Emerging Steps for a Proven Technology
Slimhole CTD Targets Shallow Plays
Get ready for a quiet revolution in drilling.
The next few years will begin a dramatic increase in coiled-tubing drilling (CTD), slimhole drilling and microbore applications. That could bring a handsome payoff to anyone who solves the downhole geology puzzle.
And the biggest prize might be existing, low-production wells.
This drilling technology can’t be used everywhere, but with the right conditions it produces substantial cost savings and minimizes both formation and environmental damage.
It’s one more reason geologists need to keep up with emerging drilling applications.
“Overall, I think this kind of drilling will replace conventional rotary drilling in the shallow depths all across the United States,” said Kent Perry, managing director for E&P research at the Gas Technology Institute (GTI) in Des Plaines, Ill.
The definition of slimhole keeps changing, and getting slimmer.
Once used to describe wellbores less than seven inches in diameter, then less than six inches, slimhole now generally includes wells completed at 4.75 inches or smaller.
Slimhole CTD carries the unusual status of being both an emerging and a proven technology. Frequently used in western Canada and on Alaska’s North Slope, CTD has been deployed to some extent around the world.
But drilling with coiled tubing never became commonplace, and a broad new range of slimhole tools and equipment is just beginning to appear.
“I think right now is prime time for moving coiled tubing equipment into the United States,” said David Wennerstrom, technical sales representative for Technicoil Corp. in Calgary.
Small drill pads, no mud pit, less drilling mud, smaller access roads, minimum noise: There are many reasons why slimhole CTD is considered environmentally friendly.
A Good Start
A CTD field test conducted by Perry and GTI, with funding from the U.S. Department of Energy, shows why.
Perry said the test employed a self-contained and fit-for-purpose -- actually more built-for-purpose -- CTD rig.
“Oftentimes in the past, you’d have some kind of derrick someone would set up and then they’d bring in a coiled tubing unit,” he said.
In this case, the highway-capable drilling rig could be quickly set up and moved from one location to another.
Perry credited DOE’s Roy Long and rig designer Tom Gipson of Advanced Drilling Technologies Inc. for making the project go.
In addition to reducing drilling costs, the 50-foot long slimhole CTD rig is highway capable -- and good at moving through fields, too.
With Rosewood Resources of Dallas as operator, the rig was deployed to drill gas wells in the mature Niobrara formation play of western Kansas and eastern Colorado.
Perry said the nature of the shallow, underpressured and low-permeability formation made economics a critical factor.
In the Niobrara, “you can’t afford to spend 20 days drilling a horizontal well,” he noted.
Slimhole CTD met the requirements for speed, low impact and minimal chance of formation damage. The test included drilling 40,000 feet of hole for 23 wells from 1,500 to 3,100 feet deep.
“Long story short, by the end of the field test they were able to drill one 3,000-foot well a day,” Perry said.
“We were out there measuring everything carefully,” he added, “because we wanted to know exactly how much time each activity was taking.”
With faster drilling and shorter on-site times, cost savings ranged from 25 to 35 percent per well.
“There were significant environmental savings,” Perry said. “Location costs were minimal. No pits were required, other than a small pit for the cuttings.”
Other advantages included:
- Good hole quality.
- Improved safety.
- Minimal cuttings.
- Low emissions and noise.
- Reduced chance of damaging the underpressured formation.
“You’re not surging,” Perry said. “Every time you make a connection (with jointed pipe) you get a surge, which can aggravate formation damage.”
The Perfect Storm
In Canada, Technicoil operates a rig fleet that includes hybrid CTD/pipe drilling rigs and CTD well service rigs.
“What developed here was a desire to drill shallow gas wells, less than 6,000 feet in depth. It’s a very suitable area for coiled-tubing drilling,” Wennerstrom said.
“Basically, it’s kind of the perfect storm,” he added. “You have a lot of things that came together at the right time” to popularize CTD.
Wennerstrom called 2-7/8-inch tubing the workhorse of the industry for drilling. At one time, he noted, quality was so low that people said CT stood for “continuous trouble” instead of coiled tubing.
“Coiled tubing used to be very unreliable -- the metallurgy has improved so much, he said. “The reliability of the product is just night and day compared to what it was 20 years ago.”
