The diverse properties of native crude oils employed in the studied blends.
Density measurements revealed anomalies of nonideality (maxima of excess density) at some compositions in binary blends of light and heavy crude oils from diverse origins. By IR absorption measurements, density anomalies were attributed to increased contents of suspended asphaltene colloidal-sized particles/aggregates in the blends. By comparison with a database of world’s native crude oils, it was concluded that density anomalies may correspond to different equilibrium structural states of asphaltene colloids that occur at several specific asphaltene contents, apparently common for petroleum media of any origin.
- crude oil blends
- excess density
- volumetric shrinkage
- asphaltene aggregates
- structural transformations
A common approach frequently used in the petroleum industry for either transportation or refining purposes is blending of heavy crude oils with lighter ones [1–5]. Therefore, studying the effect of blending on the physical and chemical properties of produced petroleum fluids has become increasingly important. It was soon realized that blends of multicomponent crude oils are nonideal systems, for which ideal “mixing rules” developed for binary mixtures of pure chemical substances [6–8] are not applicable. For example, in ideal mixtures, both mass and volume are additive parameters and the total volume of the binary blend
Consequently, the density of the ideal binary blend:
In binary blends of crude oils, deviations of the measured volume
and “excess density”:
where, by definition:
Empirical testing of crude oil blends by a number of major oil companies has produced vast amounts of volumetric shrinkage and excess density data . An API Measurement Committee correlated the data collected over several decades and in 1996 released equations, published in the API Manual of Petroleum Measurement Standards , which served as the accepted industry standard for over 20 years. In particular, the standard equation for volumetric shrinkage (in % units):
The authors of Eq. (6) made no attempts to account for the presence of any specific molecular fractions in the blended oils. Consequently, for all blends, the
More recent tendencies in discussing the measured nonideal properties of crude oil blends are attempts to reveal specific molecular substances responsible for nonideality. The most discussed nonideal petroleum constituents in crude oil blends are asphaltenes [18–27]. In particular, it was suggested that volumetric shrinkage/excess density may result from an increase in the equilibrium content of asphaltene colloidal aggregates dispersed in the blend, which occur at some specific asphaltene contents [23–27].
In this work, we describe a study of the nonideality phenomena in three blends of native crude oils of diverse origin by experimental density measurements. This study is supplemented by IR optical absorption experiments, which provide the direct proof of the crucial role of asphaltene aggregation.
2. Diversity of studied native crude oils: preparation of oil blends
To ensure that the observed nonideal effects are not specific for particular crude oils, in the described series of experiments, we employed blends of light and heavy native crude oils collected from the well heads at several Russian oilfields geographically separated by thousands of kilometers (cf. Table 1).
|Heavy oil||Light oil|
|Oilfield; well #||Kalmayursk.; 238||Korobkovsk.; n/a|
|Location||53°12′10″N; 50°08′27″E||50°19′00″N; 44°48′00″E|
|Density, 20°C, kg/m3||963.9||820.3|
|Viscosity, 20°C, cSt||3732||4.6|
|Metals (V + Ni), wt%||0.175||n/a|
|Oilfield; well #||Aznakayevsk.; 24,534||Pogranichn.; 43P|
|Location||54°32′16″N; 52°47′54″E||63°11′57″N; 75°27′02″E|
|Density, 20°C, kg/m3||893.2||818.3|
|Viscosity, 20°C, cSt||39.3||2.94|
|Metals (V + Ni), wt%||0.076||0.018|
|Oilfield; well #||Usinsk.; n/a||Potochn.; 401|
|Location||66°10′38″N; 57°21′14″E||61°15′15″N; 75°12′44″E|
|Density, 20°C, kg/m3||955.3||840.8|
|Viscosity, 20°C, cSt||962.7||7.37|
|Metals (V + Ni), wt%||0.0111||0.0137|
Samples of blends with various oil volume fractions ϕ (determined with an accuracy Δ
3. Density measurements reveal multiple “excess density” peaking in crude oil blends of varying compositions
Densities of all samples of Blend #1 were determined using a standard 10 ml pycnometer. In the studied density range of 818–964 kg/m3 (cf. Table 1), an accuracy of density measurement was ≤1 kg/m3. For samples of Blend #2 and Blend #3 measurements were performed in an oscillating U-tube densitometer (model VIP-2M, produced by TERMEX, Tomsk, Russia) with an accuracy of ±0.3 kg/m3. All measurements were performed at 24 ± 0.5°C.
