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Cement slurry retarders
The commonly used cements in well applications are API Class A, C, G, and H. These cements, as produced in accordance with API Spec. 10A do not have a sufficiently long fluid life (thickening time) for well applications above 38°C (100°F) Bottom Hole Circulating Temperature (BHCT). To extend the thickening time beyond that obtained with a neat (cement and water without additives or minerals) API-class cement slurry, additives known as retarders are required.
Types of retarders
Of the chemical compounds that have been identified as retarders, lignosulfonates are the most widely used. A lignosulfonate is a metallic sulfonate salt derived from the lignin recovered from processing wood waste. The common lignosulfonates are calcium and sodium lignosulfonate.
Three grades of lignosulfonate are available for the retardation of cement slurries. Each grade is available as calcium/sodium or sodium salts. The three grades are:
The filtered grade calcium or sodium salt is typically used at a temperature of 200°F BHCT or less at a concentration of 0.6% BWOC or less. It may be used at higher temperatures but will normally be limited by economic considerations.
The purified grade represents a class of lignosulfonates in which the sugar content has been reduced. The calcium/sodium salt is typically used at a BHCT of 200°F or lower and at a concentration of 0.5% BWOC or less.
The modified grade represents lignosulfonates that have been blended or reacted with a second component. The compounds most commonly used as blend components are boric acid and the hydroxycarboxylic acids, or their salts. Blended materials are available as calcium or sodium salts. The modified lignosulfonates are typically used at a BHCT of 200°F or above. They are more effective than the purified grade at temperatures greater than 250°F. The advantages, whether a blend or reacted product, are their improved high-temperature stability above 300°F BHCT, increased dispersing activity, and synergism with fluid-loss additives.
Two cellulose polymers are used in well-cementing applications. They are hydroxyethyl cellulose (HEC) and carboxymethyl hydroxyethyl cellulose (CMHEC). HEC is commonly considered as a fluid-loss additive. Although as a possible option, it is worth noting that at BHCT of 125°F or less, the thickening time can be extended by approximately two hours in a freshwater slurry. Traditionally, the only cellulose that is considered as a retarder is CMHEC. This is largely because it is functional as a retarder up to approximately 230°F BHCT at the same concentrations as calcium lignosulfonate, but it also provides good fluid-loss control.
The hydroxycarboxylic acids are well known for their antioxidant and sequestering properties that benefit cement-slurry performance. The antioxidant property improves the temperature stability of soluble compounds such as fluid-loss additives. Commonly used hydroxycarboxylic acids and their derivatives are:
- Citric acid
- Tartaric acid
- Gluconic acid
The commonly used hydroxycarboxylic acids are generally derived from naturally occurring sugars.
Organophosphonates, with a few exceptions, are the most powerful retarders used in cement. These materials are not widely used in well-cementing applications because of the low concentration required, difficulty of accurate measurement, and sensitivity to concentration. The advantage of organophosphate retarders is their effectiveness in ultrahigh-temperature wells (> 450°F) or in applications where extended thickening times of 24 hours or greater are desired.
The term synthetic retarder is a misnomer in that the previously mentioned retarding compounds are all, in effect, man-made. However, the term synthetic retarder has been applied to a family of low-molecular-weight copolymers. These retarders are based on the same function groups as those of conventional retarders (e.g., sulfonate, carboxylic acid, or an aromatic compound). Two common synthetic retarders are:
- Maleic anhydride
- 2-Acrylamido-2-methylpropanesulfonic acid (AMPS) copolymers.
The retardation mechanism of inorganic compounds on cement hydration is different from that for the previously discussed retarders. Inorganic compounds, commonly used as cement retarders, are borax (Na2B4O7•10H2) and other borates such as boric acid (H3BO3) and its sodium salt and zinc oxide (ZnO).
Borates are commonly used as a retarder aid for high-temperature retarders at BHCT of 300°F (149°C) and greater. At higher temperatures, the borate is a less-powerful retarder than at lower temperatures; however, it exerts a synergistic effect with other retarders such as lignosulfonates, whereby the combination provides better retardation than either retarder alone. ZnO is a strong retarder when used alone, and is normally used for the retardation of chemically extended cements.
Salt as a retarder
Water containing salt concentrations of greater than 20% BWOW has a retarding effect on cement. The gelation is evident in the thickening-time viscosity profile of saturated salt slurries by a sudden increase in Bearden units of consistency that then levels off before set. Saturated salt slurries are useful for cementing through salt domes. They also help protect shale sections from sloughing and heaving during cementing, and aid in preventing annular bridging and the lost circulation that could result. Saturated salt cements are also dispersed, and salt reduces the effectiveness of fluid-loss additives.
- API Spec. 10A, Specification for Cements and Materials for Well Cementing, 23rd edition. 2002. Washington, DC: API.