Resumo: This thesis contains the most relevant part of the work done by the author on NMR-based porous media characterization techniques at the NMR and Quantum
Information lab of CBPF. It starts with a short review of NMR applications to Petrophysics, reportedly, the science of physical processes that occur within the pore domain of a porous material. Most of NMR-based methods are concerned with geometric characterization of the
pore structure and it is discussed both the type and the extent of such descriptions. A more in-depth but also considerably biased overview of NMR diffusive transport follows, laying much of the foundation for the subsequent chapters of original content and delineating the limitations and natural difficulties of the underlying theory and methods. It is believed, nevertheless, that this chapter is still able to offer an unified and a rather fresh perspective on a topic so amply and thoroughly discussed over the past half century. In particular, the algorithmic character of the theoretical treatment of NMR diffusion is emphasized and analyzed over several formalisms in
order to establish a clear connection with essential numerical methods. The beginning of second part contains a first-principles theory on one of the most common and effective scenarios that lead to NMR relaxation enhancement of fluid in porous media, namely, the presence of adsorbed or lodged paramagnetic impurities on the pore structure boundary. The concept of active surface elements is introduced and it is argued that NMR relaxation in the immediate vicinity of these centers is dependent upon surface normal orientation. The central hypotheses of the model and
its predictions are then subjected to experimental tests for verification. In the last chapter, the singularities of high-field NMR are exploited in devising a protocol that allows experimental observation of internal field autocorrelation functions. It is discussed what sort of information regarding domain geometry can be provided by such statistical characteristics and the method is put to test over model porous samples of known grain form and size. To the best of knowledge, it is the first time such an intrinsic property of the porous system is actively observed and, though
research is still incipient, results look rather promising. |
