Downhole nuclear magnetic resonance (NMR) tool with transversal-dipole antenna configuration
In some aspects, a downhole nuclear magnetic resonance (NMR) tool includes a magnet assembly and an antenna assembly. The NMR tool can operate in a wellbore in a subterranean region to obtain NMR data from the subterranean region. The magnet assembly produces a magnetic field in a volume about the wellbore. The magnet assembly includes a central magnet, a first end piece magnet spaced apart from a first axial end of the central magnet, and a second end piece magnet spaced apart from a second axial end of the central magnet. The antenna assembly includes a transversal-dipole antenna. In some cases, orthogonal transversal-dipole antennas produce circular-polarized excitation in the volume about the wellbore, and acquire a response from the volume by quadrature coil detection.
1. A nuclear magnetic resonance (NMR) tool for use in a wellbore in a subterranean region, the NMR tool comprising:
a magnet assembly to produce a magnetic field in a volume in a subterranean region, the magnet assembly comprising:
a central magnet having a first axial end and a second, opposite axial end;
a first end piece magnet spaced apart from the first axial end of the central magnet; and
a second end piece magnet spaced apart from the second axial end of the central magnet; and
an antenna assembly comprising mutually orthogonal transversal-dipole antennas.
2. The NMR tool of claim 1 , wherein the antenna assembly comprises the mutually orthogonal transversal-dipole antennas to at least one of:
produce circular-polarized excitation in the volume; or
acquire a response from the volume by quadrature coil detection.
3. The NMR tool of claim 1 , wherein the central magnet defines a first magnetic field orientation, and the first and second end piece magnets each define a second magnetic field orientation that is orthogonal to the first magnetic field orientation.
4. The NMR tool of claim 1 , wherein the magnet assembly and the antenna assembly are configured to operate within a wellbore in the subterranean region during drilling operations.
5. The NMR tool of claim 1 , wherein the magnet assembly comprises a permanent magnet assembly, and the central magnet and the first and second end piece magnets each comprise one or more permanent magnets.
6. The NMR tool of claim 1 , wherein:
the volume comprises multiple distinct sub-volumes, the multiple distinct sub-volumes comprise a first sub-volume that is elongate in a first direction parallel to a longitudinal axis of the NMR tool, the magnetic field in the first sub-volume being substantially uniformly oriented in the first direction; and
the NMR tool comprises multiple antenna assemblies at respective locations along the longitudinal axis, each antenna assembly to detect an NMR response from a respective one of the distinct sub-volumes.
7. The NMR tool of claim 1 , wherein the central magnet, the first end piece magnet, and the second end piece magnet of the magnet assembly are all axially magnetized.
8. The NMR tool of claim 6 , wherein the distinct sub-volumes further comprise:
a second sub-volume spaced apart from a first axial end of the first sub-volume; and
a third sub-volume spaced apart from a second, opposite axial end of the first sub-volume, the magnetic field in the second and third sub-volumes having a radial orientation that is substantially perpendicular to the first direction.
9. The NMR tool of claim 6 , wherein the first sub-volume is configured for acquiring an NMR signal while tripping a drill string.
10. The NMR tool of claim 1 , comprising a transversal-dipole and monopole antenna assembly to obtain a response from the volume.
11. The NMR tool of claim 10 , wherein the transversal-dipole and monopole antenna assembly is operable to obtain a unidirectional azimuthally-selective response from the volume.
12. The NMR tool of claim 10 , wherein the magnet assembly and antenna assembly are operable to acquire an NMR signal while drilling.
13. A method of obtaining nuclear magnetic resonance (NMR) data from a subterranean region, the method comprising
producing a magnetic field in a volume in a subterranean region by a magnet assembly in a wellbore, the magnet assembly comprising:
an elongate central magnet having a first axial end and a second, opposite axial end;
a first end piece magnet spaced apart from the first axial end of the central magnet; and
a second end piece magnet spaced apart from the second axial end of the central magnet; and
producing a circular-polarized excitation in the volume by an antenna assembly comprising mutually orthogonal transversal-dipole antennas;
acquiring a response from the volume based on the circular-polarized excitation produced by the antenna assembly;
determining properties of the volume from the response, employing a computing system; and
providing, employing the computing system, the properties for a user.
14. The method of claim 13 , wherein the response is acquired by quadrature coil detection.
15. The method of claim 13 , wherein the central magnet defines a first magnetic field orientation, and the first and second end piece magnets each define a second magnetic field orientation that is orthogonal to the first magnetic field orientation.
16. The method of claim 13 , wherein a downhole NMR tool comprises the magnet assembly and the antenna assembly, and the circular-polarized excitation is produced and the response is acquired while the downhole NMR tool is disposed in a wellbore in the subterranean region.
17. The method of claim 16 , wherein the circular-polarized excitation comprises a first excitation produced in a first sub-volume by a first antenna assembly comprising the orthogonal transversal-dipole antennas, the first sub-volume is elongate in a first direction parallel to a longitudinal axis of the downhole NMR tool, and the method comprises:
producing a second excitation in a second sub-volume that is spaced apart from the first axial end of the first sub-volume;
producing a third excitation in a third sub-volume that is spaced apart from a second, opposite axial end of the first sub-volume, the magnetic field in the second and third sub-volumes having a radial orientation that is substantially perpendicular to the first direction; and
acquiring responses from the second and third sub-volumes based on the second and third excitations.
18. The method of claim 17 , wherein the NMR tool is coupled to a drill string, the first sub-volume is elongate in a first direction parallel to a longitudinal axis of the NMR tool, the magnetic field in the first sub-volume is substantially uniformly oriented in the first direction, and the response is acquired from the first sub-volume while tripping the drill string in the wellbore.
19. A drill string assembly comprising a downhole Nuclear Magnetic Resonance (NMR) tool disposed in a wellbore in a subterranean region, the downhole NMR tool comprising:
a magnet assembly to produce a magnetic field in a volume about the wellbore, the magnet assembly comprising:
a central magnet having a first axial end and a second, opposite axial end;
a first end piece magnet spaced apart from the first axial end of the central magnet; and
a second end piece magnet spaced apart from the second axial end of the central magnet; and
an antenna assembly comprising mutually orthogonal transversal-dipole antennas to at least one of:
produce circular-polarized excitation in the volume; or
acquire a response from the volume by quadrature coil detection.
20. The drill string assembly of claim 19 , wherein:
the volume comprises multiple distinct sub-volumes, the multiple distinct sub-volumes comprise a first sub-volume that is elongate in a first direction parallel to a longitudinal axis of the NMR tool, the magnetic field in the first sub-volume being substantially uniformly oriented in the first direction; and
the downhole NMR tool comprises multiple antenna assemblies at respective locations along the longitudinal axis, each antenna assembly to detect an NMR response from a respective one of the distinct sub-volumes.