Asymmetric excluder for downhole logging tool

1. A downhole tool, comprising,

a cylindrical housing;

a magnet positioned within the housing;

an antenna positioned within the housing; and

an excluder positioned around the housing, wherein a thickness of the excluder varies around the housing, wherein the thickness of the excluder increases through a first angle with respect to an axis that extends from a central longitudinal axis of the housing through a center of the antenna, wherein the first angle is from about 10° to about 120°.

2. The downhole tool of claim 1 , wherein the antenna emits radiofrequency electromagnetic pulses of energy.

3. The downhole tool of claim 1 , wherein the first angle is measured in a clockwise direction with respect to the axis.

4. The downhole tool of claim 3 , wherein the thickness of the excluder increases through a second angle from about 10° to about 120° with respect to the axis, and wherein the second angle is measured in a counterclockwise direction with respect to the axis.

5. The downhole tool of claim 3 , wherein the thickness of the excluder decreases through a second angle with respect to the axis, wherein a sum of the first and second angles is 180°, and wherein the second angle is measured in the clockwise direction with respect to the axis.

6. The downhole tool of claim 1 , wherein an outer surface of the excluder comprises a protrusion, a recess, or both.

7. The downhole tool of claim 1 , wherein a cross-sectional shape of the excluder is an oval, a prolate spheroid, a football, or a combination thereof.

8. A downhole tool, comprising,

a housing;

a magnet positioned within the housing;

an antenna positioned within the housing; and

an excluder positioned around the housing, wherein a distance between a central longitudinal axis of the housing and an outer surface of the excluder is less at a first angle than at a second angle, and wherein the first and second angles are measured with respect to an axis that extends from the central longitudinal axis through a center of the antenna, wherein the excluder comprises a first layer made of plastic, and a second layer positioned around the first layer that is made of fiberglass.

9. The downhole tool of claim 8 , wherein the first angle is at 0° with respect to the axis, and the second angle is from about 10° to about 120° with respect to the axis.

10. The downhole tool of claim 9 , wherein the distance is greater at the second angle than at a third angle, wherein the third angle is 180° from the first angle.

11. The downhole tool of claim 10 , wherein an outer surface of the excluder comprises a protrusion, a recess, or both.

12. A method for measuring properties of a formation surrounding a wellbore, comprising:

running a downhole tool into the wellbore, the downhole tool comprising:

a substantially cylindrical housing;

a magnet positioned within the housing;

an antenna positioned within the housing; and

an excluder positioned around the housing, wherein a thickness of the excluder varies around the housing; and

positioning the downhole tool such that an outer surface of the excluder is in contact with a wall of the wellbore, wherein the thickness of the excluder increases through a first angle with respect to an axis that extends from a central longitudinal axis of the housing through a center of the antenna, and wherein the first angle is from about 10° to about 120°.

13. The method of claim 12 , further comprising emitting radiofrequency electromagnetic pulses of energy from the antenna into the formation.

14. The method of claim 13 , wherein at least a portion of the outer surface of the excluder has a radius that is substantially equal to a radius of the wellbore.

15. The method of claim 12 , wherein the thickness of the excluder decreases through a second angle with respect to the axis, wherein a sum of the first and second angles is 180°, and wherein the second angle is measured in the clockwise direction with respect to the axis.