Extending the Operating Life of Mature North Sea Assets by Big-Scale Offshore Installation of Large-Diameter Chrome Coiled-Tubing Velocity Strings

Abstract

In the Southern North Sea, many fields are suffering from declining gas production due to depletion of the reservoir and associated liquid loading. At some time during the operating life of offshore production facilities, the ability for these facilities to keep operating economically at a further decline in gas production, will need to be evaluated. To avoid early abandonment of these offshore assets, well deliquification or liquid unloading techniques, like Velocity String (VS) installations, are considered for extending the operating life of these mature fields.Large diameter (2 3/8 and 2 7/8-in.) chrome coiled tubing (CT) strings are being installed in deep gas wells in the Southern North Sea, as part of a large-scale installation project, including some 20 wells over a period of two years. A self-propelled jackup work vessel is used to house all the coiled tubing, well test and associated equipment needed to enable well interventions on the small production platforms and NUI's (Normally Unmanned Installations). Critical to the success of the project was the cross-discipline approach to preparing, selecting and testing appropriate coiled tubing hardware and tools, hazard analyses and logistical coordination.In this paper the preparation of the project, the operational details and lessons learned are presented. Candidate well-selection criteria and the choice for the selected coiled tubing material and size are discussed as well. Due to the size of the project, which is a worldwide first of this magnitude and set-up, both in terms of investment and quantity of offshore installations, good case histories for large diameter offshore chrome velocity string installations are provided.At time of writing not all the velocity strings have been installed. The velocity strings installed thus far have yielded incremental gas production of the order of 50,000-150,000 sm 3 /d (1.7 -5.3 Mscf/day) per well, which is in line with expectations. Based on the initial production results and production expectations for the remainder of the wells, the production life of these assets is expected to extend with many years.