GB2251078A

GB2251078A - Method for the correction of magnetic interference in the surveying of boreholes

Info

Publication number: GB2251078A
Application number: GB9125878A
Authority: GB
Inventors: Paul Anthony Mcclure
Original assignee: Teleco Oilfield Services Inc
Current assignee: Teleco Oilfield Services Inc
Priority date: 1990-12-21
Filing date: 1991-12-05
Publication date: 1992-06-24
Also published as: NO915048L; NL9102114A; GB9125878D0; NO915048D0

Abstract

A method for correction of measurements of borehole parameters for errors caused by magnetic interference includes surveying the borehole in the absence of substantial magnetic interference to determine reference values for the components of the local gravitational and local magnetic field along three mutually perpendicular axes. The reference values so determined are used for downhole correction of geomagnetic data obtained during drilling of the borehole. The sensor 2 comprises a three-axis accelerometer and magnetometer and may be suspended from a rig slick line in an open bore hole (Fig. 3 not shown) or supported on an aluminium spacer 40 between steel drill pipe 38 and a measurement-while-drilling tool 36. A non-magnetic member 42 is also used to separate the sensor from sources of axial magnetic interference. The sensor outputs are stored and used to calculate local field magnitudes and magnetic field dip angle, which are thereafter used to correct the MWD values. <IMAGE>

Description

METHOD FOR THE CORRECTION OF MAGNETIC INTERFERENCE IN THE SURVEYING OF BOREHOLE The present invention relates to a method for characterizing a local gravitational field and a local magnetic field in earth formation surrounding a borehole and more particularly to a method for correcting measurements of borehole parameters for errors caused by perturbations of the earth's magnetic field.

Instruments for borehole directional measurement typically use a three axis magnetometer and a two or three axis accelerometer to determine the components of the earth's magnetic and gravitational fields in a coordinate system centered on the instrument. A geometric transformation is employed to determine the desired parameters defining the tools orientation, namely the azimuth, inclination and tool face reference.

Perturbation of the earth's magnetic field, referred to herein as magnetic interference, may result in an error of the calculated borehole parameters. Magnetic interference may arise from the drillstring itself, i.e. from the presence of permeable, and possibly magnetized, materials in the drillstring, or from an external source, e.g. a ferrous ore body in the formation surrounding the borehole.

The existence of error in calculated borehole parameters introduced by magnetic interference and the need to correct for the error has been recognized in the art and various correction algorithms have been proposed. For example U.S. Patent No. 4,761,889, the disclosure of which is incorporated herein by reference, is directed to a method for the detection of and correction for magnetic interference in the surveying of boreholes. In the method of U.S. Patent No. 4,761,889 the correction to the measured azimuth is based on the difference between measured and nominal, i.e. calculated from values charted for a particular region of the earth, dip angle quantities which are functions of ratios of measured or known quantities.

The method of U.S. Patent No. 4,761,889 reduces one source of potential error in the calculation of borehole parameters, i.e. the variation in scaled factors in the downhole sensor.

The accuracy of the corrections provided by the various interference correction algorithms is limited in certain geographic locations by the inadequacy of interpolated or extrapolated geomagnetic survey data, i.e. the nominal local values, used in the correction algorithms.

The objective of the present invention is to provide a method for characterizing a local gravitational field and a local magnetic field in an earth formation surrounding a borehole, which eliminates the disadvantages described above.

In order to achieve this objective, the method includes surveying the borehole in the absence of substantial magnetic interference to determine reference values for the components of the local gravitational field along each of three mutually perpendicular first axes and reference values of the components of the local magnetic field along each of three mutually perpendicular second axes.

The reference values so determined are used for downhole correction of geomagnetic data obtained during drilling.

FIGURE 1 shows a perspective view of a drillstring segment showing relationships between various axes, angles and vectors of interest in the present invention.

FIGURE 2 shows a schematic view of a reference sensor of the present invention.

FIGURE 3 shows a schematic view of the reference sensor of the present invention used in an open borehole as a wire line tool.

FIGURE 4 shows a schematic view of the reference sensor of the present invention positioned in a drillstring with a borehole.

The term H is used herein to denote the geomagnetic field. Hx, Hy, Hz are components of H in the coordinate system of the tool. G refers to the force of gravity. Gx, Gy, Gz are components of G in the coordinate system of the tool. A symbol with a bar, e.g. H, refers to a vector, that symbol without the bar, e.g. H, refers to the magnitude of the vector.

