“Subsea wells represent the area where least well intervention work has been done and where the biggest practical and technical challenges lie,” Lars Fjaertoft of Statoil said in a presentation at the 2011 SPE/ICOTA Coiled Tubing and Well Intervention Conference and Exhibition last week in The Woodlands. According to Mr Fjaertoft, Statoil is a proponent of subsea riserless light well intervention (RLWI) and uses this method as a primary means of intervention in its 460 subsea wells in 34 subsea fields on the Norwegian Continental Shelf.
Statoil has been using this form of intervention for more than 10 years and has three specialized RLWI vessels on contract. The company’s goal is to achieve an average recovery factor of 55% for its subsea fields, and this form of intervention is seen as a means to improve recovery and increase production efficiency. More than 125 RLWIs have been performed since 2000.
The RLWI process is performed from a monohull intervention vessel. The wireline blowout preventer is placed on top of the Christmas tree and is operated by a multi-bore umbilical. All operations are run in open sea through the moonpool.
The RLWI activities have been found to improve HSE in subsea well interventions, improve recovery and cut intervention cost by more than half compared with anchored rigs and riser systems. “The RLWI concept does not have any hydrocarbons back to surface,” Mr Fjaertoft said. “Hence the fire and explosion risk onboard the intervention vessel is greatly reduced. The well pressure is contained subsea so there is a limited amount of pressurized equipment topside.” Also, the equipment is handled with wire, lowering the risk of fallen objects.
“The RLWI concept has made the access to production data more cost efficient, and production logging has become a large part of the work performed,” Mr Fjaertoft said. Also, perforations made later in the life of a well are very costly. “The availability of a cost-efficient intervention method enables perforations to be staged over time to increase the recovery factor.”
With the use of RLWI, when an insert valve is required, the cost is significantly reduced because a semisubmersible is not required. Remediating scale buildup no longer requires the use of a semi. By using milling equipment on wireline in combination with a wireline tractor, small-scale bridge have been milled out and full-bore access to the wells have been reinstated.
“RLWI has become a routine operation with increasing demands and continuously improved operational efficiency as an ongoing activity to meet Statoil’s overall ambitions for RLWI,” Mr Fjaertoft said.
For more information, please see SPE 143296, “Success From Subsea Riserless Well Interventions,” by L. Fjaertoft and G. Sonstabo.