Thermal cracking of the light aromatic fraction of Safaniya crude oil – experimental study and compositional modelling of molecular classes

The thermal cracking of the light (C6–C14) aromatic fraction of a type II crude oil (Safaniya) was studied, through laboratory pyrolyses, in order to better understand and predict the behaviour of such components in reservoirs. The individual constituents of the fraction were lumped into six molecular classes: BTXN, methylaromatics, alkylaromatics, naphthenoaromatics, indenes and sulphur-containing aromatics. The changes in abundance and composition of each class were determined by closed system pyrolyses, for a large set (34) of temperature/time conditions encompassing a wide range (1–93%) of global conversion. Identification and quantitation of the compounds generated under each condition were completed and these compounds were ultimately lumped into eight product-classes: five molecular classes for gases and three classes for the C15–C20 polyaromatics, the C20+ polyaromatics and the insoluble polyaromatic residue. These pyrolysis data were used to elaborate a semi-empirical kinetic scheme of 13 stoichiometric reactions for the primary and secondary cracking of the unstable classes. The scheme was numerically optimised, with the constraints of mass balance and hydrogen conservation between the reactants and pyrolysis products, using GeoKin Compo Software. The optimised scheme yielded an excellent fit between calculated and experimental data for the conversion of the charge, the evolution of its molecular classes, the formation of the product-classes and individual gases. The optimised compositional model was extrapolated to low temperature to simulate the thermal cracking of light aromatics in reservoired oils and to predict their behaviour under geological conditions as well as the nature and abundance of the generated products, including coke and gas.