Directional Drilling
Steering a drill bit thousands of feet underground to hit a target the size of a small room. Here's how it actually works.
Why Drill Directionally?
Vertical wells only contact a reservoir where they pass directly through it. Directional and horizontal drilling let a single wellbore travel laterally through a reservoir, dramatically increasing the surface area in contact with hydrocarbon-bearing rock. This is the technology, combined with hydraulic fracturing, that unlocked shale plays like the Permian and Eagle Ford.
Directional drilling is also used to reach reservoirs positioned under obstacles — city infrastructure, protected areas, or neighboring leases — from a surface location that wouldn't otherwise have access, and to drill multiple wells from a single offshore platform in different directions.
How the Bit Is Steered
A directional well is steered using a combination of:
- Downhole (mud) motor — a turbine-like tool near the bit that converts drilling fluid flow into rotational power, allowing the bit to turn independently of the drill string above it.
- Bent sub / bent housing — a slight mechanical bend that, combined with the motor, lets the driller build angle by "sliding" (not rotating the whole string) in a chosen direction.
- Rotary steerable systems (RSS) — modern tools that steer while continuously rotating the entire string, which improves hole cleaning and ROP compared to sliding with a motor.
MWD and LWD — Seeing Underground
MWD (Measurement While Drilling) tools sit in the BHA and continuously measure the wellbore's inclination and azimuth, transmitting that data to surface in real time — typically via mud pulse telemetry, which encodes data as pressure pulses in the drilling fluid.
LWD (Logging While Drilling) tools add formation evaluation data — resistivity, density, gamma ray — to that same real-time stream, letting the directional driller and wellsite geologist actually see what kind of rock the bit is in without waiting for a wireline log run.
Well Profiles and Build Rate
Directional wells are not freehand. The path is planned as a profile, and the shape is chosen to keep torque, drag and casing wear manageable rather than to take the shortest route.
Build rate — expressed in degrees per 100 ft — is the governing number. A conventional well might build at 2–3°/100 ft. Medium and short-radius wells build far harder, and every degree of build adds contact between string and wellbore, which turns into drag on the way in and torque while rotating.
| Profile | Shape | Typical use |
|---|---|---|
| Build and Hold | Kick off, build to angle, hold to target | The standard slant well; simplest to drill and to case |
| S-Curve | Build, hold, then drop back toward vertical | Where the target must be entered near-vertically, or to avoid a shallow hazard |
| Horizontal | Build to roughly 90° and stay in the reservoir | Unconventional and thin-reservoir development; maximises reservoir contact |
| Extended Reach | High angle held over very long departure | Reaching targets far from the surface location, often from an offshore platform |
Two limits show up in practice. Torque and drag rise with angle and departure until the string cannot be rotated or slid reliably. Hole cleaning gets harder above about 50–60°, because cuttings settle out of the flow onto the low side of the hole and form beds instead of being carried out — which is a mud property and flow rate problem as much as a geometry one, and ties back to mud weight and hydraulics.
Dogleg Severity
Dogleg severity is the actual measured change in hole direction per 100 ft, combining build and turn. Planned build rate is the intention; dogleg severity is what the hole really did.
It matters because doglegs are where things wear and break. A sharp dogleg concentrates bending stress on drill pipe every single rotation, which is a fatigue mechanism rather than an overload one — the string can be well within its tensile rating and still fail there over time. The same bend drives casing wear as pipe rotates against it, and can make casing difficult to run at all.
This is one of the clearest cases where the drill string and the well path are the same problem: the profile decides the loads the string will live with for the rest of the well.
The Directional Driller Role
Directional drillers are specialists, usually employed by a service company rather than the drilling contractor. They monitor the MWD data stream continuously — often from a control cabin — making steering adjustments to keep the well on its planned trajectory. It's one of the higher-paying specialist tracks in drilling; see the salary guide for current figures.