Coiled tubing becomes light while running into a live gas well
A coiled tubing unit is rigged up on a live gas well with 4,200 psi shut-in wellhead pressure. The CT is 1.75-in OD with 0.134-in wall. The CT BOP stack has blind rams, shear rams, slip rams and pipe rams. Before running in, the crew function-tests the BOPs and pressure-tests them to 5,000 psi. At 3,000 ft, the CT string weight decreases from 8,000 lbf to 2,000 lbf as well pressure acts on the end. The injector head is rated 20,000 lbf snub force. The well starts to flow slightly at the return line. The CT supervisor wants to continue to 8,000 ft to set a plug. The well has 2 7/8-in tubing and 4½-in casing. The CT BOP is 5,000 psi rated.
1. Why does the CT string weight fall, and what does light mean?
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The weight falls because well pressure acts upward on the cross-section of the CT end. As the CT goes deeper, more of the string is surrounded by well fluid and the buoyancy effect increases. The string is light when the indicated weight is less than the force needed to push it into the well, or when the well pressure can push it back out. At 2,000 lbf the string is not necessarily floating, but the injector must hold it and provide snubbing force to keep running in. Light CT is a pressure-control concern because the well can push the string out of the hole if the injector or slips are not controlling it. The crew must not rely on string weight alone to keep the CT in the well; the injector, slip rams and pipe rams must be used according to procedure.
2. Should the supervisor continue to 8,000 ft? Give the decision and actions.
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The supervisor should not simply continue. A slight return-line flow could be normal displacement as the CT enters the well, but it could also be a kick or a sign that the well is unloading. The crew should stop running, hold the CT with the injector and slips, and perform a flow check with the pumps off. They should compare return volume with the calculated displacement and watch for increasing flow, pressure at the wellhead or gas. If the returns match displacement and pressures are stable, running can resume with close monitoring. If flow increases or the well pressure rises, the BOP should be closed and the well killed. Continuing to 8,000 ft while the string is light and the well is flowing could allow the CT to be ejected or the well to become uncontrolled.
3. What is the role of the slip rams and shear rams in this situation?
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The slip rams hang off and hold the CT so the injector can release its grip without the string being pushed out by well pressure. They are used when the string is light or when the crew needs to stop and secure the CT. The shear rams are the emergency barrier: they cut the CT and seal the well if the string cannot be controlled, for example if the injector fails and the CT is being pushed out. Pipe rams seal around the CT and allow circulation or holding; blind rams seal the well when no CT is present. In this case, the crew should be ready to use the slip rams to hold the light string and the shear rams only as a last resort, because shearing leaves a fish in the well. The BOP configuration and closing sequence must be confirmed before running continues.