Roscoe Moss Company Memo - Well Screen Open Area   -   2

Ground Water - It's Development, Use and Conservation

Author: E. W. Bennison, Editor, Johnson National Drillers
Journal, First Edition, 1947.
Published by: Edward E. Johnson, Inc.

Entrance Velocity As Related To Head Loss

Bennison states:

"Our opinion is that sufficient open area should be provided to keep the entrance velocity down to 0.10 to 0.25 feet per/second. A conservative figure would be 0. 10 foot per second after allowance was made for the portion of the open area that would be blocked off by formation particles."

Apparently this "opinion" was not based on any experimental or theoretical work, He cites no references.

Bennison also points out the velocity of flow through well screen is inversely proportional to the well diameter.

"In other words, doubling the diameter reduces velocity one-half and friction loss to one-quarter."

Restated this means friction loss through screens is proportional to the square of the velocity. This is largely true if the flow is turbulent which it will not be with velocities below 2-4 ft/second. Even with turbulent flow the exponent of velocity may not be exactly two.

In a related theory Bennison asserts in cases where,

"... The openings are small and their total open area is only a small portion of the screen surface area, then the open area in the screen is less than the open area in the voids of the formation furnishing the water and pressure is required to force water through the formation and screen openings."

Pressure is always required to force water through the formation and screen openings and the amount of drawdown per quantity produced is a function of aquifer hydraulic conductivity and transmissivity, not porosity. Possibly Bennison means effective porosity. Ground water moves to a well in a streamline (laminar flow) with each individual particle overcoming frictional resistance under a gentle but persistent hydraulic gradient. Approaching the well screen the flow accelerates. In the near well zone and through the well screen turbulent losses may occur but velocities have to exceed 2 to 4 ft/second entering the opening for significant screen losses.

Bennison continues with the statement

"...as the sand carrying capacity of water depends entirely on velocity, and once entrance velocity has been reduced sufficiently to prevent sand-carrying further reductions are not necessary. This explains why it is not necessary to make gravel packs more than one foot in thickness".

This is of course fallacious since in a gravel envelope well the filtering takes place at the interface between the formation and filter pack. Even in most naturally developed wells sand production depends on factors more important than velocity and many high-capacity wells with in place low area of opening perforations have been successfully developed.

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