The standard method of drilling a well, in which a bit attached to a rotating drill string cuts through rock while drilling fluid is circulated to remove cuttings, cool the bit and control downhole pressure.
The basic idea
In rotary drilling, weight from the drill string presses the bit into the rock while rotation grinds or shears it away. Drilling fluid — mud — is pumped down the hollow drill string, jets out through the bit, and flows back up the annulus carrying rock cuttings to surface. There it passes over a shale shaker that screens out the cuttings so the cleaned mud can be pumped down again.
Step by step
- Spud the well. Drilling begins when the bit first penetrates the ground — called the spud.
- Drill ahead. The rotary table or top drive spins the string; the bit cuts; mud circulates cuttings out.
- Make connections. As the hole deepens, new joints (or 30-ft and 90-ft stands) of drill pipe are added to the string.
- Run and cement casing. At planned depths, drilling stops and steel casing is run and cemented to line and isolate the hole before drilling continues at a smaller diameter.
- Reach total depth. The process repeats — drill, case, cement — until the bit reaches the target reservoir.
Top drive vs rotary table
Older rigs rotate the string with a rotary table on the floor that turns a kelly. Modern rigs use a top drive — a motor hung in the derrick that rotates the string from above, allowing longer drilling between connections and far better directional control.
Drilling mud isn't just for cleaning the hole — its weight provides the hydrostatic pressure that balances formation pressure and prevents a blowout.
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Frequently asked
To spud a well is to begin drilling — the moment the bit first penetrates the ground.
Casing lines and stabilizes the hole, isolates zones and protects freshwater. It's run and cemented in progressively smaller diameters as the well deepens.