NMR under Confinement: Roots in Retrospect

ROBERT J. S. BROWN, PAOLA FANTAZZINI, JORG KARGER AND RAINER KIMMICH

Nuclear magnetic resonance (NMR) has provided us with many beneficial opportunities for science and technology. Its continued use in novel fields has yielded impressive strength and attractiveness for nearly a century. This is particularly true with regards to the topic of this book, the exploration of "Fluid Transport in Porous Solids and Heterogeneous Materials".

Here, the benefit of NMR in being able to look "from the outside" into a system becomes particularly evident. NMR operates as an "ideal spy", providing information without interfering with internally occurring phenomena. NMR is able to give information on pore spaces as well as anything that might happen within them. This wide-range of information that is accessible is illustrated by the examples in this book. The origin of some of these developments can, most remarkably, be traced back over many decades, to the very beginning of NMR research. In this chapter we will recollect some of the roots of the challenges we face today with applying NMR to studying "Fluid Transport in Porous Solids and Heterogeneous Materials" — albeit with some bias by personal experiences and impressions.

The output of nuclear magnetic relaxation on pore space architecture and guest dynamics in porous materials is, generally, based on model assumptions. These assumptions are, as a rule, well established and supported by experimental evidence. In its early years, however, NMR was used for studying molecular diffusion. The information gained stands on its own. Hahn's seminal paper in 1950 provided us with an opportunity that, in subsequent years, has been extensively exploited for diffusion measurements with liquids. With the application of pulsed field gradients by Stejskal and Tanner, the gradient intensity could be chosen large enough so that, eventually, diffusion measurements with porous materials have become possible. In his seminal paper of 1965 John Tanner introduced the technique under the title "Pulsed Field Gradients for NMR Spin-Echo Diffusion Measurements". Since then, the method has found application in quite a number of different communities. Its widespread use might have contributed to a diversification in nomenclature, with currently two names in common use: pulsed field gradient (PFG) and pulsed gradient spin echo (PGSE) NMR. In either case, Tanner's original wording is easily recognized.

The development of NMR was, essentially from its very beginning, closely related with the search for its application to petrophysical studies. The oil industry became aware of the potential of this novel source of information and vigorously promoted research on logging projects. The data in Table 1.1, taken from the paper of Kleinberg and Jackson, illustrate this intense and most rewarding partnership from its beginning until 2000.

In 1948, two years after the discovery of NMR in condensed matter by Bloch and Purcell, the thesis of Bloembergen and the classical paper by Bloembergen, Purcell and Pound (BPP theory) explained many features of the relaxation of NMR signals in bulk liquids by interpreting the dependence of the relaxation times on parameters related to molecular motion, including temperature, viscosity and distance between spins. A retrospective article by Bloembergen gives a review of NMR attempts before 1946 and of early work on relaxation. It has also been recognized that fluid molecules can be adsorbed near a solid surface, resulting in a decrease in their mobility. The existence and influence of pore walls were later found to appear in the relaxation patterns of NMR. The application of the BPP theory to the adsorbed layers could have caused researchers to think that the relaxation times of molecules in the adsorbed layers could have been decreased and so decreasing the relaxation times of fluids inside the pore space of porous media; but it seems that nobody had that intuition.

However, the idea was raised to build a device to be lowered inside the wells to get the signals of oil and water from the porous rock formation outside the borehole at depths of thousands of meters. Russel Varian had demonstrated that it was possible to observe NMR by free precession (at about 2 kHz) in the Earth's field. Numerous studies about the feasibility of the application of Nuclear Magnetic resonance for well Logging (NML) by Varian Associates followed. In those pioneering researches, the now widespread use and importance of the NMR single-sided NMR devices, that led to the evolution of the concept of compact and mobile devices, able to detect NMR signal outside the magnet, outside the laboratory, in a non-destructive way, regardless of the sample sizes emerged. The key feature of NML was intended to be the possibility to exploit the different relaxation times of bulk oil and water (10 times larger for water than for oil) to distinguish their signals. Since water and oil have about the same H nuclei density, the fraction of water and oil could have been determined...