Nippleless Completion System for Slimhole/Monobore Wells
Authors: Hoprnann M.E. 
Published: October 2008

INTRODUCTION

In the current era of growing cost and environmental constraints in oilfield operations, the fact that reducing the diameter of a borehole can significantly cut drilling costs has enhanced the popularity of slimhole drilling. However, reminiscent of horizontal drilling in the '80s, the potential for industry-wide success of slimhole drilling in the '90s is contingent upon the development of completion systems that will allow long-term economic management of the reservoir.1 An emerging trend, particularly in areas where a high degree of reservoir wokover activity is anticipated, is to complete the wells as monobores-that is, to allow unlimited access to the reservoir and enable operators to leave the tubing in the well for the life of the completion. Completion of slimholelmonobore wells poses an interesting set of challenges. Those challenges are intensified in the North Sea, where water injection produces barium and strontium sulfide scale which prevents tools from being successfully set in landing nipples. To address these challenges, Baker Oil Tools is in the process of developing an integrated nippleless completion system composed of three primary products:a production bridge plug;a retrievable slimhole straddle system;a disappearing plug.

The production bridge plug is a wireline set, retrievable bridge plug that replaces the traditional lock and nipple and performs all its functions in the well with increased flexibility. The retrievable slimhole straddle system is capable of straddling and isolating producing or injection intervals in casing of up to 300 feet, while maintaining an ID large enough to allow the passage of through-tubing inflatable equipment, guns, etc.. This system is run on coiled tubing or electric wireline, and is ideal for selective or permanent zone abandonment of long or short producing intervals in any instance where anoperator requires the flexibility of running through-tubing inflatable equipment through the bore of the tool in order to allow access to the zones below the straddle. The disappearing plug, run in place as part of the tailpipe and used only for the initial completion phase, eliminates wireline trips in the well and allows the completion to be run in the hole underbalanced.

BACKGROUND

For the past 20–30 years, the conventional offshore completion in the North Seawas designedwith 9 5/8"casingfrom the surface; a 7" production linerwith aproduction packer landed on top; 5 1/2" tubing, and two different nipple sizes (i.e., 4 1/8" and 4) downhole, with a perforated flow joint between the two nipples and a sliding sleeve above, for circulation. The completion was capped by a subsurface safety valve. See Figure 1. This conventional type of completion presented problems when reservoir workovers (i.e., cement squeeze jobs, etc.) were required. Because the ID of the linerwas larger than the ID of the smallest nipple, performing workovers necessitated pulling all the tubing out of the well and milling out the packer to get the workover tools down to the formation. The only tools capable of going through the 4" nipple then expanding into the 7" liner are through-tubing inflatable products. Thru-Tubing technology has somewhat limited performance for high pressure applications caused by the fact that the element has to expand to a diametertwo to three times larger than its OD.