PRIVATE WATER SYSTEMS - Completion - Gravel Pack Design
A properly designed gravel-pack with the right ratio of formation
to gravel-pack grain sizes will provide mechanical retention of formation
sand and prevent sand from moving into the well.
Construct sieve analysis curves for the aquifer material, and determine
which stratum contains the finest material. Base selection of the grading of
the gravel pack on the analysis of the finest stratum.
(**Be sure to label curve lines**)
Multiply the 70% size of sand (from the stratum chosen in step 1) by a
factor between 4 and 6. Use 4 when the stratum is fine and uniform and 6
when it is coarse and non-uniform (multiply by a factor between 6 and 9 when
the formation is highly non-uniform). The number calculated is the 70% grain
size for the gravel in the gravel-pack.
On the same graph depicting the analyses for the aquifer, plot the 70%
grain as the first point of the curve representing the grading of the gravel
pack.
Through the point plotted in step 3, draw a smooth curve representing a
material having a uniformity coefficient of 2.5 or less.
To 4 or 5 sieve sizes that cover the size range shown by the grain size
analysis of the gravel-pack, assign a permissible range for the percent
retained on each sieve (i.e. the smallest sieve may retain from 90 to 80
percent of the material).
Select the slot size that will retain 90% or more of the gravel-pack
material.
Material should be clean and well-rounded. Limestone and other
calcium rich material should be avoided; 5% calcareous material is a
common acceptable level of calcium in gravel packs.
Shale, anhydrite, and gypsum should also be avoided.
To insure that an envelope of gravel-pack material will
completely surround the well screen, a thickness of three inches is
considered practical for installing in the field. Experience has
shown that thicknesses of more than 8 inches are not any more
beneficial but, in fact, make the development process more difficult.