Drag reducing agent

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DRA is typically avery viscous polymer. They can be formulated in milky dispersed form to reduce viscosity and improve deliverability.

A drag reducing agent (DRA), also called drag reducer or flow improver, is a long chain polymer chemical that is used in crude oil, refined products or non-potable water pipelines. It is injected in small amounts (parts per million) and is used to reduce the frictional pressure drop along the pipeline's length. It's use has allowed pipeline systems to greatly increase in traditional capacity and extend the life of existing systems. The higher flow rates possible on long pipelines have also increased the potential for surge on older systems not previously designed for high velocities. As drag reduction is a near-wall phenomenon [1], it is necessary to inject the chemicals downstream of pipeline pumps to avoid the high shear degradation that occurs within a pump.

Contents

History

The phenomena of drag reducing was first observed in 1943. It was found that by adding small amounts of certain synthetic oil soluble polymers to pipeline systems in turbulent flow there was a substantial reduction in fluid flow resistance.

DRA was first commercially applied in the Trans Alaska Pipeline in 1979[2]. The use of drag reducers accounted for about 200,000 bbl/D increase in the Trans Alaska Pipeline system [3].

Chemistry

The DRA chemistry used for crude oil and fuels are typically poly olefins, with average molecular weight of 500,000 - 10,000,000. It is typically in the form of gel or dispersion.


The DRA chemistry used for water system is typically hydrolyzed polyacrylamide emulsion polymers with average molecular weights of 5,000,000 - 25,000,000.

Benefits

The benefits of using a drag reducer are the following

  1. Increase in pipeline throughput
  2. Reduction of the waiting time for tanker loading/offloading
  3. Maintaining the throughput during MOL (Main Oil Line) pump maintenance for de-rated lines
  4. Bypassing MOL pump stations
  5. Energy Savings

The chemicals damp turbulent bursts of the oil near the pipeline wall, that way less disturbance is created during the oil flow. Minimizing turbulence in the radial direction better preserves flow in the axial direction of the pipeline.

Drag reduction effectiveness for a given concentration is based on the turbulent characteristics of the pipeline. The maximum theoretical effect is the same as a pipe in laminar flow, where all of the turbulence is eliminated by the agent. Drag reduction effectiveness is measured as a percentage of the pipeline with no DRA present. For example, 75% drag reduction is representative of a pipeline that has one quarter of the frictional pressure loss at a given flow rate.

Rules of Thumb for DRA applications

The following are the rules of thumb to evaluate whether a particular application is suitable for using DRA:

  • Crude oil system
    • Viscosity: <60 cSt
    • Pipe diameter: >1 inch
    • Pipe length: >0.5 mile
    • GOR: <2,000
    • Watercut: <60%
    • Wax content: <20%
    • Flow regime: turbulent flow
  • Water system
    • Pipe diameter: >1 inch
    • Pipe length: >0.5 mile
    • TDS: 60,000 - 90,000 mg/L ??
    • Re number: >4,000
    • Flow regime: turbulent flow
    • Must be compatible with water

Degradation

Since DRA is composed of long polymer strands, it is prone to degradation as it travels through the pipeline due to shearing of the strands. Large pressure changes through a control valve or pump result in a total loss of effectiveness. DRA may be reinjected after such equipment, but the total injection is usually limited by the product specifications or fluid limitations. DRA should never be used with any turbine fuels (such as jet fuel) because the polymer will accumulate on turbine blades and may damage the turbine.


Other factors that may result in partial degradation:

  • Diameter change
  • Pipeline meters
  • Manifolds
  • High flow velocity
  • Temperature (40F-100F range preferred)
  • Water or wax crystals present in the oil

DRA delivery and injection challenges

DRA products are typically highly viscous polymers that can be difficult to pump or inject. The carrier fluid can potentially evaporate if volatile, which would leave dried out polymer behind and plug injection equipments.

A suitable chemical injection equipment must be identified for successful DRA delivery. It typically requires a positive displacement pump with variable speed drive and accurate flow indicator.

Dispersed DRA

Milky colored dispersed DRA products are typically composed of high molecular weight polymers that are solid at normal temperatures. The polymer solid is grounded and coated to prevent agglomeration. This type of DRA typically has better pumpability and is easier to handle. However, trace amount of water contamination can cause the polymer to relax and form a thick gel and result in plugging of injection equipment.

Best practices

The following best practices are recommended:

  • Prevent water contamination during transport and application
  • Store in closed container to prevent solvent loss
  • Use large diameter injection lines and suction lines for chemical injection
  • Use two check valves to prevent water backflow
    • Chemical pump discharge
    • Injection point
  • Use strainer on fill line to prevent any precipitated polymers from entering day tank
  • Use chemical injection quills to place product directly in the flow stream
  • Use absorbent material to scoop up any spill and use diesel or mineral spirits for clean up.

External links

References

  1. Wells, C.S.Jr and Spangler, J.G.: "Injection of a Drag-Reducing Fluid into Turbulent Pipe Flow of a Newtonian Fluid." Phys. Fluids (1967).10. 1890-1894
  2. Burger, E.D., Munk, W.R., and Wahl, H.A.:"Flow increase in the Trans Alaska Pipeline Through Use of a Polymeric Drag-Reducing Additive", JPT (Feb 1982) 377-386
  3. Wahl, W.R.; Beaty, W.R.; Dopper, J.G. and Hass, G.R.:"Drag Reducer Increase Oil Pipeline Flow Rates", SPE 10446 (Feb 1982)