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Accelerators
Accelerators are chemicals that reduce the setting time of cement and increase the rate of compressive strength development. Accelerators are often used in shallow or cool wells to speed up the set of cement so less rig time is spent waiting on the cement to set.
Most operators wait for cement to reach a minimum of 500 psi compressive strength before resuming operations, but at temperatures below 100 °F neat cement may require a day or two to develop 500 psi strength. Accelerators reduce the waiting-on-cement (WOC) time. They can also counteract the effects of other additives employed for other purposes that concurrently cause a delay in setting time.
Commonly-used Accelerators
The most commonly used accelerator is calcium chloride (CaCl2) added at concentrations between 2% and 4% by weight of cement (BWOC). Its popularity is due to the fact that it is both efficient and economical.
For example, at 91 °F the addition of 4% BWOC CaCl2 reduces setting time from 4 hours to 1 hour and 15 minutes (Table 1). As the cement sets faster, compressive strength also builds more quickly, reaching 1,320 psi after 6 hours versus only 370 psi without the CaCl2, at 100 °F.
| Table 1: Effects of adding CaCl2 to neat cement (Nelson and Guillot 2006) | |||||
|---|---|---|---|---|---|
(% BWOC) | Thickening Time (hr:mm) | Compressive Strength (psi) | |||
| 91 °F (33 °C) | 113 °F (45 °C) | 100 °F (38 °C) | |||
| 6 hr | 12 hr | 24 hr | |||
| 0 | 4:00 | 2:32 | 370 | 840 | 1780 |
| 2 | 1:17 | 1:01 | 1110 | 2370 | 3950 |
| 4 | 1:15 | 0:59 | 1320 | 2560 | 4450 |
Another accelerator option, sodium chloride (NaCl), affects setting time and compressive strength development in different ways depending on concentration and temperature:
- Accelerator at concentrations up to 15% by weight of mix water (BWOW)
- No effect at concentrations between 15% and 20%
- Retarder (slows thickening time) at concentrations above 20%
When seawater is used as mix water at offshore locations, its NaCl content (about 2.5%) results in acceleration of setting time.
There are also a number of chloride-free accelerators for special situations, including calcium formate, calcium nitrate in conjunction with triethanolamine, and thiocyanate salts. However, these do not generally perform as well as calcium chloride. A complex mixture of calcium nitrate and four other compounds is marketed as an accelerator for deep water and arctic permafrost wells because it can out-perform calcium chloride at low temperatures between 40 °F and 70 °F.
