-Plume Geothermal (CPG) is a proposed technology that combines carbon capture and storage (CCS/CCUS) with geothermal energy extraction, utilising carbon dioxide () itself as a geothermal energy extraction fluid. The technology was first proposed by Dr. Donald W. Brown at the 25th Workshop on Geothermal Reservoir Engineering at Stanford University in 2000.
First, would be injected in deep and naturally permeable reservoirs, as in CCS, where the would be heated by the surrounding rock. At a nearby location, production wells would then extract the geothermally heated supercritical back to the surface, where it would be expanded in a turbine to generate electricity. The would then be cooled and condensed back to a dense phase and re-injected into the reservoir, closing the cycle and enabling all to remain sequestered. CPG has the potential to generate over twice the power of conventional, water-based geothermal systems for similar conditions: while the specific heat capacity of is less than that of water, the significantly lower dynamic viscosity of would enable higher overall energy extraction rates.
As the subsurface reservoir cools due to geothermal heat extraction, the density of in the subsurface increases, enabling a larger mass to be stored for a given formation. Other identified impacts of CPG on CCS include increased control over volumetric sweep, reduced carbon intensity of storage due to renewable energy production, 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 .
While existing equipment from enhanced oil recovery (EOR) and CCS projects could be repurposed for CPG, additional new equipment is required, primarily lower temperature supercritical turbines and high-pressure cooling and condensing units. Selecting suitable locations is a challenge.