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Autore: R. Bencini

Collana: FM - 2004 Sorrento - Third International Symposium - Energy and Environment

Note:
È ormai dimostrato che la CO2 sia non solo il maggior gas-serra, capace di alterare negativamente il clima globale se non ne viene fermata la crescita in atmosfera, ma anche che l’eccesso di CO2 non riassorbito dai sistemi naturali è di origine umana, dovuto alla combustione di combustibili fossili quali carbone, petrolio e gas naturale.
Nell’impossibilità attuale di frenare lo sviluppo delle nazioni, particolarmente quelle oggi in crescita, di eliminare l’uso dei combustibili fossili, e di poter fare ricorso esclusivamente alle sorgenti di energia rinnovabile, la cattura e stoccaggio geologico della CO2 di origine antropica si prospetta come il rimedio tecnologicamente ed ambientalmente compatibile di maggior efficacia e di costo più contenuto oggi possibile.
Dal punto di vista economico, le tecniche di stoccaggio geologico della CO2 maggiormente favorite sono quelle che permettono il recupero di prodotti utili, quali in ordine di importanza il petrolio greggio (Enhanced Oil Recovery, EOR) o il gas naturale (Enhanced Coal Bed Methane, ECBM, e Enhanced Gas Recovery, EGR), partendo magari da CO2 già concentrata, per minimizzare i costi di cattura.
Da un punto di vista tecnico, invece, va sottolineata l’enorme capacità di immagazzinamento degli acquiferi salini profondi (Deep Saline Aquifers, DSA), che potrebbero da soli ospitare, praticamente per sempre, tutta la CO2 antropogenica.
In Europa e in America sorgono nuove iniziative di ricerca e di indirizzo politico sull’argomento. Segnaliamo il “Carbon Sequestration Leadership Forum” e le importanti iniziative di ricerca internazionali appena terminate con successo (progetti SACS e WEYBURN della Commissione Europea e dell’IEA) o ai blocchi di partenza.
In Italia, l’Istituto Nazionale di Geofisica e Vulcanologia di Roma (INGV) forte della propria esperienza pratica nel progetto Weyburn, si è fatto promotore dei progetti di settore, che comprendono alcuni progetti dimostrativi delle tecniche EOR e ECBM, e un progetto nazionale di mappatura dei siti di possibile iniezione di CO2, con particolare enfasi sugli acquiferi salini profondi.




CO2 is a by-product of the combustion of carbon-based fuels (biomass, coal, oil and natural gas) for heating, power generation or transport, cement making and certain chemical carbon-based industrial processes. Carbon dioxide (CO2) is also the main contributor to the greenhouse effect. When too much greenhouse gas is emitted to the atmosphere, the extra atmospheric heating may elevate the average surface temperature of the planet by a few degrees with dangerous effects, if no remedial actions are taken.
Such global warming is modelled not to be uniform everywhere, but to be stronger in the lowest parts of the atmosphere and to be stronger at the poles, the North Pole in particular.
NOAA, the United States National Ocean and Atmosphere Agency, recently released important data that confirm the growth of atmosphere CO2 content mostly in the Northern Hemisphere, and that such CO2 is heavily influenced by fossil fuel burning in terms of Carbon isotopes change.
An acceptable way to obtain enough energy without CO2 emissions is to capture and geologically sequester the CO2 coming from the use of conventional fossil fuels (coal, oil, gas). CO2 geological storage has the potential for eliminating all the man-made CO2 emissions for the next few centuries, while waiting for the development of suitable CO2-free primary energy sources in sufficient quantity. Conventional renewable primary energy sources are in fact unlikely to provide enough low price energy to satisfy the growing energy needs of humanity for a long time to come.
CO2 capture is a pre-requisite for CO2 geological storage. Today’s technology allows capturing CO2 in three principal ways, namely post-combustion, pre-combustion and oxy-fuel combustion, each subject to possible improvements under current research. Post-combustion CO2 capture means capturing the CO2 from the exhaust fumes, after conventional fossil fuel combustion. Pre-combustion CO2 capture means capturing the CO2 after the reforming of fossil fuel in a gasification-like apparatus. The third option, capture by oxyfuel combustion, means that oxygen is separated from air, releasing a nitrogen stream to the atmosphere, then fossil fuel burns in an oxygen and recycled CO2 stream (CO2 replaces nitrogen as combustion moderator) generating a water vapour and CO2 exhaust stream, from which CO2 is cheaply extracted to go back to the combustion chamber and to compression, transport and geological sequestration.
The CO2 underground geological storage consists in injecting CO2 in a suitable geological formation that is likely to retain the CO2 for significant length of time, say several thousand years. The stored CO2 can be considered effectively sequestrated when it eventually reacts with the reservoir formation and/or with the formation brine to give stable solid or liquid products. Today’s technology allows to safely store CO2 underground in three principal ways, namely by injection in deep saline aquifers (or depleted natural gas fields unsuitable for methane storage), in deep unmineable coal beds, and in semi-depleted oil or gas fields.
It has been recently estimated that the CO2 storage capacity of known deep saline aquifers, known deep unminable coal beds and known depleted or semi-depleted oil and gas fields vastly exceed the CO2 likely to be generated by human activities in the next few centuries. Most of such storage capacity is located in deep saline aquifers.
CO2 geological storage as a concept is not new to the experts, and it is the subject of scientific and technological research since the early 90’s, when the European Union started to finance targeted research projects, the most noticeable of which are the SACS and the Weyburn projects.
The SACS I and SACS II projects demonstrated that CO2 injection in deep saline aquifers is perfectly feasible and that it is possible to monitor the whereabouts of the CO2 in the reservoir using the same reflection seismic technology commonly used in hydrocarbon exploration and production. Offshore Norway, the CO2 separated from the Sleipner field natural gas is injected through one well in the Utsira sandstone formation (the saline aquifer) a few km away from the production platform. For Statoil, the operator of the Sleipner field, this is cheaper than paying the carbon tax on CO2 emissions due to the Norwegian Government if the same CO2 was released to the atmosphere.
The Weyburn project studied the CO2 storage associated with large-scale EOR operations at the Weyburn oil field, Southern Saskatchewan, Canada, operated by EnCana. Approximately 5000 ton/day of CO2 are injected in a carbonate reservoir through multiple wells. At the end of the EOR operations, some 20 million ton of CO2 is expected to remain stored in the depleted oil reservoir. This is the largest CO2 geological storage research project in the world, and it is co-sponsored by IEA, DoE of the USA, Natural Resources of Canada and the European Commission. INGV of Rome is the Italian research partner in the project.
A useful review of European and International research and demonstration initiatives of CO2 capture and CO2 geological storage is available on www.co2sequestration.info, a site created and maintained by the GHG R&D Programme. More recently, the DoE of the USA launched the “Carbon Sequestration Leadership Forum” www.cslforum.org .
In Italy, the National Institute for Geophysics and Vulcanology (INGV) of Rome is promoting the research projects in this sector. The Italian programme includes an EOR project, and ECBM project and a national project for the mapping of all sites that are deemed to be suitable for CO2 injection.
The CO2 geological storage is the practical mean to substantially reduce industrial CO2 emissions most likely to gain acceptance and success in the near future. CO2 geological storage is well posed to become the principal tool for greenhouse gas emission reduction worldwide.