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Using a compass to steer a drill bit The critical importance of accurately steering a directional drilling operation places high demands on magnetic field measurement Alan Bailey Petroleum News
The use of the Earth’s magnetic field to make directional measurements, a technique that goes back many centuries in the form of the magnetic compass, remains a key to precisely steering a drill bit through subsurface rock strata during oil well drilling, according to an article in the fall 2013 issue of Oilfield Review.
But, given the high level of accuracy required to direct a well through thousands of feet of the subsurface and thread the well through thin layers of oil bearing rock, the space-age compass technology for drilling has orders of magnitude greater precision than the type of device used by, say, a hiker navigating a backcountry trail. And, at the levels of precision required for drilling, it becomes essential to take into account small-scale and sometimes fleeting variations in the Earth’s magnetic field, resulting from factors such as magnetic storms caused by disturbances in the solar wind, the flow of atomic particles from the sun.
Deadhorse observatory Carol Finn, co-author of the Oilfield Review article and leader of the U.S. Geological Survey Geomagnetism Group, explained that at high latitudes, such as in northern Alaska, the Earth’s geomagnetic field can be particularly active and can change quite rapidly. On the North Slope a partnership between the U.S. Geological Survey, or USGS, and oil services company Schlumberger operates a geomagnetic observatory at Deadhorse, continuously providing precise information about the local magnetic field to drillers on the Slope. The Deadhorse observatory is one of a network of 14 similar USGS observatories in the United States and its territories.
“Drill-bit positioning requires directional accuracy of a fraction of a degree, and this can be accomplished with advanced technology and expert understanding of the Earth’s dynamic magnetic field,” Finn said. “USGS operational systems measure the magnetic field on a continuous basis. These data are provided as a service to research scientists, civilian and defense government agencies, and to customers in the private sector, including the oil and gas drilling industry.”
Precision magnetometers Modern directional drilling commonly uses a technique called measurement while drilling, or MWD, to provide a continuous flow of information from near the drill bit to the drilling team at the surface. According to the Oilfield Review article, directional information for the well generally comes from a high-precision instrument called a magnetometer that acts as a compass, measuring the orientation of the downhole tool assembly relative to the local magnetic field. The magnetometer typically works in conjunction with a device for measuring the local gravitational field, the Oilfield Review article says. There are several different designs of magnetometer: One design, for example, uses the tendency of atomic nuclei with a magnetic spin to orient along the dominant magnetic field, the article says.
As with a compass used by a hiker, a user of a downhole magnetometer must adjust directional measurements to allow for the slowly changing deviation between the Earth’s magnetic north and true north. But a driller must also accommodate the local geomagnetic effects resulting from the local geology and possibly from the effect of magnetic materials in nearby oil wells. The driller must also calibrate the magnetometer readings to allow for the magnetic properties of the drill string and bottom hole drilling equipment. And then there are those relatively rapid fluctuations in the magnetic field that are reported by the geomagnetic observatory — on the North Slope, disturbance to the magnetic field from a magnetic storm can cause short term variations of several degrees in the magnetic declination, as well as significant changes in the strength of the magnetic field, the Oilfield Review article says.
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