Rigless Completions: A Spoolable Coiled-Tubing Gas-Lift System
Authors: Moore B.K. , Laflin W.J. , Walker E.J. 
Published: October 2008

Coiled tubing (CT) has been used as production tubing in a variety of remedial completion applications. In the early applications, CT was installed in existing production tubing as siphon/velocity strings. The concept was expanded to provide gas lift in conjunction with CT installations. This paper describes the development and implementation of a spoolable CT gas lift system which enhances CT gas lift capabilities.

Introduction

Standard side pocket, slim hole eccentric or concentric mandrels have been coupled to CT and installed inside existing production tubing with documented success. These standard mandrels inherently exceed the CT diameter precluding the mandrels from being run through the CT injector head and conventional annular pack-off assemblies. As a result, the CT must be cut at each gas lift station as the string is run into the well. An access window between the injector head and well control equipment is often required to facilitate installation of the gas lift mandrels in the CT string. The mandrel upset may require additional annular well control equipment which complicates running a gas lift string into a live well (See Figure No. 1- Rig-up for installation utilizing access window). Additionally, mandrel size is a limiting factor in some existing wellbores with internal restrictions.

Elimination of the mandrel upsets improves the efficiency and applicability of CT gas lift installations. A design that eliminates upsets obviously requires that the gas lift mandrel diameters equal that of the CT. Additionally, the gas lift mandrels and valves must be flexible to allow spooling with the CT.

Spoolable Gas Lift Valve and Mandrel Design

Traditionally, downhole gas lift equipment is designed to be rigid and resist material yielding under expected service loads. In contrast, CT is plastically deformed as it is spooled onto a reel. In order to produce a spoolable gas lift assembly, it is necessary to mount the gas lift valve inside the mandrel and provide flexibility to tolerate bending to the radius of a reel. Since the mandrel has the same diameter as the CT, the first compromise to be recognized was that the flow area adjacent to the valve is restricted in CT production applications. For 2-3/8 in. O.D. CT, a 1 in. O.D. gas lift valve was selected to give a good balance between valve performance and restriction in production flow area (See Figure No. 2- Spoolable gas lift valve).

Installation of the valve inside the mandrel is accomplished by permanently anchoring the valve into the receptacle section of the mandrel. The receptacle provides a single radial port allowing injection gas to enter or exit the mandrel through the valve. O-rings on the valve isolate the production fluid pressure from the injection gas pressure. The receptacle features multiple by-pass slots to allow the produced fluids or injection gas to pass axially along its length and past the gas lift valve. The receptacle and adjacent sections of the gas lift valve assembly remain rigid during spooling operations. The remainder of the mandrel will assume the radius of curvature when spooled onto a reel or over the guide arch.