A multi-dimensional experiment for characterization of pore structure heterogeneity using NMR

Model Synthetic Samples for Validation of NMR Signal Simulations

Simulations of nuclear magnetic resonance (NMR) signal from fluids contained in porous media (such as rock cores) need to account for both enhanced surface relaxation and the presence of internal magnetic field gradients due to magnetic susceptibility contrast between the rock matrix and the contained fluid phase. Such simulations are typically focussed on the extraction of the NMR T2 relaxation distribution which can be related to pore size and indirectly to system permeability. Discrepancies between such NMR signal simulations on digital rock cores and associated experimental measurements are however frequently reported; these are generally attributed to spatial variations in rock matric composition resulting in heterogeneously distributed NMR surface relaxivities (ρ) and internal magnetic field gradients. To this end, a range of synthetic sediments composed of variable mixtures of quartz and garnet sands were studied. These two constituents were selected for the following reasons: they have different densities allowing for ready phase differentiation in 3D μCT images of samples to use as simulation lattices and they have distinctly different ρ and magnetic susceptibility values which allow for a rigorous test of NMR simulations. Here these 3D simulations were used to calculate the distribution of internal magnetic field gradients in the range of samples, these data were then compared against corresponding NMR experimental measurements. Agreement was reasonably good with the largest discrepancy being the simulation predicting weak internal gradients (in the vicinity of the quartz sand for mixed samples) which were not detected experimentally. The suite of 3D μCT images and associated experimental NMR measurements are all publicly available for the development and validation of NMR simulation efforts.

Multidimensional dynamic NMR correlations in sedimentary rock cores at different liquid saturations

In Nuclear Magnetic Resonance (NMR) spin echo measurements of confined liquids, the dynamic behaviour of liquid molecules are influenced by diffusion (D), translational displacement of molecules in internal gradients (G₀), and transverse surface relaxation (T₂). In this study, an experimental approach that enables characterisation of water and oil in rock core materials is presented. The approach is based on three-dimensional D-DG₀²-T₂ correlations, but the main focus is on the two-dimensional parts that involve DG₀²-T₂ and D-∣G₀∣. In order to evaluate potential signal loss that can be introduced when going from a two-dimensional to a three-dimensional experiment, D-T₂,DG₀²-T₂ and D-∣G₀∣ correlations derived from subsets of data obtained in the D-DG₀²-T₂ experiment are compared to directly obtained D-T₂ and DG₀²-T₂ correlations. The results show that when diffusion encoding is included in a multi-dimensional correlation experiment, it may lead to a significant loss of signals from liquids with relatively high diffusivity and which is located close to the mineral surface. Furthermore, a negative correlation between D and ∣G₀∣ is observed for the confined liquids in all the saturation states. Such correlations have not been measured previously, and they results in a more detailed description of the local distribution of the confined liquids. In particular, at significantly high water saturations, the surviving signal from water is found at lower values of internal gradients compared to the main part of the oil signal, indicating that this water is located further away from the surface compared to the oil. The study shows that the impact from heterogeneity in pore geometry and surface properties on the individual liquids is described in more detail in DG₀²-T₂ and D-∣G₀∣ correlations compared toD-T₂ correlations, but that potential signal loss during diffusion encoding intervals should be monitored and verified.

Effect of Mineral Composition on Transverse Relaxation Time Distributions and MR Imaging of Tight Rocks from Offshore Ireland

