A Study of the Processes Related to Coagulation of Asphaltenes by Electronic Spin Resonance

Abstract Coagulation of asphaltenes in crude oil has been studied by electronic spin resonance (ESR). Free radicals in asphaltenes with the content of vanadium less than 0.1 mass% are found to recombine when asphaltenes precipitate. Polyaromatic fragments are shown to be localized in the inner part of the asphaltene molecules. A possibility of the formation of diamagnetic compounds between vanadyl (VO2+) complexes and free radicals of asphaltenes is investigated. Introduction The study of asphaltenes from crude oil is a matter of special interest and research in oil chemistry. The constantly increasing share of heavy crude oil and bitumen in total oil production has lead to the increasing importance of studies in this area. High concentrations of asphaltenes, resins, vanadium and sulphur are characteristic for most of the heavy crude oil and bitumen deposits. That makes oil production and refining more complicated. Petroleum asphaltenes are the highest molecular species among components of petroleum and asphaltenes only —the basic carriers of oil paramagnetysm. Basically, paramagnetic components in oils are free stable radicals and vanadyl (VO2+) complexes. The behaviour of asphaltenes under reservoir conditions, and in the preparation and transportation of crude oil, is strongly related to the interactions between paramagnetic fragments in the structure of asphaltene aggregates. There is a hypothesis that asphaltene molecules, especially the large ones, are not necessarily two-dimensional flat disks but they have the capacity, owing to the presence of long polymethylene bridges, to fold upon themselves into a complex three-dimensional globular conformer with an internal structure(1). In other works, however, it has been shown it is possible for the molecules containing alkyl fragments up to C24 (paraffins, ceresines, etc.) to co-precipitate together with asphaltenes(2,3). Hypothetical structures of the asphaltene molecules have been presented(4,5). They have a different type: polyaromatic fragments are in the centre of the molecules whereas naphtene-aliphatic groups are located on the periphery. By way of explanation of the characteristics of crude oil and bitumen, the most significant model was proposed by Juan Murgich(6) to describe asphaltenes from different crude oils. According to the model, all asphaltenes are two types: A-type and S-type. A-type asphaltenes have 15 to 20 condensed aromatic rings (in the centre), alkyl substitutes and heteroatoms on the periphery. Asphaltenes of this type are flat. S-type asphaltenes have several aromatic parts (2 to 5 rings) which are connected with each other by alkyl and sulphide bridges. The molecules are almost spherical. The aromatic parts are located on the outer surface of the asphaltene molecules. In the work by Galtsev et al.(7), a conclusion is made about the low mobility of the vanadyl complexes/ asphaltene system. This conclusion is based on the reverse correlation between the relaxation characteristics of electrons in crude oil. On the basis of the time of spin-spin relaxation, the concentration of an agent responsible for widening of the signal associated with free radicals, i.e., vanadyl complexes, is calculated. The concentration of vanadyl complexes derived from the integral intensity of the signal in a spectrum is lower than that derived based on the time of spin-spin relaxation.