NMR logging probing the internal magnetic field gradients of rocks

The internal field gradients in rocks has a big effect on nuclear magnetic resonance (NMR) transverse relaxation T_2 distribution.A two-dimensional NMR method of probing the internal magnetic field gradients of rocks and its inversion algorithms are developed using CPMG pulse sequence.Internal field gradient distributions of water-saturated sandstones and shaly sandstones with paramagnetic impurities are obtained.The results show that internal field gradients of rocks increase with the increasing of content of paramagnetic impurities.The internal field gradients of sandstones with uniform grain show unimodal distributions.The T_2 distributions of water-saturated shaly sandstones with chlorite show bimodal feature with obvious irreducible water peak.The small pores with short T_2 have higher internal field gradients than that of the big pore with long T_2.When the content of chlorite is less than 15%,the short T_2 components corresponding to irreducible water peak increase with increasing the content of chlorite,T_2 distribution becomes wider.However,when the content of chlorite is more than 15%,the higher internal field gradients result in short relaxation component decaying quickly, the short T_2 components decrease,T_2 distribution becomes narrower with increasing the content of chlorite.