Saturday, April 22, 2006

REVIEW OF BOOK ON "Geological Storage of Carbon Dioxide"

Geological Storage of Carbon Dioxide
Edited by S. J. Baines and R. H. Worden (2004)
Geological Society, London, Special Publications, 233.

Geosequestration is the geological storage of carbon dioxide (CO2) in the subsurface in saline aquifers, existing hydrocarbon reservoirs or unmineable coal seams. As the editors of this excellent publication state, it is one of the more technologically advanced (but expensive) options available for storing CO2 produced from the combustion of fossil fuels as a practical solution to CO2 pollution of the world’s atmosphere.

Many of the articles contained in this hardcover book arose from a technical session held at the 2001 European Union of Geosciences conference held in Strasbourg (France). Apart from a general introductory article by the editors, the remaining 240 pages comprise 15 articles focusing on mostly European and American geology and gas injection projects or theoretical assessments, although the content should be readily applicable to similar geological environments elsewhere on the planet.

Several articles provide useful information on existing CO2 deposits within sedimentary sequences, some many millions of years old. This suggests that geosequestration is feasible, provided potential leakage hazards are recognised, assessed and monitored to ensure an acceptably low overall risk.

While faults are an obvious potential source of CO2 migration out of the storage reservoir, chemical interactions between pre-existing minerals and the injected CO2 are shown to be an important factor in assessing long-term security of the CO2 reservoir. The editors’ article on the long-term fate of CO2 in the subsurface explains some of the possible chemical reactions between CO2 and carbonate or aluminosilicate minerals. Anorthite, zeolite, smectite and other Fe- and Mg-clay minerals are likely to react with injected CO2, so their presence must be determined prior to CO2 injection.

Rochelle et al. go further and state that chemical processes and hence chemical stability will also be dependent upon the structure, mineralogy and hydrogeology of the lithologies of strata into which CO2 is injected. Individual storage operations will therefore have to take into account local geological, fluid chemical and hydrological conditions.

Some 2.3 million tonnes of CO2 have been injected into a saline aquifer beneath the North Sea since 1999. Three papers describe aspects of results and simulations from this trail. Johnston et al. demonstrate a simulated increase in caprock integrity when CO2 is placed into the saline aquifer, suggesting that the chemistry of the sedimentary rocks into which CO2 is to be injected may assist in long-term reservoir security. Conversely, Zweigel et al. report on their simulation of the same saline aquifer, concluding that the 6.5 metre shale caprock does not fully inhibit upward migration of CO2. Finally, Arts et al. report on seismic monitoring of the injection area and express confidence that there has been no significant leakage into the overlying Pliocene shales.

To give an idea of the potential scale of geosequestration, Laenen et al. estimate that parts of a carbonate sequence in the Roer Valley graben of north Belgium could store up to 130 million tonnes of CO2, while six coalbed methane target areas containing an estimated 53 to 79 million tonnes of methane could store at least 400 million tonnes of CO2 once the methane has been extracted.

The book concludes with two papers on related but relevant topics. Bachu and Gunter discuss the injection of acid gas (hydrogen sulphide and CO2) into depleted oil and gas reservoirs in Alberta, Canada. Stenhouse and Savage then describe the lessons learned from the monitoring of a nuclear waste repository in New Mexico, USA, where geotechnical, environmental and subsidence issues were important considerations in the repository’s long-term security.

Criticisms of this publication are few and mostly minor. There is no list of abbreviations and it would have been useful to summarise the methods by which CO2 is removed from power station exhaust gas and the dangers posed by uncontrolled release of CO2 onto the land. The most serious criticism is that little space is devoted to the potential storage of CO2 in unmineable coal seams – just 4 pages relating to Westphalian coal in northern Belgium. Considering that the editors highlighted such coal seams as major potential sites for geosequestration, this paucity of coverage is surprising.

Overall, however, this book is of a high technical standard and it deserves to be widely available to policy makers and the public. At a cost of GBP75.00, it isn’t cheap but the 16 papers cover a diverse range of technical aspects of geosequestration and the publication warrants its use as an essential reference in academic and research institution libraries.

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