Internal corrosion is prevalent in offshore wellheads in the Siri field in the Danish North Sea. Certain wellhead components must be replaced with corrosion-resistant parts. During the wellhead maintenance work, local regulations require a dual barrier and each of these barriers must be tested independently. The casing below the wellhead is completed with fiberglass lining. Standard bridge plugs and standard cleaning methods of tubing are incompatible with the lining and are not considered feasible.
This paper presents an alternative solution that uses two inflatable bridge plugs as an isolation method and provides the ability to remove the scale present in the well in the setting area. It details yard tests performed to ensure the chosen technique does not damage the lining of the tubing and can efficiently isolate the wellhead. It also reviews results of several operations performed between 2006 and 2007, in which inflatable bridge plugs were successfully set and retrieved in the fiberglass-lined tubing.
The original recovery / gas disposal design for Siri field was SWAG (simultaneous water and gas injection).1 This design was derived from the lack of gas export and the ability to lower the gas compression pressure requirements by mixing it with water; however due to the mixture in the injected fluid (ca. 0.5% CO2 in the water, high pressure and therefore high partial pressure of CO2) together with oxygen content created several corrosion issues. The corrosion can severely affect both the tubulars and the wellhead components. The wells require some sort of corrosion protection in order to avoid having to change completion often; however the corrosion protection systems have not always been successful. On some wells of the Siri field parts of the wellhead needed to be changed with inconel inserts due to corrosion while the tubulars are GRE lined. It is required to pressure test the wellheads while having a double barrier in place. To protect the tubulars from corrosion the following options are available:
Install alloy completion
Perform corrosion protection treatments
Install lined tubulars
Each one of these options has advantages as well as disadvantages.
Chrome or alloy completions are the best possible solution as these are resistant to most of the wellbore fluids and have good mechanical properties as well. The main disadvantage is high product cost and ordering lead time, which is much longer than traditional steel. The use of this technique is increasing all over the world particularly in the North Sea as it guarantees long term performance.
Corrosion protection systems like cathodic protection are available and less expensive than chrome or alloy tubulars. However, the main disadvantage is the limited effect on particular types of corrosion where the electrochemical mechanism is clearly understood and in less severe environments.2, 3