Experimental evaluation of fluid flow and heat transfer in dual fracture enhanced geothermal system

The solidified and crystallized magma in the earth’s crust forms igneous rocks such as gabbro, diorite, and granite. Depending on the depth and geological age, the temperature of these formations is relatively high, and thus, they contain a considerable amount of thermal energy. These formations are often nearly impermeable and contain little to no pore fluid. However, they are brittle and can be stimulated by hydraulic fracturing to gain permeability. The stimulated igneous formations that contain a large amount of thermal energy are considered enhanced geothermal reservoirs. Extracting thermal energy from enhanced geothermal reservoirs requires the presence of a geothermal working fluid. Water is the most common geothermal fluid with excellent specific heat capacity; however, the hydraulic properties of water can create undesirable fast paths through large fractures from an injection well to a production well. This phenomenon will reduce the enhanced geothermal reservoir's heat extraction efficiency and sustainability over time. This research aims to experimentally investigate the coupled thermo-hydro mechanical fluid flow in an enhanced geothermal system (EGS). Specifically, flow distribution in a simplified dual fracture system is investigated for different fluids. A new geothermal working fluid is produced to enhance thermal energy extraction using an ionic liquid, deep eutectic solvent (DES). A novel experimental design is constructed to address the research objectives consisting of two different-sized fractures. Granite rock samples simulate fluid flow in EGS systems, where the experimental design can independently vary each fracture temperature to investigate geothermal short-circuiting. The research results show that the 66% DES concentration has a significantly higher pressure drop than water for all temperature and flow rates. In dual fracture configuration, the fluid maintains its maximum temperature for a longer duration with DES with 66% concentration. Water showed the highest fluid temperature extraction compared to DES concentrations.