Concentric Coiled Tubing Technology Revolutionizes Logging Sub-Hydrostatic, Single-Completion Wells
Authors: Ababou M. , Shote A. , Sturgeon T. 
Published: May 2016

Single-completion, sub-hydrostatic wells were a challenge for production logging runs because two strings of tubing and dual wellhead would be required, necessitating workover rig operations. However, for smaller cased holes such as 7- or 4 ½-in. casings, running a dual completion string is typically too complex or impossible. When operators had to determine water-entry points in fields with high water cuts, well intervention costs could become prohibitive to contemplate.

Hence, logging sub-hydrostatic wells to determine water entry points was restricted to a limited number of wells of a particular construction. These wells had to be dual-completion wells or 9 5/8-in. cased wells, able to accommodate dual-string completions. One string would be used for running the logging tool via coiled tubing or wireline, and the other would be used for artificial lift and produce the well while logging.

Instead, a conventional concentric coiled tubing (CCT) string can be transformed into a specialty CCT by adding an encapsulated single-conductor wire inside. The string can then be run with a modified vacuum bottomhole assembly and a logging tool to achieve production analysis and logging simultaneously.

Combining these two proven technologies into one solution is an economical way to produce the wells via the CCT and its vacuum BHA while logging the well in one run. It has economic benefits to operators by eliminating the high cost associated with installing dual completions and dual wellheads. It also enables logging in 7- and 4 ½-in. cased holes that were not accessible for production logging using prior technologies.

This paper will describe the revolutionary technology and case histories along with lessons learned. It will also describe in particular the applications of the technology for high-producing water wells.

Introduction

Logging using concentric-coiled tubing (CCT) unit was developed as a potential alternative to "logging while pumping" system, which previously used in South Oman fields to acquire production logs in sub-hydrostatic wells.

For such wells, one major Operator in South Oman had a need of a technology that would allow logging with powered production tools due to the beneficial ability to see data in real-time and flexibility of use. Some logging tools, such as pulsed-neutron tool, are not available in memory mode. Therefore, the need of a powered production tool was highlighted and the development of a new technology was requested.

In response to this need, a new system was engineered and the solution consisted of combining a conventional concentric coiled tubing (CCT) string with an encapsulated single-conductor wire inside.\

To allow a powered tool to be used, a cable must be pumped down the 1-in. CT string to allow tool activation. In addition, two adaptors are required: one to allow the cable to be passed-into the CT string at surface and the second to allow the cable to be passed-out through both 1-in. and 2-in. CT strings downhole. This would transform the string into an "intelligent" CCT.