Updated July 24, 2026 · By Drillstrings Technical Team
A bottom-hole assembly (BHA) is the lower section of the drill string that carries the bit and the tools that control direction and weight-on-bit. In directional and horizontal wells the BHA does far more than make hole — it builds angle, holds a target inclination and azimuth, drills long laterals, and streams real-time data to surface. This guide covers the essential components and how they combine to steer a well.
Why directional BHAs are different
A vertical BHA mainly needs to drill straight and keep the bit loaded. A directional BHA must also deviate the wellbore on a planned path — building angle from vertical, turning to the target azimuth, then holding a high-angle or horizontal course through the reservoir. That demands steering tools, real-time measurement, and careful management of the loads that long, bent trajectories put on the string.
The steering system: motor vs RSS
Positive displacement mud motor (PDM)
A mud motor converts drilling-fluid flow into bit rotation through a power section, and steers using a small bent housing. It has two modes:
- Slide — the string is not rotated; the bent housing points the bit to build or turn angle.
- Rotate — the whole string rotates, averaging out the bend to drill roughly straight ahead.
Motors are cost-effective and versatile, but sliding is slower, cleans the hole less well and can create tortuosity in long laterals.
Rotary steerable system (RSS)
A rotary steerable system steers while the entire string rotates continuously, using either a push-the-bit or point-the-bit mechanism. The advantages are significant in long horizontals: smoother boreholes, better hole cleaning, higher ROP and reduced stuck-pipe risk. The trade-off is higher tool cost and complexity. RSS has become the default for demanding extended-reach and unconventional laterals.
Measurement: MWD and LWD
Steering blind is impossible, so directional BHAs carry survey and evaluation tools:
- MWD (measurement while drilling) streams inclination, azimuth and toolface to surface in near real time, telling the directional driller where the bit is pointing and how the well is tracking against plan.
- LWD (logging while drilling) adds formation evaluation — resistivity, gamma ray, porosity and more — used for geosteering the lateral to stay in the most productive part of the reservoir.
Stabilization and the drilling mechanics
Where stabilizers sit in the BHA controls its directional tendency:
- Full-gauge near-bit stabilizer stiffens the assembly and promotes a building or holding tendency depending on placement.
- String stabilizers control the BHA’s stiffness and standoff, managing build, hold or drop.
- Reamers and hole openers enlarge or condition the hole where required.
Stabilizer count and spacing are tuned along with the bit and motor bend or RSS setting to hit the planned build rate, expressed as dogleg severity (degrees per 100 ft or per 30 m).
The tubular components
Below and above the steering tools, the BHA and string include:
- Drill collars — thick-walled, heavy tubulars that provide weight-on-bit and keep the lower BHA in compression. In high-angle sections their placement is managed carefully to avoid excess drag.
- Heavy weight drill pipe — an intermediate-weight, fatigue-resistant tubular used to transfer weight and manage the transition between stiff collars and limber drill pipe. In horizontal wells, HWDP is often run in the vertical section to push weight to the bit while keeping the lateral string flexible.
- Drill pipe — the main string, typically high-strength (e.g., S-135) for the torque and tension of long laterals. See our companion guide, Drill Pipe Grades Explained.
- Crossovers, subs and stabilizers — connect and stabilize the assembly.
Managing torque, drag and fatigue
Long, bent and horizontal wellbores put demanding loads on the string:
- Torque and drag rise with length and dogleg severity; string design must keep them within the capacity of the pipe and connections and the rig’s top-drive limits.
- Fatigue concentrates at connections cycling through doglegs — controlling dogleg severity and choosing fatigue-resistant components protects the string.
- Hole cleaning in the lateral depends on flow rate, string rotation (favoring RSS) and mud properties; poor cleaning drives cuttings beds and stuck pipe.
- Buckling in compression must be avoided in the horizontal section, which is why weight is transferred from the vertical HWDP rather than pushed along the lateral.
Sound BHA and string design keeps every one of these within limits at every point along the well.
Applications
Directional BHAs serve conventional directional drilling, extended-reach and horizontal development, re-drills and sidetracks in well intervention, and relief-well work. The same principles apply whether the goal is hitting a distant target or placing a two-mile lateral inside a thin reservoir.
BHA components on rental
Directional BHA components — collars, HWDP, stabilizers, subs and crossovers — vary by hole size, build rate and connection from well to well, making them well suited to rental. Renting lets you run correctly sized, freshly inspected components matched to the specific trajectory, without carrying idle specialty inventory between wells.
If you are planning a directional or horizontal well, our team can help you specify the collars, HWDP, stabilizers and high-strength drill pipe your trajectory needs. Contact us with your well plan — hole sizes, build rate and lateral length — and we will help assemble the string.