Handling and Prevention Measures for Drill String Accidents

**Handling and Prevention Measures for Drill String Accidents**

1. **Concept:**
Drill string accidents refer to various incidents involving drill pipes, drill collars, and other downhole tools and auxiliary tools. The drill string left in the well after an accident is called a “fish,” and the top of the fish is referred to as the “fish top” or “fish head.”

2. **Common Drill String Accidents and Causes**

– **(1) Drill String Breakage:**
– **Drill Pipe Breakage:** Caused by reduced strength due to poor pipe quality, thinning of the pipe wall, damaged slips causing marks, external damage, or corrosion. The pipe breaks under high tensile or torsional loads (e.g., during sticking, jamming, or forced rotation). This usually occurs within 1.2 meters of the pin end.
– **Drill Collar Breakage at the Thread:** The drill collar has high rigidity, but the threads are smaller and weaker. Breakage occurs at thread roots or stress concentration points near the tool joint, due to combined loads of compression, torsion, and bending, or poor thread quality.

– **(2) Drill String Back-off:**
Back-off refers to the unthreading of the tool joint without damage to the threads, often caused by jamming, reverse rotation, or back-off operations.

– **(3) Thread Slippage:**
Thread slippage happens when the threads disengage under load due to excessive wear, improper make-up, drilling fluid erosion, or non-standard thread profiles.

3. **Handling Drill String Accidents**
After a drill string accident, the appropriate recovery method should be used based on the nature of the issue. For back-off or slippage, fishing tools are used to recover the fish. If recovery fails, the incident is treated as a breakage, and appropriate fishing tools are used.

– **(1) Common Fishing Tools:**
– **Overshot, Spear, Taper Tap, Die Collar, Safety Joint, Impression Block, Wall Hook, Back-off Sub, Magnet Fishing Tool, etc.**

Common tools include the spear for fishing inside the fish and the overshot for engaging the outside of the fish. Magnetic tools are used to recover small metal debris.

– **(2) Fishing Tool Applications:**
– **Spear:** Used for internal fishing, effective for thick-walled sections like tool joints, drill collars, or the thickened parts of drill pipes. Not suitable for thin-walled pipes or casing.
– **Taper Tap:** Used for external fishing, applicable to thinner-walled pipes like drill pipes or tubing.
– **Overshot:** Engages the outside of the fish, suitable for strong pulling and rotation. It can handle a wide range of sizes by changing the slips inside the tool.
– **Fishing Basket/Gripper Spear:** Similar to the spear, used for internal fishing and suitable for various downhole tools and casing.

– **(3) Procedure for Handling Drill String Accidents:**
– **Preparation Stage:** Identify the accident and plan the recovery operation.
– **Fishing Preparation Stage:** Select appropriate fishing tools based on the nature of the fish.
– **Fishing Operation Stage:** Execute the fishing operation, ensuring proper use of tools and techniques.

4. **Prevention of Drill String Accidents**

– **Causes of Accidents:**
Fatigue failure, corrosion, mechanical damage, and operational errors.

– **Indicators of Potential Accidents:**
– Decrease in hook load.
– Decrease in pump pressure.
– Reduction in rotary torque.
– No progress in drilling.

– **Preventive Measures:**
– **A. Storage and Handling:**
– Drill pipes should be stored on pipe racks, not exceeding three layers, with even supports between layers.
– Assign specific crews to manage drill pipes to avoid mixing different batches. Use drill pipes with similar service times whenever possible.
– Clean the internal and external surfaces of stored drill pipes, inspect for corrosion, and apply anti-corrosion measures.

– **B. Inspection and Maintenance:**
– Mark damaged pipes clearly with paint and tags for easy identification.
– Before running drill pipes into the well, inspect specifications like steel grade, wall thickness, thread condition, and bore size.
– Regularly perform non-destructive testing (NDT) on in-service drill pipes.
– Follow strict operational procedures to avoid over-stressing the drill string.

– **C. Proper Operation:**
– Avoid sudden impacts or overloading during operations, especially during jarring operations.
– If the cause of a pump pressure drop cannot be identified on the surface, pull the drill string to inspect it.

– **D. Regular Inspection of Connections:**
– Periodically break out and inspect tool joints and connections. Replace any damaged parts, and ensure re-threaded drill pipes undergo several make-ups and break-outs to ensure smooth threads.

– **E. Handling and Transport:**
– Use thread protectors during tripping, and handle drill pipes carefully to avoid damage to connection ends.
– Clean threads thoroughly and apply thread compound before making up connections.

– **F. Torque Control:**
– Make up connections with the appropriate torque, ensuring neither under-tightening nor over-tightening.

– **G. Systematic Pipe Rotation:**
– Periodically change the position of drill pipes in the drill string to even out wear and stress.

– **H. Other Precautions:**
– Ensure proper thread alignment during make-up to avoid thread damage.
– Maintain drilling fluid pH around 9.5 to reduce corrosion.
– Use corrosion inhibitors in high-salinity environments.
– Avoid using bent pipes, especially bent square drill pipes, in the well.
– Ensure new or re-threaded pipes undergo proper breaking-in procedures.
– Rotate drill pipes regularly, typically every 2500-3000 meters for drill pipes and every 1000-1500 meters for drill collars.

By following these handling and preventive measures, the frequency of drill string accidents can be significantly reduced, ensuring safer and more efficient drilling operations.