PRIVATE WATER SYSTEMS - Completion - Gravel Pack Design

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.


    Gravel Pack Design
  1. 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**)

  2. 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.
    Gravel Pack Design
  3. 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.
    Gravel Pack Design

  4. Through the point plotted in step 3, draw a smooth curve representing a material having a uniformity coefficient of 2.5 or less.

  5. 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).

  6. Select the slot size that will retain 90% or more of the gravel-pack material.

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.


Gravel-Pack Thickness

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.

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