Effect of salt on soil moving particles

The clay minerals in the drilling fluid have negative charges on the particle surface due to the substitution of lattice, and adsorb cations to form a diffusion electric double layer. With the increasing concentration of inorganic salt cations entering the drilling fluid, the number of positive ions in the clay particle diffusion electric double layer is bound to increase, so as to compress the electric double layer, reduce the thickness of the diffusion layer and reduce the electromotive potential on the particle surface. In this case, the electrostatic repulsion between clay particles decreases, the hydration film becomes thinner, the particle dispersion decreases, and the trend of end-to-surface and end-to-end connection between particles increases. Due to the formation of flocculation structure, the viscosity, shear force and filtration loss of drilling fluid gradually increase. When the inorganic salt cation concentration increases to a certain extent, the phenomenon of compressing the electric double layer becomes more serious, and the hydration film of clay particles becomes thinner, resulting in surface to surface coalescence of clay particles and obvious reduction of dispersion. Therefore, the viscosity and shear force of drilling fluid decrease after reaching their maximum values respectively, and the filtration loss continues to rise.

Effect of salt on treatment agent

Polymer molecules can adjust mud properties and stabilize mud through adsorption on clay surface. The hydration, solubility and molecular conformation of polymer determine its adsorption characteristics and laws on the surface of clay particles. Salt greatly reduces the efficiency of polymer treatment agent through the above three aspects.

Effect of salt on hydration and solubility of treatment agent

With the increasing concentration of inorganic salt ions entering the drilling fluid, the inorganic salt counter ions can “bond” with the dissociated groups on the treatment agent molecules to release the water molecules in the hydration layer around the treatment agent molecules, so as to reduce the hydration capacity and solubility of the treatment agent. In serious cases, the molecular chain precipitates or precipitates from the solution, resulting in the decline or even failure of the treatment agent in the mud.

Effect of salt on Adsorption Properties of treating agent on clay

The presence of salt will increase the adsorption capacity of the treatment agent on clay particles. The main reasons are as follows: first, metal positive ions reduce the repulsion between clay particles and polymer molecular chain of treatment agent by reducing the negative point of clay particle surface and polymer molecular chain; Second, salt decreases the hydration and solubility of the treatment agent, and the salting out effect increases the chemical potential of the treatment agent in the solution and the tendency to escape to the solution.

Effect of salt on molecular coil size of treating agent

The treatment agent polymer of solution in water expands and fully extends the polymer chain due to electrostatic repulsion and solvation. Salt can neutralize the charge on the molecular chain, weaken the electrostatic repulsion, weaken the solvation of polymer molecules, curl and deform the morphology of polymer molecular chain, and shrink the size of coil sharply, thus affecting the regulation of polymer on mud performance.