Nuclear magnetic resonance apparatus and method having toroidal antenna windings
First Claim
1. An apparatus for determining a parameter of interest of a subsurface formation in the vicinity of s borehole comprising:
- (a) a magnet arrangement on a downhole tool for producing a static magnetic field in a region of examination in the formation and aligning nuclear spins in said region substantially parallel to a direction of said static magnetic field in said region;
(b) a transmitter coil on the downhole tool for transmitting a pulsed radio frequency (RF) magnetic field into said region of examination, said transmitter coil having windings around a toroidal (tubular) core, said toroidal core intensifying the magnetic flux in the first coil, said pulsed RF field substantially perpendicular to said static field and inducing precession of nuclear spins in said region of examination;
(c) a first receiver coil receiving a first component of echo signals from said precessing nuclear spins and producing a first signal;
(d) at least one additional receiver coil on the downhole tool for receiving a second component of echo signals from the precessing nuclear spins and producing at least one second signal, said second component substantially orthogonal to the first component, and (e) a processor for combining the first signal and the at least one second signal to determine the parameter of interest.
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Accused Products
Abstract
An NMR apparatus includes permanent magnets oppositely polarized in the axial direction and suitably arranged along the axis of the apparatus to produce a nearly radial static magnetic field in a toroidal examination region within the rock formation surrounding the wellbore. A first coil is wound around a cylindrical annulus core coaxial with the magnets, the core including high magnetic permeability material. The first coil produces pulsed RF magnetic fields oriented parallel to the axis of the apparatus and thus perpendicular to the static magnetic field in the toroidal examination region. This reorients certain nuclear spins and the bulk magnetization of the formation within the examination region resulting from them. The first coil picks up on component of the spin-echo signals of the precessing nucleii. A second coil toroidally wound around the same cylindrical annulus core in the form of a torus with windings in planes parallel to the axis of the apparatus detects a second component of the spin-echo signals. The apparatus also includes a third coil spaced apart from the first coil and having a geometry similar to that of the second coil. An electrically conducting collar shields the third coil from the magnetic fields produced elsewhere in the apparatus. A processor combines the signals from the three coils to give spin-echo signals with improved signal-to-noise ratio.
24 Citations
21 Claims
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1. An apparatus for determining a parameter of interest of a subsurface formation in the vicinity of s borehole comprising:
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(a) a magnet arrangement on a downhole tool for producing a static magnetic field in a region of examination in the formation and aligning nuclear spins in said region substantially parallel to a direction of said static magnetic field in said region;
(b) a transmitter coil on the downhole tool for transmitting a pulsed radio frequency (RF) magnetic field into said region of examination, said transmitter coil having windings around a toroidal (tubular) core, said toroidal core intensifying the magnetic flux in the first coil, said pulsed RF field substantially perpendicular to said static field and inducing precession of nuclear spins in said region of examination;
(c) a first receiver coil receiving a first component of echo signals from said precessing nuclear spins and producing a first signal;
(d) at least one additional receiver coil on the downhole tool for receiving a second component of echo signals from the precessing nuclear spins and producing at least one second signal, said second component substantially orthogonal to the first component, and (e) a processor for combining the first signal and the at least one second signal to determine the parameter of interest. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16)
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17. A method of determining a parameter of interest of a subsurface formation in the vicinity of a borehole comprising:
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(a) conveying a downhole tool into the borehole, the downhole tool having a magnet arrangement for producing a static magnetic field in a region of examination in the formation and aligning nuclear spins in said region substantially parallel to a direction of said static magnetic field in said region;
(b) using a transmitter coil on the downhole tool for transmitting a pulsed radio frequency (RF) magnetic field into said region of examination, said pulsed RF field substantially perpendicular to said static field and inducing precession of nuclear spins in said region of examination, said transmitter coil having windings around a toroidal (tubular) core;
(c) receiving in a first receiver coil a first component of echo signals from said precessing nuclear spins and producing a first signal;
(d) receiving in a second receiver coil proximate to the first coil on the downhole tool a second component of echo signals from the precessing nuclear spins and producing a second signal, said second component substantially orthogonal to the first component, and (e) using a processor for combining the first signal and the second signal to determine the parameter of interest. - View Dependent Claims (18, 19, 20, 21)
(i) receiving in a third receiver coil distant from the first coil the second component of the echo signals from the precessing nuclear spins to give a third signal; and
(ii) using the processor to combine the third signal with the combination of the first and second signals.
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21. The new method of claim 17 wherein the toroidal core has an axis substantially parallel to said axis of the borehole.
Specification