Nuclear magnetic resonance apparatus, systems, and methods

A method comprising:

• disposing a downhole tool into a wellbore, wherein the downhole tool comprises a magnet assembly that comprises 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 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 between the central magnet and the first end piece magnet, the first shim magnet disposed adjacent to or at least partially surrounded by a first magnetically permeable material, the first shim magnet to shape a first static magnetic field sub-volume provided by the central magnet and the first end piece magnet;
• outputting, using the magnet assembly, a magnetic field in a volume into a subterranean region around the wellbore;
• emitting an excitation into the volume of the magnetic field in the subterranean region;
• detecting, by at least one antenna, a nuclear magnetic resonance response to the excitation of the volume of the magnetic field;
• 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 an end of the first end piece magnet that is proximal to the central magnet or next to an end of the first end piece magnet that is distal to the central magnet;
• at least one antenna that includes,
• a transmitter to emit an excitation into the volume of the magnetic field in the subterranean region;
• and
• a receiver to detect a nuclear magnetic resonance response in the volume of the magnetic field;
• a processor;
• and
• a machine-readable medium having program code executable by the processor to cause the processor to determine a property of the subterranean region based on the nuclear magnetic resonance response.