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.