Inverse Design of RF Coil for Slim Nuclear Magnetic Resonance Logging Tool

Nuclear magnetic resonance (NMR) logging provides key parameters for reservoir evaluation and production operations. In the design of an NMR logging tool, the combination of inhomogeneous static magnetic and radio frequency (RF) fields results in a complex signal distribution. Matching the two magnetic fields allows one to accurately calculate the sensitive volume of the probe and estimate the signal amplitude. The structure of the RF coil is typically optimized to match the static field via a time-consuming procedure. In this study, a magnetic-charge model is used to calculate the static magnetic field of a simplified square-shaped magnet. Subsequently, the target-field method is used to design the RF coil for matching the specific static magnet field generated by the magnet. The designed coil provides a more homogeneous NMR signal in the azimuthal orientation. A cylindrical slice with a height of 100 and 32 mm from the axis of symmetry constitutes the sensitive volume of the probe. To assess the detection effectiveness of the probe, 1-D and 2-D NMR experiments are conducted using copper-sulfate solutions and white-oil samples with different viscosities. The high-gradient magnetic field (i.e., 3.2 T/m) of the probe is used to facilitate diffusion coefficient measurements for high-viscosity fluids.