Comparisons of Heat Extraction Performances for a Multilateral-well Enhanced Geothermal System with CO2 and Water as Working Fluids

Abstract

Increasing studies were conducted to compare heat extraction performances of CO2-EGS and water-EGS. However, these research efforts never considered CO2 expansion effect in wellbore, which could not provide accurate enough comparisons between CO2-EGS and water-EGS. Hence, based on a reservoir and wellbore coupling model, this paper comprehensively compares heat extraction performances of a multilateral-well CO2-EGS and water-EGS considering the CO2 pressure work in wellbore. The temperature, pressure and output thermal power distributions of multilateral-well CO2-EGS and water-EGS are analyzed. Multiple cases for multilateral-well CO2-EGS and water-EGS are investigated, including assessment of differences in heat extraction using varying well depths and reservoir pressures. Results show that when CO2-EGS mass flow rate is three times of water-EGS mass flow rate, the reservoir outlet thermal power of CO2-EGS and water-EGS is similar. However, the CO2-EGS has much lower wellhead thermal power than water-EGS, due to the noticeable temperature reduction in CO2-EGS wellbore induced by the CO2 expansion. Additionally, the CO2-EGS can perform better in a geothermal reservoir with a smaller well depth and higher reservoir pressure.