Wellbore Trajectory Contro

• Control of Inclination Change Rate (Build/Drop Rate): This requires a properly designed bottom-hole assembly (BHA) that manages the rate at which the wellbore angle increases (build) or decreases (drop).
• Control of Azimuth Change Rate (Turn Rate): This requires specially designed downhole tools (such as bent housing motors or rotary steerable systems) that control the rate at which the wellbore direction changes left or right.
• Control of Weight on Bit (WOB) and Formation Force: Proper selection and management of WOB controls the bit's rate of penetration (ROP) and the magnitude of formation reaction forces, directly influencing the wellbore curvature (dogleg severity).

The following tools are commonly used in conventional directional drilling operations to achieve the above control objectives.

1. Orientation Sub (Directional Sub)

Two main types exist:

• Direct Orientation Sub: Used with positive displacement motors (PDMs) that have a bent housing.
• Bent Orientation Sub: Used with straight-body PDMs. This sub incorporates a structural bend angle (λ) to provide the necessary toolface for steering.
• Structure: Both types feature a housing, a centralizing sleeve, an orientation key (for aligning the toolface with the motor), and set screws. The bent version has a built-in bend angle.

2. Non-Magnetic Drill Collar (NMDC)

• Function: Shields magnetic survey instruments (e.g., MWD magnetometers) from the magnetic interference of the steel drill string, ensuring accurate azimuth measurements. It also provides weight on bit like a standard drill collar.
• Materials: Common materials include Monel alloy (copper-nickel-chromium), chromium-nickel alloy, austenitic chromium-manganese alloy, beryllium copper, SMFI non-magnetic steel, and domestic manganese-chromium-nickel steel.
• Length Selection: Determined by consulting geomagnetic field strength maps. Higher magnetic field strength regions require longer NMDCs to effectively isolate interference.
• Inspection & Use: Straightness (≤2mm/m, ≤5mm total), surface defect limits, smooth thread surfaces, proper make-up torque (≥ specified minimum), and annual checks for relative magnetic permeability and magnetic uniformity according to standards (e.g., SY5145-86).

3. Heavy Weight Drill Pipe (HWDP)

Used at the bottom of the BHA to replace some drill collars. Its advantages in directional drilling include:

• Reduced contact area with the borehole wall, minimizing friction and torque.
• Improved sliding capability (when using a mud motor without rotating the pipe).
• Increased downhole safety.
• Serves as a transition between drill collars and standard drill pipe, easing stiffness changes.

4. Stabilizer (Centralizer)

Functions in directional drilling:

• As a Fulcrum: In building or dropping assemblies, changing the stabilizer's position alters the BHA's mechanical behavior, controlling the trajectory.
• Increasing Stiffness: In holding assemblies, reducing the distances between the bit and stabilizer(s) creates a stiffer BHA that resists changes in inclination and azimuth.
• Reaming & Conditioning: Smooths borehole irregularities, reducing the risk of keyseats and other problems.
• Inspection: Entry and exit measurements of outer diameter, wear checks (OD wear ≤2mm), and proper placement are critical.

5. Keyseat Wiper (Keyseat Breaker)

Geometrically similar to a spiral stabilizer but with a smaller OD (larger than drill collars but smaller than a full stabilizer). Its upper and lower tapered shoulders are hardfaced with tungsten carbide for cutting and reaming.

Applications:

• Dedicated Keyseat Removal BHA: Bit + small-OD collars (50-60m) + keyseat wiper + jar + HWDP. For longer keyseats, a flexible sub may be added.
• While Drilling: Run the keyseat wiper as part of the BHA in build sections, reciprocating through known doglegs to prevent keyseat formation.
• Operation: When running in, slow descent above the keyseat depth (≈100m), and ream with very low WOB (<49 kN).

6. Float Valve

Prevents reverse flow of drilling fluid (backflow) from damaging downhole survey tools (e.g., MWD electronics) and prevents bit nozzles from becoming plugged by debris.

7. Landing Sub (Hanger Sub)

Provides a secure, stable environment for mounting MWD instruments. It features a high-side scribe line on its outer wall for aligning the motor's toolface and measuring the offset (OFFSET value) between the MWD and the motor. Inside the sub, a protruding key is used for MWD latching. Caution: If not using MWD, the landing sub should be removed from the string to prevent the key from being washed out by high flow rates, which could lead to a downhole failure.

8. Jar (Drilling Jar)

Maintains balanced working conditions for the bit and BHA. Its primary function is to deliver a sharp impact (upward or downward) to help free the string if it becomes stuck.

9. Flexible Sub

Used primarily to protect the jar by providing additional elasticity in the BHA, especially during jarring operations. It also helps reduce bending stresses in stiff assemblies.

Successful wellbore trajectory control requires a systematic approach: managing inclination change, azimuth change, and weight-on-bit, while deploying a carefully selected suite of tools. From orientation subs and non-magnetic collars to stabilizers, keyseat wipers, and jars, each component plays a specific role. Understanding these tools and their interactions allows directional drillers to design effective BHAs, minimize non-productive time, and accurately guide the wellbore to its geological target. For more detailed information, please don't hesitate to contact Vigor team for more detailed product information.