Heat Production Performance from an Enhanced Geothermal System (EGS) Using CO2 as the Working Fluid

CO2-based enhanced geothermal systems (CO2-EGS) are greatly attractive in geothermal energy production due to their high flow rates and the additional benefit of CO2 geological storage. In this work, a CO2-EGS model is built based on the available geological data in the Gonghe Basin, Northwest China. In our model, the wellbore flow is considered and coupled with a geothermal reservoir to better simulate the complex CO2 flow and heat production behavior. Based on the fractured geothermal reservoir at depths between 2900 m and 3300 m, the long-term (30-year) heat production performance is predicted using CO2 as the working fluid with fixed wellhead pressure. The results indicate that the proposed CO2-EGS will obtain an ascending heat extraction rate in the first 9 years, followed by a slight decrease in the following 21 years. Due to the significant natural convection of CO2 (e.g., low viscosity and density) in the geothermal reservoir, the mass production rate of the CO2-EGS will reach 150 kg/s. The heat extraction rates will be greater than 32 MW throughout the 30-year production period, showing a significant production performance. However, the Joule-Thomson effect in the wellbore will result in a drastic decrease in production temperature (e.g., a 62.6 degrees C decrease in the production well). This means that the pre-optimization analyses and physical material treatments are required during geothermal production using CO2 as the working fluid.

Automated summary

This study builds a CO2-EGS model based on geological data in the Gonghe Basin of Northwest China to predict the long-term heat production performance. The results show an increasing heat extraction rate in the first 9 years and a slight decrease in the following 21 years. Due to the natural convection of CO2 in the geothermal reservoir, the CO2-EGS achieves a mass production rate of 150 kg/s and a heat extraction rate greater than 32 MW throughout the 30-year production period. However, the Joule-Thomson effect in the wellbore causes a drastic decrease in production temperature, requiring pre-optimization analyses and physical material treatments.