Experimental values of “excess density”
Immediately evident is a strong quantitative disagreement of model predictions with experimental data (note that in Figure 1, all model data are multiplied by the factor of 10). A fundamental qualitative difference of the model with experimental results appears even more significant. Namely, as noted in Introduction, the API model predicts a single maximum of ρex, roughly at equal contents of light and heavy oil in any blend. In contrast, experimental data in Figure 1 reveal multiple peaks of the measured excess density for all studied blends with diverse properties of the parent light and heavy oils.
In accordance with previous publications [23–27], we suggest that the main mechanism behind the observed multiple extrema of
4. IR absorption experiments confirm close relationship of excess density with the content of asphaltene colloidal aggregates in crude oil blends
Infrared optical absorption spectra were measured in the 650–4000 cm−1 range with 2 cm−1 increment using a model FT-801 FTIR Spectrometer (Simex, Novosibirsk, Russia). For convenience of presentation/discussion, the values of “transmittance”
Figure 2 show experimental IR absorption spectra for the parent light and heavy oil components of Blend #3 (cf. Table 1). By literature analysis, we could not reliably distinguish in these complex spectra any characteristic peaks representative of asphaltene colloidal aggregates.
To solve this problem, we utilized a well-known method of changing the aggregation state of asphaltenes by dilution of crude oils with
On the basis of the above arguments, we concluded that changes in the content of colloidal-sized asphaltenes in crude oil blends with varying compositions may be characterized by intensity increments of the 3041 cm−1 absorption peak Δ
5. Density maxima in the studied crude oil blends and in the database of world’s native crude oils are observed at the same special asphaltene contents in petroleum media
Previously, we have analyzed distributions of density/specific gravity in a database of several hundreds world’s native crude oils from a diversity of geographical/geological locations . Figure 5 shows the scatter plot of specific gravity (
Additional analysis of excess density measurements from Figure 1 in terms of asphaltene content has shown that in all blends peaking of
Summarizing, Figures 6–8 indicate that the observed multiple nonideal anomalies in the properties of equilibrated crude oil blends (multiple peaks of excess density/volumetric shrinkage values) apparently result from step-like changes between several equilibrium structural phases of suspended asphaltene colloids, which are observed between several particular asphaltene contents, apparently common for petroleum media of any origin.
6. Plausible constitution of multiple structural phases of asphaltenes. Apparent resemblance of asphaltene transformations to those of block copolymers
In spite of intensive experimental and theoretical studies, association and aggregation mechanisms of asphaltene colloids are still not well characterized and are subjects of ongoing debate [22–25, 29, 30]. In particular, for decades, the prevailing aggregation paradigm (now dismissed as erroneous) has been that at a “critical micelle concentration” (“CMC” ≈ 2–10 g/L) of asphaltenes in native petroleum, or in “good” solvents, such as toluene, there is a
The dismissed paradigm :
Apparently, due to their simplicity, the above
However, all evidence for the close resemblance of petroleum asphaltenes to block copolymers, both in their molecular structure and in their propensity for self-assembly into wide array of nano-sized and micro-sized structures, still remains virtually unnoticed. In fact, the existence of several aggregation stages below and above CNAC has been repeatedly demonstrated by various experiments (cf. a review in Ref. ). It appears that the corresponding characteristic concentrations
Historically, asphaltenes are defined not as members of some particular family of chemical species, but as a fraction of petroleum, which is soluble in toluene (“good solvent”), and resemblance of asphaltene molecular architecture to that of block copolymers for the first time was suggested in 1994 by I.A. Wiehe , whereas in 2002, W. Loh noticed a parallel between some aggregation processes for block copolymers and asphaltenes . Various molecular models of asphaltenes have been developed [22, 29, 30, 41]; all these models agree that asphaltene molecules contain the following principal “building blocks” : (1) rigid flat condensed systems of aromatic rings with an affinity for “good solvents” and (2) flexible linear alkyl side chains, which may contain some heteroatoms, and have an affinity for “nonsolvents.” In the “continental-type” models, aromatic regions are large, whereas in “archipelago-type” models, aromatic blocks are much smaller. Aggregation of such “multiblock” molecules may proceed via π-π stacking and H-π bonding, involving aromatic blocks, as well as via polar and hydrogen-bonding interactions between side-chain blocks. By virtue of these noncovalent interactions, asphaltenes in solutions as well as in native petroleum apparently exhibit multiple structural phases—a well-known property of block copolymers .
