NMRlogging

NMR logging, or nuclear magnetic resonance logging, is a downhole measurement that uses low-field magnetic resonance to investigate hydrogen nuclei in formation fluids and rock matrix. The resulting data illuminate pore geometry and fluid content and are used to characterize hydrocarbon reservoirs. NMR logs are typically run alongside conventional logs to provide porosity, pore size distribution, fluid typing, and permeability estimates.

The measurement relies on pulsed magnetic fields and radiofrequency excitation to induce and detect the nuclear spin response of protons. In practice, a sequence such as a Carr-Purcell-Meiboom-Gill (CPMG) train yields a T2 relaxation time distribution for each formation. The area under the signal is proportional to proton density, giving total porosity, while the shape of the T2 distribution reflects pore sizes and surface relaxation effects. Short T2 components are associated with bound water in clays; longer T2 components relate to movable fluids in larger pores. Gas-bearing zones typically show reduced signals due to the low proton density of gas-filled pores.

Interpretation uses the T2 distribution to estimate pore-size distributions and to classify fluids. Movable-fluid indices and permeability proxies (for example, T2 cut-offs and empirical relationships like Timur-Coates or Coates models) enable reservoir characterization and deliver input for saturation and production planning.

Limitations include borehole temperature and pressure effects, high clay content, and magnetic environment requiring corrections. NMR logs are valuable for distinguishing oil, water, and gas, estimating porosity independent of lithology, and providing insight into pore geometry that complements other log types.