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 constitutive law. However, very few of them are based on the hydro-mechanical interactions emerging in underground engineering applications. In this study, we propose a BI model based on the partitioning of the injection energy E_I into non-seismic deformation energy E_d associated with hydraulic fracture propagation. To calculate the E_d, we apply a model for temporal fracturing area (A_d) within the penny-shaped fracture; and 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 σ_v = 6.5 MPa, σ_H = 3 MPa, σ_h = 1.5 MPa or σ_v = 15 MPa, σ_H = 10 MPa, σ_h = 5 MPa. 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 E_d ranges between 32.4% and 90.6% of the total injection energy E_I, which is slightly higher than the value reported from field-scale data (15% to 80%), but comparable to laboratory-derived data (18% to 94%) from literature. The results show that the predictions based on our proposed 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 of geomaterials subjected to hydraulic fracturing. This study demonstrates that our BI model opens a new way for quantifying the brittleness index regarding to realistic fracture propagation scenarios in field.