Water-driven geothermal heat extraction with simultaneous CO2 injection: overview of concepts, benefits and challenges

Geothermal energy production and CCS (Carbon Capture and Storage) represent promising technological solutions to help mitigate climate change and aid the current global energy crisis. In recent years, the number of concepts that propose to combine and mutualize these technologies has risen dramatically. While a number of concepts (notably CPG and CO₂-EGS) use supercritical CO₂ as the heat vector, another promising route for hybridization is to inject dissolved CO₂ in the geothermal brine. This is the focus of our current work. An extensive literature review was carried out of the concepts, complemented by interviews of some of the developers.

A few concepts are still theoretical (only described in the literature), but most technological ones are on the way to pilot/demonstration projects at progressively increasing scale. The main projects and their associated sites are:

• CO₂-DISSOLVED technology, with potential sites identified in the Paris basin (France);
• AAT-G / Cleag technology, with a site in Croatia;
• Related projects CarbFix, GECO and SUCCEED, with sites in Hellisheidi (Iceland), Nesjavellir (Iceland), Bochum (Germany), Kızıldere (Turkey), Castelnuovo in Italy (as case study due to permitting issues);
• Reinjection of CO₂ from geothermal brines at Ngatamariki and Te Huka sites in New Zealand.

Despite similarities, these solutions are differentiated by their purpose:

• either to store CO₂ from an external industrial emitter (notably for the CO₂-Dissolved concept), thus bringing a contribution to CCS,
• or to reinject CO₂ emitted by CO₂-rich brine during geothermal exploitation, thus bringing geothermal to near-zero emissions.

Because of the CO₂ solubility limit in brine, the performance of heat extraction is generally higher than that of CO₂ storage. For instance the CO₂-DISSOLVED technology is particularly well-suited to small CO₂ industrial emitters (ca. <150,000 t CO₂/year). Unlike concepts using supercritical CO₂, those using dissolved CO₂ can be deployed at much lower depths (no need to exceed the supercritical point). The concepts still need a tight caprock, but the high solubility trapping represents a lower risk of leakage. The geothermal system behaves mainly as a water-driven system, but the adjunction of dissolved CO₂ can in some cases increase thermo-hydrological performance (pH decrease might avoid clogging and/or open porosity in carbonate reservoirs).

A number of challenges still need to be addressed, including complexity of regulations and validation of some technical aspects. Besides, considering the variety of underground configurations, there is no turnkey solution, which might hamper the economics of such small-scale projects.

Acknowledgements: This work is in part taken from a study published in the report ‘IEAGHG, “Prospective integration of Geothermal Energy with Carbon Capture and Storage (CCS)”, 2023-02, August 2023’. We are grateful to IEAGHG, especially to Nicola Clarke, for proposing and funding this topic and for interesting scientific discussions and debates as part of this work.