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Basic Knowledge
Low density cement slurry system
  • Detailed description
Usually the following situations will use low density cementing slurry: 1) low pressure oil gas layer or leakage layer, to prevent sealed oil and gas flowing into the stratum; 2) Some developed formation often only just exploit not note, and that will cause formation pressure decrease, leaking when cementing and the slurry returns height is not enough; 3) Some oil and gas wells which exist severely corrosive water or gas requested seal the well completely. That can increase the oil and gas well cementing performance and protect the casing from corrosion; 4) long seal section cementing.
At present, the low density of slurry system used in the oil well cementing basically has the following kinds
Cenosphere low density cement, the reason of cenosphere can reduce the density of cement slurry is the cenosphere’s density is lower than the cement’s and the cenosphere also has active. It can replace part of cement. The density of cement slurry prepared can reach from 1.44 to 1.08 g/cm3. Note that using 4000 rpm low speed to mix the slurry to prevent the cenosphere broken. Because the major components of the cenosphere are SiO2 and Al2O3, and has large activity at higher temperatures which is helpful for increasing strength, so cenosphere low density cement can be used in the middle and high depth well cementing. But the cost of cenosphere low density is high. Even so, as low density cement, cenosphere remains the preferred materials.
Bentonite low density cement mainly use increasing water to realize low density. But the set cement’ strength will decrease if the bentonite’s dosage increases. And at the same time, the permeability will increase. So the most appropriate mount of bentonite is from 8 %to 10%. The bentonite low density cement slurry’s density is 1.53-1.58g/cm3, and the application temperature is from 40 to 100℃. At the same time, it is wide supple, low cost and easy to use.
Fly ash low density cement is depend on the fly ash itself density. The fly ash low density is lower than cement density, and it has high activity so that it can replace part of the cement. Usually the dosage of fly ash is about 60% -120%, the corresponding slurry density is 1.68-1.55 g/cm3. The ash low density cement mixed with accelerator has the characters such as adjustable thickening time, high strength and little free fluid ect. It can be used in oil layer casing cementing operation and the use of fly ash belongs to utilization of three wastes, therefore it has a low cost, and supply wide advantages.
The slag low density cement. The slag’s density is lighter, and its density is 2.6 ~ 2.8 g/cm3. Without other lighting agents, the slag can makes the low density slurry of 1.45 g/cm3 or above. When mixed with part of lighting agent such as cenosphere, a good performance of 1.20 ~ 1.50 g/cmlow density slurry can be prepared. But because of the slag decrease the cement density by increasing water cement ratio, therefore generally this kind of cement used only for sealing non-target-zone cementing operation.