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Making Strides in Studying Seafloor CO2 Sequestration
Much Research and Development in Korea Has Zeroed in on Offshore, Underground CO2 Storage

By Cheol Huh
Senior Researcher
Seong-Gil Kang
Project Manager
Maritime and Ocean Engineering
Research Institute
Korea Ocean Research
& Development Institute
Daejeon, Korea

There are a variety of options to mitigate climate change, such as improved energy efficiency, the switch to less carbon-intensive fuels, renewable energy, nuclear power, and carbon dioxide (CO2) capture and storage (CCS). CCS technology from fossil fuel combustion could play an important role in the reduction of greenhouse gases. CCS consists of the separation of CO2 from industrial and energy-related sources and transporting it to a storage location for long-term isolation from the atmosphere. The widespread use of CCS technology would make it possible to mitigate global warming without a rapid change in energy supply infrastructure.

Technology for the large-scale capture of CO2 is already commercially available and fairly well developed. Although CO2 has been injected into geological formations for various purposes, technology for long-term storage of CO2 in marine geological structures is still in early development.

Since 2005, the Korea Ocean Research & Development Institute (KORDI) has been focusing on developing technologies for offshore, underground geological storage of CO2. The research and development (R&D) activities at KORDI include technology for transportation and storage of CO2 and the assessment of its potential effects. Various technologies have been proposed for storing CO2 to reduce its concentration in the atmosphere and the potential impacts of global warming. Especially in recent years, CO2 storage in marine geological structures has been regarded as one of the most promising options. Currently, subseabed geological storage is a more technically feasible and reliable option for longer term CO2 disposal than ocean dissolution and lake-type storage, which pose relatively high technical and environmental uncertainties.

Therefore, KORDI's efforts developing technology for CO2 storage have been focused on marine geological storage.
Transport and Storage
The physical and chemical properties of CO2 and CO2 hydrate are the most important parameters in developing CO2 transport and storage. The phase equilibrium of CO2 gas mixtures, including water, hydrate and electrolyte, has been investigated. Two and three-phase equilibria were computed for gas hydrates using the equation of state for fugacity of guest components in gas hydrates, and the van der Waals and Platteeuw model was computed. So far, an account of the inhibition effect of electrolytes in phase equilibria containing gas hydrate has been given by excess Gibbs energy models, which are less appropriate than an equation of state for high-pressure applications. In this study, phase equilibria containing gas hydrates and electrolytes were predicted using the electrolyte lattice-fluid equation of state, in which long-range electrostatic interactions were modeled using the mean spherical approximation, and salvation effects were considered with Veytsman statistics. The proposed model closely predicted various phase equilibria, including inhibition and salting-in effects.

A preliminary design of a deep-sea injection system for CO2 ocean sequestration was also created. Systems for liquid transport, liquid storage and liquid injection were conceptually determined for the functional requirement. In terms of internal flow for liquid injection, the flow rate and temperature of the liquid CO2 in the injection pipe needs to be controlled. Reducing vortex-induced vibration (VIV) is also required for dynamic stability of the injection pipe. An experiment on the VIV of a slender marine structure model in linearly sheared flow was designed in order to obtain data for tuning and confirming the 3D nonlinear analysis code for the structure. It also aided in investigating the high-mode vibration of the main slender structure. A high-level response analysis was conducted using the results of this research.

A case study for a CO2 sequestration capacity of 10 million tons per year was carried out. This study determined the total number of injection ships, the flow rate of liquid CO2 and the configuration of an injection pipe. A static structural analysis of the injection pipe was also performed.

To develop the most reliable and efficient CO2 transport technology, a front-end engineering design study for supercritical CO2 compression, CO2 liquefaction and CO2 liquid pumping (for injection) was carried out. Health, safety and environmental considerations such as a hazard and operability study, a fire and evacuation study and an equipment protection study will be carried out within the next couple of years.

The CO2 geological storage technology is aimed at storing CO2 in a geological formation safely. The best places for CO2 geological storage are depleted oil and gas fields, coal beds and deep saline aquifers—porous rocks such as sandstone located underneath a layer of impermeable cap rocks.

