A Guide To Water Well Casing and Screen Selection   -   15

6.0  WELL SCREEN REQUIREMENT

6.1  Aperature Size and Internal Design

Over the years, many different types of screens have been developed to meet the varied requirements of water wells. Common sense suggests that there is no single type best for all conditions. Well screen types have different attributes and characteristics and may be equal in some respects, but unequal in others. To assist in evaluation, the following general requirements are set forth:

The well screen should have the largest possible aperture size consistent with retaining the filter pack in a gravel envelope well, or formation material in non-gravel envelope wells.

The internal configuration of the screen should facilitate well development and redevelopment through accommodation of the most effective mechanical methods available, such as tight swabbing.

6.2  Economy

Economy is not only a general consideration, but a special one, if cost dictates the use of less screen or screening of intervals only in gravel envelope wells. These practices have had unfortunate results in many cases. Often even if the best sampling procedures are followed and electric logging performed and interpreted, selective screening can result in inadvertent blanking off of productive aquifers. Another important consideration is the bridging tendency of gravel at the top of any screen section. It is difficult and sometimes impossible to consolidate gravel behind blank casing. Possible outcomes include sand pumping or sealing off of water through sloughing of impervious material against the aquifers. This does not apply to the bottom of the well where a 10 to 20 foot section of blank casing should be installed to provide a basin for sediment material.

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