Effect of oxygen on the NMR relaxation properties of crude oils

Abstract

Nuclear magnetic resonance relaxation measurements of bulk fluids provide a sensitive probe of the dynamics of molecular motion. Dissolved oxygen can interfere with this technique as its paramagnetic nature leads to a reduction of the paramagnetic relaxation times of the fluids. We studied this effect for the relaxation properties of crude oils that are in general characterized by a distribution of relaxation times. The samples were stock tank oils that have been exposed to air. We compared T~ and ir relaxation time distributions and their correlation functions of the initial (oxygenated) samples with those from the deoxygenated samples. Oxygen was removed from the oils with a freeze-thaw technique. As expected, the effect of oxygen is most apparent in oils with long relaxation times. In these oils the effect of oxygen can be described by an additional relaxation rate 1/T~~ to the transverse and longitudinal relaxation rates that is sample dependent but does not vary within the relaxation time distribution of the oil. Values of 1/T~~ for different crude oils were found to be in the range of 2.5 to 8.3 s. For crude oils that have components with relaxation times less than 100 ras, no significant oxygen effect is observed.

Temperature-Dependent Oxygen Effect on NMR D-$$T_2$$ Relaxation-Diffusion Correlation of n-Alkanes

“…higher alkanes, narrow range of aromatics, etc. Chiarotti et al [4] and later Chen et al [12] and Mutina and Hürlimann [14] attributed a discrepancy between published bulk fluid relaxation times to paramagnetism of dissolved oxygen. Thus, the observed relaxation rate is the linear sum of two processes, bulk relaxation time of pure fluid, , defined by dipolar interactions and rotational coupling of spins and relaxation due to paramagnetic species—oxygen gas in this study, :For liquid normal alkanes under typical ambient conditions (at atmospheric pressure and temperature interval of interest, from approx.…”

“…The latter found that the modified theory of Torrey [35] describing relaxation by dipolar translational diffusion (i.e scalar coupling effect is negligible) is sufficient to describe the effect of dissolved gaseous oxygen on relaxation in water and potentially in liquids in a more broad sense. Chen et al [12] and Hürlimann et al [14] suggested a simple concept of oxygen relaxation enhancement by considering observed relaxation time as the arithmetic sum of two relaxation terms, see Eq. 4: (1) bulk relaxation of pure deoxygenated fluid, 1/ and (2) an oxygen enhanced relaxation term, 1/.…”

“…4: (1) bulk relaxation of pure deoxygenated fluid, 1/ and (2) an oxygen enhanced relaxation term, 1/. The latter work [14] reported between 2.5 and 8.3 s for ten stock tank oils exposed to the atmosphere, with virtually no oxygen effect for components faster than 100 ms. One may attempt to express the oxygen-related relaxation term as a function of oxygen molecules concentration, , oxygen diffusion transport in a host fluid, (or transfer coefficient ) and viscosity of a fluid, :Parker and Harmon [34] expressed oxygen-enhanced relaxation rate as a linear function of two terms dependent on concentration of oxygen molecules and a mutual diffusion coefficient:where is the mutual diffusion coefficient of oxygen and water, the number of oxygen molecules dissolved in water and , , combines such values like mean square magnetic moment of electron, , effective molecular diameter, , gyromagnetic ratio of proton, , and so on. The anomalously strong impact on NMR relaxation of oxygen dissolved in liquids, even though the concentration of that gas in water at normal conditions typically is about 10–15 ppm, is due to unpaired electronic spin (assuming oxygen is in a triplet ground state, ).…”

“…Furthermore, a temperature-dependent oxygen related enhanced relaxation term is reported as a second-degree polynomial of temperature. Freedman et al [13] reported diffusion-relaxation correlation of some deoxygenated alkanes and their mixtures, Mutina and Hürlimann [14] systematically studied the effect of oxygen on relaxation time of ten crude oils; a viscosity/long-chain hydrocarbon related dependence on oxygen-related relaxation rate is observed. Winkler et al [15] studied a variety of petroleum fluids containing significant amount of solution gas in oxygen-free state.…”

Oxidation of oleic acid under pulsed electric field processing

“…The deoxygenation procedure was well described in the study of Mutina and Hürlimann (2005) (Keszler, Kriska, & Németh, 2000;Mutina & Hürlimann, 2005). One cycle of deoxygenation procedure included the following steps: 1.…”