4.8.3.2. Well-Screen Slot Openings Well screen slot openings primarily depend on the size distribution of the aquifer material and also on whether the well is naturally developed or filter-packed (i.e., artificially gravel-packed). Wells in aquifers with coarse-grained (d10 > 0.25 mm) and non-homogeneous material can be developed naturally. But wells in aquifers with fine-grained and homogeneous material are best developed using a filter pack (or gravel pack) outside the well screen.
In a naturally developed well, the screen slot size is selected so that most of the finer aquifer materials in the vicinity of the borehole are brought into the screen and pumped from the well during development. The process creates a zone of graded formation materials extending 0.3 to 0.6 m outward from the screen (6). The slot size for the screen of such wells can be selected from Table 4.8.
