Deepwater fields are particularly prone to severe hydrate issues. Hydrate formation may cause significant production losses and, in worst cases, expensive offshore operations to replace completely plugged subsea flowlines. The experience hereto described is related to a deepwater oil field located in Gulf of Guinea. The field was brought on stream by means of six subsea wells directed to a Floating Production, Storage and Offloading vessel (FPSO). Reservoir pressure is sustained by two water and two associated gas injector wells.
One of two gas injectors had shown, since life inception, little to poor performance, probably due to sand control issues not allowing to reach designed rates, with detrimental effects on production. In order to reduce the skin and improve injection, remedial acid treatments in bullheading were carried out from FPSO along subsea flowline. After last pumping operations flowline was found to be completely obstructed, with lost communication between FPSO and subsea XT. Investigations, by means of different approaches, highlighted a probable presence of hydrate blockages, located at touch-down point and in XT proximity.
Decision was taken to mobilize a rig to re-enter well, assess completion status and recover it to functionality. Availability of rig facilities and equipment (such as C/WO riser, nitrogen pumping unit and well test spread) was exploited to execute an innovative procedure of de-pressurization at sea bed depth, bringing hydrates in injection flowline below stable formation pressure and completely melting the plugs.
Flowline was finally flushed and displaced with diesel, preserving it until end of well abandon and rig demob. Operation allowed strong anticipation of gas injection restoration and capital saving, with no need for further offshore operations and new flexible line purchase and deployment.
