Contents preface (VII) introduction 1—37

Table 4.8 Selection of slot size for well screen (17)

Uniformity coefficient of	Condition of the overlying	Slot size in terms of
the aquifer being tapped	material	aquifer material size
> 6	fairly firm; would not easily	^d70
	cave in
> 6	soft; would easily cave in	^d50
= 3	fairly firm; would not easily	^d60
	cave in
= 3	soft; would easily cave in	^d40

If more than one aquifer is tapped, and the average size of the coarsest aquifer is less than four times the average size of the finest aquifer, the slot size should correspond with the finest aquifer. Otherwise, slot size must vary and correspond with the sizes of aquifer material (16). A more conservative slot size should be selected if: (i) there is some doubt about the reliability of the samples, (ii) the aquifer is thin and overlain by fine-grained loose material, (iii) the development time is at a premium, and (iv) the formation is well-sorted. Under these conditions, slot sizes which will retain 40 to 50 per cent of the aquifer material (i.e., d₆₀ to d₅₀) should be preferred (6).

In filter-packed wells, the zone in the immediate vicinity of the well screen is made more permeable by removing some formation material and replacing it with specially graded material. This filter pack or gravel pack separates the screen from the aquifer material and increases the effective hydraulic diameter of the well. A filter pack is so designed that it is capable of retaining 90 per cent of the aquifer material after development. Well screen openings should be such that they can retain 90 per cent of the filter pack material (6). The filter pack material must be well graded to yield a highly porous and permeable zone around the well

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screen. The uniformity coefficient of the filter pack should be 2.0 or less so that there is less segregation during placing and lower head loss through the pack. The filter pack material should be clean and well-rounded. Clean material requires less development time and also results in little loss of material during development. Well-rounded grains make the filter pack more permeable which reduces drawdown and increases the yield. Further, the filter pack must contain 90 to 95% quartz grains so that there is no loss of volume caused by the dissolution of materials. For minimum head loss through the filter pack and minimum sand movement, the pack-aquifer ratio (i.e., the ratio of the average size of the filter pack material to the average size of the aquifer material) should be as follows (16):

	Pack-aquifer Ratio
(i) Uniform aquifer with uniform filter pack	9–12.5
(ii) Non-uniform aquifer with uniform filter pack	11–15.5

The thickness of the filter pack designed in this manner should be between 15 and

20 cm.