Carbon capture and storage

Introduction

Increasing numbers now recognise the potential devastation upon the worldwide environment climate change could have. With CO2 emissions increasing at a rate of 1.6%/Yr (1999-2005) and emissions from power production at 23,684 Mt/yr (2005)1 plus no current successor to the Kyoto Protocol*, it is clear that Carbon Dioxide is going to become an ever growing threat to our planets stability. Worryingly, not only in a climatic sense but a societal one as well. From solar and geothermal power to hydrogen fuel cells, the scientific community is working to develop ways of reducing CO2 output and one field of growing interest from both the research and business community is Carbon Capture and Storage (CCS). Serious research in this field is relatively new and many aspects of its viability, safety, efficiency and cost have still to be fully discovered. As CCS is simply storing CO2 and not actually decomposing it, many feel CCS is counter-productive and the resources should instead be channelled to focus on clean energy production. However with current emission trends, CCS will be an extremely useful tool should we see drastic changes in climate toward the end of this century and need a way to buy time to fully utilise and develop clean energy. This paper will briefly describe a range of potential CCS methods as shown in figure A as well as discuss the potential for CCS in our society. The smallest estimated potential storage for CO2 at 320Gt is worth approximately 32 years of emissions!2 Few dispute the fact that we should evolve to a more environmentally-friendly society in all senses of the word, CCS will buy the time needed for this to happen. Over the last 10-20 years several proposals have been put forward and developed such as the Sleipner oil field, Norway and ‘CarbFix’ in Iceland. We are now beginning to get live data from current CCS projects worldwide to analyse and use for the enhancement of CCS, this paper aims to synthesise this information from these projects for a brief analysis of CCS potential.

Deep Ocean or the deep seabed

Many have hypothesised on potential CCS sites. One suggestion is storing CO2 in the deep ocean or seabed as shown in figure B. As the oceans are already absorbing ~8 billion tons of CO2and negating ~50% of our anthropogenic CO2 emissions3 it is already a natural CCS site. CO2 is denser than seawater in its supercritical state (both solid and liquid, see figure C)and so will sink and pond on the seabed staying there for thousands of years as figure B illustrates. Alternatively, ships would pump CO2 into the ocean as shown in figure B where natural thermohaline currents would dissolve the CO2 upon which that slightly denser body of water would pond on the seabed. While salinity, pressure and temperature all affect the dissolution of CO2, below 600m, 41-48kg/m3 CO2 can dissolve in a 1M brine solution,2 a fairly large figure. Increase the brine concentration and this figure will drop,however, with the average molarity of the oceans at 0.5M it is clear that this store has great potential. Unfortunately immediate acidification of the local water would occur as carbonic acid forms. Therefore this storage method would probably be devastating to local ecology. The cost:benefit analysis over acidifying patches of ocean as opposed to lowering atmospheric CO2 and that’s effect upon terrestrial habitats and surface ocean marine communities could fill a thesis and resulted in much debate. This method has so far seen no field tests even though its potential storage capacity is vast and inestimable.

Mineral Carbonation

Of similar environmental concern is disposal via mineral carbonation. CO2 reacts with certain rocks to form carbonate minerals. This process is seen naturally in the form of weathering where ~1.8×108 tons CO2 are mineralised annually yet this geochemical process could also occur underground. Rather than mine and crush rocks such as basalt and peridotite to react with atmospheric CO2 on the surface, causing major environmental disruption due to mass mining operation and a great increase in sediment flux,4 CO2 would be injected into deep geological stores of: olivine; pyroxene; and plagioclase.Here the CO2 would slowly react to form its carbonates over tens of thousands of years where it would then be a near permanent store. As these reactive minerals are found in reasonable abundance in basic rock, potential CCS sites of this nature are found worldwide. The Columbia River basalt has been predicted to be able to dispose of 36-148Gt/CO2 whereas the Caribbean flood basalts could potential store 1,000-5,500Gt/CO2. Similarly, the basalt basin offshore of Washington D.C. could hold 500-2,500Gt/CO2.10 The gaseous CO2 conversion to solid carbonate involves an increase in volume and pressure. It is hypothesised this process would cause major fracturing within the basalt rock which could potentially form an escape route for the still supercritical CO2 (see figure D).8 The ‘CarbFix Pilot Project’ in Iceland is monitoring the effects and potential of this style of CCS through intensive Geophysical monitoring as ~9.4Mt/CO2 is pumped into the ground.

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Coal-bed seams

Worldwide there are many coal fields economically unviable for mining and these are potential CCS sites as figure A (4) shows. The coal seams contain natural micropores due to coal production process. These micropores currently contain methane molecules, again as a by-product of the coal creation. However, CO2 molecules adsorb to the micropores easier than the CH4.2 By pumping CO2 into these seams a volume of CH4 will be yielded proportional to the volume of CO2 injected,2 while still providing a deep underground store for CO2. This has been calculated at 20m3/ton coal from a field site in the San Juan Basin. Therefore there is an approximate minimum storage capacity of 150Gt/CO2 worldwide however exact volumes of unmineable coal are not available. Adsorption involves weak electrostatic forces to hold the CO2 molecules to the pore which are very dependent on a stable environment.2 Should any tectonic activity take place to alter the temperature or pressure of the storage site, the CO2 would detach and plume. This CO2 plume would then slow migrate to the surface through existing pore channels which figure D shows clearly. This is a worry faced in many CCS schemes, as any CO2 migration could cause interaction and dissolution into groundwater thereby polluting it, force saline groundwater to mix with freshwater and pollute the freshwater or alternatively migrate to the earth surface and plume. Also, dependent on the CCS site, CO2 could end up acidifying patches of ocean where ‘leaks’ have occurred. CO2 plumes on the earth surface have proven fatal before when 1,700 people and all fauna within a 14km radius perished in the Lake Nyos disaster when CO2 suddenly degassed from the base of the lake to the atmosphere. 14

