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Cement slurry weighting agents
Weighting agents or heavyweight additives are used to increase slurry density for control of highly pressured wells.
Requirements for weighting agents
Weighting agents are normally required at densities greater than 17 lbm/gal where dispersants or silica is no longer effective. The main requirements for weighting agents are:
- The specific gravity is greater than the cement
- The particle size distribution is consistent
- They have a low water requirement
- They are chemically inert in the cement slurry
- They do not interfere with logging tools
Types of weighting agents
This is the most commonly used weighting agent. Hematite is a brick-red, naturally occurring mineral with a dull metallic luster. It contains approximately 70% iron. The specific gravity of hematite ranges from 4.9 to 5.3, depending on purity, and it has a Mohs hardness of approximately 6.
Ilmenite (FeO TiO2)
This is not as commonly used as hematite, although it has some advantages over hematite. Ilmenite is a black to dark brownish-black, naturally occurring mineral with a submetallic luster that contains approximately 37% iron. It resembles magnetite in appearance but has only a slightly magnetic character. The specific gravity ranges from 4.5 to 5, depending on the purity, and it has a Mohs hardness of 5 to 6.
Hausmannite is being used increasingly because of its unique properties that address many of the disadvantages encountered with the other weighting agents. Hausmannite is a dark brownish-black material that is a byproduct mineral from the processing industry. The specific gravity range or Mohs hardness has not been well established. Because of its particle size and unique wetting characteristics, the material can suspended in the mix water at up to 40 wt% with a minimum of agitation, providing a liquid weighting agent. Because the average particle size of hausmannite is much smaller than that of cement, it allows the material to fit within the cement pore matrix, displacing entrained water, resulting in a lower viscosity and significantly more-stable slurry. The main disadvantage is that it is not readily available in all geographic regions, so the additional shipping cost can make it cost-prohibitive.
Barite is not normally used in cementing as a weighting agent because of its high surface area and high water demand. It is a soft, light gray, naturally occurring nonmetallic material. The specific gravity ranges from approximately 4.0 to 4.5, depending on purity, and it has a Mohs hardness of 2.5 to 3.5.