A Guide To Water Well Casing and Screen Selection   -   16

6.3  Entrance Velocity

Plugged vertical milled slot screen with greatly reduced effective area of opening.

Proper screen design minimizes frictional head losses associated with the entrance of water into the well. This point has been overemphasized in the past, with virtues claimed for screens with the highest area of opening (lowest entrance velocity) which have not been confirmed by field data. Extensive research, including mathematical analysis, review of practical experience and extensive model testing, has established that entrance velocities up to 3.5 feet per second result in minimal head losses. An important factor in determining actual entrance velocity, however, is the "effective area of opening", which may be less than the measured opening due to plugging or invasion of fine material. This is usually the result of poor gravel pack and/or slot size selection. The screen opening should be designed to minimize this possibility.

While there have also been many statements concerning the role of entrance velocity through a screen with respect to corrosion and encrustation, the body of field and theoretical evidence does not prove any relationship with respect to the usual range of velocities through commonly used screens.

Actually, velocities from one to five feet per second are considered beneficial in pipelines. Higher velocities often cause a combination of corrosion and erosion which may be very severe. This condition occurs in wells where most screen openings have been encrusted, leaving the water to be produced from a sporadic few. In addition to losses in production and well efficiency, high entrance velocities through these perforations can cause rapid enlargement and frequent failure. Proper screen configuration and sufficient area of opening mitigate this problem.

6.4  Maintenance and Future Contingencies

The screen should be designed to be as maintenance-free as possible. However, maintenance is a function not only of screen geometry, but also of well design and material selection. The screen should permit deepening or repair if required.

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