Accelerators are added to the cement slurry to shorten the time taken for the cement to set. These are used when the setting time for the cement would be much longer than that required to mix and place the slurry, and the drilling rig would incur WOC time. Accelerators are especially important in shallow wells where temperatures are low and therefore the slurry may take a long time to set. In deeper wells the higher temperatures promote the setting process, and accelerators may not be necessary. The most common types of accelerator are:
• Calcium chloride (CaCl2) 1.5 – 2.0%
• Sodium chloride (NaCl) 2.0 – 2.5%
It should be noted that at higher concentrations these additives will act as retarders.