Drill String Design & Downhole Tools

Module 2: Drill String Design & Downhole Tools 

2.1 Drill Pipe & Bottom Hole Assembly (BHA)

Drill Pipe:
The drill pipe is the primary component of the drill string, transmitting torque and circulating drilling fluid. Key aspects include:

• Grades: Classified by API standards (e.g., E-75, X-95, G-105, S-135) based on yield strength and toughness. Higher grades (S-135) are used in deep, high-stress wells.

• Tool Joints: Thick, threaded connections welded to pipe ends. They must withstand high torque and tension while maintaining a hydraulic seal.

• Fatigue Management: Repeated stress cycles can cause cracks. Mitigation includes:

• Inspection: Regular ultrasonic/magnetic particle testing.

• Proper Handling: Avoiding doglegs (sharp bends) in the wellbore.

Bottom Hole Assembly (BHA):
The BHA provides weight, stability, and directional control. Key components:

• Drill Collars: Thick-walled, heavy pipes that add weight for bit penetration.

• Stabilizers: Centralize the BHA to prevent well deviation and reduce vibration.

• Heavy-Weight Drill Pipe (HWDP): Transitional pipe between drill collars and standard drill pipe, reducing fatigue at stress points.

2.2 Drill Bits & Performance

Bit Types:

• PDC (Polycrystalline Diamond Compact): Fixed-cutters with diamond-enhanced blades. Best for soft to medium formations (shale, limestone).

• Advantages: High ROP (Rate of Penetration), durable in abrasive formations.

• Roller Cone (Tricone): Three rotating cones with teeth or tungsten carbide inserts.

• Teeth Bits: Soft formations (clay, sandstone).

• Insert Bits: Hard formations (dolomite, granite).

• Diamond Bits: Industrial diamonds embedded in matrix. Used for ultra-hard formations (chert, quartzite).

Bit Selection Criteria:

• Formation Hardness: Soft = PDC, Hard = Tricone/Diamond.

• ROP vs. Durability: PDC offers speed but may fail in hard rock.

• Hydraulics: Nozzle size affects cuttings removal and cooling.

2.3 Downhole Challenges

Stuck Pipe Prevention & Fishing Tools:

• Causes of Stuck Pipe:

• Differential Sticking: Mud pressure exceeds formation pressure, pinning the pipe.

• Mechanical Sticking: Caused by cuttings accumulation or keyseating.

• Prevention:

• Maintain proper mud weight and rheology.

• Minimize stationary time in open hole.

• Fishing Tools:

• Overshots & Spears: Retrieve stuck pipe by latching onto it.

• Jar Tools: Deliver impact force to free the pipe.

Directional Drilling Tools (MWD/LWD):

• MWD (Measurement While Drilling):

• Tracks well trajectory (azimuth, inclination) in real-time.

• Uses mud pulse telemetry or electromagnetic signals.

• LWD (Logging While Drilling):

• Measures formation properties (resistivity, porosity) during drilling.

• Reduces need for wireline logging.

Key Takeaways:

• Drill pipe grades must match wellbore stresses; fatigue management is critical.

• BHA design balances weight, stability, and directional control.

• Bit selection depends on formation hardness and drilling objectives.

• Stuck pipe is mitigated through proper mud management and operational practices.

• MWD/LWD enables precise well placement and real-time data.