The impact of surface active indigenous components on interfacial tension (IFT) of crude oil–water systems is an important parameter in many aspects of crude oil production such as emulsion stability, reservoir wettability, and capillary number calculations. These components may affect productivity across the reservoir due to variations in concentrations. In most cases simulation of IFT is not taking interfacial activity into account and is purely based on oil bulk properties. In this paper we examine two crude oils and their subfractions such as maltenes, deacidified crude, and natural acidic components. Films were prepared at the toluene–water interface with crude oil and its various fractions and studied for interfacial activity and chemical compositions. The chemical analysis of the interfacial active material indicated that carboxylic acids are preferentially adsorbed or concentrated at the oil/water interface. Infrared spectroscopic analysis of the interfacial films clearly demonstrates that carboxylic acids species (e.g., fatty acids, resins, or asphaltenes with a −COOH functionality) are concentrated at the interface. The GC-MS analysis of the interfacial film revealed the presence of a homologous series of linear chain carboxylic acids ranging from C10–C25+. 2D GC-MS analysis showed that heteroacids are also present. Acid free crude was prepared and back mixed with the original crude oil in different proportions confirming the role of the acids in decreasing IFT. The removal of these relatively small amounts of acids leads to a decrease in IFT between 1.3 and 2.2 mN/m. The results also indicate that acids can be preferentially removed by ion exchange resins without affecting the overall composition of the oil as is shown by back-mixing deacidified oil into the original oil.

Interfacial tension plays an important role in reservoir mechanisms and in the production and processing of crude oil with coproduced water. Natural surfactants adhere to the oil/water (O/W) interface, which reduces the interfacial tension and in some cases leads to a solidified interfacial film with a significant impact on emulsion stability. These technical challenges are a result of the chemistry of the petroleum system, and hence optimization of production depends on a better understanding of this. The properties and composition of the interfacial film have been the subject of many studies. In this study, films of oil-related components were generated at the interface between oil and water. The interfacial material was recovered by skimming the interface remaining after repeated solvent washings. The recovered material was analyzed using Fourier transform infrared spectroscopy by attenuated total reflectance (FTIR ATR) to obtain both quantitative and qualitative information about the composition of the film. When compared to the spectrum of the total precipitated n-heptane asphaltenes, which show many aromatic features, the interfacial film shows surprisingly few of these. There is a substantial difference between the interfacial film material and the asphaltenes, especially in the aromatic C−H vibration range between 700 and 900 cm −1 . It is observed that oxygenated compounds, sulfoxides and carbonyls, and one specific type of aromatics are concentrated in the film. The latter also shows up in the oil and in the maltenes but less prominently. In a recent communication [Andersen et al. Energy Fuels 2016, 30 (6), 4475−4485], we showed that homologous series of fatty acids can be found in the interfacial film, and the current detailed examination of the FTIR spectra also indicates that the water interface serves as a "substrate" for specific adsorption of aromatic molecules that are not representative of the bulk asphaltenes.

Fluorescence spectroscopy has been widely used in biochemistry to elucidate molecular interactions. Likewise, it has also been applied to petroleum systems and, in particular, asphaltene association by concentration effects. It is also a common method for online measurements of oil in water in the offshore oil and gas industry. As a result of the complexity of crude oils and asphaltenes, many assumptions and corrections are necessary in an adequate analysis, especially of concentration effects. Unfortunately, many studies in the literature lack appropriate incorporations of simple effects, such as reabsorption effects (inner filter) and energy transfer [fluorescence resonance energy transfer (FRET)], among molecules leading to apparent red shifts. The latter has wrongly been reported as an effect of molecular association. In the present paper, we analyze the "PetroPhase 2017" asphaltenes and crude oils using emission spectra (λ ex of 300 and 400 nm) and synchronous spectra as a function of concentrations in toluene between 0.2 and 1000 ppm in both front face and right angle irradiation geometries. Inner filter effects are seen in both configurations, leading to complete loss of fluorescence at a short wavelength. After corrections, we observe that linearity in intensity−concentration relations is almost restored, but some red shifts remain. This is mainly due to FRET and, hence, not related to true molecular aggregation. We further explore a number of Stern−Volmer (SV)-based approaches to elucidate mechanisms of molecular interaction using fluorescence spectroscopy. The SV plots indicate that fluorophores could be shielded at quite high concentrations in agreement with asphaltene aggregation. The presence of FRET indicates that some molecules contain several covalently bonded but independent fluorophores. The conclusion is that caution is needed in interpretation of concentration effects in fluorescence spectroscopy studies of petroleum fractions and all possible corrections should be made before concluding on molecular interactions, such as critical nanoaggregate concentration phenomena.

We investigate the formation and properties of crude oil/water interfacial films. The time evolution of interfacial tension suggests the presence of short and long timescale processes reflecting the competition between different populations of surface-active molecules. We measure both the time-dependent shear and extensional interfacial rheology moduli. Late-time interface rheology is dominated by elasticity, which results in visible wrinkles on the crude oil drop surface upon interface disturbance. We also find that the chemical composition of the interfacial films is affected by the composition of the aqueous phase that it has contacted. For example, sulfate ions promote films enriched with carboxylic groups and condensed aromatics. Finally, we perform solution exchange experiments and monitor the late-time film composition upon the exchange. We detect the film composition change upon replacing chloride solutions with sulfate-enriched ones. To the best of our knowledge, we are the first to report the composition alteration of aged crude oil films. This finding might foreshadow an essential crude oil recovery mechanism.

Fourier transform infrared spectrometry (FTIR) spectra were measured for 16 distillation cuts obtained from two bitumens using a deep vacuum fractionation apparatus. Three regions of the spectra were deconvoluted into peaks each associated with a known type of vibration: (1) aliphatic C−H stretching in the 2800−3000 cm −1 region, (2) aliphatic C−H scissoring/symmetric deformation in the 1350−1400 cm −1 region, and (3) aromatic C−H out-of-plane bending in the 680−900 cm −1 region. The distribution of chemical structures in the oils were assessed, and preliminary correlations were identified between measured physical properties (density, atomic H/C ratio, and molecular weight) and the quantified peak areas obtained from the FTIR spectra. A preliminary method was proposed to generate physical property distribution data for crude oils based on distillation and FTIR data. The method predicted the density, atomic H/C ratio, and molecular weight of the distillation cuts, with average deviations less than 0.8, 1.4, and 16%, respectively. Note that the method was tested on the same cuts used to generate the correlations because there were insufficient data for an independent test.