Laboratory Studies of Cryogenic Fracturing on Hot Dry Rock by Cyclic Injection

Abstract

Hot dry rock (HDR) geothermal energy exploitation is essential to meet the energy demand and achieve low-carbon solutions. Creation of complex fracture networks to enhance the thermodynamic efficiency and heat production in HDR is significant. Here, we presented a new reservoir stimulation concept, i.e. cyclic cryogenic fracturing, based on cyclic soft stimulation (CSS) which is originally put forth to mitigate the seismic events and improve the permeability enhancement process in Enhanced Geothermal System (EGS). In cyclic cryogenic fracturing, liquid nitrogen, an eco-friendly and super-cooling fluid (-196℃ at atmospheric pressure), is proposed to be an alternative fracturing fluid and injected in a cyclic manner, i.e. alternating high-injection-rate and low-injection-rate. Hence, formation rocks will be subjected to cyclic cooling, stress oscillating and fatigue failure, which is expected to improve the stimulated reservoir volume. First, laboratory cyclic cryogenic fracturing tests were conducted on 200℃ granite to investigate the fracture initiation/propagation behavior and fracture network morphology. Water fracturing experiments were also conducted as a comparison. Then, X-ray microscope (XRM) and scanning electron microscopy (SEM) in a micro-nano-scale resolution were presented to assist in understanding the cyclic cryogenic fracturing mechanisms. Finally, the implications and potential applications in the EGS field site were discussed. The results demonstrated that cyclic cryogenic fracturing can significantly lower the breakdown pressure and produce more tortuous fracture patterns. Increasing the number of cycles could generate more pronounced complex fracture networks. Besides, microstructure analysis showed that numerous thermal fatigue cracks were formed, which played a role of “fragmentizing the rocks” during fracturing. The key findings of this work are expected to provide a theoretical basis for the development of HDR geothermal resources in an efficient and safe manner.