Nuclear magnetic resonance logging tool with quadrature coil configuration

A method comprising:

• introducing a nuclear magnetic resonance (NMR) logging tool into a wellbore penetrating a subterranean formation, the wellbore containing a fluid and having a z-axis extending along a length of the wellbore, wherein the NMR logging tool has a z-coil and a transversal coil;
• generating a plurality of radiofrequency (RF) excitations each corresponding to different volumes of interest in the subterranean formation, wherein the plurality of RF excitations includes a transversal RF excitation in a transversal direction relative to the z-axis of the wellbore with the transversal coil and a quadrature RF excitation with the z-coil and the transversal coil, wherein a frequency corresponding to the quadrature RF excitation is less than a frequency corresponding to the transversal RF excitation;
• and
• detecting an NMR signal from the subterranean formation produced as a result of the transversal RF excitation or the quadrature RF excitation with one of:
• (1) the transversal coil or (2) both the z-coil and the transversal coil.

A method comprising:

• introducing a nuclear magnetic resonance (NMR) logging tool into a wellbore penetrating a subterranean formation, the wellbore containing an oil-based mud (OBM) and having a z-axis extending along a length of the wellbore, wherein the NMR logging tool having a transversal dipole magnet, a z-coil, and a transversal coil;
• generating a quadrature radiofrequency (RF) excitation with the z-coil and the transversal coil;
• generating a transversal RF excitation in a transversal direction relative to the z-axis of the wellbore with the transversal coil;
• detecting a first NMR signal and a second NMR signal from the subterranean formation produced by the quadrature RF excitation and the transversal RF excitation, respectively, with one of:
• (1) the transversal coil or (2) both the z-coil and the transversal coil;
• calculating a quality factor for each of the z-coil and the transversal coil based on the first and second NMR signals relative to the quadrature RF excitation and the transversal RF excitation, respectively;
• and
• calculating a formation anisotropy based on the quality factor for the z-coil and the quality factor for the transversal coil.

A system comprising:

• a wellbore penetrating a subterranean formation containing a fluid and having a z-axis extending along a length of the wellbore;
• and
• a nuclear magnetic resonance (NMR) logging tool extendable within the wellbore and having a z-coil and a transversal coil,
• wherein the NMR logging tool generates in a volume of investigation of the subterranean formation either (1) a transversal radiofrequency (RF) excitation in a transversal direction relative to the z-axis of the wellbore with the transversal coil or (2) a quadrature RF excitation with both the z-coil and the transversal coil, a selection between the transversal RF excitation and the quadrature RF excitation being based on a resistivity of the fluid, a diameter of the wellbore, a depth into the subterranean formation of the volume of investigation, or a combination thereof, and
• wherein the NMR logging tool detects an NMR signal from the subterranean formation produced as a result of the transversal RF excitation or the quadrature RF excitation with one of the transversal coil or both the z-coil and the transversal coil.