| dc.description.abstract | Understanding the interaction between hydraulically induced fractures and pre-existing natural fractures in geologic formations is key for optimizing subsurface energy systems that rely on fluid injection into fractured rocks. These include Enhanced Geothermal Systems (EGS), CO₂ sequestration, hydrogen storage in depleted reservoirs, unconventional oil and gas development in shale formations, and nuclear waste disposal, among others. In all these applications, controlling fracture propagation and interaction is essential for ensuring operational efficiency, safety, and long-term integrity of the system. This thesis presents a comprehensive experimental and theoretical investigation of hydraulic fracture (HF) interactions with natural fractures (NFs), using Opalinus Clayshale as a representative anisotropic material.
The experimental work involved a series of hydraulic fracturing tests on Opalinus Clayshale specimens under controlled quasi-true-triaxial stress conditions, comparing normal and dried states. Novel monitoring techniques, including high-resolution imaging, high-speed video, acoustic emissions (AE), and pressure tracking, were employed to capture the fracturing process in real-time. Three dominant interaction modes (Crossing, Arrest, and Opening) were systematically characterized and linked to key parameters, including stress ratio, fracture geometry, and injection rates. A critical stress ratio (σ₁/σ₃) of approximately 20 was identified as the threshold for achieving fracture crossing under our experimental conditions: cohesionless, “open” natural fractures, with a low viscosity injection fluid, in a toughness-dominated regime. In dried specimens, high flaw pressurization rates were necessary to overcome matrix fluid loss and achieve crossing.
To complement and interpret the experimental results, existing theoretical models were reviewed and implemented. Furthermore, a simplified version of the OpenT model (Chuprakov et al., 2014) was developed and applied for Opalinus Clayshale, incorporating stress, energy, friction, and permeability effects. By integrating laboratory results with theoretical frameworks, this thesis offers an integral approach to predictive understanding of fracture propagation in naturally fractured rocks, stating that not only the characteristics of the discontinuity or the far-field stresses involved in the process are important in determining the mechanism of interaction, but also the dynamic energy balance at the fracture tip, which is influenced by injection rate, fluid viscosity, and discontinuity properties.
Overall, this thesis bridges the gap between laboratory experiments and theoretical models, advancing a more comprehensive understanding of fracture propagation in naturally fractured media. The findings highlight the importance of considering both mechanical and hydraulic parameters, particularly in low-viscosity, toughness-dominated regimes, for accurately predicting fracture behavior. | |
