Determination of surface relaxivity from NMR diffusion measurements

Nuclear magnetic resonance (NMR) T2-decay measurements are usually interpreted in terms of pore-size distributions. The T2 relaxation time of a water-wet pore is proportional to the size of the pore via surface relaxivity. Quantitative knowledge of the surface relaxivity is important when T2 spectra are to be used for further use such as NMR derived capillary curves. s study, we demonstrate that surface relaxivity can be directly determined from NMR measurements. Diffusion of hydrogen spins is restricted by the pore size and this effect is independent of surface relaxivity. Hence, surface relaxivity can be determined by combining restricted diffusion and T2-relaxation. The latter two effects are measured simultaneously in a NMR T2 decay measurement performed in a static magnetic field gradient. This method generalises existing ones for uniform pore systems to full pore-size distributions of realistic rocks. e performed laboratory NMR diffusion measurements on a number of sandstone core plugs. The surface relaxivities found from these data are compared to those obtained from other methods. ethod of measuring surface relaxivity can in principle be applied to NMR data obtained in boreholes which leads to a new application of NMR logging in the characterisation of oil and gas reservoirs.