Reactivity of Injected CO2 with the Usira Sand Reservoir at Sleipner, Northern North Sea

This chapter summarizes results of the geochemical work carried out as part of the “Saline Aquifer CO2 Storage” (SACS) project, a European research project that aims to monitor the injection operations at Sleipner. The objective of the geochemical work within the SACS project was to determine the potential for chemical reactions between injected CO2, formation water and the reservoir rock, as these reactions may affect injection operations and long term CO2 storage potential. The chemical reactivity of CO2 with a host formation has to be assessed in any CO2 geological sequestration project, as it may affect injection operations and long term CO2 storage potential. At the Sleipner gas field (Norwegian sector of the North Sea), CO2 has been injected since 1996 into a deep saline aquifer (Utsira formation) approximately 1 km below the bed of the North Sea. This can enhance the CO2 storage capacity and have consequences on CO2 migration or immobilization. A direct approach was used, based on laboratory experiments reacting samples of Utsira sand and formation water with CO2 under simulated reservoir conditions for timescales up to 24 months. Laboratory investigations are particularly useful for the study of shorter-term processes. Although limited in scale and timeframe, laboratory experiments have the advantage that they can help to identify the key geochemical reactions on actual rock material under actual reservoir conditions, which is very important as such reactions are known to be highly site-specific. They are also helpful to test the ability of geochemical codes to reproduce the experimental observations before using them to make long term predictions over experimental timescales up to thousands of years.