Application of pulsed and high-frequency electron paramagnetic resonance techniques to study petroleum disperse systems

Abstract

The spectral and relaxation characteristics of “free” organic radicals (FR) and vanadyl-porphyrin (VP) complexes in various petroleum disperse systems (PDS) like bitumen, petroleum, their high-molecular components and solutions were studied using stationary (conventional) and pulsed electron paramagnetic resonance (EPR) techniques in two frequency ranges (X- and W-bands, with the microwave radiation frequencies of about 9 GHz and 95 GHz, respectively). The features of the pulsed approaches (electron spin echo, modulation of the electron spin echo signal decay, electronic relaxation times) and high-frequency EPR for PDS investigations were examined. W-band EPR allows to resolve spectrally the lines from the different paramagnetic centers and more accurately determine their spectral characteristics. It is shown that the electron spin echo can be observed at room temperatures even at high magnetic fields of 3.4 T demonstrating the potential of application of pulsed EPR techniques for the low-cost oilfield measurements. Analysis of the VP transverse magnetization decay curve permits to identify electron-nuclear interactions with the 14N and 1H nuclei in situ while in the EPR spectra these hyperfine interactions usually cannot be detected. It is found from the W-band EPR measurements that FR lineshape cannot be fitted with isotropic parameters in contrast to the established X-band results. The observed effect of increasing the rates of electronic transverse relaxation in asphaltenes is described in the framework of a model of spectral diffusion between the fast- and slow-relaxing paramagnetic centers in supramolecular complexes of asphaltenes.

High-Field (3.4 T) Electron Paramagnetic Resonance, 1H Electron-Nuclear Double Resonance, ESEEM, HYSCORE, and Relaxation Studies of Asphaltene Solubility Fractions of Bitumen for Structural Characterization of Intrinsic Carbon-Centered Radicals

“…The EPR method is based on the resonant (selective) absorption of microwave energy by PCs in an external magnetic field, B 0 . The EPR spectra observed in petroleum components are due to the presence of magnetic moment for unpaired electrons and are affected by several types of interactions between the unpaired electron and its environment [ 38 ]: (1) the Zeeman interaction between the unpaired electrons and B 0 , (2) the spin–orbital interaction, (3) the electron–nuclear interaction, (4) the interaction with other unpaired electrons (spin–spin interaction), (5) and the interaction with the environmental bath (spin–lattice interaction).…”

“…Asphaltenes contain intrinsic paramagnetic species (paramagnetic centers, PCs) to be studied with electron paramagnetic resonance (EPR) techniques [ 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 ]. Concentrations of PCs in asphaltenes can reach values of 10 22 spins per gram [ 13 ], meaning that every asphaltene molecule can be paramagnetic and that EPR is a suitable (valid) and unique tool for studies on PDS and PDS asphaltenes.…”

Hydrothermal In-Reservoir Upgrading of Heavy Oil in the Presence of Non-Ionic Surfactants

“…The saturation effects were hindered using a microwave power of 25 µW. The relative concentration (ratio [FR]/[VP]) was measured in all experiments due to mass variations inside the EPR cavity, probable changes of densities and viscosities of the studied species [41,42].…”