NMR (Nuclear magnetic Resonance) measurements have been extensively used for characterizing porosity and pore size distribution with assumption of constant magnetic field gradient within porous media. However, internal gradients of magnetic field generated due to the presence of paramagnetic or diamagnetic centers such as clay particles can significantly affect NMR response and the resulting interpretation for pore-size distribution and porosity. In this paper, we quantify the impact of internal magnetic gradients and spatial distribution of matrix components such as clay minerals on NMR response using pore-scale NMR numerical simulations. We also quantify the influence of the aforementioned parameters on the NMR-based evaluation of porous media.
We used finite volume method to numerically solve Bloch equations (Bloch 1946) and simulated magnetization decay in porous media. We cross-validated the reliability of numerical simulations using analytical solutions given for spherical pores in different diffusion regimes. The model is then used for simulation of NMR response in the pore-scale images of sandstone and carbonate rocks. We developed synthetic cases based on actual rock images covering a wide range of spatial distribution of clay minerals (i.e., paramagnetic or diamagnetic centers) to quantify the sensitivity of NMR decay to internal magnetic gradients. We quantified the sensitivity of NMR response for distribution of clays as random dispersion in the rock grains, as thin laminae in the rock and as thin layers on the surface of grains.
The results showed that at low concentration (e.g., 0.3%-0.7%) of dispersed clay, there is negligible impact of internal magnetic gradients on magnetization decay. At higher concentration of dispersed clay (e.g., 5.1%-7.3%), we observed significant impact of internal magnetic gradients on magnetization decay. The presence of clay minerals can cause 53% variation in the location of transverse relaxation time constant (T2) and up to 67% relative error in assessment of dominant pore sizes. We also observed that structural clays even at high volumetric concentrations of 40% produce negligible effect on NMR measurements. Shale laminations containing clay were found to produce an effect of up to 17.5% on T2 relaxation time constant that could cause a relative error of 23.1% in estimates of dominant pore size in the rock. The outcomes of this paper can potentially improve conventional techniques of pore network characterization (pore-size distribution and pore volume) in the presence of clay minerals where internal magnetic gradients are not negligible.