Geometrical instruments – Indicator of direction of force traversing natural media – Process
Reexamination Certificate
2000-11-08
2002-12-03
Gutierrez, Diego (Department: 2859)
Geometrical instruments
Indicator of direction of force traversing natural media
Process
C033S304000, C033S313000
Reexamination Certificate
active
06487782
ABSTRACT:
TECHNICAL FIELD
A method and apparatus for use in creating a magnetic declination profile for a borehole, which magnetic declination profile can be used to correct directional measurements made in the borehole with magnetic instruments.
BACKGROUND OF THE INVENTION
Measuring devices and methods used in boreholes typically make use of one or more earth fields in order to provide measurements for the inclination and direction of a borehole and for the orientation of objects located in the borehole.
The inclination of a borehole is sometimes referred to as the “drift” of the borehole and is an expression of the deviation of the borehole from vertical (i.e., from the direction of the earth's gravity vector).
The direction of a borehole is sometimes referred to as the “azimuth” of the borehole and is an expression of the direction of the borehole in a horizontal plane relative to a calibration direction such as magnetic North or true North.
The orientation of a point or object in a borehole is sometimes referred to as the “toolface” of the point or object and is an expression of the orientation of the point or object in a plane perpendicular to the longitudinal axis of the borehole relative to a reference orientation.
A variety of measuring instruments have evolved for the measurement of inclination and direction of a borehole and the orientation of points or objects in the borehole.
Inclination measurements in a borehole are commonly made with instruments that are sensitive to the earth's gravity field. Such gravity instruments include, for example, plumb bobs and accelerometers and are typically capable of measuring the amount of vertical deviation of the borehole but not the direction of the vertical deviation.
Directional measurements in a borehole are commonly made with instruments which are sensitive to the earth's magnetic field. Such magnetic instruments include, for example, compasses and magnetometers. Magnetic instruments are typically capable of providing a measurement of the direction of the borehole in a borehole coordinate system, but are unable to convert this measurement to a direction or azimuth of the borehole in a more useful reference coordinate system which is defined by the direction of the gravity vector and by compass directions in a plane perpendicular to the gravity vector.
The reference coordinate system conventionally has its X-axis parallel to the earth's surface and pointing North, its Y-axis parallel to the earth's surface and pointing East, and its Z-axis perpendicular to the earth's surface and pointing vertically down.
The borehole coordinate system is conventionally described as having its z-axis along the borehole axis, its y-axis parallel to the earth's surface and its x-axis perpendicular to both the y and z axes.
As a result, measurements taken with instruments which are sensitive to the earth's magnetic field are typically converted to values corresponding with the reference coordinate system in order to provide enhanced survey accuracy. Conventionally, this conversion is carried out by using data relating to the inclination of the borehole to transform data relating to the direction of the borehole in the borehole coordinate system to values expressed in the reference coordinate system.
Measurements of the orientation of a point or object in a borehole may be made with gravity instruments. For example, the “high side” or the “low side” of a borehole can be determined with instruments which are sensitive to the earth's gravity field.
Measurements of the orientation of a point or object in a borehole may also be made with magnetic instruments. Such measurements are sometimes referred to as “magnetic toolface” measurements and are essentially an expression of the orientation of the point or object relative to the x and y axes in the borehole coordinate system.
As a result of the above, borehole survey apparatus often include gravity instruments which are sensitive to the earth's gravity field as well as magnetic instruments which are sensitive to the earth's magnetic field so that the apparatus are capable of providing measurements of the inclination and direction of the borehole as well as the orientation of points or objects in the borehole.
Survey apparatus of the type described above are typically quite rugged and capable of enduring severe environmental conditions of vibration, heat and pressure. They are thus well suited to the rigors of use in downhole equipment such as drilling assemblies and measurement while drilling (MWD) systems.
Unfortunately, however, there are difficulties associated with the use of survey apparatus which include magnetic instruments. First, magnetic instruments take directional measurements relative to magnetic North instead of true North, with the result that magnetic directional measurements must be corrected so that they are expressed relative to true North. Such correction is typically performed empirically with the use of magnetic declination charts, thus complicating and contributing a source of error to the resulting corrected measurement.
Second, survey apparatus including magnetic instruments are also not well suited for use in circumstances where interference with the earth's magnetic field is present. For example, magnetic instruments are not commonly used for borehole surveys in boreholes which are lined with metal casing.
Furthermore, when magnetic instruments are used in drilling assemblies or MWD systems, they are typically isolated from the interfering magnetic effects of the drilling string by being contained in non-magnetic housings and by being located adjacent to non-magnetic drill collars or drill pipe. Despite such measures, error remains present in magnetic instrument measurements due to the influence of magnetic deposits (or the casing of nearby boreholes) in the formation being drilled and due to the effects of magnetism in the drilling string and the drilling assembly.
The effects of magnetism in the drilling string and the drilling assembly are reasonably well understood and numerous methods have been developed for addressing the measurement error resulting from these effects. Examples of such methods are found in U.S. Pat. No. 4,682,421 (van Dongen et al), U.S. Pat. No. 5,103,177 (Russell et al), U.S. Pat. No. 5,435,069 (Nicholson), U.S. Pat. No. 5,787,997 (Hartmann) and U.S. Pat. No. 5,806,194 (Rodney et al).
Some efforts have also been made to address the issues associated with orienting objects in a borehole using magnetic instruments where magnetic deposits or adjacent casing strings are present. Typically these methods are used to avoid adjacent boreholes during perforation operations or during drilling of a borehole and may in fact utilize the interfering effects caused. by such adjacent boreholes. Examples of such methods are found in U.S. Pat. No. 3,704,749 (Estes et al), U.S. Pat. No. 3,964,553 (Basham et al), U.S. Pat. No. 4,593,770 (Hoehn) and U.S. Pat. No. 5,582,248 (Estes).
Many of the difficulties associated with the use of magnetic instruments to take measurements in boreholes may be overcome by using instruments which take measurements which are not influenced by magnetic flux in the borehole. These instruments typically measure changes of direction relative to a calibration direction. One example of such an instrument is a conventional gyroscope, which can be oriented in a calibration direction and will then sense movement relative to the calibration direction.
Such “non-magnetic” instruments may also utilize an earth field vector to assist in establishing the calibration direction, such as the earth's inertial angular velocity vector as described in U.S. Pat. No. 4,433,491 (Ott et al). One example of a “non-magnetic” instrument which makes use of the earth's inertial angular velocity vector is a “north seeking” gyroscopic instrument, which is capable of taking directional measurements relative to true North.
Gyroscopic instruments of a variety of types are used frequently to survey existing boreho
Gutierrez Diego
Halliburton Energy Service,s Inc.
Kuharchuk Terrence N.
McCully Michael D.
Reis Travis
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