Drag reduction mechanism

Most of the drag reducing agents in oil phase are polymer with flow chain or long straight chain and few side chains. For example, ZODR-102 is a high molecular polymer - σ olefin with a molecular weight of 10-10. The polymer pure agent is a rubber like solid, as a commodity, it is generally dissolved in the solution of hydrocarbons (kerosene). 10% DRA solution is very viscous viscoelastic, which is difficult to flow and can be drawn into a long filament. Polymer drag reducer can be dissolved in crude oil or oil, but not in water, and long chain curling of molecules occurs when meeting water. The results show that the drag reducing agent solution has strong Newtonian characteristics, its viscosity is as high as 3000pa · s at low shear rate, and it will not decompose below 120 ℃. Drag reduction is a special turbulence phenomenon. Drag reduction effect is a macroscopic manifestation of the influence of drag reduction on turbulent flow field, which is a pure physical action. The drag reducing agent molecules do not interact with the oil molecules and do not affect the chemical properties of the oil, but are closely related to their flow characteristics. In turbulence, the velocity of fluid particles changes randomly, forming large and small vortices. Large scale vortices absorb energy from the fluid, deform and break, and transform into small-scale vortices. Small scale vortex, also known as dissipative vortex, is weakened and subsided under the action of viscous force. Part of the energy it carries is converted into heat energy and dissipated. In the side layer near the pipe wall, the transformation is more serious due to the effect of wall shear stress and viscous force.

After the drag reducing agent is added to the pipeline, the drag reducer is dispersed in the fluid in a continuous phase. With its unique viscoelasticity, the long molecular chain extends along the flow to the natural state, and its microelements directly affect the movement of the fluid microelements. The radial force from the fluid element acts on the drag reducer element, causing it to twist and rotate. The gravitational force between the drag reducing agent molecules resists the above forces and reacts on the fluid microelement, changing the action direction and size of the fluid microelement, so that a part of the radial force is transformed into the axial force along the flow direction, thus reducing the consumption of idle work and reducing the loss of friction resistance. In laminar flow, the fluid is affected by viscous force, and there is no eddy dissipation like turbulent flow. Therefore, it is useless to add drag reducer. With the increase of Reynolds number, the drag reducing agent will show the effect of drag reduction. The larger the Reynolds number, the more obvious the drag reduction effect. When the Reynolds number is quite large and the shear stress of the fluid is enough to destroy the molecular chain structure of the drag reducer, the drag reduction effect will be reduced or even completely lost when the drag reducer is degraded. The concentration of drag reducing agent affects the thickness of the elastic bottom layer in the pipeline. The higher the concentration is, the thicker the elastic bottom layer is, the better the drag reduction effect is. Theoretically, when the elastic bottom reaches the pipe axis, the drag reduction reaches the limit, that is, the maximum drag reduction. The drag reduction effect is also related to oil viscosity, pipeline diameter, water content, pigging and other factors.

Production technology

The key technology of drag reduction agent production mainly includes two aspects: one is the synthesis of ultra-high molecular weight, amorphous, hydrocarbon solvent soluble drag reducing polymer; the other is the post-treatment of drag reducing polymer.

Synthesis of polymers

A large number of literatures show that the most effective drag reducing polymer is poly - α - olefin. In the early stage, solution polymerization was used in the production of poly (α - olefins), and the polymerization products were directly used in oil pipelines. Due to the high viscosity and low polymer content of the solution polymerization products, it was difficult to transport and use them. It was not until the mid-1990s that the bulk polymerization method was developed, which greatly improved the monomer conversion and drag reducer performance. The key technology to implement bulk polymerization is to take away a large amount of reaction heat generated in the polymerization process in time. One of the methods is to use a reaction vessel made of polymer material and design it to release the reaction heat rapidly. In the process of polymerization, nitrogen was used to purge the reaction vessel, then monomer and catalyst were added in proportion, sealed and put into low-temperature medium for 3-6 days at low temperature. In general, the purity and molecular weight of bulk polymerization products are much higher than those of solution polymerization products.     

In addition, the solution polymerization of α - olefin drag reduction polymer has also made a new breakthrough. By adding viscosity reducing agent in the process of α - olefin polymerization, the overall flow performance and treatment characteristics of the finished product can be improved. At the same time, higher polymer molecular weight and more uniform molecular weight distribution can be obtained, and the solubility of poly α - olefin drag reducer can be improved.

Post treatment of polymer

The practical application shows that it will bring a lot of inconvenience to the oil transportation production if the solution polymerization product is directly used as the drag reducer. Therefore, this method has been eliminated. In order to improve the performance of the drag reducer, the polymer and dispersant are usually ground into powder in the environment below its glass transition temperature, and appropriate additives are added to make drag reducer products with different appearance forms. At present, water-based emulsion drag reducer is widely used in crude oil pipeline. It uses stabilizer, surfactant and other additives to suspend polymer powder in water or mixture of water and alcohol. This product has the advantages of high polymer concentration, convenient injection and good solubility in crude oil, but it also has the disadvantages of short storage time and poor stability. Low viscosity colloidal drag reducer is mainly used in product oil pipeline. It dissolves polymer powder into product oil or mixture of product oil and some solvents. This product has the advantages of low viscosity and convenient injection, but it has the defects of low polymer concentration and heavy transportation workload. In order to overcome the defects of the two products mentioned above, a non-aqueous suspension drag reducer has been developed recently. The polymer powder is suspended in alcohol fluid with the aid of suspension agent. This kind of drag reducer does not need surfactant, bactericide and complex stabilizer system. It simplifies the production process. It has the advantages of good antifreeze, high polymer concentration, good stability, and can prevent water and other impurities from entering the oil pipeline. It can also be used for the transportation of crude oil and finished oil at the same time.

Bulk polymerization products can be ground directly in low temperature environment, while solution polymerization products need to precipitate out of solvent and then comminute. Conoco company and Baker Hughes company of the United States respectively adopt the method of precipitating polymer from solution. Alcohols which can precipitate polymer but are immiscible with hydrocarbon solvent are used as precipitant, and the precipitated polymer is formed into small particles through special device. It is worth mentioning that Baker Hughes can control the diameter of the precipitated polymer particles below 0.25cm by controlling the speed of adding precipitant into the solution polymerization products and proper stirring. This kind of polymer can be directly prepared with suspension agent and liquid alcohol to form non-aqueous suspension drag reducer. In this way, the low temperature crushing process of polymer is omitted and the production process is simplified.   

To sum up, drag reducer with different appearance and performance can be obtained through different post-treatment processes.