Abstract: the friction resistance between oil and gas and pipe wall is the main reason of pressure drop in oil and gas pipeline. Drag reducer can reduce friction resistance and pressure drop. This paper summarizes the action mechanism and application characteristics of drag reducer for natural gas, crude oil and product oil pipeline, and the influence of drag reducer on the design of new pipeline and operation pipeline management. Natural gas pipeline and oil pipeline drag reducer have their own characteristics in the mechanism and application requirements.

Key words: drag reducer, natural gas, oil, pipeline

1、 Introduction

There is friction resistance between natural gas, product oil and crude oil with the pipe wall during pipeline transportation, which is the main reason for pipeline pressure drop. Reducing the pressure drop of the pipeline plays an important and positive role in improving the design and operation of the pipeline and increasing the benefit of the enterprise. However, the mechanism of action and the method of use have their own characteristics.

2、 Drag reducing agent for natural gas pipeline

The flow in natural gas pipeline is generally turbulent flow, and the gas molecules near the pipe wall will produce radial movement, resulting in additional energy loss. Natural gas drag reducer can reduce the radial pulsation of gas and reduce friction resistance. At the same time, the non-polar end and the polar end contact with the gas in the pipeline, forming a smooth film and a special gas-solid interface. This interface reduces the gas radial pulsation and reduces friction Friction resistance. In the field application, the drag reducing agent should not only have the effect of reducing rent, but also should have no influence on the gas physics and the pipeline safety

(1) There is a strong adsorption force between the drag reducing agent and the pipe wall. The drag reducing agent molecules can be firmly adsorbed on the pipe wall and form a stable film on the pipe wall. At the same time, the drag reducing agent film can be stable on the pipe wall for a long time;

(2) The drag reducer can absorb the turbulent energy of gas and reduce the energy loss of gas movement;

(3) The drag reducer itself should be non corrosive and not affect the quality of natural gas;

(4) The drag reducer should be soluble in some solvents to prepare the solution and inject it into the natural gas pipeline.Because the natural gas drag reducer must adhere to the pipe wall and form a film to play the role of pour point reduction. Due to the limitation of pipeline structure, drag reducer can only be added from one end of the pipeline in the station. Therefore, the adding process used in the pipeline should be able to ensure the formation of stable and continuous membrane in the whole pipeline between the two stations. At present, there are three ways to add drag reducing agent to natural gas pipeline, including atomization, foam injection and pigging.

a. Atomization injection

The method is to install atomization device at the inlet of the pipeline, and add drag reducing agent into the pipeline after atomization. The drag reducing agent is suspended in natural gas in the form of small droplets, and then adsorbed on the pipe wall to form a film after contacting with the pipe wall. As the liquid will settle from the gas during transportation, the concentration of drag reducing agent in natural gas decreases along the pipeline, and there may be no drag reducer in the natural gas at the end of the pipeline. This method is difficult to ensure the formation of effective membrane in the whole pipe section, so it is suitable for the pipeline with short length.

b. Foam injection

The method is to make the rent reducing agent foam into the pipe and form a foam plug. The foam plug is driven by natural gas. The drag reducing agent is absorbed from the foam and then adsorbed on the tube wall to form a drag reduction film. The range of the drag reducing agent can be covered is related to the stability of the foam. The longer the foam is in the pipeline, the larger the coverage of the drag reducing agent will be.

c. Pig coating

In the pipeline, two pigs are used to form a sealed cabin, and the drag reducer solution is added into the sealed cabin, and the drag reducing agent is continuously coated on the pipe wall with the movement of the pig. Straight plate pig is generally used in the rear part, and professional coating pig with bypass hole is used for front pig. Drag reducing agent is evenly coated on the pipe wall through the bypass hole. With this method, the thickness of DRA film can be controlled and the whole pipeline can be coated effectively. The amount of drag reducer can be calculated according to the thickness of the liquid film on the pipe wall and the accidental leakage at the pig.

2、 Drag reducing agent for oil pipeline

Oil pipeline is divided into crude oil pipeline and product oil pipeline. Although the type and molecular structure of drag reducer used in the two pipelines are different, the mechanism of action is similar to the injection mode and the influencing factors, and both are quite different from the drag reducer used in natural gas pipeline.

In the turbulent state, the liquid molecules in the turbulent buffer zone not only move along the axial direction of the pipeline, but also move along the radial direction to the turbulent core area, resulting in large friction loss. Drag reducing agent is a kind of macromolecular polymer. When it is added into the pipeline, the drag reducing agent molecule is stretched under the shear force of liquid. The radial force of liquid molecules acts on the drag reducer molecules, causing them to twist, rotate and deform. Under the action of the reaction forces of drag reducer molecules, part of the radial force of liquid molecules changes into the axial force along the flow direction, so as to reduce the loss of idle work and realize the rent reduction effect.

The drag reducing agent works because its long chain molecular structure will change the movement of liquid molecules in the pipeline, in which the liquid molecules are mainly affected by shear. The drag reducer is added into the oil by injection pump. During the filling process, the drag reducer molecules curl up, and the molecular long chain is not affected by the shear of the injection pump. However, after adding the drag reducer into the oil product, the long molecular chain is stretched under the effect of oil shear force. Under the action of severe shear, the long chain is easy to be cut off, thus losing the rent reduction effect. The shear in the pipeline mainly comes from the shear force between the fluid and the pipe wall, as well as the shear through the pump and valve. The application experience in practical pipeline shows that the shear effect of long-distance straight pipe can be ignored, but there is strong shear at the pump and filter, which may make the drag reducer lose its effect completely and increase the flow resistance in a small range. Therefore, it is better to select the filling position at the exit of the pipeline to avoid the influence of any equipment in the station. For long-distance pipeline, the long chain structure of drag reducer after passing through the pumps, valves and filters in the intermediate station may be seriously damaged, and it is necessary to re inject the DRA at the outlet of the pipeline in the intermediate station.

3、 Effect of drag reducing agent on new pipeline and running pipeline

By using drag reducing agent to reduce the operating pressure of the pipeline, the design scheme of the new pipeline can be improved and the operation condition of the running pipeline can be improved.

（1） New pipeline reduces investment and increases station spacing.

After the use of drag reducer, the friction pressure drop of the pipeline will be reduced. When the outlet pressure of the pipeline remains unchanged, the pressure along the pipeline will decrease, and the pipeline with smaller wall thickness can be used. At the same time, the station spacing can be appropriately increased to reduce the number of pump stations (compressor stations for natural gas) along the pipeline, so as to reduce the expenses of equipment and staff of the pump station.

（2） Depressurization of operation pipeline, hydraulic crossing station and increase of transportation.

For the pipeline in operation, the operating pressure along the pipeline can be reduced, so as to increase the safety factor of the old and corroded pipeline; when the intermediate station is shut down or other conditions occur, the hydraulic crossing station can be realized; under the condition of maintaining the original operating pressure, the pipeline can be increased.

4、 Summary

Drag reducer can effectively reduce the pressure drop in oil and gas pipelines. For natural gas pipelines, it is necessary to ensure that the drag reducing agent forms a continuous and complete film along the pipeline; for oil pipeline, it is necessary to avoid sharp shear of drag reducer. By reducing the pressure loss of the pipeline, the design of the new pipeline and the operation of the pipeline in operation can be optimized.