For more than a decade, the industry has been focusing on slimhole drilling and completion techniques to increase efficiency and reduce costs. The challenge has been to engineer equipment and techniques to successfully complete the wells in the small clearances available.
Slimhole tools are increasing in availability, driven by the demand for multilateral completions and for more cost-efficient workovers.
Whether slimholes are drilled with rotary systems or coiled-tubing (CT) systems, the smaller bit sizes will be less expensive than those used in conventional-sized boreholes. Cutting a smaller diameter wellbore reduces drilling time and produces fewer well cuttings, lessening the cost of treating and transporting cuttings. It also uses less drilling fluid, lowering overall well cost and reducing the potential impact on the environment.
Microhole drilling, formation evaluation, and completion systems using coiled tubing can reduce the environmental impact of industry operations by significantly reducing the surface footprint. Coiled-tubing rigs are small and highly efficient, providing a rapid rate-of-penetration.
Southeast Asia
Unocal has been actively drilling smaller diameter boreholes in Indonesia and Thailand. The company claims to have “pioneered” slimhole drilling in Indonesia.
Unocal Thailand Ltd., a subsidiary of Unocal Corp., drills and completes slimholes in the Gulf of Thailand. In November 1996, the company announced that by using slimhole drilling technology, it averaged only 8 days/well for drilling, logging, and testing, at less than $900,000/well.
Thakun Jiwalai, a petroleum engineer who worked for Unocal Thailand for 8 years, summarized the results of slimhole drilling in the Gulf of Thailand through 1996.1
In May 1997, Unocal announced that it had drilled its 1,000th well in Thailand. Brian Marcotte, president of Unocal Thailand, said that using slimhole drilling and innovative, environmentally friendly drilling fluids had increased the pace and efficiency of the company’s drilling.
Before 1981 Unocal said it spent more than $5 million and 68 days drilling a typical gas well to 11,000 ft. But by 1997, the same well would cost only $800,000 and could be drilled in fewer than 6 days.
In March 1999, Unocal Thailand announced a new record drilling natural gas development wells at the Funan J platform in the Gulf of Thailand, drilling the Funan J-13 well at 5,145 ft/day. Tara Tiradnakorn, Unocal Thailand’s vice-president of operations, said that the 14 wells on the Funan J platform were drilled at $800,000/well ($77.05/ft). The J-13 well cost $636,000 ($64.50/ft).
After achieving these successes with slimholes in the 1990s, Unocal moved on to other innovations, such as SX drilling and the use of surface BOPs.
BJ Tubular Services provided well services for Chevron Offshore (Thailand) in Chevron’s Tantawan, Benchamas, and other fields in the Gulf of Thailand, beginning in late 2000. Jeff Hibbler, with BJ Services International (Thailand) Ltd. reported on slimholes in the Gulf of Thailand at a 2004 SPE conference.2
Alaska’s North Slope
While completing more than 450 reentry sidetracks through 41⁄2 and 31⁄2-in. production tubing in North Slope fields, BP Exploration (Alaska) Inc., Orbis Engineering Inc., and ConocoPhillips Alaska Inc. developed several innovative slimhole completion designs.3
The teams used coiled-tubing drilling (CTD) and through-tubing rotary drilling (TTRD) with cemented liners. The target reservoirs occurred from 8,600 to 9,200 ft true vertical depth, and cemented liners were required for vertical and zonal isolation.
The most common CTD sidetracks involved drilling 33⁄4-in. (less commonly 41⁄8-in.) open “big hole” sections through 41⁄2-in. production tubing, then setting and cementing 33⁄16-in. production liner.
The companies developed a CTD slimhole sidetrack technique using a downsized bit and steerable BHA, drilling 2.7 x 3.0-in. bicenter open holes through 31⁄2-in. production tubing, and then setting 23⁄8-in. production liner.
The small-diameter completions improved production, isolated the producing zones, and preserved the parent wellbore for additional sidetracks.
