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Injection of carbon dioxide into depleted oil and gas reservoirs is one of the most promising solutions for its long-term capture and storage, and is thereby directly connected to global reduction in emissions of atmospheric greenhouse gases. However, carbon dioxide is a corrosive gas which makes the selection of casing, downhole equipment and cement very important in the well design. Casing and downhole equipment should be made out of steel alloys with a high chrome content or should be protected by regular injection of corrosion inhibitors in the case of steel casing pipe. The cement based on Portland cement is susceptible to corrosion in the CO2 laden fluids or in CO2 supercritical state. The corrosion process commences with the carbonisation of free Ca(OH)2 and the C-S-H gel. Upon the consumption of all available quantities of Ca(OH)2, the resulting calcium carbonate is converted into a soluble bicarbonate, which is then released from the set cement matrix. Consequently, porosity and permeability of set cement increases and its compressive strength concomitantly decreases. Thus, special types of cement should be used. These could comprise of non-Portland based cement or cement blends based on Portland cement with the addition of materials which increase set cement CO2 corrosion resistance. Cement slurries made of these special cement systems have to be optimised in order to obtain rheological and filtration properties required for their application under well conditions. This paper describes the CO2 corrosion of Portland-based set cement and gives an overview of the current knowledge of special cement systems suitable for application in the carbon dioxide rich environment. In addition, well cementing technology of CO2 injection wells is presented and illustrated with two enhanced oil recovery case studies related to a CO2 injection well in the Žutica and Ivanić oil fields. |
