The principle of nuclear magnetism logging
Under the action of the geomagnetic field, the hydrogen nuclei in the formation whose spin axis does not completely coincide with the geomagnetic field spun around the geomagnetic field. If a polarization coil is used in a downhole tool to generate a strong pulse magnetic field perpendicular to the geomagnetic field (compared to the geomagnetic field), the spin axis of the hydrogen nuclei is forced to leave the direction of the geomagnetic field. The magnetic field precesses and gradually returns to its original state. The precession of hydrogen nuclei produces a gradually attenuating radio frequency signal in the induction coil, the amplitude of which depends on the number of hydrogen nuclei of the free fluid in the formation, called the free fluid index.
However, bound water or dead oil has no obvious effect on nuclear magnetic logging. The signal produced by the borehole decays quickly, and its effect can be minimized by delaying the measurement time. The free fluid porosity of the rock can be obtained according to the free fluid index, and the permeability can be calculated with other data. If you then measure the thermal relaxation time, you can distinguish between oil and water.