# US 10,768,335 B1 · Saddle point nuclear magnetic resonance tool for measurements at multiple depths of investigation

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- Number: US 10,768,335 B1
- Document ID: US10768335B1
- Status: Granted
- Kind: B1
- Application number: 16/289,063
- Filed: 2019-02-28
- Published: 2020-09-03
- Granted: 2020-09-08
- Earliest claimed: 2019-02-28
- Assignees: Schlumberger Technology Corporation
- Applicants: Schlumberger Technology Corporation
- Inventors: Anatoly Dementyev; Henry Bachman
- Web page: https://patentgrep.com/patent/US10768335B1

## CPC

- G01V 3/32: operating with electron or nuclear magnetic resonance
- G01N 24/081: {Making measurements of geologic samples, e.g. measurements of moisture, pH, porosity, permeability, tortuosity or viscosity}
- G01R 33/3808: {Magnet assemblies for single-sided MR wherein the magnet assembly is located on one side of a subject only; Magnet assemblies for inside-out MR, e.g. for MR in a borehole or in a blood vessel, or magnet assemblies for fringe-field MR}
- G01V 3/34: Transmitting data to recording or processing apparatus; Recording data
- H01Q 1/04: Adaptation for subterranean or subaqueous use
- G01R 33/383: using permanent magnets
- G01V: GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS (means for indicating the location of accidentally buried, e.g. snow-buried, persons A63B29/02)
- G01N: INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES (measuring or testing apparatus or processes other than immunoassay, involving enzymes or microorganisms C12M, C12Q)
- G01R: MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES (indicating correct tuning of resonant circuits H03J3/12)
- H01Q: ANTENNAS, i.e. RADIO AERIALS (radiators or antennas for microwave heating H05B6/72)

## Abstract

The present disclosure relates to generating multiple depths of investigation (DOI) measurements with a saddle point design nuclear magnetic resonance sensor. In general, an NMR sensor in accordance with the present disclosure includes a first magnet at a first radial distance from a radial center, and a second magnet at a second radial distance from the radial center. The first magnet at least partially cancels out a magnetic field gradient produced by the second magnet. Further, the NMR sensor may include an antenna that generates a first set of NMR data from a first DOI by operating at a first frequency and, generates a second set of NMR data from a second DOI by operating a second frequency. Additional frequencies are possible and envisioned.

## Claims

### Claim 1

A device for generating nuclear magnetic resonance (NMR) data, the device comprising:
a first magnet configured to generate a first magnetic moment that is along a first direction;
a second magnet disposed at a first position separate from the first magnet at a distance along a direction of the device, wherein the second magnet is configured to generate a second magnetic moment that is along a second direction, wherein a first angle between the first direction and the second direction is greater than 90 degrees, wherein a net magnetic field produced by at least the first magnet and the second magnet has a magnetic-field profile that is suitable for multiple depth of investigation measurements (DOI);
a third magnet disposed at a third radial distance from the radial center of the NMR sensor, wherein the third magnet is configured to generate a third magnetic moment along a third direction that is opposite of the first direction, and wherein a second angle between the third direction and the first direction is about 192 degrees;
a fourth magnet disposed at a fourth radial distance from the radial center of the NMR sensor, wherein the fourth magnet is configured to generate a fourth magnetic moment along a fourth direction that is opposite of the first direction, and wherein a third angle between the fourth direction and the first direction is about 168 degrees; wherein the first magnet, the second magnet, the third magnet, and the fourth magnet are separate from each other, wherein a net magnetic field produced by the first magnet, the second magnet, and the third magnet has a magnetic field profile that is suitable for multiple depth of investigation measurements (DOI); and
an antenna, wherein the antenna is configured to:
generate a first set of NMR data from a first DOI operating at a first frequency; and
generate a second set of NMR data from a second DOI operating at a second frequency.

### Claim 2 (depends on 1)

The device of claim 1, wherein a first cross-sectional area of the first magnet is less than half of a second cross-sectional area of the second magnet.

### Claim 3 (depends on 1)

The device of claim 1, wherein the first radial distance is greater than the second radial distance, the third radial distance, and the fourth radial distance.

### Claim 4 (depends on 1)

The device of claim 1, wherein the distance between the first magnet and the second magnet is less than a radius of the device.

