Cement extenders are routinely used to accomplish one or both of the following.
Reduce slurry density: A reduction of slurry density reduces the hydrostatic pressure during cementing. This helps to prevent induced lost circulation because of the breakdown of weak formations. In addition, the number of stages required to cement any oil & gas well may be reduced.
Increase slurry yield: Extenders reduce the amount of cement required to produce a given volume of set product. This results in a greater economy.
Extenders can be classified into one of three categories, depending upon the mechanism of density reduction and/or yield increase. Often more than one type of extender is used in the same slurry.
Water extenders: Clays and various water viscosifying agents allow the addition of more water to achieve slurry extension. Such extenders maintain a homogeneous slurry and prevent the development of excessive free water.
Low-density aggregates: This varied category consists of materials with densities lower than that of Portland cement (3.20 g/cm3). The slurry density is reduced when significant quantities of such extenders are present.
Gaseous extenders: Nitrogen or air can be used to prepare foamed cement with exceptionally low densities yet sufficient compressive strength.