M.O. Johnson, a senior CTD engineer at BP, said, “Completion success is critical to the longevity of any through-tubing sidetrack program.”
DOE project in Alaska
Projects have been underway since the 1990s, evaluating shallow, organic-rich shales adjacent to Cominco Alaska Inc.’s Red Dog Mine in Kotzebue, northwest Alaska. Located in the DeLong Mountains of the Brooks Range province, Red Dog is the world’s largest lead-zinc mine, with a great need for energy. Early exploration results focused in on the Mississippian-Pennsylvanian shales of the Lisburne Group, found at 1,000-2,000 ft.4
In 2001, the US DOE funded a pilot project to develop techniques to drill and complete small-diameter wellbores in order to provide a local source of energy for mine development. Teams drilled and cored slimholes and studied the effects of ethylene glycol on permeability.5
As a follow-up, the DOE’s National Energy Technology Laboratory (NETL) funded half of a $1.28 million project to develop new well-testing tools, techniques, and algorithms for use in wellbores 3-in. diameter or less, under arctic conditions, and to design a miniature, mobile system for completing, stimulating, and producing low-cost gas wells in remote arctic locations (e.g., Canada, Russia).6
Partners included Anchorage-based Northwest Alaska Native Association (NANA) Development Corp., Arlington, Va.-based Advanced Resources International Inc., and Cominco Alaska.
This project adapted mineral core drilling equipment to drill 3.05-in diameter wellbores. All equipment was heli-transportable. The team used biodegradable propylene glycol (15% by weight) as an antifreeze additive to the drill mud.
NANA and Cominco estimated that conventional drilling would be 2.5-5 times the cost of slimhole drilling in remote arctic locations and require a footprint three times as large.
Other examples
In November 2000, after 5-in liner became stuck above TD in a well in the South Timbalier Block 21 field, Gulf of Mexico operator J.M. Huber Corp. and Schlumberger revised the well-completion strategy.
They cemented the 5-in. liner in place to serve as intermediate casing and ran 31⁄2-in. liner. Because they could not use a conventional gravel pack (with 23⁄8-in. screens) in the new slimhole sidetrack, they substituted a screenless completion, in four steps:
• Perforate at maximum of 6 shots/ft perforation density.
• Acidize and chemically consolidate with K300 furan resin via coiled tubing.
• Frac with ceramic proppant and PropNET fibers.
• Clean out wellbore using coiled tubing.
The clean, upper Miocene sandstones in the Block 21 field often required sand-control measures. The resin helped prevent sand production through the perforations; none was seen in the first year after the completion was finished.7
Saudi Aramco has begun drilling single and dual-lateral horizontal wells using smaller well diameters (57⁄8-in.). In June of this year, Miller et al. reported that the company has drilled seven of these “slimhole” horizontals to date.8
Slimhole drilling, logging
Several contractors offer slimhole rotary steerable systems (RSS), certain to be used a great deal in the ongoing redevelopment of mature fields.
Baker Hughes INTEQ used its 31⁄8-in. AutoTrak rotary closed-loop drilling system on Aug. 25, 2004, to drill the first ultraslimhole with a rotary steerable system.
In late October 2005, Halliburton Energy Services Group announced commercialization of a new slimhole (43⁄4-in.) testing while drilling suite (GeoTap), using Sperry’s Geo-Pilot slimhole rotary steerable system, with RT resistivity, gamma, neutron, density, sonic, and vibration sensors. Sperry’s new Geo-Pilot 5200 RSS series is available for 6 to 63⁄4-in. holes.
Precision Energy Services has developed a rotary steerable service marketed under the name “Revolution,” available for 43⁄4-in., 63⁄4-in., and 81⁄4-in. holes. Precision also offers a slim monopole array tool, a slim sector bond tool, and a tubing-conveyed perforating system.
