Role of asphaltene and its sub-fractions in the stability of acid-oil emulsion.

[Display omitted] • Asphaltene emulsifying ability is primarily due to IAA sub-fraction. • Weight fraction of IAA sub-fraction is increased in the presence of HCl. • IAA sub-fraction has higher aromaticity and contains more polar functional groups. • RA sub-fraction forms a less stable acid-induced emulsion. • Detrimental interactions of asphaltene and acid ions was hindered by emulsified acids. Acid stimulation is potentially a tangible approach to improve recovery from oil reservoirs. Nonetheless, there is a possibility of forming unwanted acid-oil emulsions with considerable stability. Among natural surfactants in the reservoir, asphaltene is known to greatly impact the stability of emulsion. It has been reported that a small portion of asphaltene, named interfacial-active asphaltene (IAA), has the primary stabilizing influence on water–oil emulsions, while the remained asphaltenes (RA) seem to have meagre impact. In this paper, two different oil-extracted asphaltene samples (named A and B) were fractionated into IAA and RA sub-fractions based on their different interfacial activity through emulsification with water, and also HCl for the first time. Then, the elemental (CHNSO) and FTIR analysis were performed to characterize their chemical composition. Next, the stability of the acid-synthetic oil emulsion was investigated by conducting bottle tests using various acid solutions such as HCl, acetic acid, and emulsified acid. The results showed that sample A had higher H/C, i.e. less aromaticity, and its total heteroatom content (O, N, and S) was 7.92 % more than sample B. Furthermore, sample B had almost 3 times greater IAA than sample A in both cases of using water and HCl for fractionation. In addition, FTIR analysis confirmed that IAA contained more heteroatoms and functional groups like sulfoxide, carboxylic acids, and nitrogen-containing aromatic rings, which can interact with acid ions, and have higher aromaticity than RA. For the HCl solution, the minimum stability of emulsions was at 10–15 wt% of HCl whereas the stability increased for acetic acid, which is a weak organic acid. It was most likely due to co-precipitation or co-adsorption of acetic acid molecules along with asphaltene molecules. Moreover, the spent HCl acid formed an emulsion with less than 10 % stability. Similar experiments with RA resulted in less stable emulsions and confirmed that the emulsifying ability of asphaltene samples originated from a small sub-fraction (IAA). Finally, various emulsified acids (HCl in diesel, kerosene, light naphtha, and xylene) have been investigated. Xylene, as an asphaltene solvent, formed the least stable acid-induced emulsion by increasing the aromaticity of the continuous phase of the emulsion. The diesel-based emulsified acid showed the most stable acid-induced emulsion because of its higher molecular weight and attractive van der Waals force with asphaltene. It can be concluded that the unwanted interactions between asphaltene in synthetic oil and acid ions can be mitigated by the utilization of a suitable emulsified acid in the acidizing process. Light naphtha cannot be a suitable alternative for diesel for preparation of effective emulsified acid because of its resultant unstable acidizing system. [ABSTRACT FROM AUTHOR]