Retarders

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    Retarders inhibit hydration and delay setting, allowing sufficient time for slurry placement in deep and hot wells. The technology of retarders is well developed and several types are used. Why they work is something of an enigma, although several theories have been developed.

    The most common retarders are derived from wood pulp. They are comprised of sodium and calcium salts of lignosulfonic acids and contain some saccharides. These retarders are thought to absorb onto the initial layer of C-S-H gel, rendering it hydrophobic and prolonging the induction period. Added in concentrations of 0.1 to 1.5% BWOC, they retard hydration at temperatures up to 250°F (122°C) (see following chart). When treated with other chemicals such as borax, lignosulfonates can be used to 600°F (315°C).

    Thickening time is prolonged when retarders such as lignosulfonate are added to cement.

    Hydroxycarboxylic acids, such as gluconate and glucoheptonate salts, also retard hydration but are not used when the bottomhole temperature is below 200°F (93°C). Otherwise, thickening times become excessively long. These compounds attach themselves to calcium ions and as a result are thought to inhibit nucleation and growth of hydration products.

    Cellulose derivatives such as carboxymethyl hydroxyethyl cellulose (CMHEC) have been used for many years as cement retarders. They are generally effective to 250°F (120°C). Like the lignosulfonates, they slow hydration by rendering the C-S-H gel hydrophobic. CMHEC imparts some secondary effects such as improved fluid-loss control, which may be desirable, and higher slurry viscosity, which may be undesirable.

    A relatively new class of retarders, organophosphonates, are effective at bottomhole circulating temperatures as high as 400°F (204°C). They tend to tolerate variations in cement composition and can lower the viscosity of high-density cement slurries. Little is known about their mode of action.