A typical, basic drilling array for CTD includes a mud motor, measurement tool, bit sub and PDC bit. Wennerstrom said Technicoil also adds a deviation tool.
Drilling begins with a larger hole and casing to isolate water zones, typically seven-inch casing down to 425 feet for Technicoil, and the company most often completes the drilled well with 4-1/2-inch casing, he said.
Limitations still affect the depth of CTD-drilled wells, including tubing strength and weight. While the CTD market may include wells up to 10,000 feet deep in the future, practical commercial drilling depths are now less than 6,000 feet.
“For physical reasons, we have a bit of a logistical barrier,” Wennerstrom said. “For us to be able to put a coil on one of our trucks and not break all the highway weight limits, we’re pretty well limited to 2,000 meters of coiled tubing on our truck at any one time.”
Let’s Get Small
In addition to drilling improvements and the advent of larger and stronger coiled tubing, slimhole technology has seen advances in tools and equipment.
And these can be teeny-tiny tools.
Jack Kollé, president of Tempress Technologies in Kent, Wash., said his company’s smallest tool measures 1.165 inches.
“The reason it’s so small is that it’s a re-entry tool designed to go out of casing sideways for a couple of hundred feet. It’s really a technology that will compete with fracing,” he said.
CTD helps deploy that kind of tool by making it easier to get in and out of the hole. Using the re-entry tool would require multiple pipe trips, Kollé observed.
“Every one of these is a trip; then you have to set the shoe, deploy the milling tool, mill it and do the application,” he said.
To improve the effectiveness of water jets in work like descaling, Tempress has developed a gas separator tool and a downhole intensifier for use with CTD, Kollé said.
Some cleaning work requires 10,000 psi pressure, but the pressure across the jets is normally about 5,000 psi, he noted.
“With the downhole intensifier, we can take that 5,000 and essentially double it,” he said.
“We’re also interested in jet drilling tools. You need to be up to 15,000 psi to start drilling sandstone and rock like that,” Kollé added.
Even with recent improvements, Kollé thinks it will take time for slimhole CTD to penetrate the U.S. drilling market.
“It’ll take five to 20 years for that to occur,” he said. “It’s expensive equipment, and people are reluctant to make that investment until they see it used extensively the next county over,” he said.
Slimhole CTD will have to overcome other objections to gain wide acceptance:
- With minimal weight on bit, CTD can develop push-pull and torque problems. That also limits horizontal drilling applications.
“The ability to go horizontal with the lightweight coil is limited, but there are some things being done to overcome that,” including the use of downhole tractors, Perry noted.
- The drilling industry has relatively little experience with CTD outside of western Canada and Alaska.
- Production engineers worry about the ability to rework wells and the future use of wellbores in slimhole drilling.
- Even the speed of slimhole CTD can be a problem for an industry accustomed to much longer drilling times.
“A lot of that burden has fallen on the geologist and the landman. The whole infrastructure has had to adapt to the faster drilling,” Wennerstrom said.
But it’s the speed and cost savings that give CTD an edge in shallow-depth drilling for the worldwide oil and gas industry.
“If you take that 25 percent cost savings that we documented, it adds up to billions of dollars,” Perry said.
Slimhole CTD’s low impact also gives the technology a major advantage. In the DOE-GTI project “we wanted to investigate a zero-discharge drilling system. In this case, you would just take the cuttings with you,” he explained.
Even essentially zero-impact drilling could be possible with CTD.
“A booted tarp would be laid down,” Perry said, “and they’d just drive the rig on top of that.”
Well re-entry and workover provide another area for CTD growth, in applications that can be both fast and highly effective.
“They (major service and supply companies) have all developed rapid re-entry systems,” Kollé said. “They’ve certainly been promoting that as a business.”
One promising area for CTD, and a significant reason for funding of recent CTD research, is its potential use in boosting low-production wells -- primarily stripper wells and low-pressure gas wells.
If that use takes off, “the market will just explode,” Wennerstrom said.
Knowledge of downhole geology will point the way for the spread of slimhole CTD. New drilling technology and techniques present a learning-curve challenge for the entire industry.
Perry thinks the Niobrara demonstration project shows that slimhole CTD has a bright future, with cost only a minor barrier.
“This rig cost about a million, a million-and-a-half dollars to put together,” he said. “And there’s no reason someone couldn’t put together another one tomorrow.”