The thickening time of a cement slurry is the time during which the cement slurry can be pumped and displaced into the annulus (i.e. the slurry is pumpable during this time). The slurry should have sufficient thickening time to allow it to be:
• Pumped into the casing
• Displaced by drilling fluid until it is in the required place
Generally 2 – 3 hours thickening time is enough to allow the above operations to be completed. This also allows enough time for any delays and interruptions in the cementing operation. The thickening time that is required for a particular operation will be carefully selected so that the following operational issues are satisfied:
• The cement slurry does not set whilst it is being pumped
• The cement slurry is not sitting in position as a slurry for long periods, potentially being contaminated by the formation fluids or other contaminants
• The rig is not waiting on cement for long periods.
Wellbore conditions have a significant effect on thickening time. An increase in temperature, pressure or fluid loss will each reduce the thickening time and these conditions will be simulated when the cement slurry is being formulated and tested in the laboratory before the operation is performed.