Referring to FIGURE 1, one can see the relationship between the tool-related axes and those fixed to the earth.

For clarity, the origin of the tool fixed axes has been displaced from 0 to 0' and the Z (tool) axis is shown as a double line. The inclination angle INC is defined as the angle between the vertical line OD and the tool axis OZ.

The gravity tool face reference angle GTF is defined as the angle between the vertical plane containing OD and OZ and the plane containing O'Z and O'Y. At low values of inclination, the magnetic tool face angle MTF (not shown) is employed, this being the angle between the vertical plane through OD and ON and the plane through O'Z and O'Y.

The azimuth angle through OD and ON and the vertical plan through OD and OZ. The relationships between the sensor readings and the angles, INC, AZ and GTF (or MTF) are well known in the literature. The following relationships exist: INC = TAN-1 ((Gx2 + Gy2 / Gz) (1) (0 # INC # 180 ) GTF = TAN 1 (Gx/Gy) (2) (0 # GTF # 360 ) MTF = TAN-1 (H4Hy) (3) (0 # MTF # 360 ) G* (HxGy - HyGx) Az = TAN1 2 2 (4) Hz* (Gx2 + Gy2) + Gz* (HxGx + HyGy) (0 # AZ # 360 ) Where G = (Gx2 + Gy2 + Gz) (5).

In evaluating these equations, the value INC is taken between 0 and 1800, and the values of GTF, MTF and AZ lie between 0 and 3600.

It should be noted that, although the gravitational vector G lies along one of the earth-fixed coordinate axes OD, the magnetic field H will not, in general, coincide with the axis ON (i.e. the magnetic field will not be in the horizontal plane of ON and OE. The angle that the magnetic field makes with the horizontal plane containing ON and OE is the dip angle h . This angle is positive in the northern hemisphere (i.e., the vertical component of H is downward) and negative in the southern hemisphere. The equations for the dip angle, the magnetic field vector magnitude H, and the axial field strength Hz in term of the six sensor readings, are as follows:

( K -90" ) 900) H = (Hx2 + Hy2 + HZ2) 1/2 (7) Hz = Hz (8) In the absence of magnetic interference, the first two of these quantities are independent of the orientation of the tool. Furthermore, equation (4) given above for azimuth angle is not dependent upon either the dip angle or total field strength. It only depends upon the assumption that the horizontal component of the earth's field strength, dip angle and magnetic declination (i.e., the difference in heading between the geographic North and geomagnetic North) are known for any latitude and longitude.

In the presence of magnetic interference, any or all of the above magnetic quantities (e.g., (6)-(8)) may be affected. The only component of the interfering field which will influence the measured azimuth is that in the eastwest direction, i.e., along the axis OE.

Referring to Figure 2, the sensor unit 2 includes a triaxial accelerometer 4, and a triaxial magnetometer 6.

The axes of the instruments 4 and 6 coincide so that Z axes of the instruments are aligned with the longitudinal axis of the sensor 2 and with the Y axes of the instruments perpendicular to the Z axes. The X axes are oriented perpendicular to both the Y and Z axes to define a lefthanded coordinate system.

The sensor unit 2 also contains a temperature sensor 8 for providing temperature compensation for outputs of sensors 4 and 6, an analog to digital converter, and filter 10 and a microprocessor 12. The outputs of instruments 4 and 6 are filtered, digitized and delivered to the microprocessor 12 for storage in memory. The sensor unit includes a battery 14 for providing electrical power to the components of the sensor unit 2.

The sensor 2 may be used to determine reference values as a wire tool as shown in FIGURE 3. The sensor 2 is suspended from a rig slick line 20 in an open borehole 26.

A nonmagnetic member 24 is provided to separate the sensor from potential sources of axial magnetic interference.

The sensor 2 may be used to determine reference values by dropping the sensor 2 into a drillstring 30 at the end of a run within a borehole 31 as shown in FIGURE 4, the drillstring 30 includes a bit 32, a motor 34, and a measurement while drilling (MWD) tool 36, and steel drillpipe 38. The sensor 2 is placed in the drillstring between the drillpipe 38 and the MWD tool 36 and is supported on aluminium space 40 and surrounded by nonmagnetic member 42 to separate the sensor 2 from potential sources of axial magnetic interference.

In either of the above described configurations, the sensor is used to survey the borehole and determine reference values for the local gravitational field and local magnetic field in each of three mutually perpendicular axes under conditions characterized by the absence of substantial magnetic interference. Sensor outputs are stored in memory.

