| Abstract |
COâ‚‚ Plume Geothermal (CPG) is a COâ‚‚-based geothermal electricity generation system that circulates COâ‚‚ into the geologic reservoir instead of water. When COâ‚‚ is circulated through a deep ( greater than 2 km) sedimentary reservoir, it extracts more heat per unit pumping power than water due to the decreased viscosity of supercritical COâ‚‚. Additionally, the compressibility of COâ‚‚ generates a pressure differential at the surface, allowing electricity to be generated directly from the produced geofluid instead of with an Organic Rankine Cycle. Therefore, CPG has the potential to generate geothermal electricity from reservoirs where water geothermal is uneconomical. However, the steps necessary to develop a CPG pilot depend on the field demonstration of the circulation of geologically stored COâ‚‚ between a sedimentary reservoir and the surface. To date, the COâ‚‚ circulation test at the SEACARB Cranfield site has been the only demonstration. While the test showed that sequestered COâ‚‚ may be re-produced and circulated from the subsurface, the test did not develop a non-zero, steady, thermosiphon-generated recirculation rate, as is expected when COâ‚‚ is used as the subsurface working fluid. In this paper, we analyze the publicly-available test results for the COâ‚‚ circulation test at the SECARB Cranfield site and describe the underlying physics which produced those results. We employ a numerical wellbore heat loss model to test the effect of wellbore fluid heat loss on the thermosiphon-generated fluid mass flowrate. We find that the accumulation of high-density fluid (i.e., water) within the production wellbore could cause the thermosiphon-generated recirculation flowrate to asymptote to zero, as it did during the field test. Most importantly, we find that a self-sustaining thermosiphon is not necessary for the successful operation of a COâ‚‚-based geothermal system and that the inclusion of a COâ‚‚ injection pump would have allowed for sustained circulation rates regardless of the production well fluid composition. |