Shear-flow characteristics of rock fractures and implications for injection-induced seismicity

• Main Author: Ji, Yinlin
• Other Authors: Wu Wei
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Civil engineering
• Online Access: https://hdl.handle.net/10356/143989
• Institution: Nanyang Technological University

Harvesting heat trapped in geothermal reservoirs has the potential to offer an affordable and sustainable solution for reducing the current dependence on fossil fuels. Hydraulic stimulation has been employed to enhance the permeability of geothermal reservoirs and to maintain the productivity of renewable heat. However, the effectiveness of heat extraction depends not only on the permeability of fractured granite but also on the injection-driven instability of pre-existing and hydraulically induced fractures. Over the past decades, induced seismicity resulted from fluid injection has sparked huge concerns for the safety and efficiency of deep geothermal systems. Therefore, understanding the mechanisms of injection-driven fracture instability is critical to the optimization of hydraulic stimulation strategies. The objectives of this study are to uncover the mechanisms of injection-driven fracture instability and to develop new hydraulic stimulation strategies for mitigating injection-induced seismicity. The injection-driven shear tests are conducted on sawcut and natural fractures in granite using the triaxial shear-flow setup. The design of the triaxial shear-flow setup ensures that the frictional slip purely occurs along the fracture and the injected fluid solely flows through the fracture without interacting with the adjacent granite matrix. The jacket deformation and fracture area reduction of the triaxial shear-flow setup are also systematically corrected to accommodate the dynamic changes of the test system and sample assembly. The experimental study reveals that the fracture surface heterogeneity and fluid pressure heterogeneity play significant role in controlling the injection-induced fracture instability. In the injection-driven shear tests, the sawcut and natural fractures exhibit dynamic slip (seismic) and quasi-dynamic sliding (aseismic), respectively, which are controlled by several physical processes, such as thermal pressurization and stress imbalance. The laboratory-derived fluid pressure heterogeneity is visualized in COMSOL simulations and increases with normal stress and injection rate, leading to the increase in fluid pressure at fracture failure. To explore the causal link between fluid pressure heterogeneity and fracture slip propagation, the PFC2D modeling is developed and demonstrates that non-uniformly pressurized fractures exhibit global and localized drops of shear stress during the aseismic and seismic slip, respectively, and the slip transition dominates the fluid injection period. The fluid pressure front lags behind the slip front and shear stress transfer front during the aseismic slip. The results also suggest that the prediction of seismic moment release should consider not only injected volume but also fluid pressure distribution, especially for non-uniformly pressurized fractures. The optimization of hydraulic stimulation strategies is finally inspired by the in-depth understanding of injection-induced seismicity. Cyclic fluid injection promotes the fluid pressure diffusion on fractures, but the reduction in seismic moment release depends on several cycle-related factors, such as critical fluid pressure and injection frequency. Proper design of injection parameters is thus essential to balance the energy budget between seismic energy and hydraulic energy. This study also indicates that the activation of nearby blind, critically stressed faults in some circumstances may be inevitable and the resulted moment magnitude is mostly controlled by the tectonic stresses.