Recent advances in the development of lightweight CO{sub 2}-resistant well cements

Regardless of the properties of the cured material, the practical use of advanced cementitious materials for geothermal well completions depends primarily upon the ability to predictably extend the thickening times for the precursor slurries at elevated temperatures to times sufficient to permit conventional placement. Ongoing work at Brookhaven National Laboratory being conducted with assistance from the geothermal industry, indicates that lightweight calcium phosphate cement slurries being developed as CO{sub 2{minus}} resistant well completion materials, can be conventional pumped at temperatures > 100{degrees}C without the addition of retarding admixtures. These slurries consist of mullite-shelled hollow microspheres, calcium aluminate cements and polybasic sodium phosphate. When cured in hydrothermal environments, the slurries yield high strength, low permeability cements which bond well to steel casing. Two compositional factors that affect the thickening times were identified. One is the calcium aluminate cement species that serve as the base reagent in the slurry formulation, and the other is the rate of hydrothermal reaction between the microspheres and the polybasic sodium phosphate solution.