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Cement slurry accelerators
Accelerators speed up or shorten the reaction time required for a cement slurry to become a hardened mass. In the case of oilfield cement slurries, this indicates a reduction in thickening time and/or an increase in the rate of compressive-strength development of the slurry.
Types of accelerators
Acceleration is particularly beneficial in cases where a low-density (e.g., high-water-content) cement slurry is required or where low-temperature formations are encountered.
Calcium cloride (CaCl2)
Of the chloride salts, CaCl2 is the most widely used, and in most applications, it is also the most economical. The exception is when water-soluble polymers such as fluid-loss-control agents are used. The major benefits of the use of CaCl2 are the significant reduction in thickening time achieved and that, regardless of concentration, it always acts as an accelerator. The normal concentration range of use for CaCl2 is 1 to 4% by weight of cement (BWOC). Above a concentration of 6% BWOC, the results will become unpredictable and gelation can occur.
Potassium chloride (KCl)
The acceleration performance of KCl is similar to that of NaCl. KCl has two advantages over other accelerators:
- Its stabilizing effect on shale or active clay-containing formations
- Its minimal effect on the performance of fluid-loss additives
As an accelerator, KCl may be used at concentrations up to 5% BWOW; for formation stabilization, concentrations of 3% BWOW are effective.
Sodium silicate (Na2SiO3)
Sodium silicate is normally considered to be a chemical extender, although it is also functional as an accelerator. The effectiveness depends on the concentration and molecular weight. The low-molecular-weight form may be used at concentrations of 1% BWOC or less to accelerate normal-density slurries. The high-molecular-weight form is an effective accelerator at concentrations up to 4% BWOC. Sodium-meta-silicate also provides excellent lost-circulation control when used with cement or CaCl2 brines.
Seawater. Seawater is a naturally occurring mixture of alkali chloride salts, including magnesium chloride. The composition of seawater varies widely around the world. For example, the equivalent chloride salt content can vary from 2.7 to 3.8% BWOW.
Alkali hydroxides[Ca(OH)2, NaOH]
Alkali hydroxides are commonly used in pozzolan-extended cements. They accelerate both the pozzolanic and the cement component by altering the aqueous chemistry.
Mono-calcium aluminate (CaO.Al2O3 = CA)
Calcium aluminate is used as an accelerator in pozzolan- and gypsum-extended cements.