Characterization of porous media structure by non linear NMR methods (original) (raw)
Related papers
Probing the structure of porous media using NMR spin echoes
Magnetic Resonance Imaging, 1994
gator of the molecules of a fluid diffusing in the pores of a porous medium. For small values of k = 'yg&, where g is the gradient strength, 6 is the duration of the gradient pulse, and y the gyromagnetic ratio, the PFGSE amplitude gives the diffusion coefficient o(t). The exact short-time diffusion coefficient, D(t)/&, = 1 -(4m,S)/ (9& &?) -D&L!V(12 5) + ptS/6Y,, provides an important method for determining the surface to pore-volume ratio S/V,. Here the mean surface curvature H = (l/R1 + l/R&. Combining early II(f) with the magnetization decay one obtains the surface relaxivity p. The long-time effective diffusion constant derived from PFGSE gives information on the tortuosity of the connected space. The diffusion coefficient measured by PFGSE equals that derived from electrical conductivity only when p = 0. Exact solutions with partially absorbing boundary conditions for a periodic structure are used to illustrate the influence of p on the diffusion coefficient. M(k,t) can be well represented by a convolution of the structure factor of the connected pore space with an appropriate Gaussian propagator. This ansatz provides a model-independent way of obtaining the structure factor. Kevwords: Restricted diffusion: Porous medium: NMR surface relaxation; NMR pulsed field gradient; Surfaceto-volume ratio; Formation factor. 12(2):279-284; 1994. Kleinberg, R.L. Pore-size distribution, pore coupling, and transverse relaxation spectra of porous rocks. Magn. Reson. Imaging 12(2):271-274; 1994. Callaghan, P.T.; MacGowan, D.; Packer, K.J.; Zelaya, F.O. Diffraction like effects in NMR diffusion studies of fluids in porous solids. Nature 351:467-468; 1991.
NMR imaging applied to various studies of porous media
Magnetic Resonance Imaging, 1991
We present results of various studies performed in porous media by NMR hnaghtg: discontinuity in the water spatial repartition during drying of a limestone block, drying of cement pastes, progressive melting of a natural clay soil, and quantitative evaluation of dispersion coefficients in glass bead packs. Moreover, the technique has been as well used to obtain the spatial distribution of the relaxation times. The results have been obtained at a magnetic field of 0.1 tesla with samples of approximately 10 cm sire. The 3-D spatial distribution has been obtained with a resolution of the order of 1 mm using the standard method of a spin echo in coincidence with a gradient echo for an echo time of 6 to 20 ms and field gradients of 1 G/cm on a whole body access.
A multi-dimensional experiment for characterization of pore structure heterogeneity using NMR
Journal of Magnetic Resonance, 2016
In a liquid saturated porous sample the spatial inhomogeneous internal magnetic field in general depends on the strength of the static magnetic field, the differences in magnetic susceptibilities, but also on the geometry of the porous network. To thoroughly investigate how the internal field can be used to determine various properties of the porous structure, we present a novel multi-dimensional NMR experiment that enables us to measure several dynamic correlations in one experiment, and where all of the correlations involve the internal magnetic field and its dependence on the geometry of the porous network. (Correlations: internal gradient-pore size, internal gradient-magnetic susceptibility difference, internal gradient-longitudinal relaxation, longitudinal relaxation-magnetic susceptibility difference.) It is always a spatial average of the internal magnetic field, or one of the related properties, that is measured, which is important to take into consideration when analyzing the obtained results. We demonstrate how these correlations can be an indicator for pore structure heterogeneity, and focus in particular on how the effect from spatial averaging can be evaluated and taken into account in the different cases.
