ABSTRACT:

When set Portland cements are exposed to high temperatures, typically above 110 to 120°C (230 to 250°F), their properties will change. This change causes a noticeable increase in porosity and a decrease of strength known as strength retrogression. This phenomenon increases with temperature and time, thus potentially impairing zonal isolation on a long term. Commonly, cement strength retrogression is controlled by the addition of silica (quartz) sand or more often silica flour pre-blended with the cement. However, in some cases, the use of pre-blended admixtures in the bulk cement is not practical. Also it is desirable to minimize the exposure of the workers mixing cement at well site to dry crystalline silica. Early in 2003 in the Norwegian sector of the North Sea, an operator and a service company chose to use a recently developed liquid additive, rather than pre-blended silica flour, to help prevent strength retrogression. This liquid strength retrogression prevention additive (LSRPA) was selected because it could provide adequate-quality slurry and was more convenient to use than dry-blended silica flour. High-quality cement slurry was mixed without problems, and 9000 litres (2378 gal) of the LSRPA was used on this job. Pressure tests performed after the cementing operation indicated excellent cement-to-pipe bonding on this perforation squeeze job. A second job was performed successfully a few weeks later, this time on a primary cementing application. This paper provides the case history details of the methods used to achieve the first successful cementing with the use of an improved LSRPA. The benefits of using an LSRPA are outlined and the conclusions provide methods for enhancing LSRPA stability

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