Water Well Drilling Practices and Equipment
Core concepts
A candidate must know the main drilling methods and when each is used: rotary air (fast in hard rock, needs a stable hole), mud rotary (good through unconsolidated sands and clays; mud holds the wall and lifts cuttings), cable tool (slow but reliable in boulders and gravel; casing is driven as you go), and auger/hollow-stem methods for shallow unconsolidated work. Understand the rig components - derrick/mast, draw works, rotary table or top drive, mud pump or air compressor, and the bit types (tricone roller, drag, hammer). Know the purpose of drilling fluid: cool the bit, carry cuttings, and maintain hydrostatic pressure against formation pressure. Penetration rate, circulation velocity (cuttings must be lifted - rule of thumb: annular velocity high enough to carry the largest cuttings), and hole cleaning are recurring themes.
Traps
Questions love to invert cause and effect: lost circulation (fluid escapes into the formation) versus a kick (formation fluid enters the hole). Another trap is confusing bit selection with formation: a tricone is not for soft clay; a drag bit is not for hard rock. Watch for questions about blowout prevention and the difference between the drilling fluid's weight and its viscosity. Also beware of "fastest method" questions - speed depends on formation, not method alone.
Worked example / rule of thumb
If a hole is sloughing and cuttings are not returning, suspect low annular velocity or insufficient mud weight. Rule of thumb: keep the mud weight slightly above formation pore pressure but below fracture pressure; if you lose returns, the weight is too high or the formation is fractured - reduce weight and add lost-circulation material.