If the slot openings to be selected on the basis of a
naturally developed well are less than 0.01 inches wide,
construction of a gravel-pack should be considered. With a gravel pack,
the slot openings can then be much wider, improving the efficiency of
the well.
If the water is extremely alkaline (incrusting), a lower
limit of 0.015 tp 0.02 inches may be used.
It is also helpful to remember that experience may dictate a much
different set of limits. For example, on the Coastal Gulf Plain,
experience has shown that the best design width for naturally
developed wells is 0.006 inches.
In thick artesian aquifers where a long screen is required
and the pump is to be positioned above the screen, a smaller diameter
screen can be centered in the borehole, and the annulus filled with
gravel. This is preferable to using a shorter screen of a diameter
about the same as that of the borehole
In formations exhibiting extensive lamination or alternating
layers of fine and coarse material, it is difficult to determine the
precise position and thickness of each individual layer and to choose
the proper length of each section of a multiple-slot screen
corresponding to the stratification. Designing the well for a gravel
pack reduces the chance of making an error. The size of gravel-pack
material should be based on analysis of the finest material in the
water-bearing portion of the aquifer.
In boreholes in loosely cemented sandstones, sand particles
tend to slough from the walls of finished open holes. Since most
sandstones are fine-grained, 0.005 inch or smaller screen openings
may be called for. Providing a gravel-pack will allow wider screen
openings.
Another reason for installing a gravel pack is that in loosely
cemented sandstones, the formation material usually provides little
or no lateral support for the screen - the formation does not readily
cave or slump against the screen as in unconsolidated formations.
Instead some void spaces are left around the screen leaving open the
possibility that chunks of material could fall off and damage the
screen.