Geothermal energy combined with CO2 sequestration: An additional benefit

In this transition period from a fossil-fuel based society to a sustainable-energy society, it is expected that CO2 capture and subsequent sequestration in geological formations plays a major role in reducing greenhouse gas emissions. An alternative for CO2 emission reduction is to partially replace conventional-energy for heating and cooling buildings (e.g., cogeneration units) with geothermal energy. A mixture of CO2 with cold return water injected into geothermal reservoirs can be the integration of geothermal-energy production and subsurface CO2 storage. In this process, mixed CO2-water is injected, while hot water is simultaneously produced from the production well. The process may end when CO2 either in the aqueous phase or in the CO2-rich phase breaks through. It depends on the function of the CO2 as being a stored medium or being a source for gasdrive water production. In this study, we discuss the influence of various CO2 injection volumes on low-enthalpy production mechanisms/storage of CO2 in relation to the production of hot water from the geothermal aquifer systems. Furthermore, we provide injection-screening conditions for optimal geothermal recovery, maximal storage of CO2 and/or re-use of a CO2-water cycle. For any energy source, one important attribute is the recovery efficiency, i.e., how much energy can be extracted from this source with respect to the amount of energy invested during the process of energy extraction. In this work, we estimate the total amount of energy invested for mixed CO2-water injection into the geothermal reservoir, using an “effectiveenergy” analysis. Furthermore, we provide a cursory evaluation of the economics of the proposed project assuming that we can relate the energy balance to an economic analysis. In such a conversion, the notion intensity of embodied energy plays a central role. We also introduce a plot of the heat-energy extraction and the storage capacity, which can be used to locate optimal in-situ conditions. These results, which are plotted in a heat-energy/storage-capacity diagram, are discussed in detail. Aspects regarding specific geoand technical infrastructure are ignored.Geoscience & EngineeringCivil Engineering and Geoscience