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Coiled tubing drilling proves its worth on the North Slope The pioneering efforts of Alaska drillers have resulted in breakthroughs in oilfield development and drilling productivity Alan Bailey PNA Contributing Writer
The SLB/Nordic-Calista rig#1 recently completed its 100th coiled tubing well. This landmark occasion signaled the success of a program in which three coiled tubing drilling rigs on the North Slope have revolutionized the economics of oilfield development. The people from Nabors, Nordic-Calista and Schlumberger who have pioneered these rigs continue to extend the life of the Prudhoe Bay oilfield.
Coiled tubing drilling involves the use of a single length of up to 60,000 feet of coiled, narrow-diameter tubing: the coiled tubing replaces the 30 or 90 foot lengths of rigid piping used in conventional drilling. A motor, powered by drilling mud, turns a drill bit at the bottom end of the tubing, while other tools in the bottom hole assembly control the direction in which the hole is drilled. While the drill bit gouges its way through the rock formations, the operators in the drilling rig slowly unwind the coiled tubing into the well hole. Increased drilling efficiency The use of coiled tubing provides immediate benefits by reducing the time taken to drill a well. By avoiding the need to assemble and disassemble lengths of pipe, it becomes possible to trip tubing in and out of a well quickly. Udo Cassee, operations manager at Schlumberger, explained that it normally takes two and a half to three hours to drill 10,000 feet using a conventional rig. “(With coiled tubing drilling) we do that in an hour and a half,” he said.
However, the ability to do precision directional drilling through an existing well bore provides the really big payoff. Drillers can insert the narrow diameter coiled tubing down into a well, right through the well head assembly. As a result, there is no need to drill a completely new well, or to fully disassemble the well head. In a typical operation, the tubing cuts out at an angle through the side of the older well, to create a so-called sidetrack well. Drillers can steer the sidetrack well several thousand feet horizontally to penetrate a small, isolate pocket of oil.
“Coiled tubing drilling provides us with a very cost-effective way of access to diminishing pockets (of oil) that are left in Prudhoe Bay,” said Mark Stanley, coiled tubing drilling superintendent for BP Exploration (Alaska) Inc. Stanley explained that coiled tubing proves particularly effective in Alaska, because of the depth and shape of the reservoir. “The sheer geometry of what we’ve got ... has made coiled tubing drilling possible for us,” he said.
Cassee also emphasized the use of coiled tubing to drill into small oil pockets. “If you get a large drilling rig on the slope with jointed pipe, you need to have a certain size of oil pocket to drill into to make it worthwhile,” he said. With the efficiency of coiled tubing drilling we can penetrate small oil pockets in an economical way, he explained. Development in the early 1990s Although drill crews have employed coiled tubing for well remedial work since the 1970s, no one considered using it for drilling until the early 1990s. People had been using coiled tubing to install cement plugs when they got to thinking that they could use coil to do drilling, said Cassee.
Cassee said that between 1991 and 1993 ARCO and Schlumberger made several attempts to prove that coiled tubing could be used to drill sidetracks out of existing well bores. In 1994 Nabors started a full time coiled tubing drilling program for ARCO, using a work over rig.
While these early ventures at the new drilling technique progressed, equipment manufacturers quickly latched onto the need for new equipment - coiled tubing required special small diameter tools, such as drill bits and drill motors.
“People didn’t believe you could drill with (coiled tubing) ... then when they saw that it was easy to drill with S they started developing the tools for it,” Phil Snisarenko, operations manager for Nordic-Calista, said.Building an integrated rig ARCO’s first forays into coiled tubing drilling involved operating a coiled tubing unit with an existing drilling rig.
However, coupling a coiled tubing unit with a conventional rig can prove problematic, because of differences in the design of the equipment. “A lot of times when they needed a coiled tubing unit on a drilling rig they had a hard time getting it on,” Snisarenko said.
When the need arose for an additional coiled tubing rig, people started thinking about integrating the coiled tubing unit right into the rig. ARCO, Schlumberger and Nabors had proved the drilling technique. An integrated rig design would make rig operation more efficient.
Nordic-Calista and Schlumberger came up with the idea of converting SLB/Nordic-Calista rig#1, a self-propelled, work over completion rig, built in 1986. The two companies risked funding the conversion, while BP provided the incentive of an initial 10 well contract for the rig.
The team which converted the rig in 1996 broke new ground. “At the time when it was built, worldwide, it was unique,” said Cassee. Snisarenko explained that they needed to engineer the rig cantilever with new equipment and then ensure that the rig could move around when carrying the coiled tubing unit. The companies empowered the construction team to find creative solutions to the technical problems involved.
With the initial drilling contract enabling only one chance to prove the converted rig, success became imperative. “I told our people that this was going to be a hell of a rodeo and we’d better come out of the chute running,” Snisarenko said.
“It was fun,” said Dave Smith, Schlumberger maintenance manager. “And the people that worked on that rig ...on the Nordic side and the Schlumberger side had to know that they’re making their own future,” he said. Records started falling Drilling records started falling as soon as the converted rig went into operation. “The very first well we did was the least expensive that we had done in Prudhoe,” said Smith. The second or third well was the fastest that had been drilled.
With more and more work to do, the efficiency of the rig continuously improved. Eventually the rig was drilling so fast, that the well planners struggled to keep up with the drilling schedule.
Several factors contributed to the improved efficiency: increased drilling experience, more reliable equipment and a better understanding of where to use the coiled tubing drilling technique. “The equipment we’re using is becoming very, very reliable,” Stanley said. “The scope of the wells we drill is (also) well understood.”
And SLB/Nordic-Calista rig#1 continued to excel. “We drilled 40 percent more wells in the year 2000 with that particular (coiled tubing drilling) rig than we did in 1999,” said Stanley. “I think in terms of exceptional performance, that was it.”
Three coiled tubing drilling rigs now operate on the North Slope: the SLB/Nordic-Calista rig#1, Nabors CDR-1 and Nabors 3S/Dowell Unit No. 4. These rigs all provide a crucial component of the development of Prudhoe Bay.
“We’ve drilled over 250 coiled tubing sidetracks in Prudhoe Bay already and plan to probably drill another 500 (coiled tubing) wells in the future,” Stanley said. “We would not be able to drill these wells using conventional technology.”
Stanley also said that BP used coiled tubing to drill over 80 percent of the new oil which the company developed in the year 2000.
But, the story of coiled tubing drilling is really the story of the people who developed the technology and who continue to operate the rigs. The majority of the development of coiled tubing drilling occurred in Alaska. Many of the Alaskans who developed the rigs still operate them. The skill and experience of the drilling teams underpin the success of the drilling program.
With the search for smaller and smaller pockets of economic oil in Prudhoe Bay, coiled tubing drilling is likely to continue play a key role in extending the life of the field. “Coiled tubing drilling will probably put the lights out on the slope,” Snisarenko said. “They’ll be the last rigs to leave — they’ll shut the gate and lock the door.”
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