The Britannia field has been on production for approximately 10 years. Produced fluids are retrograde gas condensate with some associated water. In recent years, as the reservoir pressure has declined, some of the wells have entered a region where flow has become unstable due to the phenomenon of liquid loading. In a small number of cases the effects of liquid loading are so pronounced that the wells remain largely shut in and are not considered to be part of the “production pool”. In others a cyclic clean up program is employed which involves routing the wells periodically through the test separator at low pressure to unload the associated liquids.
In 2007 a Low Rate Well Strategy was developed in order to identify possible solutions to address this issue of liquid loading. One such strategy was that of continuous foam injection at or just above the producing interval. This chemical application works by lowering the minimum liquid unloading rate of an individual well, thus enhancing the ability of that well to lift liquids from the wellbore.
The method selected by Britannia to enable continuous injection of the chemical foamer was via a permanently installed thru- tubing capillary string. Capillary tubing is similar in appearance to coiled tubing. However it is much smaller in size, the outer diameters ranging from 1/4” to 5/8” and is more akin to the control line used to contain fluid for the operation of sub surface safety valves.
The process by which a suitable chemical foamer for Britannia was selected and tested, both from an effectiveness and compatibility standpoint, will be discussed.
The first operation to install a 3/8” capillary string on a Britannia well was successfully completed in May 2009. This was followed by a second installation in July 2009. The procedure included the installation of a modified tubing bonnet, which allows chemical injection at the wellhead, and deployment of a capillary tubing system which contains an integral sub surface safety valve. This system provides a flow path for produced fluids, and allows continuous injection of the chemical foamer from surface to the producing interval, whilst maintaining full well integrity.
This paper will discuss the first installations of this system in the UK sector of the North Sea. The planning and design activities, followed by the offshore implementation, will be described in detail. Candidate selection and a review of well performance, both pre and post capillary string installation, will also be included.
Whilst the “Wellhead Modification System” has been employed for the initial installations, simplified designs are being reviewed for future Britannia wells and these will also be discussed.