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.
If a formation has naturally occurring, well-sorted, clean, fairly large sands and/or gravels, a filter pack may not be necessary. The well can be developed by allowing the formation material to collapse against the screen and develop the natural sand pack by pumping and/or surging to remove the finer grains from the formation and filter pack.
The filter pack size should be selected based on the sizing of the formation grains where only about 5 percent to 20 percent of the finer portion of the formation material is allowed to pass through the filer pack and the screen slot size during development.
Proper well development by using a surge block (a topic for another discussion) is critical to the short- and long-term yield of the well. Ideally, a well should be surged until the water becomes crystal clear and no fines settle in samples collected in clear glass containers. Effective removal of the fines is necessary to prevent plugging of the filer pack and screen.