Installation of 2 7/8-in. Coiled-Tubing Tailpipes in Live Gas Wells
Authors: Campbell J.A. 
Published: December 2007

Summary
This paper describes a technique for installing 2 7/8-in. coiled tubing astailpipe extensions below existing production packers in live gas wells. It also covers the use of coiled tubing as a way to complete wells. Large savings in rig time and deferred production have been realized with this technique. Fluid losses to the formation do not occur, and no expensive rig time is needed to kill or clean up the wells, as required for conventional workovers below existing production packers. This technique is particularly applicable in depleted reservoirs that could be impaired by traditional workover methods.

Introduction
The Leman gas field, discovered in the early 1960's, has been producing gas from the Rotleigendes sandstone since 1969 (Fig. 1). As cumulative production has increased, reservoir pressures have declined to the point where the formation cannot support a column of fluid, causing problems whenever workovers that require kill fluid across the perforated interval are attempted. These operations can result in large fluid losses to the formation, leading to formation damage and delayed reinstatement of production. In most cases, any tubular changes or work associated with completions is performed by setting isolation plugs in the existing tailpipes, which prevents any associated problems with loss of kill fluids to the producing interval. As production rates have declined, the ability of individual wells to lift produced fluid has been reduced, so liquids accumulate in the wellbore. Liquid loadup in the wellbore has resulted in increased hydrostatic pressure on the reservoir, further reducing the energy available to produce gas.
A number of Leman wells have been worked over recently with a cantil ever jack up rig to replace the original 5 1/2-in. production string with a 31/2-in.-diameter string to improve production velocities and fluid removal from the wells. Although these operations have continued, it was recognized in 1991that the existing tailpipes would have to be extended several hundred feet down to the perforated interval to optimize production velocities in the casing sections below the production packers.
With the known problem of performing workovers across the perforated interval, the decision was made to use wireline to install 2 7/8-in. tailpipe extensions with the wells still live. Any technique to install tailpipes would be financially beneficial only if it could be done without introducing kill fluid into the producing formation. The maximum length of tailpipe that could be run on wireline with current techniques was about 50 ft. Post commissioning results have shown that tailpipes of this length do not provide sub stantial benefits. It was recognized that the tailpipe length would be crucial for maximizing the lifting capacity of the produced gas below the production packer.
Therefore, it was decided to initiate a program to install 27/8-in.-diameter tailpipes below the existing production packers in lengths of up to 700 ft. The length of the tailpipes to be installed was dictated by the distance between the existing tailpipes and the perforated intervals, which inmost cases was from 500 to &gt;700 ft. Installation of these tailpipes would ensure increased gas production velocities in the entire casing sections (Fig.2).

Workover Procedure and Implementation
Combining the installation of these tailpipe extensions with replacing the original larger-diameter production string with a smaller-diameter production string more suited to the reduced production rates of the field appeared cost-effective. As discussed, because of well kill problems, we decided to install the tailpipes on coiled tubing with the wells live. The coiled-tubing-installed tailpipes would be left in the completions with wireline plugs in place, effectively leaving the well isolated and ready for arig to perform a conventional tubular replacement.
This technique would have the following benefits.The coiled-tubing installation could be carried out on the production platform before the workover rig was on location, thus saving rig time.The operation using coiled tubing to install the tailpipes could be performed with the wells live.The addition of wireline-retrievable plugs in the bottom of the installed tailpipes would mean that the wells could be left suspended, ready for the workover rig to perform the tubing replacement (Fig. 3).After replacing the tubing, the rig could be released immediately and the wells brought on production by use of platform- based nitrogen equipment to evacuate the workover fluid before the wireline plugs in the 2 7/8-in. tailpipes were pulled, which was the previous workover practice.
An installation procedure involving the following steps was designed.From the production platform, rig up 2 7/8-in. and 1 1/2-in. coiled-tubing blowout preventer (BOP) equipment. Run the required length of 27/8-in. coiled tubing into the well; complete with wireline plugs in the bottomhole assembly (BHA) (Fig. 4).Hang off the required length of 2 7/8-in. coiled tubing in the BOP system. Close both sets of tubing rams/slips and cold-cut the pipe on surface. Install a permanent packer on 2 7/8-in. coiled tubing (Fig. 5).Rig down the 2 7/8-in. coiled-tubing injector; rig up the 1 1/2-in. coiled-tubing equipment; and attach the 1 1/2-in. coiled tubing to packer and packer running tool (Fig. 6).Run 2 7/8-in. tailpipe to depth on 1 1/2-in. coiled tubing. Tag the no-gonipple and space out for depth correlation (Fig. 7).
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