Richard S. Jayne: Heat Predicts CO2 Breakthrough
As atmospheric CO2 concentrations continue rising, there is an increasing, no…urgent, need for political, social, economic and technological solutions to mitigate anthropogenic climate change that is caused by CO2 accumulation in the atmosphere. One promising technological solution is carbon capture and storage (CCS), which is the process of stripping CO2 from point-source generators and pumping it into deep geologic reservoirs. In theory, this will isolate CO2 from atmosphere. The most obvious geologic formations for CO2 storage are depleted oil and gas reservoirs because they have traps that held petroleum resources for millions of years before production. Deep sedimentary basins are also attractive reservoirs because they are characterized by alternating layers of high permeability sandstone and carbonate formations and low permeability shale, the latter of which models suggest can trap CO2 over 1,000+ year time scales. While depleted oil/gas reservoirs and deep sedimentary basins may prove invaluable for CCS deployment, they generally occur in the same locations, and thus impose geographic constraints on where CCS can occur.
Over the last 10 to 15 years, basalt formations have been proposed as target reservoirs for CCS because they are highly reactive and may facilitate rapid and permanent CO2 storage through mineralization reactions. In this process, the CO2 dissolves in water to release hydrogen and bicarbonate ions. The hydrogen acidifies the formation water, which dissolves the basalt and releases divalent cations, such as calcium, iron, and magnesium. These divalent cations then react with the bicarbonate ions to produce new minerals, such as calcite, magnesite and siderite, among others. Basalt sequestration has been shown to work at small-scale field tests in Iceland and Washington State, USA, and it holds tremendous promise for decreasing CO2 emissions in regions, such as India, where basalt formations are widespread and coal-fired electricity is ubiquitous. Despite these promising attributes of CCS in basalt reservoirs, there remain fundamental challenges in achieving CO2 injection rates required for industrial operations.
One critical challenge is that basalt formations are pervasively fractured, which presents a paradox – on one hand, the fractures provide reactive surface area for mineralization reactions to proceed, but on the other the fractures provide pathways for CO2 to escape the storage reservoir. Another critical challenge is developing methods to monitor CO2 migration within and above the disposal formation. These challenges are exacerbated by the fracture heterogeneity in basalt formations because we do not currently have the technology to accurately map fracture networks at depths where CO2 injections occur, i.e., 800+ m below ground. In new research published this month in Geophysical Research Letters, Richard S. Jayne and colleagues used numerical simulation to show that the thermodynamics of the CO2-water system may provide a much-needed strategy for monitoring CO2 breakthrough in highly heterogeneous geologic formations, such as basalt. This research shows that heat is released when CO2 dissolves in water, and while this has been known for some time, Jayne’s study shows that within heterogeneous formations this heat is concentrated in high permeability conduits and migrates well ahead of the CO2 plume. In fact, Jayne’s research shows that the thermal signature is a predictor of CO2 breakthrough, and that reservoir temperature begins increasing weeks or even years before the CO2 plume arrives. This is a significant breakthrough in our understanding of multi-phase, CO2-water flow in heterogeneous geologic formations, and paves the way for the development of effective measuring, monitoring, and verification strategies for industrial-scale CCS operations in highly fractured basalt formations.