Coupling geothermal energy with geological carbon storage: A holistic review of enhanced geothermal systems using CO₂ as a working fluid

CO 2 -based enhanced geothermal system (CO 2 -EGS), also known as CO 2 plume geothermal, has emerged as a promising avenue to address the growing global energy demand and mitigate global climate concerns by exploiting renewable energy from geothermal reservoirs while concurrently sequestering CO 2 . In this method, CO 2 , in a supercritical state or dissolved in brine, is used as a working fluid to harness the geothermal energy held in hot reservoir rocks, with part of the CO 2 being trapped in the reservoir. Despite their rapidly growing popularity, the integration assessment of CO 2 -EGS studies, fragmented into various subjects such as thermodynamics, heat transfer, multiphase flow, reservoir hydraulics, geomechanics, and geochemistry, remains insufficiently explored. Thus, a critical review that consolidates conducted studies, identifies gaps, and directs future research in this coupled technology is crucial. This review aims to provide a comprehensive assessment of CO 2 -EGS, emphasizing its significance, the major challenges affecting its performance and mitigation strategies, the thermophysical properties of CO 2 as a working fluid, and CO 2 storage while extracting geothermal energy. The study revealed the key benefits of CO 2 -EGS, including reducing corrosion and scaling effects in the wellbore, maintaining reservoir pressure, storing CO 2 , increasing sweep efficiency of the reservoir, lowering pumping power, and addressing water scarcity for geothermal systems. Despite its significance, CO 2 -EGS encounters major challenges, such as cost, drilling and operating wells in harsh geological conditions, CO 2 leakage, lost circulation, premature thermal breakthrough, lower specific enthalpy, and incomplete heating. Key factors influencing its performance include properties of the reservoir, natural fractures and faults, geochemical and geomechanical factors, well design, type of thermodynamic cycle used, and CO 2 -related factors such as injection rate, injection pressure, temperature, and impurities. Overall, this review provides insights into significant advancements achieved and highlights future research to leverage CO 2 -EGS for reducing CO 2 emissions while extracting geothermal energy. • Renewable energy generation and geological carbon sequestration can be achieved in CO 2 -based EGS. • CO 2 -based EGS outperforms water-based EGS in low-permeability reservoirs with a minimum recovery of 50%. • Key factors affecting heat extraction and CO 2 storage in CO 2 -EGS are discussed. • Mitigation mechanisms for the challenges in CO 2 -EGS are analyzed. • Future research directions to optimize CO₂-EGS are provided.