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Breaking records ConocoPhillips pushes worldwide drilling limits on Alaska’s North Slope Alan Bailey Petroleum News
Drillers, the unsung heroes of the North Slope oil patch, are continuing to push the technology envelope. Precision horizontal drilling threading through thin reservoir sands is enabling high well production rates and the production of heavy oil. Highly deviated and extended reach drilling is pushing access from single well pads to reservoir targets dispersed across wide areas of the subsurface.
And recent record breaking drilling by ConocoPhillips exemplifies this drilling trend.
ConocoPhillips drilling engineer Dennis Hartwig talked to Petroleum News about the 1J-174 D-sands well in the West Sak field — this particular well has recently broken the ConocoPhillips worldwide extended reach drilling record by achieving an extended reach drilling ratio of 6.05 to 1. That’s close to the limits of extended reach drilling ratios achieved by anyone, Hartwig said.
The extended reach drilling ratio is the ratio of the horizontal departure of the well to the true vertical well depth. The departure is the horizontal distance measured along the well path from the well head to the bottom of the well, Hartwig said.
Shallow, heavy oil The West Sak field contains heavy oil, requiring wells that pass horizontally through the reservoir to ensure adequate well production for field viability. Located above the Kuparuk oil field, the West Sak reservoir is at a relatively shallow depth, thus requiring wells that bend quite sharply between the vertical surface wellheads and the horizontal configuration of the well bores in the pay zones.
The bottom of the 1J-174 D-sand well lies in the West Sak reservoir at a vertical depth of 3,055 feet, with a horizontal departure of 18,472 feet, Hartwig said.
The well trajectories also have to bend horizontally into alignment with the array of parallel well bores in the reservoir formation — the overall well pattern looks a bit like a giant grass rake, with the handle at the surface drilling pad and each prong representing an individual well.
“A lot these wells have big sweeping turns in them to get the laterals lined up correctly,” Chris Alvord, drilling team leader for the Alpine field, explained.
The West Sak reservoir zones that ConocoPhillips is now targeting lie a long way from the well pad — zones closer to the pad have already been drilled, Hartwig said. But to reach the more distant zones, the middle sections of the wells need to deviate very precisely to hit the required target, at the point where the horizontal well section needs to start.
“The challenge on a lot of these wells is the intermediate hole section … to land the pipe in the zone,” Hartwig said. “So we drill a long, long high angle trajectory that’s starting at the surface and going out there to 3,000 feet TVD (true vertical depth). Once we land in the zone we set casing. That’s the most difficult and challenging part of our wells.”
Just to compound the technical complexities, wells like 1J-174 involve multilateral completions, in which several horizontal well sections extend from a single well bore that connects to the surface, Alvord said.
North Slope record The recently drilled CD4-07 well in the Alpine field has achieved a horizontal departure of 22,512 feet, a new record for the North Slope, Alvord said. Like West Sak, the Alpine field uses arrays of horizontal wells lined up in the reservoir sands. However, the Alpine reservoir lies deep underground — the vertical depth of the CD4-07 well is 7,436 feet.
The use of state-of-the-art rotary steerable technology is enabling the fast drilling of highly deviated wells such as this and thus minimizing drilling costs, Alvord said. The record-breaking Alpine well took just 32 days to drill, he said. The use of rotary steerable technology also leads to a smoothly shaped well bore trajectory and that reduces the casing cost, Hartwig said.
In a rotary steerable system, rotation of the drill string in the well turns the drill bit as it augurs its way through the rock. Sophisticated technology at the bottom end of the drill string enables the drillers to steer the bit while the string and bit are turning.
And in the case of the 7,500-foot horizontal section of the Alpine well, the drillers had to steer the bit up and down, to thread the well bore precisely through the reservoir formation.
“I think we stayed in zone 100 percent,” Alvord said.
Permafrost West Sak wells face a particular challenge in that the permafrost zone extends from the surface down to a depth of about 1,500 feet: it is much more difficult to steer a well in the permafrost than elsewhere, Hartwig said. But drilling vertically down through the permafrost becomes problematic, because the base of the permafrost occurs halfway to the total vertical depth of a West Sak well. The subsequent deviation of the well would then have to be particularly acute to bend the well trajectory to the horizontal by the time the drilling bit reaches the target reservoir zone.
So, instead of drilling vertically through the permafrost, the drillers have been perfecting the technique of deviated drilling through the permafrost using a mud motor to drive the drill bit, Hartwig said. The flow of the mud that is pumped down the well to remove rock cuttings powers the mud motor.
Once clear of the permafrost, the drillers switch over to rotary steerable technology to finish the well Hartwig said.
But the directional drilling in the permafrost demands great skill.
“So we really work hard at improving our permafrost drilling techniques with standard equipment,” Hartwig said. “We really pay close attention to … experienced personnel … directional drillers.”
What are the limits? Given the record-breaking drilling in West Sak and Alpine, what are the ultimate limits to extended reach drilling?
One of the main constraints relates to the drilling mud hydraulics, Hartwig said. Pumping mud around a long well bore requires high mud pressures to overcome frictional forces that impede the mud flow.
“We have to pump down the drill string and back up the annulus to carry out cuttings,” Hartwig said. “The farther you get out there the more frictional force you have to pump.”
But the maximum mud pressures are limited by the capabilities of the mud pumps and the characteristics of the reservoir formation.
“If you put too much pressure on the formation it breaks down and fractures, and then you tend to lose mud,” Hartwig said.
Pipe strength The material strength of the drill pipe is also an issue, because the longer the drill string the higher the torque that the drill rig has to impart to the string to overcome frictional resistance in the well bore, Hartwig said. That issue drives a need for increasingly large pipe diameters.
With high levels of deviation, servicing the bottom of the well after it goes into production also becomes an issue. Once the drilling rig moves off the well it may not be possible to slide a well intervention tool to the bottom of the well bore. For a lot of the bigger wells companies are waiting for new well intervention technology, Hartwig said.
But, from the perspective of the drilling, Alvord sees the use of the existing conventional technology to reach huge well deviations as a major achievement.
“The key message for me is that it’s done with conventional rigs, conventional mud pumps, conventional hole sizes,” Alvord said.
Doyon Drilling drilled the two record breaking wells, with Sperry Drilling Services providing specialized services, including the downhole tools for navigation, formation evaluation and directional control.
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