March 1998 Vol. 219 No. 3  Production

Thomas R. Wright, Jr.,  Editorial Director

Floater-based snubbing unit works over a subsea well

Workover operations normally aren't considered "romantic," but as offshore fields come onstream in ever-increasing water depths, well intervention is demanding more attention. A recent example of the increasing complexity of deep water workovers was a snubbing job performed from a floating vessel in the Gulf of Mexico.

Details of the job were presented in a paper by Tony W. Moak and Terry Prater of Enserch Exploration Inc. (Houston) and Randall Lagendyk and Bernt E. Olsen HWC Inc. (Houma, La.) at World Oil's recent Coiled Tubing and Well Intervention Conference, Houston, Feb. 9 – 11, 1998. Following are some highlights of that presentation.

Enserch and HWC developed a new system for using a hydraulic snubbing jack on the Enserch D/R Garden Banks semisubmersible — a combination drilling and production vessel. Last summer, the snubbing system was successfully used to wash out sand from a subsea oil well produced from the vessel. The snubbing unit stayed rigged up for 17 days, including several days while production testing, with no problems encountered.

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Workover objective. Garden Banks is in 2,100 ft of water in the Gulf of Mexico. Field produces about 12,200 bopd at 1,348 GOR. Wells are gravel packed and use Cameron Spool-type subsea trees. The problem well was producing from two zones, but began making sand and eventually plugged off.

The main objective was to clean out tubing to evaluate the failure mechanism, isolate the problem interval and place the well back on production. Secondary objectives were to pull the primary barrier (Halliburton SRP wireline plug) at the swab entry of the tree, mill out a ceramic flapper valve located at 8,160 ft and reset the primary barrier in the tree when work was completed.

The motivation for selecting a prototype snubbing system over a proven coiled tubing system was the snubbing work string's higher reliability. The 1.66-in. Hydrill CS, grade P-105 work string used has a tensile yield strength over 74,000 lb, compared to 44,000 lb for 1-1/2-in. CT. Higher strength pipe and snubbing rotating capabilities provides increased tolerance for unexpected events.

System design. To compensate for motion, the snubbing unit was suspended in the rig's drill string compensator (DSC). This allowed use of a rigid high pressure (HP) riser and the rig's existing drilling package.

Well control equipment included the standard 18-3/4-in., 10,000-psi subsea BOP with marine drilling riser, which was latched to the swab connection of the tree. This formed a shielded conduit for the HP riser when it was run within the marine riser. In an emergency, the shear/seal rams would be able to shear the HP riser with the snubbing work string inside.

The HP riser connects the wellbore and snubbing system. At bottom, a standard Cameron production tubing landing and testing tool was used to connect the HP riser to the tree. About 2,150 ft of 5-in. drill pipe was used as the riser. At the top, a special cross-over (SXO) to the snubbing well control system was integrated into the tension leg base plate. The SXO incorporates an external swivel that allows the HP riser to rotate while under tension. This function was required in order to lock the riser to the tree connector.

Operation. After debris prevented a wireline pulling tool from engaging the Halliburton SRP plug, a special washing spear was manufactured and run on snubbing pipe to retrieve the plug. A viscous pill, with standard and reverse circulation, was used to bring debris to surface. The plug was retrieved on the first run.

The snubbing BHA, including a 2-1/4-in.-OD blade mill, was run and used to wash with sea water from 4,008 to 8,160 ft where the ceramic plug was located. An attempt to shatter the plug by setting down at high speed was unsuccessful, so the plug was milled using the blade mill. Washing then proceeded to the top of the top gravel pack screen.

The well, which had developed nearly 2,000 psi SIWHP, was killed and the snubbing string was pulled to replace the blade mill with a 2-1/2-in. string mill above a mule shoe to minimize damage to the screens. Washing continued past the upper screens to an R-nipple between the two gravel packed sections.

Kill fluid was displaced with sea water, the snubbing work string retrieved and an isolation plug was set on wireline. The well was flow tested with production reaching as high as 5,000 bpd during a 2-1/2-day period. The amount of sand produced was such that the assumption of a failed lower gravel pack was considered as confirmed.

Copyright © 1999 World Oil
Copyright © 1999 Gulf Publishing Company