Thermodynamic analysis of enhanced geothermal systems using impure CO2 as the geofluid

Numerical simulations have indicated that pure CO2 is superior to water for wellbore hydraulics and for recovering heat from hot fractured rock. However, CO2 captured from power plants may contain impurities such as nitrogen, oxygen and particles. Rather than treating CO2 as merely a waste fluid to be stored in the deep subsurface, impure CO2 may also be used as a geofluid in EGS to extract heat energy from the hot dry rock while permanently storing the CO2 in the rock. Thus, the system performance with impure CO2 must be evaluated for large-scale CO2 utilization. This paper presents an entire cycle model for the flow in the wellbore, the heat extraction and the flow in the reservoir to model the system based on the reservoir parameters and ambient temperatures. The pressure and temperature profiles of the impure CO2 in the injection well and the production well, the power output and the thermal efficiency for an impure CO2-EGS were simulated by a FORTRAN code. The result clarifies the basic principles for the system optimization and the ORC working fluid selection for the impure CO2-EGS. (C) 2016 Published by Elsevier Ltd.