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Water insoluble materials in cementing
Bentonite is not typically used as the primary fluid-loss agent in normal-density slurries. In low-density slurries, where higher concentrations can be used, it may provide sufficient fluid-loss control (400 to 700 cm 3 /30 min) for safe placement in noncritical well applications. Fluid-loss control, obtained through the use of bentonite, is achieved by the reduction of filter-cake permeability by pore-throat bridging. Fluid-loss rates can be erratic, because of:
- The concentration of use at a given density
- Variations in platelet disassociation caused by shear
- Stacking arrangement in the filter cake
Microsilica imparts a degree of fluid-loss control to cement slurries because of its small particle size of less than 5 microns. The small particles reduce the pore-throat volume within the cement matrix through a tighter packing arrangement, resulting in a reduction of filter-cake permeability.
PolyVinyl alcohol (PVA)
PVA is a white to cream-colored powder with a density range of 1.27 to 1.31 g/cm3 . It is a water-soluble polymer derived from polyvinyl acetate, and is chemically reactive with acids and alkalis. It is not listed in the water-soluble polymers section, because it loses solubility in alkaline environments such as the aqueous phase of a cement slurry. PVA also provides gas-migration control, and enhances cement bonding and acid resistance.
This is an oil-in-water emulsion system consisting of a dispersed phase of a water-insoluble elastomer, surfactants, and a water exterior phase. These emulsions are characterized by their milky-white appearance. Their density is typically approximately 1 g/cm3 . The most common emulsion used is styrene-butadiene rubber (SBR), which provides:
- Exceptionally low fluid-loss control
- Gas-migration control
- Acid-solubility resistance
The surfactant system plays a key role in the use of latex in well-cementing applications. In cement slurries, the emulsion system readily disperses and exhibits time-, shear-, and temperature-dependent stability. The emulsion stability can be improved by the addition of additional surfactant, and the emulsion stability may be controlled to above 300°F (149°C) BHCT, depending on the surfactant type and concentration. The surfactant system also acts as a dispersant in the cement slurry, resulting in low slurry viscosity. Control of emulsion stability is critical to slurry performance, because the rate of inversion of the emulsion controls slurry viscosity and thickening time. Inversion of the emulsion system results in an almost instantaneous conversion to a rubberized mass (set) that is reported as the pumping time for the slurry.
Latex is typically used at a concentration of ≥ 0.8 gal/sk (~ 3.5% BWOC dry-weight equivalent) to obtain a fluid loss of less than 100 cm3 /30 min. Fluid-loss values of less than 20 cm 3 /30 min are possible at 1.5 gal/sk in nonsilica slurries and 2 to 3 gal/sk with 35% BWOC silica slurries. Fluid loss is controlled in the cement slurry by particle plugging.