New Muscle for Coiled Tubing
Authors: Kilgore M.D. 
Published: October 2008

This paper describes a downhole retrievable hydraulic force generator for coiled tubing. The purpose of this system is to hydraulically generate downhole push and pull forces, thereby eliminating the need to stretch and compress the string of coiled tubing to accomplish mechanical work downhole. This system allows forces to be safely generated downhole which could exceed the physical limits of the coiled tubing if applied in the conventional manner, which is pushing and pulling with the surface injector assembly. Hole deviation and frictional drag do not detract from the forces generated. The work produced by the tool downhole is therefore entirely predictable since it is simply pressure acting upon a system of engineered, hydraulic devices.

Engineering laboratory and horizontal force testing results are presented, as well as functional test results in a vertical test well. Measured shear tests of control specimens confirm calculations of force based on pressure times area. Shearing and shifting action of downhole tools are smooth and controlled since impact forces are eliminated. Applied stress on the coiled tubing is limited primarily to internal pressure. This system effectively extends the capability of coiled tubing beyond its normal limits for performing push or pull operations. It permits use of standard slickline completion equipment and techniques regardless of well deviation.

INTRODUCTION

Since its beginning in the mid 1960's, coiled tubing has been used with success for nitrogen kickoffs, well cleanouts, acidizing, and cementing. These services, for the most part, do not tax the physical limitations of the coiled tubing. 1,2 With the advent of highly deviated wells, and more recently horizontal and extended reach wells, coiled tubing has been used for push/pull operations such as running, pulling, and shifting downhole flow control devices, including slickline plugs and sliding-sleeve circulating tools. 3 This is because the rigidity of the coiled tubing permits it to be used in angled and horizontal wells where loss of the assistance of gravity makes slickline operations impossible.

Using coiled tubing for push/pull operations has its limitations. The lateral reach attainable for coiled tubing is dramatically educed as the ratio of production tubing I.D. to coiled tubing O.D. gets larger4. As wells get deeper and more deviated, much of the strength and capacity of coiled tubingto do work downhole is lost to frictional drag in the wellbore.5 Hydraulic jars and impact tools have been used in an attempt to overcome these frictional losses. However, friction can become so great that jars cannot be adequately loaded to perform properly. This is especially true for jarring-down operations. Helical lockup of the coiled tubing inside the well tubulars can prevent weight from reaching bottom.

What was needed was a device or system which could locally generate forces downhole, thus reducing the total load the coiled tubing had to provide. The coiled tubing could then assume the primary role of providing hydraulic pressure, and delivering and retrieving tools used to engage or actuate downhole devices or "workstations."