(2) The principle of the fluid loss control agent The fluid loss control agent refers to a chemical agent that can reduce the fluid loss of the drilling fluid. Fluid loss additives are mostly water-soluble polymer compounds, which can reduce the fluid loss of drilling fluid through the following ways.
1. The role of stabilizing colloidal particles As mentioned above, the clay particles in the drilling fluid are required to have a proper size distribution, and at the same time, it is required to have more fine clay particles. The drilling fluid with a large fluid loss has more coarse particles on the one hand, and on the other hand, the fine particles have been flocculated into coarse particles, and there are fewer fine sol particles (<100pLm) in the system. The filter cake formed by this drilling fluid with more coarse particles and few fine particles is loose and has large pores, so the filtration loss is large. Fluid loss additives are polymer compounds that can dissociate negatively charged groups in water. On the one hand, it can be adsorbed on the surface of the clay to form an adsorption layer to prevent the flocculation of the clay particles; on the other hand, it can stabilize the fine particles dismantled under the action of the circulating stirring of the drilling fluid through adsorption, and no longer bind into large particles ( Figure 1-5). In this way, a sufficient amount of fine particle ratio can be ensured, so that the drilling fluid can form a thin and dense filter cake and reduce the filtration loss. This effect is also known as the protective glue effect. It is worth noting that the concentration of the fluid loss agent in the drilling fluid must be high enough to help enclose the disassembled clay particles, so that on the one hand, it will bring a higher negative charge density to the surface of the clay particles, increase the f potential, and increase the f-potential. The repulsion between large particles; on the other hand, a thicker hydration film is formed due to the hydration of the hydration group, which makes the clay particles difficult to merge and become larger. If the concentration of the added fluid loss agent is lower than the concentration required for protection, the fluid loss agent will not only have no protective effect on the colloidal particles, but will make the clay particles easier to agglomerate.
2. Increasing the viscosity of the filtrate is known from the static fluid loss equation, and the fluid loss is inversely proportional to the one-half of the filtrate viscosity of the drilling fluid. The addition of macromolecular fluid loss additives to the drilling fluid can increase the viscosity of the filtrate and reduce the fluid loss. But it is worth noting that the increase in viscosity will reduce the drilling speed. Therefore, it is generally required that fluid loss additives do not increase the viscosity greatly.
3. The pore plugging effect of fluid loss agent as a high molecular compound fluid loss agent, its molecular size is within the range of colloidal particles. Adding these treatment agents is equivalent to increasing the content of colloidal particles in the drilling fluid, and they block the filter cake. The role of pores. These macromolecules block pores in two ways. Let the diameter of the polymer random coil be d. , Pore diameter d,. If 0.3d, <d.d, the polymer cannot enter the pores, but can block the pore entrance, which is called clogging, which can also reduce fluid loss.