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Cement slurry fluid-loss-control additives (FLAs)
Fluid-Loss-Control Additives (FLAs) are used to maintain a consistent fluid volume within a cement slurry to ensure that the slurry performance properties remain within an acceptable range.
Effects of water content
The variability of each of these parameters (slurry performance properties) is dependent upon the water content of the slurry. For example, if the water content is greater than intended, the following will normally occur:
- Thickening time
- Fluid loss
- Free fluid
- Permeability and porosity will be increased
- Density, viscosity, and compressive strength will be decreased
If the water content is less than intended, the opposite will normally occur. The magnitude of change is directly related to the amount of fluid lost from the slurry.
Because predictability of performance is typically the most important parameter in a cementing operation, considerable attention has been paid to mechanical control of slurry density during the mixing of the slurry to assure reproducibility. Of equivalent importance is the slurry density during displacement, which is directly related to fluid-loss control.
Cement slurries are colloidal suspensions consisting of distinct solid and liquid phases. During the cementing operation, there are several opportunities for the fluid phase to separate from the cement slurry. This can occur when the slurry is passing through small orifices or ports, and within the annulus. When the slurry is passing through orifices, the fluid phase can be accelerated, resulting in particle bridging. In a wellbore annulus, fluid can be displaced from the slurry while it is passing though constricted areas, or to the formation, resulting in an increase in the equivalent circulating density (ECD), which can lead to formation fracture (lost circulation) or flash set (dehydration). After placement, the fluid phase will filter to permeable formations, resulting in a reduction in the slurry volume and effective hydrostatic pressure, creating the potential for the migration of formation fluid into and through the cement column. FLAs are, therefore, used to prevent solids segregation during placement, and to control the rate of fluid leakoff in the static state.
Factors affecting the use of fluid-loss-control additives (FLA)
Neat cement slurries normally exhibit an uncontrolled API fluid loss of at least 1,500 cm3/30 min. This value is excessive for most cementing operations, where permeable formations are encountered or where long columns of cement will be used. The amount of fluid-loss control required for a particular operation varies widely, and is largely dependent upon:
- Slurry density
- Water content
- Formation properties
- Annular clearance
Fluid-loss-control additives (FLA) materials
Several materials are effective as FLAs. The materials that are currently in use can be loosely categorized in two groups according to their solubility characteristics:
With the exception of bentonite, the water-insoluble materials are polymer resins. All of the water-insoluble materials function as permeability reducers.
The water-soluble materials are modified natural polymers, cellulosics, and vinylinic-based polymers. The polymeric materials, whether water-insoluble or -soluble, are all synthetic (manmade) materials.
The action of FLAs depends on their solubility. The water-insolubles function by reducing the permeability of the filter cake developed.