Draft:CO2-Plume Geothermal

Technology
CO2-Plume Geothermal (CPG) combines Carbon Capture and Storage (CCS) with geothermal energy extraction, utilising the CO2 itself as a geothermal energy extraction fluid. First, liquid-like CO2 is injected in deep and naturally permeable reservoirs. In the reservoir, the CO2 is heated up by the surrounding hot rocks. At another location, production wells extract the geothermally heated supercritical CO2 back to the land surface, where the it is expanded in a CO2 turbine to generating electricity. The CO2 is then cooled and condensed back to a liquid state and re-injected into the reservoir. All CO2 is therefore permanently sequestered. CPG has the potential to generate up to two to three times the power of conventional, water-based geothermal systems for similar conditions. While the specific heat capacity of CO2 is less than that of H2O, the significantly lower dynamic viscosity of CO2 enables higher overall energy extraction rates. CPG is therefore a true CCUS technology, combining both Utilisation and Storage.



16 peer reviewed publications have been published on CPG since its invention by Martin Saar and Jimmy Randolph in 2011.

Relation to CCS projects
CPG can be seen as an add-on technology that is superimposed on top of an underlying CCS project. A mass balance argument can be made to illustrate that all CO2 is stored just like in the underlying CCS project. Given a source stream of CO2 such as that from a CO2 capture plant, as the mass of CO2 in the surface network is negligible, all CO2 is contained subsurface: storage occurs immediately.

As the subsurface reservoir is cools due to geothermal heat extraction, the density of CO2 in the subsurface increases, enabling a larger mass to be stored for a given formation. Other identified impacts of CPG on CCS include reduced carbon intensity of storage due to renewable energy production, increased control over CO2 volumetric sweep, additional monitoring data from production wells, flexibility to repurpose producer wells to injectors, avoiding injector downtime with associated halite deposition risks, and providing communities with power produced using CO2. One drawback of CPG is the need for additional penetrations through the reservoir caprock.

Research needs
While existing equipment from CO2 EOR and CCS projects can be repurposed for CPG, new equipment is required, primarily lower temperature supercritical turbines and high-pressure CO2 cooling and condensing units. A commercial field demonstration to increase the TRL of CPG has not yet been executed.