different mechanical phases that is derived from these two different models is quite comparable. Overall, considering the simplicity of the calculations and the limitations in the approach that is based on pop-in events, our recommendation would be to use the whole forcedisplacement curve to obtain the fracture toughness of geomaterials.