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

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.

Asphaltenes as Nano Objects Regulating Macrostructure of Solid High-Carbon Substances

High-carbon substances and carbon itself are important structure-forming elements. Of special interest are substances obtained from petroleum feedstock by means of high-temperature treatment. Such substances could include bitumen, pitch, coke, fullerenes, carbon nanotubes and some composition materials. The present article provides a literature review, devoted to structuring in complex multi-component systems, which demonstrated to obtain quasi-crystalline substances in macroscale. In petroleum disperse systems the regulating factor leading to the formation of hierarchy structures is presence in the medium of asphaltenes exerting paramagnetic properties. It was experimentally showed that change of the equivalence ratio of such components as core, salvation shell, and dispersion medium leads to intentional obtainment of various configurations of quasi-crystalline systems. Understanding of macrostructures formation mechanisms under condition of change of medium properties at nanolevel allows constructing new carbon-based constructional materials.