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Microhole Downhole Drilling Tractor
Project Number

The project goal is to design, manufacture, and demonstrate a reliable and economical hydraulically powered coiled tubing (CT) drilling tractor that will transport the drill bit and bottomhole assembly into long horizontal well sections and drill effectively for as much as 3,000 feet.


Western Well Tool, Inc., Anaheim, CA


Although CT drilling offers many advantages, one area of needed technology development is a method to exert enough weight on the bit to drill through rock. Coiled tubing easily buckles, which makes it difficult to impart weight to the drill bit. One technology that has been recently developed to overcome this problem is a down-hole tractor that thrusts the drill bit into the formation while pulling the coiled tubing along behind.

The baseline design of a 3¼-inch microhole drilling tractor (MDT) is complete. Experiments were conducted with low-solids drilling mud that verified component functionality and operational life.

Work on a redesign of the MDT with a 3⅜-inch outside diameter is complete; this required a complete redesign of the tool. The new design allows a greater inside diameter for flow to the downhole motor and drill bit for returning cuttings to the surface and for improved hole cleaning.

Other design improvements were made to the MDT. An improved start-stop valve design is complete; this feature provides tractor control while performing drilling operations. An improved gripper design is complete; detailed analyses predicted improved downhole fatigue life. A new operational life criterion for grippers was investigated and proven experimentally.

Design of a tractor test fixture to demonstrate the operational characteristics of the tractor is complete.

A new tractor diagnostic device was designed, built, and verified as operational. This device allows predrilling check-out of the tractor without running downhole and without need for high-capacity pumps, thus speeding field operations.

The MDT will allow drilling of horizontal holes up to 3,000 feet beyond conventional CT drilling. Using CT and the MDT can be 25-50% less expensive than rotary rigs in some applications, especially in environments where set-up time is costly. Controls are simple and direct when using the injector and pump pressure, thus eliminating need for expensive electrical controls. The MDT is capable of high loads (>3,500 lbs) through dog-legs up to 15 degrees per 100 feet. Grippers for tractor movement are highly reliable, have a long life (>175,000 ft traveled before maintenance), are highly debris- and fluid-tolerant, can efficiently traverse washed-out hole sections, do not damage casing, and are proven to operate in both very soft and hard formations. Using a simple principle of operation, power is provided by differential pressure of the drilling mud at the tool.

The MDT to be designed and manufactured is a drilling fluid-powered unidirectional downhole CT tool. The tractor outside diameter will be 3⅜ inches to accommodate 3⅝-inch holes, and the tractor will be able to drill >3,000-ft horizontal well sections.

The MDT builds on previously developed Western Well Tool Tractor technology that has demonstrated the capability to operate downhole with a variety of drilling muds, operating parameters, and drilling equipment. The tractor consists of a central control assembly that directs the mud flow and provides the pull and thrust of the tractor and a forward and aft shaft assembly with patented grippers that operate successfully in soft and hard formations.

Profile of Microhole Drilling Tractor Gripper Expanded Against Hole.
Profile of Microhole Drilling Tractor Gripper Expanded Against Hole.

The MDT’s walking process consists of several steps. A forward roller-toe gripper is expanded (inflated) against the walls of the hole, thrusting the bit forward and pulling the coiled tubing as the tractor progresses. The forward roller-toe gripper deflates while the aft roller-toe gripper expands against the hole wall, pushing the bit forward and pulling the drillstring into the new position. This process repeats, allowing the tractor to “walk” down the hole while drilling in front of the tractor and pulling the drill string behind it.

Microhole Drilling Tractor walking process.
Microhole Drilling Tractor walking process.

In the first phase of the project, a drilling tractor will be designed that is reliable, economical, and field-ready and that can drill horizontal well sections of 3,000 ft or more. In the second phase of the project, a prototype drilling tractor will be built and field-tested to demonstrate its performance with a CT rig by drilling multiple inclined and horizontal holes.

Current Status

(December 2008)
This project has been completed and the final report is available below under "Additional Information".

A field Demonstration of the MDT was arranged with a major oil company operating in Alaska. Functional and operational tests were run with the MDT inside 4.5 inch tubing at depths of 800-950 ft. The MDT successfully demonstrated On-Off capability, pulled with up to 1465 lbs force, and verified its capability to transmit torque though it from the Orienter. The MDT operated as expected. With the availability of the Microhole Drilling Tractor the maximum horizontal displacement could be increased by up to 3000-ft beyond existing technology, thereby increasing the number of CT wells drilled by as much as 30%.The Microhole Coiled Tubing Drilling Tractor was a finalist for the World Oil Awards in the category of "New Horizons Idea".

This project was selected in response to DOE’s Oil Exploration and Production solicitation DE-PS26-03NT15392, Microhole I, September 30, 2003.

Project Start
Project End
DOE Contribution


Performer Contribution

$129,084 (25 percent of total)

Contact Information

NETL - Virginia Weyland ( or 918-699-2041)
Western Well Tool - Bruce Moore ( or 714-632-0810)

Additional Information

Final Project Report [PDF-581KB]

Moore, N.B., Bloom, D., Krueger, E., Mock, P.W., Veselka, A., Delivering Perforation Strings in Extended-Reach Wells with Coiled Tubing and Hydraulic Tractor, SPE 94208, delivered to SPE/ICoTA meeting, April 12-13, 2005.