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Low shear basics - 2017



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Low shear basics - 2017

  2. 2. Low shear basics Challenges caused by shear forces in petroleum industry Effect on Separation Oil in separated water Water in separated oil Effect on Transportation Tighter emulsion Increased drag Effect on Injection Broken chemicals Loss of viscosity Intense Turbulence Causes Fluid Shearing
  3. 3. Low shear basics • There are more than 900,000 oil producing wells in the world. (Oil & Gas Journal 2010) • During the life of a well, 4-5 barrels of water are produced for every barrel of oil. (Collins, A.G. 1987. Properties of Produced Waters. In Petroleum EngineeringHandbook, H.B. Bradley ed, Ch 24. Richardson, Texas: SPE) • Daily worldwide oil production is around 97 million barrels per day; while water production is roughly three times more, and increasing (International Energy Agency) • Water handling costs are in range from 5 to more than 50 cents per barrel of water, i.e., for a oil producing well with 80% water cut, the cost of handling water is up to 4$ per barrel of oil produced. (Baily, B. et.al. 2000. Water Control. Oilfield review (Schlumberger)) • Polymer flooding is the most important chemical EOR method used in sandstone reservoirs. The sweeping of oil by polymer flooding may enhance the oil recovery by 5 – 11 %. (De Bons, F.E. and Braun, R.W. 1995. Polymer Flooding: StillViable IOR Technique. European IOR Symposium) Some Statistics in Petroleum Industry
  4. 4. Low shear basics Fluid Flow in Oil Production Nearly all macroscopic flows in engineering practice in general and in oil industry in particular have turbulent nature. Turbulence is a dissipative flow state characterized by nonlinear fluctuating three- dimensional vorticity. Oil, water and gas are flowing together from the reservoir in a common stream. The dynamics of several phases is called multiphase flow. At low velocities the different fluids are separated as in stratified flow. At high velocities fluids become mixed. Slug flow is an example of a flow regime in between, representing both separation and mixing.
  5. 5. Low shear basics Shear Forces in Turbulent Flow Shear forces are normally present in the fluid flow because adjacent layers of the fluid move with different velocities compared to each other. In turbulent flow there are no well-defined layers. Instead, eddies (vortexes) of many sizes appear and interact with each other. The eddies introduce velocity fluctuations and create shear forces, which cause the bigger droplets and bubbles of the dispersed phase to deform and possibly break-up, and smaller droplets to collide and possibly coalesce. The kinetic energy cascades from large scale eddies to smaller ones, eventually reaching smallest structures where it dissipates in the form of heat. The intermediate scale eddies between the largest and the smallest scale eddies have direct impact on the bubbles and droplets in the flow as they have similar sizes as average droplets and bubbles.
