Abstract
The primary focus of this paper is to compare the thermal performance of a fracture in an enhanced geothermal system (EGS) with supercritical carbon dioxide (CO2) and water as working fluids. The novelty in this study is the possibility that fractures may have spatial variations. Hence we investigated the impact of the heterogeneous aperture distribution on EGS performance when using CO2 as a working fluid in place of water. We modeled a doublet enhanced geothermal system with a single horizontal fracture connecting the two wells.By choosing a mass flow rate that ensured the same energy input for CO2 and water, a comparative study could be done for the two fluids. The study showed that if the fracture aperture is uniform, for the same energy injected, CO2 results in higher energy produced compared to water, similar to previous findings comparing CO2 and water as working fluids. However, if the fracture has spatial variations in aperture, CO2 may not always be a better working fluid for EGS. The study showed that the energy extracted from the enhanced geothermal system with water as working fluid exceeds that of CO2 for heterogeneous fracture aperture distributions with large correlation lengths and considerable mass flow rates.This study demonstrates that there are conditions under which either fluid can be used as a working fluid to maximize heat extraction from EGS.
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