Non-contaminating cryogenic fluid access to high-temperature resources: Liquid nitrogen fracturing in a lab-scale Enhanced Geothermal System

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 in HDR by hydraulic fracturing to enhance the thermodynamic efficiency and heat production is significant. However, the economic and environmental burden of freshwater sourcing, transportation, and treatment raises concerns. Here, we investigate liquid nitrogen, an environmental-friendly and super-cooling fluid, as an alternative fracturing fluid for potential applications in Enhanced Geothermal System. We studied the fracturing performances of liquid nitrogen in high-temperature granites under true triaxial-confining stresses. Cryo-scanning electron microscopy and 3D X-ray micro-computed tomography were applied to illuminate the fracture-network patterns. Results show that liquid nitrogen fracturing exhibits the lowest breakdown pressure compared with water fracturing and nitrogen gas fracturing. The fracture morphology displays a 3-D volumetric pattern comprised of branched fractures and thermally-stimulated zones. Higher fluid-rock temperature difference and lower stress anisotropy cause more complex fracture networks. This study, for the first time, shows potential benefits of liquid nitrogen fracturing in high-temperature crystalline rocks, paving the road towards waterless fracturing in Enhanced Geothermal System. It is expected to provide a viable alternative for the sustainable development of deep geothermal resources in an efficient and clean way.