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*Drag Reduction - SNF Drag Reduction 6 FLOPAM DR Drag Reduction The shear degradation of the polymer*

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FLOPAM DR

Drag Reduction

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Drag reduction can be defined as an increase in pumpability due to the addition of polymer to a fluid in turbulent flow. Polymers added for this purpose are called drag reducer additives commonly referred to as DRA. For pipe flow conditions the effectiveness of DRA can be calculated as follows:

In turbulent flow the simple relationship between shear stress and rate of strain through viscosity is no longer valid and the velocity profile, head loss and friction factor, etc. cannot be derived analytically and must be found experimentally. These quantities are functions of both Reynolds number and the relative roughness (e/D) of the pipe wall.

It is observed experimentally that viscous turbulent flow in pipe can be subdivided into three layers: a very thin layer near the pipe wall where the flow is essentially still laminar in which the adjacent pipe wall suppresses turbulence, a turbulent boundary layer existing in the transition from the laminar wall layer to the fully turbulent layer, and the fully turbulent layer.

Scientists have studied fluid flow characteristics with soft compliant surfaces, suction to control boundary layer growth, heat transfer through pipe, multi-phase flow and a wide array of conditions.

(ΔPwithoutDRA - ΔPwithDRA )DR (%) = _________________ x 100 ΔPwithoutDRA

During the 1970s and 80s when the cost of energy was high, there was a large research effort to study drag reduction and EOR within the petroleum industry. There is a large volume of literature on drag reduction with polymer and there are many commercial applications, especially in the oil industry where it is used to aid in the transport of crude oil through pipelines, to increase waterflood injection rates and for fracturing fluids. In this document the topic of drag reduction relative to pipe flow of aqueous fluids is presented.

When a fluid moves relative to a solid surface, frictional drag is created which results in a corresponding dissipation or loss of energy. Scientists in many industries have studied methods to minimize this effect.

Reducing the amount of friction in pipeline flow leads to reduced energy consumption or increased flowrate for the same pump pressure. The benefit is improved performance and economics and in a global sense, a more efficient and environmentally sensitive use of energy resources.

Polymer Drag Reduction Effect

Drag reduction

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Numerous techniques for reducing friction at the pipe wall have been proposed and studied for several decades but since most of the pressure loss occurs in the turbulent boundary layer it is the most interesting to study and has the greatest potential for reducing friction loss with additives.

Researchers discovered that certain fluid additives modify the structure of the boundary layer and cause a reduction of skin friction during turbulent flow. These may be either insoluble particles (clays) or fibers, associated colloids or soluble long chain polymers. Studies have shown that skin friction on marine animals was reduced by viscous secretions. The first record of drag reduction was in 1883 but not until 1948 was the mechanism of drag reduction first studied by B.A. Toms, who saw that polymethyl methacrylate could lower the pressure gradient compared to the solvent alone for the same flow rate in turbulent flow.

The first field applications of drag reducer were made in oilfields in Texas in 1950 where fracturing with guar gum was established.

In 1961 the US Navy Research Laboratories began studying drag reduction with a rotating disc apparatus. They investigated a wide range of water soluble polymers and saw that in some cases very large friction reduction was possible with just a trace amount of polymer added to the solution. Some very dilute polymer solutions had viscosities only slightly greater than water and did not behave like pseudo-plastic polymer fluids.

Numerous synthetic and natural polymers have been proven to reduce frictional drag in turbulent flow conditions; the ones that achieve the best drag reduction effect have the following properties: ■ very long chain, flexible molecular structure. ■ very high molecular weight (MW) molecules. ■ rapid solubility in water. ■ solutions exhibit viscoelastic properties.

In the early studies of synthetic polymers, polyethylene oxide having a MW around 4 to 8 million was widely used. Since then, Polyacrylamide became the most widely used polymer for drag reduction due to its low cost, good mechanical stability, and high MW. The SNF emulsion drag reducer polymer, called DR7000, has a molecular weight in the range of 15-17 million.

Associative colloids produced by mixing anionic and cationic polymers can be more stable to shear degradation but require very specific conditions for dispersion.

History

Additives for drag reduction

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■ Friction red uction as a function of velocity and concentration

Pipeflow can generally be described as laminar or turbulent based on the flow structure. In the laminar flow regime, the fluid moves in laminae or layers and the flow velocity profile can be predicted analytically. In turbulent flow, the fluid particles move in a random 3 dimensional motion superimposed on the average motion of flow and the frictional pressure loss must be determined based on experimental results. The pressure loss can be calculated once the friction factor, f, is determined from the Moody plot which shows the data for friction factor as a function of Reynolds number. The Reynolds number is calculated with the following equation:

Nre = 928.dρυ/μ

d = diameter (inches) υ = velocity (ft/sec) ρ = density (lb/gal) μ = viscosity (cp)

Drag reduction in pipe flow

For a Reynolds number less than 2000, flow is laminar and there is relatively little frictional pressure drop, but at higher Reynolds numbers there is a transition from laminar to turbulent flow and the pressure loss due to friction is large. This is where a DRA is highly effective i.e., fluids in the turbulent flow regime.

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Friction Reduction vs Velocity and Concentration for PAM with a Molecular Weight of 12 million

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This can be seen in the following curves.

At Reynolds numbers above 4000 the flow is fully turbulent and the effect of pipe roughness begins to influence the pressure loss. As the Reynolds number increases, the boundary layer thickness decreases and at extremely high Reynolds numbers the roughness elements protrude through the boundary layer into the turbulent core region. This is known as the fully-rough flow regime and the friction factor becomes a function of only the pipe roughness and no longer a function of the Reynolds number.

When a drag reducer polymer is added to a fluid in the highly turbulent fully-rough flow regime, one effect is to increase the thickness of the boundary layer which reduces the friction factor as if the pipe was smoother.

Older pipelines have scale buildup and corrosion which increase the roughness factor over time by as much as 2 or 3 times, causing the friction factor to increase by 50%.

■ Pressure drop reduction as a function of the Reynolds number

The following conclusions can be drawn for pipe flow: ■ drag reduction occurs only in fluids that are in the turbulent flow regime, above a Reynolds number of 2000. ■ drag reduction is greater in small diameter pipe due to the roughness factor relative to the pipe diameter. ■ drag reduction in dilute solutions increases with increasing polymer concentration up to a limit. ■ at higher polymer concentrations the mechanism of drag reduction is complicated by the increase of viscosity and shear thinning properties. ■ drag reducer polymers can loose their effectiveness due to shear degradation which occurs over time in the pipeline. ■ the higher the turbulence, the higher the shear rate and the more degradation there is.

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Pressure Drop Reduction vs Reynolds Number for PAM with a Molecular Weight of 12 million

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The shear degradation of the polymer is limited even at very high speed (25 m.s-1 or 75 ft.s-1) on 3 to 6 inches pipes. The shear is lower very often than 2000 s-1 in the pipe.

But a part of the process is dependant on surface equipment sometimes not very adapted to the case of t