Surveying of boreholes
A borehole drilling and survey assembly includes a drill string of magnetic material, a non-magnetic drill collar, and a non-magnetic rotary drilling system including a drill bit. A near-bit survey instrument is located in the rotary drilling system at a fixed distance from the junction of the drill string and the drill collar. A second survey instrument is located in the drill collar at a fixed distance from said junction. The survey instruments measure the local values of the component of the earth's magnetic along the borehole axis, and these values are processed to remove the magnetic effects of the drillstring. The survey instruments optionally also measure gravity vector components to enable borehole heading to be derived.
1. A method of deriving the true magnitude of the terrestrial magnetic field BZe in the direction of the longitudinal axis OZ of a borehole penetrated by a bottom-hole assembly comprising a drill string including a substantially non-magnetic drill collar to which is attached a drilling bit assembly, said method comprising the steps of measuring the longitudinal magnetic field BZa (the component of the magnetic field B in the direction OZ) at a first predetermined point which is along the length of the drill string at a distance a from a fixed point along the length of the drill string, and measuring the longitudinal magnetic field BZb (the component of the magnetic field B in the direction OZ) at a second predetermined point which is along the length of the drill string at a distance b from said fixed point along the length of the drill string, to provide a longitudinal-position-dependent pair of longitudinal magnetic field measurements BZa,BZb, and calculating BZe from the relationship:
BZe =( BZa.a 2 −BZb.b 2 )/( a 2 −b 2 ),
on the assumption that a longitudinal magnetic field error is induced by a single notional magnetic pole in the drill string substantially at the attachment of the drill string to the substantially non-magnetic drill collar.
2. A method of surveying the heading of a borehole penetrated by a bottom-hole assembly comprising a drill string including a substantially non-magnetic drill collar to which is attached a drilling bit assembly; the method comprising:
deriving the true magnitude of the terrestrial magnetic field BZe by the method of claim 1 ;
measuring the magnetic fields BX and BY in two axes which are orthogonal to the longitudinal axis and to each other;
measuring gravity vector components in each of said three axes to produce respective gravity vector components GX, GY and GZ; and
solving the function [GX,GY,GZ,BX,BY,BZe] to determine said heading.
3. Apparatus for deriving the true magnitude of the terrestrial magnetic field BZe in the direction of the longitudinal axis OZ of a borehole penetrated by a bottom-hole assembly comprising a drill string including a substantially non-magnetic drill collar to which is attached a drilling bit assembly, said apparatus comprising magnetic field measuring means for measuring the longitudinal magnetic field BZa (the component of the magnetic field B in the direction OZ) at a first predetermined point which is along the length of the drill string at a distance a from a fixed point along the length of the drill string, and for measuring the longitudinal magnetic field BZb (the component of the magnetic field B in the direction OZ) at a second predetermined point which is along the length of the drill string at a distance b from said fixed point along the length of the drill string, to provide a longitudinal-position-dependent pair of longitudinal magnetic field measurements BZa,BZb, and means for calculating BZe from the relationship:
BZe =( BZa.a 2 −BZb.b 2 )/( a 2 −b 2 ),
on the assumption that a longitudinal magnetic field error is induced by a single notional magnetic pole in the drill string substantially at the attachment of the drill string to the substantially non-magnetic drill collar.
4. Apparatus according to claim 3 , in which said magnetic field measuring means comprises first magnetic field measuring means for mounting at a first predetermined point on said drill string, and second magnetic field measuring means for mounting at a second predetermined point on said drill string.
5. Apparatus according to claim 4 , said apparatus further including:
third magnetic field measuring means for measuring the magnetic fields BX and BY in two mutually orthogonal axes each also orthogonal to the longitudinal axis; and
gravity vector component measuring means for measuring gravity vector components in each of said three axes to produce respective gravity vector measurements GX, GY, and GZ.
6. Apparatus according to claim 5 , further comprising solving means constructed to solve the function [GX,GY,GZ,BX,BY,BZe] to determine said heading.
7. Equipment for drilling a borehole and for surveying said borehole, said equipment comprising the operative combination of a substantially non-magnetic drill collar, a drilling bit assembly, and apparatus according to claim 3 .
8. Equipment for drilling a borehole and for surveying said borehole, said equipment comprising the operative combination of a substantially non-magnetic drill collar, drilling bit assembly, and apparatus according to claim 5 .
9. A method of deriving the magnetic azimuth of a borehole penetrated by a bottom-hole assembly comprising a drill string including a substantially non-magnetic drill collar to which is attached a drilling bit assembly; said method comprising the steps of:
measuring the uncorrected azimuth angle AZa at a first predetermined point which is along the length of the drill string at a distance a from a fixed point along the length of the drill string;
measuring the uncorrected azimuth angle AZb at a second predetermined point which is along the length of the drill string at a distance b from said fixed point along the length of the drill string;
thereby providing a longitudinal-position-dependent pair of uncorrected azimuth angle measurements; and calculating the corrected magnetic azimuth AZc from the relationship:
cot( AZc ).( b 2 −a 2 )= b 2 .cot( AZb )− a 2 .cot( AZa )
on the assumption that a longitudinal magnetic field error is induced by a single notional magnetic pole in the drill string substantially at the attachment of the drill string to the substantially non-magnetic drill collar.
10. Apparatus for deriving the magnetic azimuth of a borehole penetrated by a bottom-hole assembly comprising a drill string including a substantially non-magnetic drill collar to which is attached a drilling bit assembly, said apparatus comprising azimuth angle measuring means for measuring the uncorrected azimuth angle AZa at a first predetermined point which is along the length of the drill string at a distance a from a fixed point along the length of the drill string, and for measuring the uncorrected azimuth angle AZb at a second predetermined point which is along the length of the drill string at a distance b from a fixed point along the length of the drill string, to provide a longitudinal-position-dependent pair of uncorrected azimuth angle measurements AZa,AZb, and means for calculating the corrected magnetic azimuth AZc from the relationship:
cot( AZc ).( b 2 −a 2 )= b 2 .cot( AZb )− a 2 .cot( AZa )
on the assumption that a longitudinal magnetic field error is induced by a single notional magnetic pole in the drill string substantially at the attachment of the drill string to the substantially non-magnetic drill collar.
11. Apparatus according to claim 10 , in which said azimuth angle measuring means comprises first azimuth angle measuring means for mounting at a first predetermined point on said drill string, and second azimuth angle measuring means for mounting at a second predetermined point on said drill string.
12. Apparatus according to claim 11 , said apparatus further including:
magnetic field component measuring means for measuring magnetic field components in each of three mutually orthogonal axes to produce respective magnetic field measurements BX, BY and BZ; and
gravity vector component measuring means for measuring gravity vector components in each of said three axes to produce respective gravity vector measurements Gx, Gy and Gz.
13. Apparatus according to claim 12 , further comprising solving means constructed or adapted to solving the function [GX,GY,GZ,BX,BY,BZ].
14. Equipment for drilling a borehole and for surveying said borehole, said equipment comprising the operative combination of a substantially non-magnetic drill collar, a drilling bit assembly, and apparatus according to claim 10 .
15. Equipment for drilling a borehole and for surveying said borehole, said equipment comprising the operative combination of a substantially non-magnetic drill collar, a drilling bit assembly, and apparatus according to claim 14 .