What is the purpose of specific capacity when sizing a well screen?

• A. The pump capacity
• B. The casing diameter
• C. The appropriate screen size for optimal flow and minimal drawdown
• D. The drilling mud weight

Answer: (C)

Specific capacity shows how productive the well is for every unit of drawdown, and it guides how big the screen needs to be to get water with the least resistance. It’s calculated as the pumping rate divided by the drawdown at that rate, giving a measure of flow per foot (or per unit head) of drawdown. When sizing a screen, you want enough open area and appropriate slot size so water can enter easily without causing large drops in the water level. A higher specific capacity means you can achieve your target yield with less drawdown, so you select screen characteristics (open area, slot size, and length) that deliver that efficient flow. If the specific capacity is too low, the screen and formation are resisting flow too much, and you’d adjust the screen design to reduce head loss.