1.Common Crude Oil Drag Reducer

As drag reducer, it is generally an oil soluble polymer with molecular weight ≥ 106. It is required to have good solubility, shear resistance, oxidation resistance, etc. Generally, the larger the molecular weight is, the worse the shear resistance is. There are short side chains in the molecule, which enhance the shear resistance. However, the side chain should not be too long, which makes the flexibility of the molecule worse and the drag reduction reduced. At present, drag reducers used at home and abroad include:

(polyisobutylene) (polymethacrylate) (polycyclopentadiene) (polystyrene) (polyalphene) (ethylene propylene copolymer)

Crude oil flow improver has been successfully applied in many oilfields at home and abroad. For example, adding flow improver 50mg / L to Zaire crude oil in West Africa reduces the freezing point of crude oil by 15.5 ℃; adding flow improver to Rotterdam Rhine pipeline in Europe transports Libyan crude oil, reduces the pour point of crude oil from 21 ℃ to - 14 ℃. The use of flow improvers in the land and subsea crude oil and product pipelines in the United States, Australia, the United Kingdom, Mexico, the Middle East and Southeast Asia has achieved good results.

The research of flow improver in China's oil field is late, but it has made rapid progress. For example, Zhongyuan Oilfield added 100 Mg/L flow improver polyacrylate can reduce the freezing point of crude oil from 30 ℃ to 7.5 ℃ and the viscosity by 97.5%; adding 100mg / L flow improver in Shengli oilfield can reduce the freezing point of crude oil from 30.5 ℃ to 10.5 ℃ and the viscosity by 93.5%; Changqing Oilfield began to use the flow improver of Exxon company of the United States in December 1988, which has a very significant effect on the Mahuining oil pipeline The line has become the first pipeline in China to realize normal temperature transportation throughout the year.

At present, the development trend of crude oil long-distance transportation technology is to use crude oil flow improver to realize normal temperature transportation. However, it should be noted that due to the different properties of crude oil in various oilfields, a flow improver can not be effective for each crude oil. Therefore, it is necessary to select suitable reagents through experiments. In addition, the flow improver only works when the crude oil begins to separate wax, so the temperature of adding agent should be the temperature when all wax in the crude oil dissolves, that is, the temperature of adding agent should be higher than that of separating wax. It should also be noted that flow improvers should not contain substances harmful to petroleum processing and product performance.

2.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.

3.Action Principle of Crude Oil Drag Reducer

The task of drag reduction mechanism research is to explain many characteristics of drag reduction from mechanism, such as pipe diameter effect, concentration effect, turbulence velocity distribution change, polymer characteristics, so as to grasp the law of drag reduction essentially. At present, there are many theories about drag reduction mechanism, such as Toms pseudoplastic hypothesis, Virk's effective slip hypothesis, viscoelastic hypothesis, turbulence suppression theory and so on.

(1)Toms' pseudoplastic hypothesis

After Toms found the drag reduction phenomenon in 1948, he put forward the hypothesis of drag reduction mechanism. He thinks that the polymer drag reducer solution has pseudoplasticity, that is, the shear rate is inversely proportional to the apparent viscosity, and the increase of shear rate leads to the decrease of the apparent viscosity, which leads to the decrease of the resistance. However, with the development of non-Newtonian fluid mechanics, the Toms hypothesis has been gradually denied. As long as simple experiments are carried out, it can be found that the measured value of friction resistance of drag reducer solution in turbulent flow has a great error with the calculation of pseudoplastic fluid, and the pseudoplasticity of dilute drag reducer solution is very weak, even no pseudoplasticity at all. Its rheology is almost the same as Newtonian fluid, but the drag reduction rate is large. Walsh's test shows that the expanded plastic fluid also has a strong drag reduction effect.

(2)Virk's effective slip hypothesis

According to Virk, when the fluid is turbulent in the pipe, the layer close to the wall is viscous bottom layer, followed by elastic layer, and the center is turbulent core. He measured the velocity distribution through experiments, and found that the velocity in the turbulent core area of the drag reducer solution was a certain value larger than that of the pure solvent, but the velocity distribution was the same, and the velocity gradient of the elastic layer increased, resulting in the decrease of the resistance.

According to Virk's hypothesis, when the concentration of drag reducer increases, the thickness of elastic layer also increases. When the elastic layer extends to the tube axis, the drag reduction reaches the limit. This hypothesis can explain the maximum drag reduction and the pipe diameter effect. The chemical agent that can reduce the resistance of crude oil pipeline is called crude oil drag reducer.

(3)Viscoelasticity hypothesis

The viscoelasticity hypothesis suggests that the drag reduction of polymer solution is the result of the interaction between viscoelasticity and turbulent vortex. Many researchers have carried out time experiments on specific polymer drag reducer dilute solution, and found that the relaxation time of polymer molecules is longer than the duration of turbulent vortex, indicating that the elasticity of polymer molecules does play a role. Therefore, part of the kinetic energy of the turbulent vortex is absorbed by polymer molecules and stored in the form of elastic properties, which reduces the kinetic energy of the vortex and achieves the effect of drag reduction.

(4)Turbulence suppression hypothesis

According to the turbulence retention inhibition hypothesis, after the drag reducer is added to the pipeline, the drag reducer will stretch naturally along the long chain of the molecule due to its viscoelasticity, and its micro elements will directly affect the movement of the fluid micro elements. The radial force from the fluid micro element on the drag reducer micro element causes it to twist and rotate. The molecular gravity of drag reducer resists the reaction of the above forces on the fluid micro element, changes the magnitude and direction of the force of the fluid micro element, changes part of the radial force into the axial force along the flow direction, thus reducing the consumption of idle work, and plays the role of reducing the friction loss on the macro level. That is to say, polymer molecules inhibit the generation of turbulent vortices, thus reducing the intensity of pulsation and ultimately the energy loss.

Based on the above hypotheses, it can be concluded that the drag reducer added to the oil flow depends on its unique viscoelasticity, and the macromolecular chain naturally stretches along the flow direction, which will affect the movement of the fluid particle. The radial force of the fluid particle causes the drag reducer molecules to twist and rotate. The drag reducer molecules rely on the interaction between molecules to resist the force of the fluid particles, change the direction and size of the action of the fluid particles, so that a part of the radial force for reactive work is converted into the axial force in the direction of the flow, thus reducing the consumption of reactive work, and the macroscopic performance reduces the friction loss of the fluid, that is to say, it plays the role of drag reduction.