Recently, it was experimentally proven [48–52] that basic asphaltene molecules (unimers, monomers) typically include very small, 1–3 ring, aromatic systems, in contrast to the popular notions about the predominance of large multiring fused systems [41, 42]. These basic molecules become predominant equilibrium species only after dissolution of solid asphaltenes in “good” solvents (benzene, toluene, etc.) to asphaltene concentrations
Plausible types of some of the multiple structural transformations of asphaltenes are schematically illustrated in Figure 9.
At asphaltene contents of 0.6–0.7 mg/L, apparently there is a transition from a solution of small asphaltene basic molecules to a solution of voluminous primary aggregates.
At concentrations of 100–130 mg/L, soft primary aggregates are transformed to solid-like “nanocolloids” with individual nanoparticles 2–4 nm in diameter.
This group of two close characteristic concentrations (3a,b) in the range of 1.7–8 g/L (cf. Table 2) may reflect the appearance of elongated colloidal clusters of solid-like nanoparticles, 15–20 nm in length.
At concentrations of 25–30 g/L, elongated clusters presumably form fractal aggregates ≥100 nm in size.
At 55–70 g/L, there start to appear micron-sized asphaltene flocs still suspended in the liquid media.
To conclude this section, it should be emphasized that revealing multiple asphaltene structural phases in solutions and in native petroleum was largely facilitated by a seemingly trivial but a crucial improvement in analysis of experimental data. Namely, previously overlooked details of analyzed correlations become perceptually well separated only at data plots with log concentration scales, common in surfactant chemistry, while in conventional petroleum studies, linear-scale plots are still employed [29, 30, 41].
In contrast to the predictions of the standard API model, the described density measurements revealed multiple anomalies of excess density at several compositions in all studied blends of light and heavy crude oils from diverse origins. It was suggested that density anomalies are caused by increased contents of suspended asphaltene colloidal-sized particles/aggregates in the blends. In IR absorption experiments, the most intense absorption peak representative of asphaltene colloids appeared to be that at 3041 cm−1, by literature analysis, ascribed to excitation of the aromatic sp2═C─H stretching vibrations. Intensity variations of this peak correlated with variations of excess density, supporting the conclusion of the governing role of asphaltene colloids in the observed density anomalies. By comparison with an extended database of world’s native crude oils, it was further concluded that different density anomalies may correspond to different equilibrium structural states of asphaltene colloids, which occur at several specific asphaltene contents, apparently common for petroleum media of any origin.
It is hoped that the insight gained into the nonideal behavior of the densities of crude oils, upon mixing and the explanations proposed, will assist in a better scientific understanding of the mechanism of the changes that may occur in mixtures. This should help oil producers and refiners in a better utilization of the existing oil resources, i.e., crude oils and petroleum products.
We acknowledge the support from the Nedra-Test Research and Testing Establishment, LLC, Moscow, Russia, in purchasing the experimental equipment.