  6. 6. Low shear basics Considerable shear forces acting on the fluids passing through the valves and pumps cause emulsification and droplet break-up which have a detrimental impact on downstream separator efficiency. Even a moderate or low pressure differential over a choke valve can cause strong emulsification of the petroleum fluids. Conventional centrifugal pumps tend to shear process fluids significantly. Some rotary positive displacement pumps, though, have good low shear performance. Small droplets of dispersed phase are subject to the competing processes of break-up and coalescence. Moderate amounts of shear and mixing might be beneficial for droplet growth. Shear Forces in Process Equipment
  7. 7. Low shear basics • Simultaneous flow of water and oil in turbulent regime leads to fluid mixing and shearing. • In every process plant there are sources of unwanted turbulence. • Shear forces acting in oil-water flow cause fluid emulsification. • Strong emulsification results in: – Reduced efficiency of separation equipment as the small droplets are harder to separate. – Need for additional use of chemicals and/or heat treatment to achieve the desired levels of separation. – Water treatment systems may become the bottleneck for overall process plant operation. Effect on Separation Shear in Separation Process
  8. 8. Low shear basics Stokes’ Law describes the maximum vertical velocity for a droplet or particle in a fluid. The equation can be used to determine the time required for separation. The diameter of the droplet has the largest impact on the rising velocity. Hinze described the maximum stable droplet size dmax that can exist in turbulent flow regime. Wcrit, ρ, and σ are dependent on the fluid properties, and can be regarded as constant. In order to increase dmax, energy dissipation rate, ε, has to be decreased. For the flow through a valve ε can be defined as: Effect on SeparationEffect on Separation Separation Theory
  9. 9. Low shear basics The maximum droplet diameter dmax can be plotted as a function of the turbulent energy dissipation rate according to the equation. Three values of interfacial tension are considered (1, 5 and 30 mN/m), where the larger interfacial tension is shown as the solid line. [1] The graph shows the range of turbulent energy dissipation rate associated with straight pipe, high-shear centrifugal pumps, and control valves. [1]. Walsh, J. The effect of Shear on Produced Water Treatment. The savvy separator series: Part 5. Effect on Separation Droplet Development
  10. 10. Low shear basics • Water and oil flow in turbulent regime during the pipe transport. • Turbulence and shear forces in valves cause fluid emulsification. • Tight emulsions in a pipe results in: – Increased apparent viscosity of the mixture compared to the viscosities of separate fluids. – Increased viscosity lead to higher friction around the pipe walls. – Increased friction lead to higher pressure losses during the fluid transport. Effect on Transportation Shear During Transportation
  11. 11. Low shear basics The pressure gradient dP/dx in pipe flow depends on: Pipe diameter, D, fluid viscosity, μ, fluid density, ρ, and flow velocity, U. In addition, wall roughness and pipe inclination are important. Total pressure gradient in the pipe is composed of three different terms: Shear forces and emulsification in the multiphase flow affect fluid viscosity, μ. Frictional pressure gradient accounts the pressure losses due to fluid friction at the pipe wall. Effect on Transportation Pressure Loss in Pipes
  12. 12. Low shear basics Effect on Transportation Emulsion viscosity can be substantially greater than the viscosity of either component. The apparent viscosity of very tight oil-water emulsion is shown as a function of water cut and shear rate: (viscosity of oil alone is 20cp, viscosity of water <1cp)
  13. 13. Low shear basics • When added to water, either as a solution or as powder, polymers increase the water viscosity. • During polymer flooding more oil will be pushed out and recovered from the reservoir compared to only water flooding alone due to higher viscosity of the polymer solution. • Polymer solutions are shear sensitive. • High shear forces during polymer injection results in: – Polymer degradation. – Loss of solution viscosity. Effect on Injection Shear in Polymer Injection
  14. 14. Low shear basics Polymer flooding involves injection of polymer over an extended period of time, until 1/3 – 1/2 of the reservoir pore volume has been injected. The most used polymer for polymer flooding is hydrolyzed polyacrylamide (HPAM). Polymers are very sensitive to mechanical degradation. Shear degradation breaks the polymer macromolecular chain which induces strong reduction in macromolecular size and solution viscosity. Polymer such as HPAM is especially susceptible to degradation at high fluxes and flow through valves, orifices, and in low permeability formations. Effect on Injection Polymer Flooding
  15. 15. Low shear basics When polymers are subjected to high shear, they may readily be broken down. It is only when the polymer is dissolved that the shear forces are harmful and degradation of the polymer occur. The potential shear locations are: - The polymer dissolution facilities: static mixers and pumps - The injection lines: well head chokes and downhole valves - The well bore entry: perforations and sandface. At low shear rate, γ, the viscosity, η, is almost constant (A). The point at which the polymer starts to degrade is called critical shear rate. When shear rate increases, the viscosity is reduced due to shear degradation (B). Effect on Injection Mechanical Degradation
  16. 16. Low shear basics Low Shear Technology – Control Valves Control valves are designed to regulate process specific parameters, such as flow rate, pressure drop, temperature and liquid level. Control valves resemble an orifice of some sort, which can change the opening depending on a signal from a controller. In a conventional control valve, the highest fluid velocity gradients occur right after the restriction. It is where intense turbulence is generated and energy is dissipated. The effective way to reduce shear forces, and hence, the energy dissipation rate in a control valve is to increase the volume Vdis involved in the energy dissipation.
