Runhua Feng, Joel Sarout, Jeremie Dautriat, Yousef Ghuwainima, Reza Rezaee, Mohammad Sarmadivaleh

Brittleness Index (BI) is a critical parameter characterising the deformation regime of geo-materials, covering the range from purely brittle (fractures) to ductile (plastic flow). A variety of BI models have been developed based on rock properties such as mineralogy, elastic parameters, or stress-strain data. However, very few of them are based on the deformation induced by hydro-mechanical interactions emerging in a wide range of underground engineering applications. In this study, we develop a BI model based on the partitioning of the injection energy EI into non-seismic deformation energy Ed associated with hydraulic fracture propagation. To calculate the Ed, we apply a model for temporal fracturing area (Ad) within the penny-shaped fracture; we also correlate the wellbore pressure and the three-dimensional strain induced by hydraulic fracturing of the different types of rock samples subjected to true triaxial stress conditions (TTSC), either σv = 6.5 MPa, σH = 3 MPa, σh = 1.5 MPa or σv = 15 MPa, σH = 10 MPa, σh = 5 MPa for all tests. As a comparison, the BI is also quantified based on the existing models: (i) acoustic measurement from Rickman et al (2008), and (ii) the Mohr-Coulomb’s criteria from Papanastasiou et al (2016). The non-seismic deformation energy Ed ranges between 32.4% and 90.6% of the total injection energy EI, which is slightly higher than reported from field-scale data (15% to 80%), and is comparable to other laboratory-derived data (18% to 94%). The results show that the predictions based on our newly proposed hydro-mechanical energy-based BI model are qualitatively consistent with Papanastasiou et al.’s, but less so with Rickman et al.’s. Our BI model is shown to be stress-dependent and capable of capturing the brittle-to-ductile behaviour within a wide range of rheological samples subjected to hydraulic fracturing. This study demonstrates that our BI model opens a new way for quantifying the brittleness index regarding to realistic propagation scenarios, showing its superior robustness for such underground applications.