Leonard LeBlanc
Houston
- CORRECTION: This figure is a corrected version of the one appearing in the June issue. The number of unmanned platforms scheduled to be installed for each sector of the North Sea. A critical factor in reducing costs is platforms that can be operated from shore.
- One of the first applications of downhole wheel traction, well logging, for horizontal wells is shown with coiled tubing and hydraulics to power the wheels downhole. The tool was developed by Welltec of Aller d, Denmark with Statoil assistance.
Downhole tractors expand extended reach possibilities
Powered traction wheels that can expand out to casing or borehole diameter are getting a wider examination for drilling extended reach or horizontal wells that are constrained by drag problems.
Well tractors or powered traction wheels can now pull wireline well logging tools into horizontal or high angle boreholes, and have been designed to assist coiled tubing applications. The wheel motors can be powered by electrical or hydraulic sources. Wireline applications make use of available electrical energy, while coiled tubing units can be designed to use either. Hydraulics powering positive displacement motors are favored because they generate more wheel torque.
Drillers engaged in horizontal or extended reach wells often cannot make effective use of rotary drill pipe weight unless there is sufficient vertical string length, few doglegs, and a long radius kickoff. Even for coiled tubing with a hydraulic motor, the modest weight of the tubing combined with injection force at the surface reduces to zero thrust and helical lockup past a certain distance. In both cases, frictional drag of drillpipe and tools on the wellbore walls and casing limit lateral penetration.
Traction wheel units inserted at any point in a non-rotating drill string, as long as it is within the casing, can positively assist in drilling progress. Multiple tractor units can keep thrust on the drill bit through much of the drilling process, especially if the tubing is injected with some force at the surface.
Open hole operations are a different matter, however. Very little research has been done in this area. The following are some questions that must be answered:
What outward force must be placed by tractor wheels against a fragile borehole wall in order to obtain forward thrust? Can that force be adjusted? Should it be a concern, especially if multiple tractor units are placed strategically in the drillstring?
Will drillstring vibration and torquing prove too much for traction units or will continual development provide solutions in the same way as measurement-while-drilling mud pulsing systems?
If multiple wheels or traction tool units malfunction or do not operate in unison, how much of a problem would ensue?
Is there enough flexibility in the hydraulic control spectrum to accommodate simultaneously measurement-while-drilling instrumentation, borehole directional adjustment, and tractor motor drives?
One major benefit of downhole tractor units is that they can be reversed on surface order to assist tripping operations. Certainly inside the casing, but to a lesser extent in open hole, reversing the traction wheels provides an upward force on the drillstring.
Traction wheels can be extended outward from the drillstring to make contact with a borehole that has nearly twice the diameter of the tractor tool body. Further extensions are possible, but at the cost of full retraction. The tractor wheels can be provided with all types of surfaces and treads to gain purchase. The same type of wheel that maximizes traction inside casing may work poorly when placed against formation surfaces, so drillers may have to design a traction tool strategy for each well.
Once fully developed, wheeled traction units may create an even greater demand for coiled tubing and wireline units. The envelope for extended reach wells can be greatly expanded if traction can be applied against the borehole in both directions to push and pull the drill bit and tubing. With traction power applied throughout the drillstring, parting of the coiled tubing string is reduced considerably. In addition, coiled tubing strings will experience a longer life, even under the duress of extended reach operations.
Shear wave logging overcomes tube waves and weak signals
A shear wave seismic system that can map structures as far away as 100 meters from the borehole has been developed by Etrema Products of Ames, Iowa (USA) and Read Well Services of Bergen, Norway.
The system produces low frequency, low harmonic distortion signals, providing in turn a clean reflection of shear waves. The result is a discernible wave front for mapping geology. Current technology uses a seismic source and receiver in a borehole, which tend to create disruptive tube waves and diminishes any shear wave sensing.
Etrema has created the Terfenol-D low harmonic distortion device which is positioned downhole with Read's clamping system. The Etrema actuator generates radial forces as high as 3,500 N with a frequency of 10-300 Hz.
The transducer material is magnetostrictive and changes length in proportion to the applied magnetic field by as much as 50% more than conventional materials. The material is now being used in noise cancellation and anti-vibration technologies.
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