Comparison of NMR simulations of porous media derived from analytical and voxelized representations
Journal of Magnetic Resonance, 2009
NMR response Random-walk method Grain pack Voxel image Surface-area effect a b s t r a c t We develop and compare two formulations of the random-walk method, grain-based and voxel-based, to simulate the nuclear-magnetic-resonance (NMR) response of fluids contained in various models of porous media. The grain-based approach uses a spherical grain pack as input, where the solid surface is analytically defined without an approximation. In the voxel-based approach, the input is a computer-tomography or computer-generated image of reconstructed porous media. Implementation of the two approaches is largely the same, except for the representation of porous media. For comparison, both approaches are applied to various analytical and digitized models of porous media: isolated spherical pore, simple cubic packing of spheres, and random packings of monodisperse and polydisperse spheres. We find that spin magnetization decays much faster in the digitized models than in their analytical counterparts. The difference in decay rate relates to the overestimation of surface area due to the discretization of the sample; it cannot be eliminated even if the voxel size decreases. However, once considering the effect of surface-area increase in the simulation of surface relaxation, good quantitative agreement is found between the two approaches. Different grain or pore shapes entail different rates of increase of surface area, whereupon we emphasize that the value of the ''surface-area-corrected" coefficient may not be universal. Using an example of X-ray-CT image of Fontainebleau rock sample, we show that voxel size has a significant effect on the calculated surface area and, therefore, on the numerically simulated magnetization response.
Recent Fourier and Laplace perspectives for multidimensional NMR in porous media
Magnetic resonance …, 2007
Multidimensional NMR techniques used in the measurement of molecular displacements, whether by diffusion or advection, and in the measurement of nuclear spin relaxation times are categorised. Fourier-Fourier, Fourier-Laplace and Laplace-Laplace methods are identified, and recent developments discussed in terms of the separation, correlation and exchange perspective of multidimensional NMR spectroscopy. D
Bone tissue and porous media: common features and differences studied by NMR relaxation
Magnetic Resonance Imaging, 2003
Despite significant differences between bone tissues and other porous media such as oilfield rocks, there are common features as well as differences in the response of NMR relaxation measurements to the internal structures of the materials. Internal surfaces contribute to both transverse (T 2 ) and longitudinal (T 1 ) relaxation of pore fluids, and in both cases the effects depend on, among other things, local surface-to-volume ratio (S/V). In both cases variations in local S/V can lead to distributions of relaxation times, sometimes over decades. As in rocks, it is useful to take bone data under different conditions of cleaning, saturation, and desaturation. T 1 and T 2 distributions are computed using UPEN. In trabecular bone it is easy to see differences in dimensions of intertrabecular spaces in samples that have been de-fatted and saturated with water, with longer T 1 and T 2 for larger pores. Both T 1 and T 2 distributions for these water-saturated samples are bimodal, separating or partly separating inter-and intratrabecular water. The T 1 peak times have a ratio of from 10 to 30, depending on pore size, but for the smaller separations the distributions may not have deep minima. The T 2 peak times have ratios of over 1000, with intratrabecular water represented by large peaks at a fraction of a ms, which we can observe only by single spin echoes. CPMG data show peaks at about a second, tapering down to small amplitudes by a ms. In all samples the free induction decay (FID) from an inversionrecovery (IR) T 1 measurement shows an approximately Gaussian (solid-like) component, exp[Ϫ 1 ⁄2 (T/T GC ) 2 ], with T GC Ϸ 11.7 Ϯ 0.7 s (GC for "Gaussian Component"), and a liquid-like component (LLC) with initially simple-exponential decay at the rate-average time T 2-FID for the first 100 s. Averaging and smoothing procedures are adopted to derive T 2-FID as a function of IR time and to get T 1 distributions for both the GC and the LLC. It appears that contact with the GC, which is presumed to be 1 H on collagen, leads to the T 2 reduction of at least part of the LLC, which is presumed to be water. Progressive drying of the cleaned and water-saturated samples confirms that the long T 1 and T 2 components were in the large intertrabecular spaces, since the corresponding peaks are lost. Further drying leads to further shortening of T 2 for the remaining water but eventually leads to lengthening of T 1 for both the collagen and the water. After the intertrabecular water is lost by drying, T 1 is the same for GC and LLC. T 2-FID is found to be roughly 320/␣ s, where ␣ is the ratio of the extrapolated GC to LLC, appearing to indicate a time of about 320 s for 1 H transverse magnetization in GC to exchange with that of LLC. This holds for all samples and under all conditions investigated. The role of the collagen in relaxation is confirmed by treatment to remove the mineral component, observing that the GC remains and has the same T GC and has the same effect on the relaxation times of the associated water. Measurements on cortical bone show the same collagen-related effects but do not have the long T 1 and T 2 components.
