NMR can determine porosity and pore size distributions easily and quickly. Essentially, NMR signals are generated from liquids (oil or brine) when the sample is placed in a magnetic field and then excited with a brief pulse of radio frequency (RF) energy. Immediately after the pulse, an NMR signal appears and then dies away with a characteristic relaxation time or decay rate, known at T2. The amplitude of the signal immediately following the pulse is an indication of total fluid present, while theT2 of the signal gives valuable information about the physical environment of the fluid (ie. size of pore where the fluid is located).
The NMR data is then inverted to process the composite NMR signal to separate it into its component parts. Because long T2s come from large pores and short T2s from small pores, this T2 distribution is a model of the pore size distribution in the core.