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Basic Knowledge
7.6 Salt resistant cement slurry
  • Detailed description
The NaCl as the oil well cement additive has been widely used in cementing project. It is used as early strength accelerator to balance electrolyte and prevent formation stratum from collapsing and the dissolution of stratum. In the area of lack of fresh water resources, the seawater is use directly to prepare the slurry. According to the salt content, the slurry can be divided into fresh water, salt water and saturated salt water cement slurry. The research about the salt resistant cement slurry additive at home and abroad mainly focuses on the fluid loss additive. This is because the properties and dissolution of polymer in the general fluid loss additive will have great change in the salt water. Seriously and even it will have nothing to prevent fluid loss or influence the other performance of slurry.
The influence of the salt to the cement
1) the influence of the salt to the thickening time: 1. under the temperature of 52℃, if the dosage of salt is less than 7.5%, the thickening time will become shorter; If the dosage of the salt is larger than 10%, the thickening time will increase along with the increasing of the dosage. 2. when the temperature is 62℃, the influence of the salt to the thickening time is not obvious but the trend is the same.
2) The influence of the salt to the thickening performance. The cement slurry with salt not only will appear the phenomenon of short thickening time with small dosage and long thickening time with big dosage, and the initial consistency  becomes large, bubbles increases. At the same time, it is easy to produce flash set, bulge and the curve become worse easily at thickening- time experiment. These phenomenons for cementing operation are harmful or even dangerous.
3) The influences of the salt to the fluid loss performance. Along with the increasing of salt content in the cement slurry, the volume of fluid loss also increases. Many fluid loss additives used in the fresh water can’t reduce the volume of fluid loss with the affect of salt. Such as the PVA series fluid loss additive.
4) The influence of the salt to the set cement. The test shows that: under the condition of 60℃ and normal pressure, when the dosage of salt is lower than 5%, there is a obvious early strength effect, and when the dosage is higher than 10%, the strength of set cement with salt is lower than that of neat set cement. To the neat cement slurry, the strength increases quickly after thickening. The strength of the slurry with salt develops more slowly than the neat slurry.
Against the above, a lot of salt resistant additives are developed in domestic. Such as the kelioil company’s AMPS fluid loss additive CG610 series. They can be used in fresh water, half-saturated salt water and saturated salt water and have a good control to the fluid loss. At the same time, they have no affect to the other properties of the cement slurry.