NMR longitudinal surface relaxation phenomena of metal oxide nanoparticles in porous media
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1H Nuclear Magnetic Resonance (NMR) has long been applied in downhole logging and laboratory analyses to investigate pore size distributions of rocks through correlation with measured relaxation time distributions. However, due to the inherent chemical heterogeneity of pore surfaces in rock, the pore surface relaxivity, which links relaxation time and pore size, varies throughout the pore system. I seek to modify and control the surface relaxivity in natural porous media through coating of paramagnetic nanoparticles so that NMR measurements can be used to compute pore sizes directly. I chose zirconia nanoparticle dispersions with opposite surface charge but similar size. The absence of surface coating on zirconia nanoparticles simplified the calculation of nanoparticle surface relaxivity and interactions between nanoparticles and pore walls. Glass bead packs and Boise sandstone cores were saturated with positively charged zirconia nanoparticle dispersions in which nanoparticles can be electrostatically adsorbed onto pore surfaces, while negatively charged zirconia nanoparticle dispersions were employed as a control group to provide the baseline of nanoparticle retention due to non-electrostatic attraction. When 1.114 vol. % positively charged zirconia nanoparticles dispersion was used to saturate a glass bead pack, 11.6% of the nanoparticles were adsorbed to the bead surfaces and modified the glass bead surface relaxivity. I performed core flushing with DI water, pure acid and alkali, and compared properties of zirconia nanoparticles before and after exposure to Boise sandstone. After 2 pore volumes of core flooding, there was around 3% of negatively charged nanoparticles trapped in Boise sandstone core while around 30% to 40% of positively charged nanoparticles were retained in Boise sandstone cores. The results indicated that besides van der Waals attraction, electrostatic attraction is the driving force for retention of nanoparticles with positive surface charge in sandstone cores. Full coverage of nanoparticles onto sandstone surface was not achieved. The attachment of nanoparticles onto sandstone surface changed the mineral surface relaxivity. After contact with Boise sandstone, nanoparticles themselves exhibited increased relaxivity due to interactions between nanofluids and mineral surface under different pH conditions. The complicated interactions between nanofluids and pore surfaces make it difficult to predict sandstone surface relaxivity with attached nanoparticles. Since adsorption of nanoparticles changed the pore surface relaxivity, it is crucial to know nanoparticle relaxivity and factors that may affect the relaxivity of nanoparticles. T1 values of zirconia nanoparticle dispersions before and after mixing with various Fe(III) solutions were measured and compared. Adsorption of iron onto zirconia nanoparticles was confirmed based on measurements of aqueous Fe remaining in supernatants. Adsorbed iron increases zirconia nanoparticles’ surface relaxivity, as the relaxation rate of zirconia nanoparticles increased with the amount of adsorbed Fe(III). Besides adsorbed paramagnetic species, surface coatings also play a role in changing nanoparticle surface relaxivity. Since organic surface coatings usually give a small value of relaxivity, it is better to use a nanoparticle core with high relaxivity as to investigate the effect of organic surface coatings. I examined the relaxation properties of (3-Aminopropyl)triethoxysilane (APTES) coated Fe3O4 nanoparticles in mixtures with different D2O volume fractions. Fe3O4 nanoparticles exhibited decreased relaxivity with more APTES coating. The presence of D2O affects proton-proton relaxation but not electron-proton relaxation. Comparison of relaxivity of APTES coated Fe3O4 nanoparticles with different coating amount and D2O volume fractions indicated that at relatively high Fe concentration, when electron-proton interaction dominates surface relaxation, hydrogen atoms in the APTES did not significantly alter the surface relaxation mechanism of nanoparticles. At lower Fe3O4 concentration, proton-proton relaxation brought by APTES also played a role in the overall relaxation mechanism on nanoparticle surfaces, as more APTES coating showed lower apparent surface relaxivities with higher D2O volume fractions in the mixture.