Polyanionic cellulose (PAC) is mainly used as fluid loss reducer, viscosity enhancer and rheological regulator in drilling fluid. This paper briefly describes the main physical and chemical indexes of PAC, such as viscosity, rheology, substitution uniformity, purity and salt viscosity ratio, combined with the application indexes in drilling fluid.
The unique molecular structure of PAC makes it show excellent application performance in fresh water, salt water, seawater and saturated salt water. When used as a filtrate reducer in drilling fluid, PAC has efficient water loss control ability, and the mud cake formed is thin and tough. As a viscosifier, it can quickly improve the apparent viscosity, plastic viscosity and dynamic shear force of drilling fluid, and improve and control the rheology of mud. These application properties are closely related to the physical and chemical indexes of their products.
1. PAC viscosity and its application in drilling fluid
PAC viscosity is the characteristic of colloidal solution formed after dissolving in water. The rheological behavior of PAC solution has an important influence on its application. The viscosity of PAC is related to the degree of polymerization, solution concentration and temperature. Generally speaking, the higher the degree of polymerization, the higher the viscosity; The viscosity increased with the increase of PAC concentration; The solution viscosity decreases with the increase of temperature. NDJ-79 or Brookfield viscometer is usually used to test the viscosity in the physical and chemical indexes of PAC products. The viscosity of PAC products is controlled according to the application requirements. When PAC is used as tackifier or rheological regulator, high viscosity PAC is usually required (the product model is usually pac-hv, pac-r, etc.). When PAC is mainly used as fluid loss reducer and does not increase the viscosity of drilling fluid or change the rheology of drilling fluid in use, low viscosity PAC products are required (the product models are usually pac-lv and pac-l).
In practical application, the rheology of drilling fluid is related to: (1) the ability of drilling fluid to carry drilling cuttings and clean the wellbore; (2) Levitation force; (3) Stabilizing effect on shaft wall; (4) Optimization design of drilling parameters. The rheology of drilling fluid is usually tested by 6-speed rotary viscometer: 600 rpm, 300 rpm, 200 rpm, 100 rpm and 6 rpm. 3 RPM readings are used to calculate the apparent viscosity, plastic viscosity, dynamic shear force and static shear force, which reflect the rheology of PAC in drilling fluid. In the same case, the higher the viscosity of PAC, the higher the apparent viscosity and plastic viscosity, and the greater the dynamic shear force and static shear force.
In addition, there are many kinds of water-based drilling fluids (such as fresh water drilling fluid, chemical treatment drilling fluid, calcium treatment drilling fluid, saline drilling fluid, seawater drilling fluid, etc.), so the rheology of PAC in different drilling fluid systems is different. For special drilling fluid systems, there may be a large deviation in evaluating the impact on the fluidity of drilling fluid only from the viscosity index of PAC. For example, in the seawater drilling fluid system, due to the high salt content, although the product has high viscosity, the low degree of substitution of the product will lead to the low salt resistance of the product, resulting in the poor viscosity increasing effect of the product in the process of use, resulting in the low apparent viscosity, low plastic viscosity and low dynamic shear force of the drilling fluid, resulting in the poor ability of the drilling fluid to carry drilling cuttings, which may lead to sticking in serious cases.
2.Substitution degree and uniformity of PAC and its application performance in drilling fluid
The substitution degree of PAC products is usually greater than or equal to 0.9. However, due to the different needs of various manufacturers, the substitution degree of PAC products is different. In recent years, oil service companies have continuously improved the application performance requirements of PAC products, and the demand for PAC products with high degree of substitution is increasing.
The substitution degree and uniformity of PAC are closely related to the salt viscosity ratio, salt resistance and filtration loss of the product. Generally, the higher the substitution degree of PAC, the better the substitution uniformity, and the better the salt viscosity ratio, salt resistance and filtration of the product.
When PAC is dissolved in strong electrolyte inorganic salt solution, the viscosity of the solution will decrease, resulting in the so-called salt effect. The positive ions ionized by the salt and - coh2coo - The action of H2O anion group reduces (or even eliminates) the homoelectricity on the side chain of PAC molecule. Due to insufficient electrostatic repulsion force, PAC molecular chain curls and deforms, and some hydrogen bonds between molecular chains break, which destroys the original spatial structure and specifically reduces the viscosity of water.
The salt resistance of PAC is usually measured by salt viscosity ratio (SVR). When the SVR value is high, PAC shows good stability. Generally, the higher the degree of substitution and the better the uniformity of substitution, the higher the SVR value.
When PAC is used as a filtrate reducer, it can ionize into long-chain multivalent anions in drilling fluid. The hydroxyl and ether oxygen groups in its molecular chain form hydrogen bonds with oxygen on the surface of viscosity particles or form coordination bonds with Al3 + on the bond breaking edge of clay particles, so that PAC can be adsorbed on clay; The hydration of multiple sodium carboxylate groups thickens the hydration film on the surface of clay particles, prevents the aggregation of clay particles into large particles due to collision (glue protection), and multiple fine clay particles will be adsorbed on a molecular chain of PAC at the same time to form a mixed network structure covering the whole system, so as to improve the aggregation stability of viscosity particles, protect the content of particles in drilling fluid and form dense mud cake, Reduce filtration. The higher the degree of substitution of PAC products, the higher the content of sodium carboxylate, the better the uniformity of substitution, and the more uniform the hydration film, which makes the stronger the gel protection effect of PAC in drilling fluid, so the more obvious the effect of fluid loss reduction.
3. Purity of PAC and its application in drilling fluid
If the drilling fluid system is different, the dosage of drilling fluid treatment agent and treatment agent are different, so the dosage of PAC in different drilling fluid systems may be different. If the dosage of PAC in drilling fluid is specified and the drilling fluid has good rheology and filtration reduction, it can be achieved by adjusting the purity.
Under the same conditions, the higher the purity of PAC, the better the product performance. However, the purity of PAC with good product performance is not necessarily high. The balance between product performance and purity needs to be determined according to the actual situation.
4. Application performance of PAC antibacterial and environmental protection in drilling fluid
Under certain conditions, some microorganisms will cause PAC to decay, especially under the action of cellulase and peak amylase, resulting in the fracture of PAC main chain and the formation of reducing sugar, the degree of polymerization decreases, and the viscosity of the solution decreases. The anti enzyme ability of PAC mainly depends on the molecular substitution uniformity and degree of substitution. PAC with good substitution uniformity and high degree of substitution has better anti enzyme performance. This is because the side chain linked by glucose residues can prevent enzyme decomposition.
The substitution degree of PAC is relatively high, so the product has good antibacterial performance and will not produce putrid smell due to fermentation in actual use, so there is no need to add special preservatives, which is conducive to on-site construction.
Because PAC is non-toxic and harmless, it has no pollution to the environment. In addition, it can be decomposed under specific microbial conditions. Therefore, it is relatively easy to treat PAC in waste drilling fluid, and it is harmless to the environment after treatment. Therefore, PAC is an excellent environmental protection drilling fluid additive.
Post time: May-18-2021