BP Exploration's program of high angle wells on the periphery of Miller field in the U.K. North Sea has resulted in the most demanding well undertaken by the company's drilling department.
The seven wells are being drilled from a template with the Santa Fe 135 semisubmersible prior to installation of the platform next summer. Miller is to start up in early 1992 and produce about 113,000 b/d.
DAUNTING HURDLES
The fifth well in the program, 16/8b-AO5(37), a water injector, generated torque that stretched the rig's 38,000 ft/lb capacity top drive to its limit.
As a contingency, BP considered subsea completions for water injection, but they were not required because well AO5 demonstrated that the high angle wells were achievable from the template.
In addition to operating at unprecedented torque levels, the drilling department had to deal with a number of unscheduled problems. A differentially stuck pipe forced a time consuming plugback and sidetrack, followed by a broken logging cable and a small well kick.
Net result was a 126 day well instead of the planned 82 days and a demonstration that the torque problem could be overcome.
HIGH TORQUE DRILLING
The 16/8b-AO5(37) was drilled to a total depth of 20,755 ft with a 56 inclination and true vertical depth of 13,875 ft.
The two final high angle wells are now under way and scheduled to reach a total depth of about 19,500 ft.
The first four wells in the program indicated the torque problem could be acute in the fifth well, early seen as the most demanding in the program.
To handle expected problems, BP's Aberdeen drilling department planned a smooth profile for the 171/2 in. section to minimize torque loss. In the 121/4 in. section, torque was carefully monitored and drill pipe rubbers were fitted-a move that in previous wells led to a 25% torque saving at the surface.
BP said the 30,000-32,000 ft-lb of torque experienced was not too dissimilar to predictions from the company's drillstring simulator. Torque was always seen as critical in the high angle wells, and there was concern whether the motor would be able to turn the pipe.
ANTICIPATING PROBLEMS
To handle expected problems, the rig was fitted with a 38,000 ft-lb rated top drive and 6,500 ft of 6-5/8 in. pipe used to ensure this size pipe was run throughout the buildup zone.
Fitting rubbers to the 65/s in. pipe is still experimental. In the earlier high angle wells, nine rubbers per stand were fixed down to a maximum of 4,000 ft TVD-basically a temperature limit. Previously the rubbers came off the pipe too readily even though the 2,000 lb sideload limit was not exceeded.
BP said nine rubbers might see a lot. But they stayed on the pipe and allowed drillers to get to total depth with some torque to spare.
The drilling department also kept an eye on the bit profiles, because some developed more torque than others.
"When you are talking about the difference between being able to drill and not being able to drill, every 100 ft-lb is critical." BP said.
TOP DRIVE PROBLEMS
High drilling torques that in certain formations were extremely erratic tended to stall the top drive. The top drive motors, adapted from locomotive motors accustomed to more constant loads, had problems coping with these varying loads.
At one stage, with torque constantly above 30,000 ft-lb, the brushes on the top drive motor were being changed every 50 hr. A new cooling system has been developed for the final part of the program and seems to have eliminated this problem. For similar wells on BP operated Bruce field, a 44,000 ft-lb top drive will be used.
As well as handling the problems of torque, BP said there were other notable achievements. The 13% in. casing was set at 8,924 ft at 52. At the same angle, the 121/4 in. hole was drilled to 19,794 ft, leaving 10,870 ft of open hole to be cased. BP thinks this could be a North Sea record.
A directional correction run was also undertaken at 18,500 ft, the deepest the services companies on the well had ever seen.
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