Nuclear magnetic resonance apparatus, systems, and methods

A method comprising:

• disposing a downhole tool into a wellbore, wherein the downhole tool comprises:
• multiple antennas disposed at respective locations along a longitudinal axis of the downhole tool;
• and
• a magnet assembly comprising:
• a central magnet having a first axial end and a second axial end that is opposite the first axial end,
• a first end piece magnet having a proximal end and a distal end, the proximal end spaced apart from the first axial end of the central magnet,
• a second end piece magnet spaced apart from the second axial end of the central magnet,
• at least one first shim magnet spaced apart from the distal end of the first end piece magnet;
• and
• a first magnetically permeable material disposed adjacent to or at least partially surrounding the longitudinal axis of the downhole tool, the first permeable material disposed between the first end piece magnet and the first shim magnet;
• generating, using the magnet assembly, a magnetic polarization in a volume in a subterranean region about the wellbore;
• emitting an excitation in the magnetic polarization in the volume in the subterranean region;
• detecting, by each of the multiple antennas, a nuclear magnetic resonance response to the excitation of the volume of the magnetic field from a respective one of multiple corresponding distinct magnetic field sub-volumes defined by the central magnet and first and second end piece magnets;
• and
• determining a property of the subterranean region based on the nuclear magnetic resonance response.

A system, 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 axial end that is opposite the first axial end;
• a first end piece magnet having a proximal end and a distal end, the proximal end spaced apart from the first axial end of the central magnet;
• a second end piece magnet spaced apart from the second axial end of the central magnet;
• and
• at least one first shim magnet-disposed adjacent to or at least partially surrounded by first magnetically permeable material, the at least one first shim magnet disposed next to the proximal end of the first end piece magnet or next to the distal end of the first end piece magnet;
• and
• multiple antennas disposed at respective locations along a longitudinal axis of a downhole tool, wherein the multiple antennas include,
• a transmitter to emit an excitation into the volume of the magnetic field in the subterranean region, and
• a receiver in each of the multiple antennas to detect a nuclear magnetic resonance response in the volume of the magnetic field from a respective one of multiple corresponding distinct magnetic field sub-volumes defined by the central magnet and first and second end piece magnets.