Depleted oil and gas reservoirs or saline aquifers

One of the most promising and researched suggestions is storage in depleted oil and gas reservoirs or saline aquifers. Figure A (1,3a,3b) illustrates these are both on and off shore and deep geological area of rock with high porosity and low permeability. The gas field ‘Sleipner West’ in the North Sea just off the Norwegian coast is an actual working CCS site where much research into CCS is being conducted and monitored. 1×106 tons of CO2/Yr2 are being pumped into a space of 5.5x1011m32 previously occupied by predominantly methane gas. The CO2 is stored in the pore spaces in rocks identical to how groundwater is stored in aquifers. In the case of saline aquifers, while pumping in CO2, saline water is removed as well as forced into surrounding rock. These Porous rocks are commonly sedimentary rocks found in basins normally 600-1200m deep. Pressure increases with depth as well as temperature, by about 28°C/km2. This means CO2 would need to be stored in its supercritical state (figure C) which is more compact than normal, 1 ton of CO2 occupies 6m3 rock2. Once injected, the CO2 will naturally migrate through the pore spaces trying to reach ground level (figure D). During this process the CO2 will become ‘trapped’ and well in pore routes which do not actually lead to the surface. The inevitable migration makes choosing a CCS site difficult. Any site needs an impermeable rock layer above it or a low permeability rock where the migration time will be equal to the sites desired lifespan to act as a ‘cap rock’. Without a cap rock, the CO2 could migrate back to the surface in decades making the entire operation an epic fail. However, storing CO2 in these fields is not just about pocketing it underground. The geochemical processes of dissolution and mineral precipitation would also occur adding to the favourability of depleted reservoirs as the optimum CCS technique. For any single site 3 different forms of CCS would be occurring. Dissolution would take a few thousand years dependant on the surface area to volume ratio of water to CO2 and mineralisation would happen along similar timelines. Therefore, four factors will affect the usefulness of any CCS depleted reservoir site: immobilisation of CO2 in any traps or wells; geochemical reactions between the rock and CO2; dissolution into groundwater or saline water resident in the rock; and migration back to the surface.2 The benefits of this method of CCS do not stop here though! The process of pumping CO2 into the ground forces out the dregs of what was previously there, beneficial if it was gas or oil. Shows this as a separate process but it can easily be paired with depleted fossil fuel stores. This can be collected and sold, providing a slight economical offset to the cost of the project. This is referred to as Enhanced Oil Recovery (EOR). EOR has been embraced in the Americas and is in use at Pan-Canadian’s Weyburn field in Saskatchewan, another field example of CCS in use today. Only 18Mt/CO2 is its expected capacity2 however data on this specific technique will be invaluable. It does raise questions into the economics as it would take thousands of these sites worldwide to have a significant impact upon atmospheric levels and with each new site, the risks of a CO2 disaster associated with the storing of CO2 increases. Worldwide there is great uncertainty into the potential volumetric storage capacity of CO2 in underground reservoirs ranging from 400-10,000 Gt/CO2 according to Hendriks and Blok, 425Gt/CO2 was proposed by Van De Meer whereas Koide and team calculated it at 320Gt/CO2. There is such variation not only because worldwide high resolution mapping of the subsurface is scarce, but the presence of micropores is undetectable and the question of how do you incorporate figures from processes such as dissolution and geochemical mineral precipitation is raised. Figure E shows the IPCC calculation of potential worldwide storage sites. Either way, CCS can clearly buy the time we may need to avoid major global climate change.

Conclusion

As natural gas itself contains a fraction of CO2, this proves it is possible to store CO2 in a geological setting for millions of years, the exact purpose of CCS technology. Unfortunately, with all the ambition and optimism over CCS, its true benefits must be realistically analysed. Yes CO2 is a powerful greenhouse gas and the one most accountable to anthropogenic sources,1 but it is only one of many. CCS is completely unable to deal with methane, sulphur oxides and of apparent increasing importance, water vapour. Critics are also completely correct with their feelings of it only being a temporary fix. The earths surface system is dynamic enough that the stored carbon will eventually make its way back into the atmospheric carbon cycle . Although we will be far gone, is that a responsible excuse? CO2 migration will occur within the store and so will need constant monitoring. Is the economic cost of initiation, monitoring and potential clean-up should leaks develop enough to justify the project? Current estimates reckon in the cement industry, it will cost $50-250/ton CO2 to be avoided2 and that electricity prices will have to double at minimum2, the lower figures representing technology advancement. The CO2 could pollute groundwater sources with saline water and Cox et. al. have perceived that a fault during late stage CO2 injection could produce a CO2 plume similar to that seen at Lake Nyos.[19] While this risk could be mitigated by placing CCS sites offshore it would still be an ecological disaster.2 Finally, who would be responsible for the CCS site? The purpose of the site is to store CO2 for 10,000 years or more.19 It is highly unlike any company will be around for its lifetime. While CCS is technically possible, it undoubtedly requires more research and development to convince not only the rest of the scientific community, but the general public as well.

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