Potential storage sites should be carefully selected and managed in order to minimize any chance of CO2 leakage.

As candidate sites, oil and gas fields around Korea are very rare. Therefore, present research work has focused on deep saline aquifers characterized by porous media. Although current knowledge suggests a few potential areas in the southern Ulleung Basin for possible CO2 storage, more work needs to be done to assess these sites.

It has been found that the injection efficiency of CO2 significantly depends on its phase, because the density and viscosity of CO2 is significantly varied with pressure and temperature near the critical point. Although the injection rate of liquid CO2 is slower than the injection rate of gaseous CO2, the injection efficiency of liquid CO2 is much higher than the injection efficiency of gaseous CO2, considering the density and viscosity of CO2 in its gas and liquid phases.

Biological and Ecological Impacts
The oceans could absorb almost all anthropogenic CO2; however, the pH of the oceans has been found to change as the concentration of atmospheric CO2 increases. Fossil fuels' CO2 emissions not only accelerate global warming, but also lower the pH of the ocean environment—both of which consequently may result in adverse biological and ecological impacts.

Despite the well known roles of CO2 in biochemical responses and ecological processes, its effects on the metabolic activities and biological rhythm of marine organisms' oxygen consumption have not been well described. To analyze the biological and physiological effects of CO2 on various marine organisms, experimental investigations for the manila clam, Ruditapes philippinarum, the black rockfish, Sebastes schlegeli, the fleshy prawn, Penaeus chinensis, the sea urchin, Strongylocentrotus nudus, and the deep sea shrimp have been carried out.

The influence of increasing CO2 concentration in seawater on various marine organisms was assessed with regard to the impacts of anthropogenic CO2 introduced into the ocean. The adverse biological effects of CO2 on various marine organisms were investigated. These results will be useful in predicting the potential risks of increasing CO2 concentrations in seawater due to the leakage of CO2 from a marine geological structure.

Leakage Prevention Research
Due to these possible negative impacts, stored CO2 leakage is a major concern. However, the Intergovernmental Panel on Climate Change estimates storage sites that are well selected, designed and managed could trap CO2 for millions of years and retain more than 99 percent of the injected CO2 over more than 1,000 years.

To assure the safety of CO2 sequestration, the monitoring of CO2 in geological formations is necessary. For this purpose, a technology to monitor CO2 in geological structures with elastic waves has been developed.

Current, temperature and salinity around CO2 storage sites could be monitored to evaluate potential risk caused by leakage of CO2. The migration of CO2 leakage has been predicted by considering the physical properties of the current around prospective storage sites.

KORDI has carried out comprehensive R&D work on CO2 storage in marine geological structures. One of the main goals of the R&D on CCS is to establish a process for the storage and transport of CO2. The development of the pilot and commercial plant for CO2 marine geological storage is ongoing. As the initial step toward this purpose, various research activities were carried out. Basic and detailed research to demonstrate the feasibility of CO2 storage in marine geological structures has been conducted with collaborative efforts from national research institutes and universities. The results will contribute to understanding not only how commercial-scale deployment of CO2 storage in marine geological structures can be feasible, but also how more reliable and safe CCS could be achieved. The study also suggests that there may be a strong need for systematic and scientific research to ensure safe and secure storage of CO2 in marine geological structures.

This article is based on the paper 'Development of Technology for CO2 Storage in Marine Geological Structures,' which was conducted with financial support from the Republic of Korea's Ministry of Land, Transport and Maritime Affairs. The authors would like to thank the members of our collaborative research teams.

Cheol Huh earned a Ph.D. in mechanical engineering in 2006 from Pohang University of Science and Technology in Korea. He researches heat transfer, ocean engineering and energy systems, and he is currently working on the investigation of carbon dioxide transport and storage in marine geological structures.

Seong-Gil Kang earned a Ph.D. in 2000 from the department of oceanography at Seoul National University in Korea. He is the project manager for development of technology on CO2 storage in marine geological structures in Korea. The work has been financially supported by the Korean government since 2005 and funding is planned until 2015.

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