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Surface Roughness/Friction


Surface Finish of Thermoplastic Liner Products

The installation of a thermoplastic liner that is robust enough to withstand common oilfield handling creates an undesirable ID constriction in downhole tubulars. Because of this diameter reduction, it is common to imply that the mass transfer capacity of the tubular is reduced and/or the pressure drop to move the same amount of fluids will increase. Historical fluid dynamics research by Osborne Reynolds in the 1880’s proved that this is not always the case. Reynolds concluded in his research that the surface roughness of a pipe is one of the five primary variables that determine the capacity and pressure drop of fluid flow in that pipe. He concluded that this influence is greater at higher velocities and in turbulent flow regimes. Measurements using a surface profilometer prove that thermoplastic lined tubulars are approximately 30 times smoother than new (at the mill prior to installation in service conditions) bare carbon steel OCTG: 1.5 X 10-3 mm for the thermoplastic lined ID surface versus 4.6 X 10-2 mm RZDIN values for the ID of new steel OCTG. This difference can be significant. When modeling the flow regimes for producing wells, the surface roughness alone (even when also taking into account the smaller ID caused by the thermoplastic liners) can produce a decrease in the friction component of the pressure drop of over 35 percent (this example assumes high flow rates in tubing restricted conditions). This is just one example showing high rate increases simply by utilizing a theoretically smooth pipe ID surface in a turbulent flow regime. By placing the correct values for surface roughness and nominal ID in modern nodal analysis programs that incorporate accepted fluid flow models using the appropriate flow regimes in pipe, each individual case can be analyzed to verify if a benefit is present and predict the expected magnitude of that benefit. It is important to note that bare steel surfaces will typically corrode or form a passive film on the ID that will cause the surface to become rougher once they are run downhole. However, a properly selected thermoplastic liner is inert to the operating environment and should maintain the smooth surface while in operation. Because of this deterioration of the steel surface, the pressure drop in many production wells can actually increase over time without any other changes in flow conditions. One operator found that using a thermoplastic liner in comparison with bare steel, had experienced a significant decrease in electric usage due to surface roughness of a thermoplastic liner. The reduction was so significant that the cost savings paid for a 4000 foot string of 5½” steel and the liner in an 18 month time frame.

ConocoPhillips performed and funded a Friction Factor Testing. The initial purpose of the test is to determine values needed by rod design software to calculate rod loads using thermoplastic liners since bare steel factors are already known and used. The data showed a reduction in friction between the rod box and tubing ID of 30% to 50% depending on the temperature.

Operators have also concluded that in some instances, they are able to reduce intital capital costs by reducing their pump sizes when using thermoplastic liners to prevent corrosion in flowlines and injection wells.