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Basic Knowledge
7.2 High density cement slurry system
  • Detailed description

In the oil gas cementing, high porosity pressure, unstable well wall and the plastic following formation are all need to be controlled with the help of high hydrostatic pressure. So that has high requirement to the slurry density. In order to keep the well stable, increasing the slurry density is necessary.
Decreasing the water cement ratio can increase the cement slurry density, but that can make the fluidity of cement slurry becoming worse. The highest density of this method can prepared is 2.16 g/cm3. When higher slurry density is needed, high specific gravity materials must be added. The selection scope of high specific gravity materials is not very wide, and the most commonly used cement weighting agent is titanium iron ore, hematite, barite powder, etc.


Several weighting agent materials performances comparison

Weighting agent

Appearance

Density (g/cm3

Fineness

Affection to cement slurry

Density of prepared cement slurry

Barite

White powder

4.1~4.4

<75μm

Increased, larger water, thickening

2.28 g/cm3

Hematite

Dark red powder

4.8~5.2

<75μm

Increased, less water,slightly thickening

2.60 g/cm3

Titanium iron ore

Dark particle

4.4~4.5

<75μm

Increased, less water

2.40 g/cm3

The design rule of high density cement slurry
1) Fluid loss property: For the use of the weighting agent materials, the fluid loss of slurry is generally required less than 100 ml.
2) The stability of slurry settlement: the big difference density among the materials is easy to make the weighting agent materials segregation in the slurry. The slurry is designed to add some suspension at it. The suspension is influenced by temperature seriously, and that lead to the settlement stability becomes poor with the increasing of the temperature.
The workability of cement slurry: The weighted cement slurry with high density can make the liquidity decreasing, so the dispersant is needed to adjust the slurry liquidity.
The slurry free fluid control: the weighted cement slurry has high requirements to the content of free fluid. So it needs to decrease the content of free fluid as soon as possible when designing.
When designing the high density cement, it need not only consider the workability property but also give consideration to the slurry stability. But to the fluid loss, free fluid, compressive strength performances, the design rule is the same with normal cement slurry. Note that the silicon powder is needed to join to prevent strength degradation when using high density of slurry under high temperature. Because the silicon powder density is lower than cement density, it is need to increase the weighting agent materials. So the gelled materials content decrease in the cement slurry, it is adverse to strength development