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It is possible to make slurries ranging in density from 4 to 18 lbm/gal using foamed cement. Foamed cement is a mixture of cement slurry, foaming agents, and a gas. Foamed cement is created when a gas, usually nitrogen, is injected at high pressure into a base slurry that incorporates a foaming agent and foam stabilizer.
Use of nitrogen
Nitrogen gas can be considered inert, and does not react with or modify the cement-hydration-product formation. Under special circumstances, compressed air can be used instead of nitrogen to create foamed cement. In general, because of the pressures, rates, and gas volumes involved, nitrogen-pumping equipment provides a more reliable gas supply. The process forms an extremely stable, lightweight slurry that looks like gray shaving foam. When foamed slurries are properly mixed and sheared, they contain tiny, discrete bubbles that will not coalesce or migrate. Because the bubbles that form are not interconnected, they form a low-density cement matrix with low permeability and relatively high strength.
Candidates for foam cementing
Advanatages of foamed cementing
Although its design and execution can be more complex than standard jobs, foamed cement has many advantages that can overcome these concerns, because it:
- Is lightweight
- Provides excellent strength-to-density ratio
- Is ductile
- Enhances mud removal
- Helps prevent gas migration
- Improves zonal isolation
- Imparts fluid-loss control
- Is applicable for squeezing and plugging
- Stabilizes at high temperatures
- Is compatible with non-Portland cements
- Simplifies admix logistics
- Enhances volume
- Has low permeability
- Is stable to crossflows
- Forms a synergistic effect with some additives, which enhances the property of the additive
Disadvantges of foamed cementing
The disadvantage of foamed cement is the need for specialized cementing equipment both for field application and for laboratory testing.