Read Thomas Kwader's suggestions for selecting gravel packs for water wells.
Ideally, the material placed outside of a screen (often called the sand pack, filter pack, gravel pack, etc.) of a production well (much less critical for monitoring or observation wells) should be selected to match the size of the grains in the naturally occurring adjacent formation. A sieve analysis should be performed on granular formation samples for production wells prior to selecting the screen slot size or drilling the actual production well. The best producing formations generally are uniform in grain size and have an average (or median) large grain diameter. Potential well yield begins to decline as the grain sizes vary and the grains become progressively smaller. For example, poorly sorted clays, silts and fine sands are poor producers of water, however, clean, well sorted sands and gravels produce large volumes of water.