Concentric Coiled Tubing Technology Revolutionizes Logging Sub-Hydrostatic, Single-Completion Wells

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.