1. Research status of action mechanism of fluid loss additive

At present, the exact mechanism of fluid loss additive is not known in our country and abroad, but several processes are recognized. Once the water loss begins, it will precipitate on the well wall to form cement solid filter cake. Fluid loss additive can reduce fluid loss by reducing the permeability of filter cake and increasing the viscosity of liquid phase. At present, acceptable loss of water control mechanisms include:

(1) The particle size distribution of cement slurry is improved, so as to improve the permeability of filter cake. One factor that has a great influence on water loss is the dispersion of cement particles in cement slurry. It has proved difficult to control water loss unless a dispersant is used. This is because when dispersing cement particles, the dispersant also makes them gather more closely in the filter cake, reducing the permeability of the filter cake. According to Darcy's law, the filtration rate decreases. Therefore, we believe that some fluid loss additives can improve the particle size distribution of cement slurry and play the role of dispersant, so as to achieve the purpose of water loss reduction.

(2) Increase the viscosity of cement slurry phase, which also has an adverse impact on the cement slurry itself. According to Darcy's law, the filtration rate will decrease with the increase of filtrate viscosity.

(3) The polymer fluid loss additive forms an impermeable membrane at the interface between the filter cake and the porous filter medium.

(4) For the mechanism of water loss reduction of synthetic polymers, there are also such assumptions: synthetic polymer molecules are adsorbed on cement particles, and molecular links and tails extend into the surrounding solution. When the cement particles are bound in the filter cake, their aggregation density is much higher than that in the cement slurry. Under this condition, the molecular links are twisted with each other to form a complex polymer network to prevent filtrate filtration.

In fact, scholars of our country and abroad have many different understandings on the mechanism of water loss reduction:

Ma Xiping believes that fluid loss additive mainly reduces water loss by improving the quality of filter cake and increasing the viscosity of filtrate, and the quality of mud cake plays a leading role mainly depends on the dispersion and gradation of particles. Anionic polyelectrolytes containing amide hydrophilic groups and carbonyl adsorption groups can adsorb with positively charged cement particles in aqueous solution to form a grid structure. At the same time, due to the hydration of hydration groups, a firm and thick hydration layer is formed to disperse the particles. If the macromolecular chain of the polymer forms a grid structure with the cement particles, it can avoid the contact of the cement particles, prevent the cement particles from coalescence, change its gradation and form a dense mud cake, so as to reduce the water loss. The adsorption group and hydrophilic group on the fluid loss additive molecule should also have an appropriate ratio (i.e. molecular weight, monomer ratio and distribution) in order to make the cement particles have an appropriate gradation and hydration layer, so as to reduce the water loss.

Chen Li et al. Studied the action mechanism of copolymer precipitation agent with acrylamide, acrylic acid and acrylonitrile as the main body, and considered that it mainly includes two aspects: the synergistic effect of functional groups and the improvement of filter cake quality. In alkaline cement slurry, some amide groups (- CONH)) are hydrolyzed into carboxyl groups (- co0h). The original carboxyl groups and newly formed carboxyl groups are dissociated into anionic groups (- co0 I). They can form coordination bonds with calcium ions on the surface of cement particles through carboxyl oxygen. The complexation of this coordination bond greatly enhances the affinity of polar groups. Therefore, the fluid loss additive molecules adsorbed on the surface of cement particles are not easy to desorb when affected by the outside world, the formed cement slurry network structure is relatively stable, and the trapped free water is not easy to release, so the water loss is very small. Based on the above analysis, the introduction of strong hydrophilic groups similar to carboxyl groups into the polymer fluid loss additive molecules is conducive to reducing the water loss of cement slurry. It is inferred that if the polymer containing a large number of carboxyl groups is introduced into this kind of fluid loss additive, it may have a better water loss reduction effect. However, if the carboxyl content is too much, the adsorption on the surface of cement particles will be too strong. In addition, the wrapping effect of macromolecular chain on cement particles may cause the super retarding phenomenon of cement slurry. In addition, if the proportion of adsorption groups on the molecular chain of fluid loss additive is too large, the molecular chain is close to the surface of cement particles, the bridging effect between cement particles is weakened, and the water loss of cement slurry will increase. The three-dimensional spatial network structure of the cement slurry system of fluid loss additive not only hinders the coalescence between cement particles to a certain extent, but also hinders the settlement of large particles and the thermal movement of small particles, so that the cement particles can be well dispersed, and the large and small particles are evenly distributed in the whole condensed network structure . Anionic macromolecular electrolytes, when ionized in cement slurry, release a large number of low valence cations and become complex high valence anions. This kind of high valence anion is adsorbed on the surface of cement particles, so that the positive charge on the surface changes from positive to negative. With the increase of adsorption capacity, the absolute value of charge increases. The repulsion force of the same charge between particles is conducive to the uniform distribution of particles with different particle sizes. The formation of filter cake is the result of water loss due to the compression of the cement grid structure close to the well wall or filter screen. In the formed filter cake, small particles are embedded in the pore channel between large particles, and the gap between large and small particles is affected by the physical blockage of macromolecules firmly adsorbed in the gap through chemical bonds and the liquid resistance effect of water molecules adsorbed by macromolecular chains in the pores. Therefore, the permeability of filter cake is very low.

If there is no firm adsorption of water molecules by macromolecules in the gap, the water molecules in the filter cake will not be stably retained in the pores, and the water molecules will push through the microporous channel under the action of differential pressure to form the filtrate "jet" water loss. The scanning electron micrograph observation of the cement filter cake formed immediately after the addition of fluid loss additive shows that this pore blocking effect can keep the water in the cement filter cake until the cement slurry solidifies, so as to maintain a very low permeability of the cement filter cake.