| Abstract |
Cyclic injection in hydraulic fracturing has been proved to be an effective way for generating complex fracture networks while reducing the level of induced seismicity in the development of Enhanced Geothermal Systems. However, enormous pre-existing natural fractures commonly exist in deep geothermal reservoirs, affecting the hydraulic fracture extension, which should not be ignored during the stimulation. In this work, a series of laboratory experiments were conducted on granite samples with natural fractures to investigate the interaction mechanism between hydraulic fractures and natural fractures under various cyclic injection schemes. The seismic risks and permeability enhancement performance of cyclic injection fracturing were revealed by acoustic emission events and nuclear magnetic resonance, respectively. The results indicate that natural fractures tend to be activated under cyclic hydraulic fracturing compared to conventional continuous fluid injection. The cumulative AE energy decreased with increasing cementation of natural fractures due to shorter shear-slippage distance occurring at strongly sealed natural fractures. The evolution of pore structure indicated that micro-meso pores tend to be induced under cyclic injection, while macro pores easily occur after continuous injection, which might promote the early thermal breakthrough. Frequent shear stress perturbation can be caused at the surface of natural fractures, which is the primary reason for natural fractures to be activated by cyclic hydraulic fracturing. To mitigate seismic risks of fractured geothermal reservoirs, it is recommended to apply lower cyclic injection pressure with longer injection duration. This work provides theoretical guidance for the application of cyclic hydraulic fracturing in deep geothermal reservoirs. |