Once the sensor is brought to the surface, reference values are then calculated from the stored sensor outputs according to:

wherein: GX = average value of the local gravitational field along the X axis; GY = average value of the local gravitational field along the Y axis; GZ = average value of the local gravitational field along the Z axis; GXi ~ a measured value of the local gravitational field along the X axis; GYj = a measured value of the local gravitational field along the Y axis; GZk = a measured value of the local gravitational field along the Z axis; m, n, p = number of measurements along the respective axis.

wherein: HX = average value of the local gravitational field along the X axis; HY = average value of the local gravitational field along the Y axis; HZ = average value of the local gravitational field along the Z axis; HXi = measured value of the local gravitational field along the X axis; Hvj = measured value of the local gravitational field along the Y axis; HZk = measured value of the local gravitational field along the Z axis; q, r, s = number of measurements along the respective axis.

Reference total fields and a reference dip angle may then be determined according to:

Reference Dip Angle =

Once the reference total fields and dip angle have been calculated those values are input into the microprocessor of an MWD tool for use as reference; i.e. "true", values in algorithims for correcting measurements made while drilling.

For example, the reference dip angle determined according to the present invention may be substituted for the "true" dip angle, i.e. a dip angle derived from tabulated values, in the azimuth correction described in the above cited U.S. Patent No 4,761,889, wherein: dAz = ( o ) (sin INC INCsin AZ) (18) cos (sin INC cos AZ sin ## - cos INC cos where: AZ = azimuth (measured) dAZ = azimuth error k = dip angle (measured) #o = dip angle (true) INC = inclination.

It will be clear to one skilled in the art that the application of the reference values determined by the method of the present invention is general and by no means limited to a particular correction algorithm.

Claims

1. A method for characterizing a local gravitational field and a local magnetic field in an earth formation surrounding a borehole, said method being characterized by surveying the borehole in the absence of substantial magnetic interference to determine reference values of components of the local gravitational field along each of three mutually perpendicular first axes, and references values of components of the local magnetic field along each of three mutually perpendicular second axes.

2. The method as claimed in claim 1, characterized in that said first axes comprise an X axis, an Y axis and a Z axis and the reference values of the local gravitational field are average values determined, according to:

wherein: GX = average value of the local gravitational field along the X axis; GY = average value of the local gravitational field along the Y axis; GZ = average value of the local gravitational field along the Z axis; GXi = a measured value of the local gravitational field along the X axis; GYj = a measured value of the local gravitational field along the Y axis; GZk = a measured value of the local gravitational field along the Z axis; m, n, p = number of measurements along the respective axis.

3. The method as claimed in claim 2, characterized in that said second axes coincide with said first axes and the reference values of the local magnetic field are average values determined according to:

wherein: HX = average value of the local gravitational field along the X axis; HY = average value of the local gravitational field along the Y axis; HZ = average value of the local gravitational field along the X axis; HXi = measured value of the local gravitational field along the X axis; HYj = measured value of the local gravitational field along the Y axis; HZk = measured value of the local gravitational field along the Z axis; q, r, s = number of measurements along the respective axis.

4. The method as claimed in claim 3, characterized by calculating a reference dip angle according to Reference Dip Angle =

where:

5. The method as claimed in claim 1, characterized in that the survey is conducted using a reference sensor, said sensor comprising: three axis magnetometer means (6) for determining measured values for the components of the local magnetic field; three axis accelerometer means (4) for determining measured values for the components of the local gravitational field; and memory means (12) for recording the measured values.

6. The method of claim 5, characterized in that the sensor further comprises means (24) for protecting the magnetometer means from axial magnetic interference.

7. The method of claim 4, characterized by suspending the sensor in the borehole from a sLick line (20).

8. The method of claim 4, characterized by positioning the sensor (2) in a drillstring (30).

9. The method as claimed in claim 1, characterized by surveying the borehole during drilling to determine measured values of the components of the local magnetic field along each of the second axes; and comparing the measured values to the reference values to determine the magnitude of magnetic interference during drilling.

10. The method as claimed in claim 4, characterized by surveying the borehole during drilling to determine local geomagnetic parameters; calculating a local dip angle for the measured values; and comparing the local dip angle to the reference dip angle to determine the magnitude of magnetic interference during drilling.

11. A method for characterizing a local gravitational field and a local magnetic field in an earth formation surrounding a borehole substantially as described with reference to and as illustrated in the acommpanying drawings.