Multi-faceted numerical investigation into heat extraction performance of horizontal doublet fractured geothermal reservoirs with carbon dioxide working fluid

Global greenhouse gas emissions, especially from CO2, and the resulting environmental issues present opportunities for geothermal reservoirs to serve as carbon sinks and renewable energy sources. This study presents fully coupled thermo-hydraulic-mechanical numerical modeling of a horizontal doublet fractured geothermal reservoir with a discrete fracture network (DFN) and supercritical CO2 as the working fluid. We explore the effects of operational parameters (such as well configuration, injection pressure/temperature, and production pressure) and reservoir characteristics (such as fracture parameters, geomechanical parameters, and matrix porosity/permeability) on heat extraction evaluation metrics, including production temperature, heat extraction ratio, injection/production flowrate, flow impedance, and thermal output power. The findings demonstrate that injection pressure, well length, well spacing, well diameter, matrix porosity, matrix permeability, fracture aperture and geomechanical properties directly influence flowrate, heat extraction ratio and thermal output power, with inverse impacts on production temperature and flow impedance. Increasing injection temperature raises production temperature after thermal breakthrough occurs but has minimal effect on the temperature breakthrough. Nitrous oxide and carbon dioxide outperform water in terms of thermal output power and flow impedance, while water yields higher production temperatures, benefiting electricity generation efficiency at the surface equipment. Horizontal wells yield higher thermal outputs than vertical wells, even when the vertical wells are intersected with hydraulic fractures. DFN characteristics (except fracture porosity) significantly affect energy production evaluation metrics. Amongst well parameters, well diameter and well spacing have the lowest and highest effect on the performance indicators, respectively, except heat extraction ratio over which well length is the most effective parameter. Amongst geomechanical parameters, Biot's coefficient and initial rock elastic modulus has the lowest and highest effects on performance metrics, respectively, except for heat extraction ratio and average production temperature over which thermal expansion coefficient is the most effective factor. Amongst fracture parameters, fracture porosity exerts virtually no influence on the performance indexes while fracture aperture is the most significant factor controlling the heat extraction efficiency within the constructed fractured geothermal model fully saturated with supercritical CO2 heat transfer fluid.