Downhole Nuclear Magnetic Resonance Tool with Active Compensation for Motional Effects

A nuclear magnetic resonance (NMR) tool for use in a wellbore, comprising:

• an antenna assembly comprising an antenna winding at least partially disposed around a soft magnetic core comprising an upper axial end opposite a lower axial end;
• a magnet assembly comprising an upper end magnet spaced apart from the upper axial end of the soft magnetic core and a lower end magnet spaced apart from the lower axial end of the soft magnetic core;
• a compensating assembly comprising an upper electromagnet disposed between the upper end magnet and the soft magnetic core and a lower electromagnet disposed between the lower end magnet and the soft magnetic core;
• wherein the antenna assembly and the magnet assembly produce a static magnetic field;
• and
• wherein the compensating assembly reduces the static magnetic field that has an increased field due to lateral motion of the antenna and magnet assemblies and increases the static magnetic field that has a decreased field due to lateral motion of the antenna and magnet assemblies.

A method for obtaining nuclear magnetic resonance (NMR) data in a wellbore, comprising:

• introducing an NMR tool into the wellbore, wherein the NMR tool comprises:
• an antenna assembly operable to generate a radio-frequency magnetic field;
• a magnet assembly operable to generate a static magnetic field;
• and
• a compensating assembly comprising at least one electromagnet;
• producing a static magnetic field using the magnet assembly;
• measuring well logging data via the NMR tool;
• moving the NMR tool through the wellbore;
• and
• reducing the effect of lateral motion of the antenna and magnet assemblies on the NMR data by operating the compensating assembly.

A logging system, comprising:

• a conveyance;
• and
• a nuclear magnetic resonance (NMR) tool for use in a wellbore and coupled to the conveyance, the NMR tool comprises:
• an antenna assembly comprising an antenna winding at least partially disposed around a soft magnetic core comprising an upper axial end opposite a lower axial end, the upper and lower axial ends are axially aligned by a common axis;
• a magnet assembly comprising an upper end magnet spaced apart from the upper axial end of the soft magnetic core and axially aligned with the common axis and a lower end magnet spaced apart from the lower axial end of the soft magnetic core and axially aligned with the common axis;
• and
• a compensating assembly comprising an upper electromagnet disposed between the upper end magnet and the upper axial end of the soft magnetic core and a lower electromagnet disposed between the lower end magnet and the lower axial end of the soft magnetic core, the upper and lower electromagnets are axially aligned with the common axis;
• wherein the antenna assembly and the magnet assembly produces a static magnetic field in a volume within a subterranean region;
• and
• wherein the compensating assembly reduces or increases the static magnetic field that has a changed field due to lateral motion of the antenna and magnet assemblies.