Cement Additives

A neat slurry is a mixture of water and cement only. Special chemicals are often added to the slurry to achieve some desired purpose(s). These additives may fall into the following categories:

  • accelerators
  • retarders
  • density adjusters
  • dispersants
  • fluid loss additives

Some chemicals such as sand and salt have multiple beneficial effects on the cement slurry.

Accelerators. Most operators wait for cement to reach a minimum of 500 psi compressive strength before resuming operations. At temperatures below 100°F, common cement may require a day or two to develop 500-psi strengths. Accelerators are useful at reducing the amount of waiting-on-cement (WOC) time.

Low concentrations of cement accelerators, usually 2^1% by weight of cement, shorten the setting time of cement and promote rapid strength development. Calcium chloride is perhaps the most widely used chemical for this purpose. Fig. 9-11 shows the effect of 3% calcium chloride (A-7) on Class A cement. Other chemicals are often used as accelerators in addition to CaCL.

Curing Time, hr

  1. 9-11 Effect of curing time on strength at 90°F with A-7 using class A cement (Courtesy of BJ-Hughes Services)
  2. High formation temperatures associated with increased well depths necessitate the use of chemicals that retard the setting time of the cement, i.e., increase the pumping time. The chemicals provide sufficient retarding effects to allow slurry mixing and displacement into the well with a safety margin for unforeseen occurrences.

The most common retarder may be calcium lignosulfonate. Its effectiveness is limited in temperatures above 200°F. Concentrations of 0.1—1.0% are used in most slurry applications to give both predictable thickening times and compressive strengths. Amounts above \% do not add appreciably to slurry retardation.

Other retarders can be planned for the slurry design if temperatures exceed 200°F, Carboxymcthyi-hydroxyethylcellulose (CM H EC) can be used to about 240°F BHCT (bottom-hole circulating temperature). Organic acids can be used from 200-400°F, Borax added to the organic acids can be used normally from 300-500°F BHCT. Each slurry should be pilot tested.

Determining the actual bottom-hole temperature is an important aspect of selecting retarders. Isothermal gradient maps (Fig. 9-12) provide general guides in an area. Eq. 9.1 can be used with the geothermal gradients to estimate BHT:

Geothermal Gradient

Curing Time, hr

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Responses

  • Christine
    Is the above geothermal gradient?
    7 years ago

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