The commonly used cationic polymer polyacrylamides are mainly organic polyquaternary ammonium salts, such as polyamine methacrylamide (CPAM cationic polyacrylamide), cationic starch, etc. It not only has the strong coalescence effect of high-valent cations, but also has the adsorption and film-forming effect of high polymers on the surface of swelling clay, so it is an ideal inhibitor. The mechanism of action of the cationic polymer treatment agent is as follows.
The adsorption of polymers on the particles in the drilling fluid is the premise of its function. When a macromolecule is adsorbed on several particles at the same time, and a particle can adsorb several macromolecules at the same time, a network structure will be formed, and this effect of cationic polymer is called bridging. When a polymer chain is adsorbed on a particle and covers it, it is called coating. Bridging and coating are two different adsorption states of polymers in drilling fluids. In the actual system, these two adsorption states cannot be strictly separated, and generally exist at the same time, but only one of them is dominant. Different adsorption states have different effects. For example, the bridging effect can easily lead to flocculation and viscosifying action, while the coating effect is beneficial to inhibiting the dispersion of drill cuttings.
When the cationic polymer mainly undergoes bridge adsorption in the drilling fluid, some fine particles will agglomerate together to form particle clusters, which is called flocculation, and the corresponding polymer is called flocculant. The formed flocs are easy to be removed by gravity settling or solid control equipment, which is beneficial to maintain the low solid phase of the drilling fluid. Therefore, flocculation is the key to achieve low solid phase and non-dispersion in drilling fluid.
According to the flocculation effect and the influence on the performance of drilling fluid, flocculants can be divided into two categories: one is full flocculant, which can flocculate drill cuttings, inferior soil and montmorillonite at the same time. For example, nonionic polymer PAM belongs to this category; the second is a selective flocculant, which only flocculates drill cuttings and inferior soil, and does not flocculate montmorillonite, such as ionic polymers PHPA and VAMA. When the flocculant can increase the viscosity of the drilling fluid, it is called a synergistic selective flocculant, and when it has little effect on the viscosity, it is called a non-synergistic selective flocculant.
Tackifiers are mostly used in low-solid and solid-free water-based drilling fluids to improve suspension and carrying capacity. The mechanisms of viscosifying effect of cationic polymer: one is that free (unadsorbed) polymer molecules can increase the viscosity of the water phase, and the other is that the network structure formed by the bridging effect of the polymer can enhance the structural viscosity of the drilling fluid. Commonly used tackifiers include PHPA with relatively high molecular weight and high-viscosity carboxymethyl cellulose (CMC).