Comparative analysis of thermal extraction performance of EGS model with N2O, H2O and CO2 as working fluids based on Voronoi fractures

The working fluid in the enhanced geothermal system (EGS) is the subject of heat convection and transfer in the fractures, and its physical properties have an important influence on the heat extraction. In this paper, N2O, H2O and CO2 are proposed as the working fluids of EGS, and their thermal extraction properties in EGS are studied respectively. Based on discrete fracture network (DFN) and Voronoi theory, a thermal-hydraulic-mechanical (THM) coupling model with Voronoi fractures is established. The influence of different Voronoi fracture blocks on N2O-EGS is compared and analyzed, and the optimum fracture parameters are selected. Then the distribution characteristics of different physical fields of N2O-EGS, H2O -EGS and CO2-EGS are investigated. It is found that the physical field distributions of N2O-EGS and CO2-EGS are similar due to the similar physical properties of the working fluid. In addition, the fracture morphology has an influence on the diffusion form of the physical field because it is the main flow channel of the working fluid. In this model, the physical field diffuses from the inside to the outside in the form of a circle in the Voronoi fracture block. Meanwhile, the effects of different well spacing, injection temperature and injection mass flow rate on EGS with different working fluids are compared. The results indicate that the thermal extraction performance curves of N2O-EGS and CO2-EGS are basically the same, and the production temperature was higher than that of H2O-EGS. The viscosity and heat capacity of H2O are high, and the change of parameters in EGS has obvious influence on H2O-EGS. Thus, N2O and CO2 as working fluid have higher production stability. Among the parameters, the change of mass flow rate has the greatest influence on the heat extraction performance of the model. Therefore, the production parameters should be selected reasonably in actual production. This paper provides an effective reference for selecting suitable working fluid in complex fracture networks.