In this paper, we investigate the effect of magnetic field strength on the transverse relaxation time constant (T2) in six distinct core plugs from four different rock types (three sandstones, one basalt, one volcanic tuff and one siltstone), retrieved from offshore Ireland. The CPMG pulse-sequence was used at two different magnetic field strengths: high-field at 4.70 T and low-field at 0.28 T. Axial images of the core plugs were also acquired with the RAREst sequence at high magnetic field strength. Thin-sections of the core plugs were prepared for optical imaging and SEM analysis, and provided qualitative information on the porosity and quantification of the elemental composition of the rock material. The content of iron varied from 4 wt. % to close to zero in the rock samples. Nevertheless, the effective T2 distributions obtained at low-field were used to successfully predict the porosity of the core plugs. Severe signal attenuations from internal magnetic gradients resulted in an underestimation of the porosity at high-field. No definitive trend was identified on the evolution of discrete relaxation time components between magnetic field strengths. The low-field measurements demonstrate that NMR is a powerful quantitative tool for petrophysical rock analysis as compared to thin-section analysis. The results of this study are of interest to the research community who characterizes natural gas hydrates in tight heterogeneous core plugs, and who typically relies on MR imaging to distinguish between solid hydrates and fluid phases. It further exemplifies the importance of selecting appropriate magnetic field strengths when employing NMR/MRI for porosity calculation in tight rock.

Characterising oil and water in porous media using correlations between internal magnetic gradient and transverse relaxation time

A method for characterising water and oil in a rock core plug using correlations between diffusion decay in internal magnetic gradient and transverse relaxation time (DG₀²-T₂) is presented. The method is evaluated at different saturation levels and is compared with the measurement of correlations between diffusion and transverse relaxation time (D-T₂). It is shown how signals from water and oil can be separated based on their difference in diffusion decay in internal gradients. The obtained results show that the impact from heterogeneity in pore geometry and mineralogy on the individual liquids is revealed in more detail in DG₀²-T₂ correlations compared to the more established D-T₂ correlations. Measurements of DG₀²-T₂ correlations should be included in the toolbox of NMR experiments performed in the laboratory analysis of rock core plugs, and could then potentially lead to more detailed estimations of saturation levels and surface wettability properties.

Investigating pore geometry in heterogeneous porous samples using spatially resolved G0-Δχapp and G0-Δν correlations

This study presents a two-dimensional NMR pulse sequence for obtaining spatially resolved correlations between magnetic susceptibility induced internal gradients (G₀), and both the apparent difference in magnetic susceptibility (Δχ_app) and spectral frequency (Δν). G₀-Δχ_app correlations were utilized to generate spatially resolved pore size distributions, while the G₀-Δν correlations were used to additionally evaluate sample heterogeneity. The spatially resolved measurements were performed on a water saturated heterogeneous porous sample which contains one layer of 5-50 μm glass spheres (top layer) and one layer of 140-165 μm glass spheres (bottom layer). The slice selection was validated by applying the pulse sequence on a liquid mineral oil and water sample as well as on the porous sample. The resulting spatially resolved pore size distributions show very good agreement with results from our previously published non slice selective pulse sequence in the 140-165 μm glass spheres and good agreement in the 5-50 μm glass spheres. The G₀-Δν correlations correctly indicate a slightly higher degree of heterogeneity in the 5-50 μm glass spheres compared to the 140-165 μm glass spheres.

Direct correlation of internal gradients and pore size distributions with low field NMR

Internal magnetic field gradients Gint, which arise from the magnetic susceptibility difference Δχ between solid matrix and fluid in porous media relate to the pore geometry. However, this relationship is complex and not well understood. Here we correlate internal-gradient distributions to pore-size distributions directly to examine internal gradients in detail at low field NMR. The pore-size distributions were obtained by the method of Decay due to Diffusion in the Internal Field (DDIF), and the internal-gradient distributions were measured with the Carr-Purcell-Meiboom-Gill (CPMG) method. The internal-gradient-pore-size distributions correlation maps were obtained for water in packs of glass beads with different diameter and in a sandstone sample. The relationship between internal gradients and pore structure is analyzed in detail by considering the restricted diffusion of fluids in porous samples. For each case diffusion regimes are assigned by plotting normalized CPMG data and comparing the diffusion lengths, the dephasing lengths and pore diameters. In the free-diffusion limit, the correlation maps reveal the true relationship between pore structure and internal gradients so that Δχ can be approximated from the correlation maps. This limit is met most easily at low field. It provides information about porous media, which is expected to benefit the oil industry, in particular NMR well logging.