When the temperature, pressure and permeability are constant, the main factors affecting the water loss of cement slurry are: (1) the shape and size of cement particles; (2) the viscosity of liquid;

(3) Charge relationship between cement particles and liquid medium components. There are generally four methods to reduce the water loss of cement slurry: (1) reduce the water cement ratio of cement slurry; (2) Reduce the particle size of cement particles:

(3) Adding clay or other particulate materials 10-100% smaller than cement particles; (4) Add natural or synthetic polymer fluid loss reducer.

Fluid loss additive is a kind of additive that can control the filtration rate of cement slurry to maintain its water solid ratio unchanged.

There are three main purposes of using fluid loss reducer. First, ensure safe construction; Second, improve cementing quality; Third, to protect the oil and gas reservoir, the fluid loss agent can reduce the water loss by controlling the filtration rate of the water in the cement slurry into the adjacent permeable formation. Its main functions include two aspects

(1) When the cement slurry is filtrated to the formation, the cement filter cake is formed on the formation surface. The function of fluid loss reducer is to improve the structure of filter cake to form a compact filter cake with low permeability, so as to reduce water loss;

(2) Common polymer fluid loss reducer can increase the viscosity of cement slurry filtrate, increase the resistance to formation filtration, and reduce the water loss of cement slurry.

The methods to reduce the water loss of the cement slurry by reducing the water cement ratio of the cement slurry, reducing the particle size of the cement particles and adding clay or other particulate materials that are 10-100% smaller than the cement particles are all to increase the suspension of the cement particles, that is, to increase the viscosity of the cement slurry, which makes it difficult to dehydrate the cement slurry, but the liquid phase flow resistance increases, which makes it difficult to pump the cement. Polymer fluid loss reducer is superior to the above three methods because the fluid viscosity and the charge between cement particles and fluid medium components can be adjusted by adding fluid loss reducer. The polymer synthesized by the structure can be designed in advance, and the appropriate oil well cement can meet the needs of the shape and size of the cement particles. Therefore, when other factors are certain, adding the polymer fluid loss reducer can significantly reduce the fluid loss of the cement slurry, which has the following five meanings:

(1) It is helpful to prevent early dehydration of cement slurry in permeable formation, especially in the case of tail pipe cement injection or irregular well diameter, premature dehydration will lead to annulus blockage and failure to complete cementing operation;

(2) Prevent oil and gas layer pollution, which is conducive to improving crude oil recovery next

(3) Enhance the water retaining function, prevent the rheological property of cement slurry from deteriorating, and improve the displacement efficiency;

(4) Effectively prevent annular air channeling and improve cementing quality;

(5) Protect water sensitive formation and prevent well diameter expansion and collapse.