Nuclear magnetic resonance relaxation measurements in porous reservoir rocks are often used to produce 'pseudo-pore size' distributions of the systems. These are commonly correlated with mercury injection capillary pressure data, allowing estimation of a constant surface relaxivity for the NMR measurements. However, this approach is not an accurate comparison as the mercury injection measures pore space accessibility at a given pressure and produces a distribution of pore throat size as opposed to pore body size.
Characterization of porous solids by NMR
Physical Review Letters, 1993
We are using a novel NMR method, that has been developed in our laboratory and employs the depression of the freezing point (AT,) of confined liquid within porous media, to investigate the effect of pre-drying silica with average pore diameter ranging from 60 A to 1000 A. Cyclohexane was the confined liquid. Pre-drying was found to affect only the smallest pores. A study to compare partially filled and over-filled samples showed that the average AT, for partially filled samples is greater than for over-filled. We have also investigated the use of water as the absorbed liquid and compared results with those obtained from cyclobexane studies. Reasonable agreement was found but cvclohexane was more sensitive. The method is fast and is suitable for monitoring pore size distributions in the range of 50-1000 A.
The Effect of Clay Content on the Spin–Spin NMR Relaxation Time Measured in Porous Media
ACS Omega
Clays, hydrous aluminous phyllosilicates, have a significant impact on the interpretation of physical measurements and properties of porous media. In particular, the presence of paramagnetic and/or ferromagnetic ions like iron, nickel, and magnesium in clays can complicate the analysis of nuclear magnetic resonance (NMR) data for porous media characterization. This is due to the internal magnetic field gradient induced by the clay minerals. In this study, we aim to investigate the impact of clay content on spin−spin relaxation time (T 2), which is strongly influenced by the pore surface chemistry. Seven rock core plugs, characterized with variable clay content, were used for this purpose. The clay mineralogy and volume were determined by means of quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN). The T 2 relaxation time was measured using a Carr−Purcell−Meiboom−Gill (CPMG) sequence with variable echo spacing (T E). The maximum percentage difference in dominant T 2 values (MRDT 2) between shortest and longest echo spacing was subsequently correlated with clay content obtained from QEMSCAN. Our results show that the reduction in T 2 distribution with increasing echo time T E is more significant in samples characterized by higher clay contents. The MRDT 2 was found to be strongly correlated with clay content. An analytical equation is presented expressing MRDT 2 as a function of clay content providing a quick and non-destructive approach for clay content estimation. Moreover, the MRDT 2 −clay content relationship showed a nonlinear behavior: MRDT 2 increases drastically as the clay content increases up to 15%, beyond which the rate of MRDT 2 change with clay content diminishes. This behavior could be attributed to the clay distribution. At higher clay contents (above 15%), it is more likely for clay to form clusters (structural clays), which will not significantly increase the clay surface in contact with the pore fluid. Further, experimental data suggests that ignoring the impact of clay on internal magnetic gradients and T 2 signal may result in considerable underestimation of the actual pore size distribution.
Pore geometry information via pulsed field gradient NMR
Magnetic Resonance Imaging, 1994
Studies of echo attenuation at long diffusion times in pulsed field gradient NMR experiments on a variety of rock core samples are interpreted in the light of recent theoretical analysis of the effect of pore geometry and surface relaxation. This study is motivated by the need to test the applicability of that theory to real rock systems.