Coiled Tubing: From Well Cleanout to Blowout Risk

Lesson Summary

A CT well cleanout scenario and how a routine operation can turn into a hazard if barriers fail.

This lesson is part of the Well Intervention track and lasts 4:11 minutes. Watch the video first, then review the key points and test your understanding with the questions below.

Key Points

1. 1Barriers work as a system, not as a single component
2. 2Continuous monitoring of pressure and weight

Written Lesson

The Scenario

This is an educational scenario based on general principles, not a specific incident. A producing gas well's production is declining due to sand accumulation at the bottom covering part of the perforations. Decision: Clean the well using CT while it is live, without killing it, to avoid formation damage from kill fluids. Shut-in wellhead pressure is 3,000 psi, and the CT diameter is 1.75 in. A routine operation performed daily in many fields. So, where is the hazard?

Barriers in this Operation

In a CT operation on a live well, we rely on two sets of barriers:

• Around the pipe: The Stripper during movement, and the Pipe Rams and Slip Rams in the BOP as backup barriers.
• Inside the pipe: Double check valves in the BHA.
• In extreme cases: Shear Rams and Blind Rams to cut the pipe and seal the well.

Above all, the crew and procedures: Pre-job testing, continuous monitoring, and emergency response planning.

How does the operation become hazardous?

A major accident is rarely a single failure, but rather a chain of small failures. Let's assume the following:

• The Stripper element is worn from a previous job and was not replaced because the test barely passed.
• During RIH near the surface, the pipe is "light," so the injector head is snubbing, and the chain grip pressure is slightly low.
• A small leak appears from the Stripper, so the operator increases its energizing pressure, increasing friction and the force the injector head must push.
• The chains slip. If the Slip Rams and Pipe Rams are not closed immediately, the pipe could be ejected upward out of the well, leaving the path wide open.

Calculated Example: Why is the pipe light? The force exerted by the well pressure to push the pipe out = 3,000 × (π ÷ 4) × 1.75² ≈ 3,000 × 2.405 ≈ 7,216 lbf. The pipe weight is about 2.31 lb/ft, and in a gas well, buoyancy is low; the weight does not overcome the pressure force until about 7,216 ÷ 2.31 ≈ 3,120 ft of pipe is in the well. This means the first 3,000 ft or so is the most dangerous zone, where any slip pushes the pipe out.

Second Scenario: Leak Inside the Pipe

Suppose the pipe is old and near its fatigue limit, and a small pinhole appears above the Stripper at the surface. Well fluids reach this hole through the inside of the pipe only if the check valves fail. If they are working, the leak remains limited to what is in the pipe, and we can pull the pipe above the hole or take a safe decision. However, if debris and sand have prevented the flappers from closing, the hole becomes a direct path for gas to the surface, and the final solution is to cut with the Shear Rams and then close the Blind Rams.

Lessons Learned

• Barriers are a system: Every barrier must be tested and its status known; do not rely on a single barrier.
• Acceptance criteria must be clear: An element that "barely passes" must be replaced.
• Continuous monitoring: Weight, depth, wellhead pressure, circulating pressure, and Stripper leakage. Any unexplained change requires stopping and thinking.
• Know the pipe light zone and calculate the balance point before the operation.
• Pipe fatigue life is not an administrative number; it is part of barrier integrity.
• Emergency drills: The operator must know the shut-in sequence by heart, as seconds matter.

What if?

Think about these questions before watching the solution in the video: What is the first Ram you close if the chains slip and the pipe moves upward? When is cutting the pipe the right decision even though it leaves tools in the well? And who has the authority for this decision on site?

The figures and examples in this lesson are for educational purposes. On the job, always follow your company procedures and accredited training center instructions.

Lesson Glossary