  17. 17. Low shear basics In order to increase the volume over which the velocity gradients are developed, Typhonix’ low shear valve technology (Typhoon® System) utilizes the hydrocyclone principle of the swirl motion of the fluids. The swirl motion in the Typhoon® System leads to maintaining the higher fluid velocities created by the flow through the orifices. In addition, conical shape of valve’s downstream volume accelerates the fluids further in spiral direction, thereby utilizing this volume for the pressure decrease across the valve. Overall pressure reduction happens over a larger volume, thus reducing the shear forces developed in the fluid flow. Low Shear Technology – Control Valves
  18. 18. Low shear basics Typhoon® System performance in simulated and live field conditions has demonstrated that separation efficiency of downstream separators can be improved tremendously. Results show that a 50 to 90% improvement of the oil-in-water (OiW) quality downstream a gravity based separator is possible when replacing a conventional choke valve with the low shear alternative. The graph below shows the water quality as a function of water cut for both the Typhoon ® System and a conventional control valve. Low Shear Technology – Control Valves
  19. 19. Low shear basics The low shear pump technology is highly relevant in produced water applications. High shear pumps may break down oil droplets in produced water, thereby increasing the difficulty of separating these droplets from the water. Following guidelines are usually followed when selecting a low shear pump: - Reciprocating pumps can be low shear, if they are equipped with low shear check valves. - Progressive cavity pumps are usually low shear, but may require high maintenance. - Centrifugal pumps tend to break droplets, but is often considered due to simplicity and low cost. Typhonix has developed a multistage centrifugal pump with specially designed impellers and diffusers. This pump exhibits low shear characteristics and is even capable to increase average droplet size in a typical produced water application. Low Shear Technology – Pumps
  20. 20. Low shear basics When fluids move from high to low intensity turbulence regions, the energy dissipation rate becomes more favorable for droplet-droplet coalescence and respectively - droplet growth. Physics of the droplet size development inside a pump are complex, and require extensive numerical analyses to be predicted. The general observation is that both mechanisms of droplet break-up and droplet coalescence can take place inside the pump housing. The coalescence process in a turbulent flow consists of two sub-processes: collision of droplets and drainage of the fluid film between them. The equation of collision frequency, ωcol , for equally sized droplets is: This equation determines how often two droplets collide. The collision frequency increases if the number of droplets, n, the size of droplets, d, or the energy dissipation rate, ε, increases. Low Shear Technology – Pumps
  21. 21. Low shear basics Pump test results for oil-water mixtures show that the Typhonix multistage centrifugal pump consistently increases the average droplet size (Dv(50)out) for various process conditions. Additional conclusions are that the coalescing effect of this pump is increased when the dispersed phase concentration is increased and the average inlet droplet size is reduced. The coalescing effect is more pronounced for lighter oils as well. Low Shear Technology – Pumps
  22. 22. Low shear basics Low shear technologies aim to reduce level of emulsification in petroleum flows. Less emulsified fluids lead to:  More effective separation process  Reduced need for additional amount of chemicals and heat treatment  Cleaner oil production  Increased flexibility in design of produced water treatment systems  Reduced problems with recycling of water streams  Increased field’s lifetime due to reduced OPEX Benefits of Low Shear Technologies
  23. 23. Low shear basics Low Shear Technologies can be used in the following areas in petroleum production industry:  Choke valves  Level control valves  Produced water treatment, reject, drain/slop  MEG/TEG regeneration Potential Areas of Application
  24. 24. Low shear basics Thank you for your attention!