Reeves Wireline, a division of Precision Energy Services, has a compact set of logging tools with 21⁄4-in. OD (pressure tool is 23⁄4-in. OD). It also offers through-tubing tools and tubing-conveyed perforating equipment.
In September 2003, Shell UK Ltd. and BP Exploration Operating Co. Ltd. contracted with Schlumberger to develop a new RSS for slimhole drilling and through-tubing rotary drilling (TTRD), to be commercialized in 2005. The new system was to be based on Schlumberger’s PowerDrive Xtra RSS.
The PowerDrive Xtra rotary steerable systems are available in 43⁄4, 63⁄4, 9, and 11-in. sizes for drilling 53⁄4 to 181⁄4-in. wellbores. The project is assigned to Schlumberger’s Global Drilling Technology Center in Stonehouse, Gloucestershire, UK.
Schlumberger also offers slimhole logging tools. The company’s SlimAccess string is available in 21⁄2-in. OD, the neutron tool has 23⁄4-in. OD. The Vision475 is a 43⁄4-in. MWD/LWD system.
It’s not unusual to run a frac down 8-9,000-ft of 1.9-in. OD (1.6-in.-ID) tubing. Schlumberger’s CoilFRAC techniques allow fracture stimulation with coiled tubing, available for slimhole completions, 27⁄8-31⁄2-in. diameter and in casing or liner 4-7-in. diameter.
Completion tools
Among the tools, jewelry, and hardware needed for slimhole completions are:
• Setting tools
• Packers (retrievable, removable, expandable)
• Screens or slotted liners (with or without gravel pack)
• Tubing-conveyed perforating (TCP) systems
• Through-tubing motors
• Injection, fracture, and stimulation equipment
• Cementing systems (centralizers, top and bottom plugs, cementing shoes).
Slimhole systems will be needed to recomplete wells in aging fields, to access deeper, untested formations, or to stimulate current perforated intervals. Through-tubing and through-bit tools are destined to become more robust and routinely deployed, as Baker Oil Tools, Halliburton, Schlumberger, Weatherford, and other oilfield service companies marshal their R&D efforts. ✦
References
1. Jiwalai, T. “Slimhole Wells in the Gulf of Thailand,” University of Texas at Austin, masters report, 1996.
2. Hibbeler, J., Seymour, R., Rae, P.J., “Faster, Deeper, Cheaper: Slimhole Well construction in the Gulf of Thailand,” poster 90999, presented at the 2004 SPE ATCE Conference, Sept. 27-29, Dallas.
3. Johnson, M.O., Hyatt, P.G., Stagg, T., and Gantt, L.L., “Unique ‘Through-Tubing’ Completions Maximize Production and Flexibility,” SPE/IADC Drilling Conference, paper 92392, Amsterdam, Feb. 23-25, 2005.
4. Reeves, S.R., “Shale Gas Exploration at the Red Dog Mine, Alaska,” presented at the West Coast PTTC, Anchorage, Mar. 3, 2000.
5. Clough, J.G., “Evaluation of effects of drilling and completion fluids in shalebed methane reservoirs-Red Dog Mine,” Alaska Div. Geol. & Geophys. Surveys, 2003, http://wwwdggs.dnr.state.ak.us/Briefing03/FY03RedDog.pdf.
6. “Demonstration of mobile, small footprint exploration and development system-Developed for Arctic unconventional gas resources,” US DOE Project facts sheet, 2004, www.netl.doe.gov/scngo/Petroleum/publications/projfactsheets/E&P/NANA04.pdf
7. Acock, A. et al., “Screenless Methods to Control Sand,” Schlumberger Oilfield Review, Spring 2003, pp. 48-49.
8. Miller, C., Aljezany, M., and Al Khamees, S., “Slimhole Horizontal Wells in Saudi Arabian HTHP Gas Wells,” presented at IADC’s World Drilling 2005 conference, Rome, June 9-10.
