Field Experience With Riserless Light-Well Intervention
Authors: Mathiassen Erling , Skeels Harold Brian 
Published: October 2008

Over the last several years StatoilHydro has established demanding ambitions with respect to increased oil recovery from our subsea fields as these fields mature and new prospects become scarcer and harder to access while market demand continues to rise. From the recent past, a gap (12% - 14%) in recovery from our fixed platforms relative to the subsea fields has emerged, even though the quality of the reservoirs is basically the same. To a large extent this gap is related to the high cost in subsea operations in general and well operations in particular. Many operators claim their wells are designed as ‘intervention free’ using sophisticated well designs to avoid future well operations cost. Experience however shows that efficient (i.e. low cost) well intervention solutions are of the utmost importance for increased field recovery as well; and riserless light well intervention (RLWI) is one tool helping to close the recovery gap. RLWI well interventions are steadily gaining industry acceptance since its introduction 2003. This activity has significantly increased since 2005 in the North Sea, requiring at least two concurrent vessels for planned operations in 2008.

RLWI operations are now common in shallow water up to 600 m (2000 ft). However, the current concept is not the ultimate technical solution worldwide, just the first step. Considerable efforts have to be made to improve the concept with respect to efficiency, weather sensitivity and last - but not least - deep water compatibility. We envisage 3000 m (10000 ft.) water depth is well within reach. Technical improvements on the first two fronts are being introduced, building on the campaign results from the pioneering vessels which first deployed RLWI technology. Most likely the ‘step change’ technical improvements will appear when going deep. The paper will discuss important elements in deepwater RLWI operations from:operations and experience in ‘shallow’ waterstechnology solutions required to perform operations efficiently in deep waters

Experience

The number of workovers using RLWI has steadily doubled over the last three years, first to prove the concept's viability then, over the last two campaigns, evaluating operational and hardware performance. In 2006, eleven wells were accessed in five different fields in water depths ranging from 140-310 m (450-1000 ft.) over a 241 day campaign. In 2007, seventeen wells were accessed in eight different fields in water depths ranging from 190-390 m (600-1300 ft.) over a 335 day campaign. Average intervention time per well was nineteen days in 2006 and seventeen days in 2007. These average intervention times included transit time (mobilization and de-mobilization between wells, and crew changes) during periods of poor weather. Typical wireline workover operations during these campaigns included:Running calipers and gauging toolsRunning production logging tools including leak detectionSetting tubing plugs to isolate intervals with high water contentAdded perforation in new production intervalsInstall insert replacement downhole safety valves.