Laboratory validation of a new hydro-mechanical energy-based brittleness index model for hydraulic fracturing

Abstract

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.

Hydraulic fracturing: Laboratory evidence of the brittle-to-ductile transition with depth

“…Finally, the correlation between fractured area and brittleness index (BI) is reported in Section 4.4. According to the quantification of brittleness index (BI) (Feng et al (2022)) and the failure characteristics of samples subjected to triaxial compression tests (Figs. A2 and A3), the six types of samples are classified into: brittle PMMA (BI=0.97), semi-brittle quartz-rich S1 (BI=0.68), semi-brittle mixed-average S4 (BI=0.57), semi-ductile calcite-rich S3 (BI=0.44), the ductile clay-rich2 S5 (BI=0.38) and clay-rich S2 (BI=0.35) under the low confinement.…”