document

Abstract

Maintaining well control is a key component of well integrity. Keeping all the valves on the wellhead working and “moving” is important for well control. But ever so often due to calcium carbonate build up, exposure to extreme weather conditions, stem breaking as a result of corrosion and other causes there have been reported cases of valve malfunctioning.

Milling out a gate valve is a very tedious operation and could be risky if the proper well control and procedures are not put in place. To date there are not many documentations on this procedure. Experience indicates there are two typical methods of milling out a faulty gate valve.

• Using a gate valve drilling/milling machine

• Using a coiled tubing unit

Well A and B are both producing wells on the North Slope, Alaska. In well A it was discovered that the master valve would not close fully and the swab valve could not open from the closed position. Well B had a master valve that was damaged in the closed position. Well A’s gate valve was milled out using the milling machine and well B’s faulty valve was mill out using the coiled tubing unit.

The objectives of this paper are to examine these two methods of gate valve milling and the procedures of the operations. After drilling out the valve, fluid can be pumped down hole to kill the well or wireline tools can be run in the well to set back pressure valves and plugs in order to well control.

The risks that would be typical in a gate valve drilling operations will be examined. The risk analysis should be reviewed prior to the start of the job and documented.

Introduction

Well A (Picture- 1) is a gas lifted producer that failed a state issued pressure testing of the surface safety valve because it would not fully close during activation of the automation system. Upon further investigation it was discovered that the master valve below the SSV would only close 27 turns not the 34 turns needed to fully close. In addition, the swab valve, which is currently in the closed position and located above the SSV, would not open even after a substantial force was applied. The possibility exists that a 3-3/8” IBP set downhole for zone isolation has released and causing problems with all the three valves.

In order to be able to replace and/or repair the tree, the well needed to be secure. Two working swab valves were installed on top of the faulty swab to comply with double valves policy. With the two working swab valves in place, and the malfunctioning swab valve drilled out, wireline would proceed to secure the well with two retrievable plugs after which the tree could be safely removed.

Well B is also a gas lifted producer whose 6-3/8” master valve was damaged while in a closed position. The stem threads were stripped preventing any remedial repairs and making the valve inoperative. The master valve was milled out with a coiled tubing conveyed mud motor.

An additional rig up of two extra swab valves as shown in the schematic diagram (see Picture- 2) is necessary in order to maintain appropriate control of the well during the milling operation. The success of the operation is dependent on proper pre-work and design analysis. Equipment selection and draft procedures with risk analysis must be reviewed and discussed prior to the commencement of the operation to ensure the safety of all personnel involved.