Underbalanced Perforation and Completion of a Long Horizontal Well: A Case History

A clastic reservoir in central Oman is to be developed with some 100 wells. The rock is sensitive to formation damage and experience shows that productivity is improved by a factor of 10 by perforating with deep-penetrating charges underbalanced and avoiding contact of brine with the formation. The wells will be horizontal to drain a 24.5m oil rim, and zonal isolation for future water and gas shutoff is desirable. An interval of 600 meters was perforated with 4 ½" OD guns. The guns were retrieved under pressure and a lower completion packer was run to isolate the reservoir from the surface. The upper completion was run with crude in the hole to ensure that no brine should contact the formation. To perforate underbalanced with a drilling rig, the well was constructed with a 7" liner at the start of the build section to total depth of the well. A 7" tie-back was run with a surface controlled downhole isolation valve (DHIV) as a barrier after perforating. The well was displaced to crude to create underbalance. A 7" predrilled liner joint was run beneath the DHIV in the work string to allow circulating out well bore fluids so that the guns could be retrieved safely. A rotating control head and underbalanced drilling surface equipment package was utilized for stripping with 8,000 kPa surface pressure bleed off volumes for tripping and manage the crude circulating system. The tie-back was retrieved and the upper completion was run after setting a plugged packer in the 7" liner. This method is unique because it utilized a conventional drilling rig for completion operations along with an underbalanced drilling package to perforate and complete the well underbalanced.