The most commonly used method for controlling water loss in cement slurry is to add water-soluble polymer fluid loss reducing agents. The role of polymer molecules is mainly manifested in the following aspects:
(1) Adsorption dispersion effect
After adding high molecular weight loss reducing agents to cement slurry, the polar groups on these polymer chains adsorb on the surface of cement particles (ionic polymers rely on electrostatic adsorption, while non-ionic polymers rely on hydrogen bonding and induction), forming an adsorption hydration layer to prevent the aggregation of cement particles and maintain a moderate distribution of particles. Meanwhile, according to the theory of diffusion double layer structure, the suspension stability of cement slurry solid particles is related to the repulsive and attractive forces between cement particles. After the adsorption of macromolecules on the surface of cement particles, it causes a change in the double layer structure, increases the potential on the surface of cement particles, and makes the electrostatic repulsion between cement particles much greater than the van der Waals attraction, thereby preventing the occurrence of flocculation and sedimentation, and making the distribution of cement particles uniform.
(2) The protective effect of polymers
The different links of AMPS polymer molecules can adsorb on cement particles, and due to the varying lengths of the molecular chains, many different sized micelle particles are formed, changing the gradation of cement particles. And through the bridging of these particles, a spatial network structure is formed throughout the entire system. Some water molecules in the cement slurry are surrounded by this spatial structure, and the stability of these bound free water depends largely on the stability of the spatial structure network. In the early stage of cement hydration, the surface of cement particles is positively charged, and polymers containing negatively charged groups such as COO - and - SO3 are added to the molecular chains. These groups can form coordination bonds with calcium ions on the surface of cement particles. The complexation effect of these coordination bonds greatly enhances the affinity of polar groups to the solid, making the water loss agent molecules adsorbed on the surface of cement particles less likely to desorb under external influence. The resulting cement slurry network structure is more stable, and the free water in the surrounding groups is not easy to precipitate, so the water loss is very small. The adhesive protective effect of the hydration layer and polymer is conducive to the formation of thin and dense filter cakes.
(3) Polymer mechanical properties
The a-bonds between atoms on linear macromolecular chains can rotate freely, allowing the relative positions of each link of the macromolecule to constantly change. This is why the flexibility of polymer chains is reflected in their high elasticity and viscoelasticity. Due to the high elasticity and viscoelasticity of the polymer adsorption layer on the surface of cement particles, a loosely structured gel aggregate is formed in the cement slurry, which stably wedges into the filter cake under pressure difference, resulting in pore blockage in the filter cake and good compressibility. Its permeability further decreases, making it denser and more tough.
(4) Increase the viscosity of the slurry
After adding polymer water loss reducing agents to cement slurry, some functional groups in the molecules can be adsorbed on the surface of cement particles, while the remaining functional groups can combine with water to form adsorbed water. The adsorbed water, together with the adsorbed hydration film on the surface of cement particles, forms a high viscosity and high elasticity adsorbed water network structure. This adsorbed water network has higher viscosity and flow resistance than free water, which can prevent cement particle aggregation, maintain appropriate particle size distribution, reduce the water loss rate of cement slurry, and facilitate the reduction of water loss.
In short, water-soluble polymer fluid loss agents can adsorb onto the surface of cement particles through polymer chains, causing cement particles to bridge and form a network structure, binding more free liquids, and achieving the goal of water loss reduction; The flexibility of linear polymer chains also endows the filter cake with high elasticity and viscoelasticity, forming a gel like aggregate wedged into the microstructure of cement filter cake, effectively reducing the permeability of the filter cake. Therefore, it can be considered that the synergistic effect of adsorption and gelation is the main mechanism of action of polymeric fluid loss agents.