Numerical investigations of CO2 and N2 miscible flow as the working fluid in enhanced geothermal systems

In this study, the heat extraction from geothermal reservoirs with the application of CO2 and N2 miscible flow as the novel working fluids has been investigated based on the discrete fracture network model. The miscible flow with different CO2 and N2 proportions and highly pressure and temperature dependent properties have been integrated into reservoir simulations. The heat production processes for CO2 and N2 miscible flow have been simulated with two different discrete fracture networks, which also take the thermal-hydraulic-mechanical (THM) coupling mechanisms into consideration. Based on simulation results, it is revealed that the heat extraction efficiencies show an irregular trend with the increasing proportions of N2 in the mixture. It is found that the miscible flow with two different N2 proportions (20% and 40%) can be a more efficient working fluid than the one with larger N2 proportions. Based on simulation results, cumulative heat production curves that represent ten CO2 and N2 proportions can be divided into three categories. Evaluations of relevant permeabilities and effective normal stresses at sample points were made, which contributed to the comprehensive analysis of the heat transfer processes. It is also found that there are reasonable ranges for the miscible flow with different CO2 and N2 proportions as working fluids that allows higher heat extraction efficiencies, which are validated by a series of comparisons. Thus, in order to define the humps on the heat extraction efficiency curves, a new concept of the optimized heat extraction efficiency range is proposed and validated. It is proved that fluid properties of the miscible flow with different CO2 and N2 proportions and the reservoir temperature determine the optimized heat extraction efficiency range directly. This study proposes the miscible flow of CO2 and N2 as the working fluid, which provides a new alternative for geothermal energy production.