Experimental and mathematical model evaluation of asphaltene fractionation based on adsorption in porous media: Dolomite reservoir rock

Abstract

In this study, a whole asphaltene sample was first aged with five dolomite core samples. Then the core samples were washed with toluene; this asphaltene was named the bulk sub-fraction. Thereafter, they were washed with tetrahydrofuran, so that the n-adsorbed sub-fraction was dissolved in solvent. At the last step, the core sample was washed with an azeotrope of methanol/chloroform, which corresponded to the h-adsorbed sub-fraction, and the residual asphaltenes within the porous media were irreversibly adsorbed. According to the elemental analysis of whole asphaltene and its sub-fractions, sulfur was the most abundant element in the molecular structure of asphaltene. The presence of sulfur near dolomite increased the interaction between asphaltene and the rock surface; thus, the h-adsorbed asphaltene had the highest sulfur content. Nitrogen and oxygen content did not show a clear trend in the sub-fractions; therefore, their placements in the molecule were examined using more precise tools, such as Fourier-transform infrared spectroscopy (FTIR). Oxygen was found in asphaltene fractions in the form of a carboxylic acid functional group. From the calculation of the value of the RO–H index, the h-adsorbed asphaltene had more ability to establish a hydrogen bond (RO–H = 26.88) than the other fractions. Furthermore, by calculating the aliphatic and aromaticity indices, the sub-fraction adsorbed to the dolomite surface (h-adsorbed) had the least aliphatic compounds and the highest amount of aromaticity among the sub-fractions. In addition, the h-adsorbed sub-fraction had shorter side alkyl chains than the bulk and n-adsorbed asphaltenes. Carbon number analysis was also performed to determine the number of carbons present in each molecule and to measure the molecular size of each asphaltene sub-fraction. The results showed that the C29+ fractions in the bulk sub-fraction were high (41.39%), while the C14–C26 and C15–C24 fractions had the highest frequencies in the n-adsorbed and h-adsorbed sub-fractions, respectively. A new mathematical model was tuned to determine the porosity and permeability of a resulting asphaltene deposition. The main reason for the high accuracy and innovation in the proposed relationship was the use of the fluid velocity parameter in the porous media and the volume of fluid injected into the core. In addition, the dependence of porosity and permeability reductions on parameters such as the ratio of the contribution of each asphaltene sub-fraction and the amount of the heteroatoms in each sub-fraction was studied for the first time.