Extenders

(Please select the product you want by clicking on the menu to the left)

    Cement extenders reduce slurry density and lower hydrostatic pressure during cementing operations. This helps prevent the breakdown of weak formations and loss of circulation. They also reduce the amount of cement needed for the cementing operation. Because they are less expensive than cement, they bring considerable savings. Three types of extenders are water extenders, low-density aggregates and gas. Often more than one type is used in the same slurry.

    Water extenders allow the addition of water to the slurry while ensuring that solids remain in suspension. The most common is bentonite, a clay mineral that has the unusual property of expanding several times its original volume when placed in water. This increases the slurry's viscosity and its ability to suspend solids. Bentonite is added in concentrations as high as 20% BWOC. Slurry density quickly decreases with bentonite concentration. However, there is a price to be paid in terms of compressive strength. Another water extender is sodium silicate. This reacts with the calcium hydroxide in the cement slurry to produce a viscous C-S-H gel allowing large volumes of water to be added to the slurry.

    Extenders such as bentonite, foamed cement and microspheres decrease the cement slurry density are used to cut down on cementing costs and protect against the breakdown of weak formations. But they also decrease final compressive strength. The bentonite and foam data were obtained on cement cured at 100°F (38°C), and the microsphere data on cement cured at 80°F (27°C). Curing time in all cases was 24 hr.

    Low-density aggregates are materials of density less than that of Portland cement, which is 3.15 g/cm3. The most commonly used are pozzolans, finely-divided siliceous and aluminous materials. They are obtained from volcanic ash, diatomaceous earth and fly ash from coal-burning power stations. Pozzolans not only reduce cement-slurry density, but also increase its compressive strength by reacting with the calcium hydroxide in the slurry.

    At present, the most efficient low-density aggregates are microspheres, small gas-filled beads with specific gravities as low as 0.2. Since they are lighter than water, slurry density is substantially reduced without adding large quantities of water. As a result, compressive strength is preserved. The improvement is dramatic when compared with bentonite.

    Gases such as nitrogen or sometimes air are used to prepare foamed cement with exceptionally low density. As with microspheres, using an extender requires no additional water.