Well Construction
Core concepts
Well construction ties together hole diameter, casing, screen, gravel pack, and the seal. Know the sequence: drill, set casing, grout the annulus, develop the well, and test. Understand the difference between a screened well (unconsolidated aquifers) and an open-hole or bedrock well. Screen slot size is selected from the grain-size analysis of the formation; the gravel pack is sized to the formation (rule of thumb: gravel pack 4–6 times the formation grain size, screen slot smaller than the smallest gravel). Know the purpose of the sanitary seal, the annular space, and the wellhead. Understand development (surging, jetting, airlifting) to remove drilling fines and improve yield.
Traps
Questions often confuse screen slot size with gravel pack size - the slot must retain the pack, not the formation. Another trap is the order of operations: grouting must happen before development, and the seal must extend from the surface down to the required depth. Watch for confusion between static water level and pumping water level, and between specific capacity and yield. Also, a "sanitary seal" is not the same as the grout column, though they overlap.
Worked example / rule of thumb
If formation grains are 0.5 mm, a gravel pack around 2–3 mm (4–6×) and a screen slot smaller than the pack (e.g., 1.0–1.5 mm) is a reasonable starting point. Specific capacity = pumping rate ÷ drawdown; a declining specific capacity signals clogging or formation damage.