### Claim 5 (depends on 1)

The device of claim 1, wherein the first magnet is disposed within a recess of the antenna.

### Claim 6 (depends on 1)

The device of claim 1, wherein the distance is a lateral distance.

### Claim 7 (depends on 1)

The device of claim 1, wherein the second frequency is pulsed sequentially after the first frequency.

### Claim 8 (depends on 1)

The device of claim 1, wherein there are more than two frequencies of operation.

### Claim 9

A downhole tool for measuring properties of a geological formation, the downhole tool comprising:
a nuclear magnetic resonance (NMR) sensor, wherein the NMR sensor comprises:
a first magnet disposed at a first radial distance from a radial center of the NMR sensor, wherein the first magnet is configured to generate a first magnetic moment that is along a first direction;
a second magnet disposed at a second radial distance from the radial center of the NMR sensor, wherein the second magnet is configured to generate a second magnetic moment along a second direction, wherein a first angle between the first direction and the second direction is greater than 90 degrees;
a third magnet disposed at a third radial distance from the radial center of the NMR sensor, wherein the third magnet is configured to generate a third magnetic moment along a third direction that is opposite of the first direction, and wherein a second angle between the third direction and the first direction is greater than 90 degrees;
a fourth magnet disposed at a fourth radial distance from the radial center of the NMR sensor, wherein the fourth magnet is configured to generate a fourth magnetic moment along a fourth direction that is opposite of the first direction, and wherein a third angle between the fourth direction and the first direction is greater than 90 degrees;
wherein the first magnet, the second magnet, the third magnet, and the fourth magnet are separate from each other, wherein a net magnetic field produced by the first magnet, the second magnet, and the third magnet has a magnetic field profile that is suitable for multiple depth of investigation measurements (DOI), wherein the first radial distance is greater than the second radial distance, the third radial distance, and the fourth radial distance; and
an antenna configured to:
generate a first set of NMR data from a first DOI at a first frequency; and
generate a second set of NMR data from a second DOI at a second frequency.

### Claim 10 (depends on 9)

The downhole tool of claim 9, wherein the second angle is 192 degrees and the third angle is 168 degrees.

### Claim 11 (depends on 9)

The downhole tool of claim 9, wherein at least one cross-sectional area of the second magnet, the third magnet, and the fourth magnet is different.

### Claim 12 (depends on 9)

The downhole tool of claim 9, wherein a cross-sectional area of the first magnet is less than half of at least one cross-sectional area of the second magnet, the third magnet, and the fourth magnet.

### Claim 13 (depends on 9)

The downhole tool of claim 9, wherein at least one magnet of the first magnet, the second magnet, the third magnet, and the fourth magnet is a magnet section that comprises multiple magnets.

### Claim 14 (depends on 9)

The device of claim 9, wherein the second angle is greater than 180 degrees, and the third angle is less than 180 degrees.

### Claim 15 (depends on 9)

The device of claim 9, wherein the first angle is greater than 190 degrees and less than 270 degrees.

### Claim 16 (depends on 9)

The device of claim 9, wherein there are more than two frequencies of operation.

### Claim 17

A method of manufacturing a nuclear magnetic resonance (NMR) sensor for a downhole tool, comprising:
providing an antenna configured to generating at least one set of NMR data;
providing a first magnet at a first radial distance from a radial center of the NMR sensor, wherein the first magnet is configured to generate a first magnetic moment that is along a first direction; and
providing a second magnet at a second radial distance from the radial center of the NMR sensor, wherein the second magnet is configured to generate a second magnetic moment along a second direction, wherein a first angle between the first direction and the second direction is greater than 90 degrees;
providing a third magnet at a third radial distance from the radial center of the NMR sensor, wherein the third magnet is configured to generate a third magnetic moment along a third direction that is opposite of the first direction, and wherein a second angle between the third direction and the first direction is greater than 90 degrees;
providing a fourth magnet at a fourth radial distance from the radial center of the NMR sensor, wherein the fourth magnet is configured to generate a fourth magnetic moment along a fourth direction that is opposite of the first direction, and wherein a third angle between the fourth direction and the first direction is greater than 90 degrees, wherein the first radial distance is greater than the second radial distance, the third radial distance, and the fourth radial distance.