Three methods, honestly compared
Mud rotary. A rotating bit, with drilling mud pumped down the rods to carry cuttings up and hold the hole open. The workhorse for sand, clay, soft shale and deep sedimentary wells. Struggles in hard rock — progress slows and bits wear fast.
DTH (down-the-hole) hammer. A pneumatic hammer pounds the bit at the bottom of the hole while air flushes cuttings out. Fast and straight in granite, basalt and hard limestone — the fractured-rock specialist. Needs a serious air compressor, and soft collapsing ground can defeat it.
Combination rigs. One rig that can run both mud rotary and DTH, switching as the formation changes. For mixed or unknown geology — the common case for a contractor who travels between districts — a combination rig avoids owning two machines.
| Ground | Method that works | Why |
|---|---|---|
| Clay, sand, soft shale | Mud rotary | Mud carries cuttings out and holds the hole open in loose ground |
| Granite, basalt, hard limestone | DTH (air hammer) | The hammer fractures rock at the bit; rotary grinds slowly and wears bits |
| Mixed or unknown formations | Combination rig | Switch between mud and air as the formation changes |
Sizing: buy depth margin, not brochure depth
A rig's rated depth assumes favourable ground. The working rule used across this site: choose a rig rated 30–50% deeper than your deepest planned well. A 120 m target well points to a 180 m class rig. Margin is what finishes the wells that turn out deeper or harder than planned — and finished wells are the ones that get paid.
Then match the rest of the system: hole diameter against your casing programme (the hole must clear the casing you will set), a mud pump or compressor sized to the method, and rods and bits for the full depth. A cheap rig with undersized support equipment is not cheap — it is parked.
Power: the electric question
Modern electric rigs drill on external power (grid or a generator) with battery-driven travel gear — zero exhaust at the rig, low noise, and electricity costs well below diesel per metre in most markets. Where sites have grid access or a generator is already on site, electric is usually the economical choice; an emission-compliant diesel configuration remains an option where power is genuinely unavailable. Our homepage works through the comparison, and the models page lists every rig with its reference price.
Go deeper with the two guides this lesson builds on: how to choose a water well drilling rig and mud rotary vs DTH drilling. Water-well models in the Aurora line: Aquilon 180T (entry crawler), Aquilon 300E (mid-size electric), Aquilon 450C (deep combination work).
Get the 8 lessons by email
The free email course is being prepared — one lesson every three days, plus the starter pack (guide PDF, well acceptance checklist, drilling log template). Message us and we will put you on the list for launch day.