1. Factors Influencing the Efficiency of Drag Reduction

Polymer drag reducers have attracted extensive attention in the field of oilfield chemistry since they were studied by Toms. In the past 50 years, a large number of theoretical and experimental studies have been carried out at home and abroad. In theory, information about the relationship between polymer structure and drag reduction performance has been accumulated, which is of great significance to the application design of drag reducers and the improvement of application technology level. The drag reduction performance of the drag reducer is affected by the flow rate of the fluid. When the fluid does not reach turbulence, there is no obvious drag reduction effect. With the increase of the fluid speed, the drag reduction effect begins to show. When the flow rate reaches a certain value, the drag reduction effect is the best. The higher the molecular weight of the drag reducer, the less the branch chain, the better the solubility, and the higher the drag reduction efficiency. In addition, drag reduction is related to molecular weight distribution, conformation of macromolecule in solvent, structure and strength of chain.

(1) Nature of crude oil

The lower the viscosity and density of crude oil, the easier the turbulence conditions are to be achieved, and the more favorable the role of crude oil drag reducer is. The high water content of crude oil affects the dissolution of drag reduction agent and its drag reducing efficiency.

(2) Pipeline transportation condition

The higher the pipeline temperature is, the lower the viscosity of the oil is, and the more favorable the drag reducer is. The faster the flow velocity, the smaller the pipe diameter, the larger the Reynolds number, the higher the turbulence degree, the better the function of the drag reducer. But when the flow velocity is too fast, which causes the degradation of the drag reducer, the drag reduction efficiency of the drag reducer will be reduced.

(3) Molecular weight and molecular weight distribution of polymer drag reducer

The molecular weight m of polymer is one of the important structural parameters that affect the drag reduction performance. The molecular weight of polymer must exceed a certain molecular weight MC (called the initial molecular weight) before drag reduction. Many researchers try to describe the relationship between DR and molecular weight by mathematical expression. Kim proposed the relationship between characteristic drag reduction rate [DR] defined by Virk little relation and m.

K is a constant, [DR] represents the drag reduction rate per unit concentration of polymer solution at infinite dilute concentration. It is a measure of drag reduction capacity. From the above formula, the drag reduction performance increases with the increase of molecular weight. The drag reduction performance of hexyl methacrylate in kerosene and kerosene / acetone was studied systematically by daijialin et al.

Where A, B and D are constants under certain flow conditions. According to the above formula, the drag reduction rate increases with the increase of molecular weight, and finally reaches an equilibrium value.

Most of the polymer samples have a certain molecular weight distribution (MWD). Under certain conditions, not all polymers have drag reduction function, and the drag reduction ability of polymers is related to the molecular weight distribution. Strictly speaking, the relationship between molecular weight and drag reduction performance is not very close when the molecular weight distribution is not clear. So far, there are two different explanations for the molecular weight distribution, one is that DR is related to the average molecular weight of the sample, the other is that DR is related to the high molecular weight part of the sample.

(4) Main chain structure of polymer

The chemical composition of polymer links, the type of bonds, the geometry and flexibility of the chains have great influence on the drag reduction efficiency. So far, most of the effective drag reducers are flexible polymers with linear or spiral structure. Gramain et al. Studied the drag reduction efficiency of linear, star and comb PS in toluene solution. The experimental results showed that the branching of molecular chain greatly reduced the drag reduction efficiency of polymer. The structure of double bond and conjugate double bond in the main chain is not conducive to drag reduction. Meier studied the drag reduction efficiency of hydrogenated polyisoprene on crude oil, and found that its solubility, drag reduction rate and shear resistance in crude oil increased with the increase of hydrogenated degree. With the increase of hydrogenation degree, the flexibility of molecular weight increases, and the drag reduction performance improves naturally. Linear polymers with the same molecular weight have more branches than linear polymers, such as star polymers have higher drag reduction rate, but the shear resistance of linear polymers is not as good as that of branched polymers. Liaw studied the relationship between drag reduction function and chain rigidity of polymers with different structures from another point of view. It was found that drag reduction performance decreased with the increase of chain rigidity. Under the same conditions, the polymer with better flexibility has higher drag reduction performance, which may be related to the extension degree and conformation change of the polymer in the flow field.

(5) Side chain structure of polymer

The side chain structure of polymer also affects drag reduction. Generally speaking, a small amount of long side groups on the main chain of polymer will increase its drag reduction performance. Wade studied the effect of branch chain length on the drag reduction performance. It was found that the drag reduction performance decreased after the short side group was generally connected to the main chain of polymer. However, the drag reduction performance is enhanced by adding a small amount of long side base. On the other hand, the side group structure should also be considered from the factors such as the flexibility of the molecule, the solubility in the solvent and the shear resistance. The short side base is not good for drag reduction, the long side base is easy to crystallize, which is not conducive to the dissolution of drag reducer, but affects the drag reduction performance. The side base with appropriate length is good for shear resistance. Therefore, many factors should be considered in the design of the molecular structure of drag reducers to achieve a proper balance.

(6) Drag reducer concentration

In a given flow system, DR value of drag reducer with a certain molecular weight increases with the increase of concentration. When the concentration reaches a certain value, DR tends to a constant value DRM, and no longer increases with the increase of concentration. Naiman found that when the concentration of drag reducer exceeded a certain value, the drag reduction rate began to decline. This phenomenon had been found when kowalik studied SVP/zn-s-epdm system. Kowalik explained that the relative high concentration of polymer molecules inhibited the full extension of the polymer chain, thus reducing the drag reduction effect.

The higher the concentration of drag reducing agent is, the higher the drag reducing efficiency is, but when it exceeds a certain value, the increase range of drag reducing efficiency is reduced. Therefore, crude oil drag reducer should be the best use of concentrated friction.