Improved Concentric Workover Techniques
Authors: Frank Wallace J. 
Published: December 2007

The concentric tubing workover rig has alleviated considerably the high cost of offshore well servicing; and dual, triple and quadruple conventional completions in upper zones can be dealt with economically. But there are a lot of problems yet to be solved, and the inventive technologist still can have his field day.

The Concentric Tubing Concept Basic Technique

The offshore concentric tubing rig is a completely self-contained, highly transportable, skid-mounted workover unit that has been designed for offshore workover operations. The concentric technique calls for leaving the production string in place and running small concentric pipe inside the production tubing to perform a variety of workover operations (Fig. 1) perform a variety of workover operations (Fig. 1) requiring a circulating flow path. The primary reason for performing concentric tubing workovers is economic. Savings result from reduced workover time and reduced rig cost by avoiding the necessity to retrieve and re-run the installed well tubing and packers.

Rig Equipment

A typical rig (Fig. 2) consists of drawworks, mast, two triplex plunger pumps, manifolded measuring and mixing tanks, 1-in. nominal tool-jointed pipe, blowout-preventers, and the necessary handling tools, controls and accessory equipment required for workover operations. The hook-load capacity of these rigs varies from 50,000 to 100,000 lb. The rig is broken down into small packages for fast rig-up and for offloading and onloading with small "stiff-leg" platform cranes. A rig breaks down so that it can be handled in 12 to 17 lifts with an 8,000-lb capacity crane.

Evolution of Use

Concentric tubing techniques are a logical extension of wireline permanent-type completion methods, which used a tubing extension landed and retrieved by wireline. A concentric tubing set-up can be described as a tubing extension that reaches all the way to the surface. Continuous tube to the surface, and the rigs specifically designed to handle it, have resulted in the substitution of concentric tubing workovers for many potential, permanent-type well completion workovers.

The widespread application of tubingless completions where only small concentric pipe strings or wireline tools can be used gave additional impetus to concentric techniques. The experience and confidence gained with this technique in tubingless completions had a material influence on the expanding use of concentric tubing in conventional wells.

After these techniques had been proved on land and in straight holes, the economics made their use in offshore directional wells especially attractive. This discussion covers Humble's experience in Louisiana offshore operations where about four or more concentric rigs have operated continuously for over 5 years. In the past 2 years, the level of this work offshore for Humble has increased to as many as 14 rigs.

Offshore Experience

Tubingless Wells and Lower Zones of Conventional Wells

During this time, 287 jobs were completed offshore on tubingless wells and lower zone completions of conventional wells (see Table 1).

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