Characterisation of porous materials by means of nuclear magnetic resonance

Characterisation of porous materials by means of nuclear magnetic resonance

Abstracts / Magnetic Resonance Imaging 19 (2001) 569 –589 cyclopentane exposition. The solvent penetration profile was obtained through a series of im...

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Abstracts / Magnetic Resonance Imaging 19 (2001) 569 –589 cyclopentane exposition. The solvent penetration profile was obtained through a series of images which were then analysed allowing extraction of diffusion parameters. Due to the higher segmental mobility of the network chains, the diffusion rate of the solvent was adequately described by a Fickian regime [3] (Case I). Determination of translational diffusion coefficients (D) obtained by analysis of the solvent front advancement [4] allowed a comparison among the materials. The diffusion rates observed could be related to different cross-link density of the polymeric chains. The method revealed to be very effective in the evaluation of resistance of this type of materials to solvent aggression. REFERENCES [1] Bluemich B. NMR imaging of polymers: methods and applications. Magnetic Resonance Microscopy 1992;167– 85. [2] Webb AG, Hall LD. NMR imaging of solvent ingress into polymers. Polymer 1991;32:2926 –38. [3] Alfrey A. Diffusion in glassy polymers. J Polym Sci 1966;12:249. [4] Grinsted R, Koenig JL. Study of multicomponent diffusion into polycarbonate rods using NMR imaging. Macromolecules 1992;25:1229 – 34. PII: S0730-725X(01)00330-7

Pore-scale simulation of dispersion: comparison with NMR experiments Maier RSa, Kroll DMb, Bernard RSc, Howington SEc, Peters JFc, Davis HTd. aArmy HPC Res. Ctr., Univ. of Minnesota, Minneapolis, MN, USA; b Dept. of Med. Chem. and Minnesota Supercomputer Inst., Univ. of Minnesota, Minneapolis, MN, USA; cWaterways Experiment Station, U.S. Army Corps of Engineers, Vicksburg, MS, USA; dDept. of Chem. Eng. and Mat. Sci., Univ. of Minnesota, Minneapolis, MN, USA. Tracer dispersion has been simulated in three-dimensional models of random sphere packings for a range of Peclet numbers. The simulation results illustrate the time evolution of dispersion, and corroborate theoretical results on the scaling of asymptotic dispersion with Peclet number. Comparisons with NMR spectroscopy experiments show agreement on transient, as well as asymptotic, dispersion rates. These results support recent NMR findings that longitudinal dispersion rates are lower than reported in earlier experimental literature, and that asymptotic rates are observed in relatively short times by techniques which employ a uniform initial distribution of tracers. Simulation results also suggest that NMR dispersion measurements in sphere packings are not significantly affected by surface relaxivity. PII: S0730-725X(01)00331-9

MRI of pore size distributions in fault sealing oil reservoir rock core samples by cryoporometric filtering techniques Mallett MJDa, Clennell MBb, Strange JHa, Fisher Qc. aSchool of Physical Sciences, University of Kent, UK; bCentro de Pesquisa em Geofı´sica e Geologia-IGEO, Universidade Federal da Bahia, Brasil; c Department of Earth Sciences, University of Leeds, UK. NMR cryoporometry can be used as an imaging filter to select particular pore sizes within a material. This technique is applied to the magnetic resonance imaging of representative fault sealing sandstone core samples. An in-depth knowledge of pore size distributions and pore connectivities within rock is of great importance to the petroleum industry. It is particularly important for predicting hydrocarbon column heights when producing models of pressure, flow rate, flow pathway and sweep efficiencies. This work presents the results of a study undertaken at the Universities of Kent and Leeds. At the University of Kent we previously have devel-

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oped a technique, known as NMR cryoporometry [1], which can measure the pore size distribution within a porous material by monitoring the depression of the melting point of a pore filling fluid obeying the GibbsThomson equation. The technique is combined with 3D MRI methods to generate information on the spatial distributions of pores. The results from analysing a number of samples from North Sea oil and gas fields and outcrop analogues are presented. Both 2D and 3D images are presented using NMR cryoporometry (acquired at the University of Kent) and optical and electron microscopy (acquired at the Rock Deformation research group at the University of Leeds) methods. The results demonstrate the ability to determine the variation of pore size and location within a number of fault sealing rocks and the capability of NMR cryoporometry to compliment the results of capillary percolation measurements such as by mercury injection. REFERENCE [1] Strange JH, Webber JBW. Meas Sci & Tech 1997;8:555– 61. PII: S0730-725X(01)00332-6

Characterisation of porous materials by means of nuclear magnetic resonance Martı´n CA, Ramia ME, Pusiol DJ, Fin˜ana A, Gayol MF, Alvarez ME, de los A´ngeles Krenz M. NMR Laboratory, Department of Physics, University of Co´rdoba, Ciudad Universitaria, 5000 Co´rdoba, Argentina. Porous materials are of great interest, both theoretically and experimentally, in various scientific and technical areas (micelles, sponges, rocks, construction materials, etc.). These reasons show it is important to have at hand experimental techniques to properly characterise them. Among the most relevant characteristic parameters we have: porosity, distribution of pore sizes or, more precisely, the volume/surface ratio of the pores, removable and/or irreducible fluids, and permeability. Nuclear Magnetic Resonance (NMR) is an adequate technique to these purposes. We have developed and built all of the necessary parts to carry on the required analyses in rocks: basic theory, spectrometers, pulse sequences, sample treatment, signal acquisition, its analysis and results, and finally, the conclusions. It is the aim of this work to present, as an example, the results we have so far obtained in plugs from cores obtained in the logging of oil wells. To this end we built a 2 MHz, for 1H, NMR spectrometer, and measured the decay of the echo amplitude using the CPMG pulse sequence, in samples fully or partially brine saturated. To properly describe these signals a linear combination of decaying terms had to be used. Each of these terms has an exponential characterised by a time constant (T2) which, it may be shown, accounts for the volume-to-surface ratio of the pores, and by a coefficient which indicates how abundant the corresponding pore size is. In order to fit these signals a regularised FORTRAN computer program was developed to find out the various exponential decays present in the acquired signals. These information provide the pore size distribution, and comparing the signals from the samples fully and partially saturated, and these with that from a sample of known porosity, it is possible to determine the NMR porosity, and how much of the saturating brine could be removed and how much is irreducible. Also, from a phenomenological point of view, it has been shown in two separated approaches by Kenyon and Coates that, with the above pore size distributions obtained from the fully and partially saturated samples, it is possible to estimate the permeability of the rocks. We have, in parallel, developed a quality system based on the standard ISO 17025, in order to make sure that the entire measurement procedure is under full control. Financial support provided by CONICOR and CONICET is gratefully acknowledged. PII: S0730-725X(01)00333-2