| Title | Fracture Propagation in Anisotropic Rocks During Hydraulic Stimulation: Experimental Results and Theoretical Predictions |
|---|---|
| Authors | Morteza NEJATI, Thomas DRIESNER |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | Anisotropic rock, fracture toughness, maximum tangential stress, maximum energy release rate, minimum strain energy density, Grimsel Granite |
| Abstract | Pilot and demonstration projects in deep underground laboratories enhance our understanding of how fracture permeability may be engineered in enhanced geothermal systems (EGS). The recent in situ hydraulic stimulation and circulation experiments in the deep underground laboratory at the Grimsel Test Site (GTS) in Switzerland demonstrated that the anisotropy of the rock mechanical properties plays a critical role in the rock mass's deformation, stability and failure. Future predictive modelling for routine engineering therefore requires a rigorous quantitative understanding of how fracture processes are affected by rock anisotropy. In this paper, we present our modeling scheme used to simulate the process of fracture growth in anisotropic rocks. This numerical scheme is based on a re-meshing approach that employs unstructured meshes in order to update the geometry of explicitly-represented fractures. We use the concept of linear elastic fracture mechanics to model the onset of crack growth using the following fracture growth criteria: maximum tangential stress, maximum tangential strain, minimum strain energy density and maximum energy release rate. The crack path obtained from all these criteria are compared to the results of a set fracture toughness tests. The fracture toughness tests are conducted on semi-circular samples of Grimsel granite. This type of rock shows an anisotropic behavior in elasticity, strength and fracture toughness, with a clear foliation plane that can form an isotropy plane in the transverse isotropy elasticity model. The samples are tested in pure mode I loading condition, with the initial crack forming different angles with the foliation plane. The critical load at the onset of fracture propagation was recorded for all samples. The crack path to the complete failure in each sample was also recorded. These experiments show a fracture toughness anisotropy ratio of about two. This significant anisotropy of fracture toughness can considerably influence the fracture growth path. |