Magnetic susceptibility contrast effects on NMR T₂ logging

When a rock sample or formation is subjected to a static magnetic field, magnetic susceptibility contrasts between rock grains and pore fluids induce local magnetic field heterogeneities. These local field fluctuations cause increases in the transverse (T2) relaxation rate. Under specific conditions, the increase in the relaxation rate is proportional to the squares of the averaged field strength, the susceptibility contrast, and the pore dimension. The rate increase is also a function of t (half the echo spacing), and is proportional to l-(tanh λτ)/λ.τ. Here X is an eigenvalue of the diffusion equation which is proportional to the reciprocal of the pore dimension squared for a periodic porous medium, or can be regarded as the dominant relaxation rate in a disordered system. Laboratory NMR core measurements indicate that an apparent 1/λ can be determined experimentally by varying the echo spacing, and can be correlated to permeability. Such a correlation may not always be as sensitive as T_2G/F, where T₂o is the logarithmic average of T₂ and F is the formation factor, but it offers additional petrophysical information for formation evaluation.