VEE-WIR -JOHNSON - Screens
Screen design is the most critical factor for the efficiency of a successful well. The following criteria should be considered to enable the correct screen to be selected:
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Selected Screen must be resistant to:
- Tensile forces relating to supporting its own weight in the whole screen/ casing installation.
- Radial compressive forces created by the formation, which will be higher on completion of well development.
Resistance to Corrosion
Corrosion can occur in instances where materials used are not conjucive with the prevailing well conditions. Water quality is important, but not the only factor. Johnson Screens offer a wide range of technology related materials and surface treatments which minimize the corrosion phenomena.
Selection of Slot Size
The screens purpose is to:
- Prevent sediment from entering the well.
- Support unconsolidated formations or gravel pack.
- Allow formation development
- Minimize clogging.
Selected slot size must give optimum open area: ie. Too small could easily block, too large would allow particles through.
Flow rates and pressure drop
It is generally accepted that the water velocity as it ascends inside the screen should not exceed 1,5m/sec, and that the through-slot water velocity (for sandy aquifers) should not be greater than 3cm/sec so that the pressure drop will be less than negligible when
compared with friction losses as flow ascends in the screen.
Gravel pack selection
Coarse and non homogeneous material can be developed naturally, whereas fine and homogeneous material is best developed using a filter pack - Rule of thumb is that if at least 60% of particles are less than 0,75mm then a filter pack is required, with the screen slot needing
to retain 90 % of the filter pack.
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