Horizontal Well Completion, Oseberg Gamma North
Authors: Bjørkesett Halvard , Saetersmoen Eli 
Published: October 2008

Completion and well-cleanup of the Gamma North horizontal well are described.

The 8 1/2" horizontal borehole was drilled with a saturated salt solution as drilling mud. A 6 5/8" prepacked screen was ran on drillpipe with 4 1/2" washpipe inside the screen before the mud-cake was removed.

The prepacked screen was plugged inside with a kill-pill to maintain well control before the remaining completion string was run.

The kill-pill from inside of the screen was produced back when opening the well for clean up flow. Analy8is of the test results from the well indicate that different reservoir zones contribute to flow. The calculated skin is approximately zero. These results confirm that the well clean up operations were successful.

INTRODUCTION

The Gamma North Structure is located on the Western Flank of the Oseberg Field off the western coast of Norway (see figure 1).

The structure consists of a 26 m thick oil zone underneath a gas cap. The reservoir is located in the Upper Statfjord formation, which in this area is multilayered sandstone.

The primary objective of the Gamma North well-is to-produce gas, after first having produced what is technically and economically the recoverable reserves of the oil leg.

After an estimated period of 3 to 4 years of production, it is forecast that the watercut will have become too high to continue oil production. At this stage the plan is to mechanically plug off the horizontal section and open up the gas zone without pulling the completion.

The 800 m horizontal section of the Gamma North well was drilled using a saturated sodium chloride brine polymer mud.

The prepacked screen was run on drillpipe with washpipe located inside the screen.

The washpipe and screen assembly had circulation ports in the upper and lower ends which allowed for circulation and squeezing of undersaturated brine to remove filtercake from drilling. In addition, the washpipe allowed for circulation out through the bottom of the screens while running into the hole.

Only undersaturated brine without breakers or acids were used during filtercake removal. Before retrieving the drillpipe and washpipe the well was killed with a saturated salt pill inside the screen.

The inside screen kill-pill wag designed such that it bridged off against the screen and the gravel to keep the well stable during the completion operations.

To obtain underbalance, the tubing string was displaced with diesel before stinging the tailpipe into the sealbore of the screen packer. When opening the well, the kill-pill was produced back. The clean-up flow was performed until a certain level of cleanliness was achieved. Pressure gauges were run on a wireline string before the main flow started.

PLANNING

During the planning phase it became evident that coiled tubing could not be used for filtercake removal.

The reason being that the coiled tubing cutter valve in the lower riser package could not cut 1 3/4" coiled tubing with cable inside and seal adequately afterwards. A cut and seal valve was an absolute requirement to maintain well control in an emergency situation. A new valve could not be delivered in time.