Adsorption/Occlusion Characteristics of Asphaltenes:  Some Implication for Asphaltene Structural Features

Abstract

In the crude oil, the other fractions such as hydrocarbons could be adsorbed and/or occluded inside asphaltene structures. In this work, the deuterated compound n-C 20 D 42 has been used to simulate the adsorption/occlusion characteristics of asphaltene in toluene solution. Based on the highly polydispersed structures of asphaltenes, the experimental results indicate that the adsorbed hydrocarbons could be exchanged with the outside bulk phase, whereas the occluded ones inside the core of asphaltenes could not be exchanged. These results suggest that substantial microporous units exist inside asphaltene structures, and these structural units are suitable to adsorb and occlude the other fractions in the crude oil. The carbon number of the occluded hydrocarbons could be up to C 40 , and the weight could be more than 0.5%, based on the initial asphaltene weight. The adsorption should occur at the periphery of asphaltene aggregates (the loosely packed ones), and then they are liable to be exchanged with the outside bulk phase. If the asphaltene molecule is large enough, the occlusion could occur at the asphaltene molecular level. However, this type of occlusion perhaps just occurs inside asphaltene aggregates (the close aggregating ones); if this is the case, this type of asphaltene aggregate should be considered to be the stable unit in toluene solution, and then the occluded hydrocarbons could not be exchanged with the bulk phase. Regarding the adsorption/occlusion properties of asphaltenes, the aforementioned results should be valid, even extrapolated into the crude oil reservoir system.

Multiple-sourced features of marine oils in the Tarim Basin, NW China – Geochemical evidence from occluded hydrocarbons inside asphaltenes

“…Well protected from secondary alteration events, these occluded hydrocarbons represent the original oil derived from kerogen (Liao et al, 2006). A H 2 O 2 /CH 3 COOH oxidative system was used to release occluded hydrocarbons from the asphaltenes (Liao et al, 2005(Liao et al, , 2006Yang et al, 2009;Zhao et al, 2010). The procedure, details of which can be found in Liao et al (2006), included the preparation of asphaltenes by n-hexane precipitation, acetone extraction of n-hexane-asphaltenes to obtain the adsorbed fraction, and H 2 O 2 /CH 3 COOH oxidation of the acetone-extracted asphaltenes to release the occluded fraction.…”

“…However, the occluded fraction inside the corresponding asphaltenes shows different distribution characteristics, without evidence of biodegradation, but slight even carbon number predominance and the presence of even carbon number terminal n-alkenes (Liao et al, 2006;Yang et al, 2009;Zhao et al, 2010). Liao et al (2005Liao et al ( , 2006 proposed that these hydrocarbons, including n-alkenes, were retained in the skeleton of asphaltenes during the early stage from kerogen to oil. They represented the original oil from kerogen, and were protected by the molecular structure of the asphaltenes from post-alterations.…”

Waxes in asphaltenes of crude oils and wax deposits

“…However, the traditional methods used to isolate asphaltenes by adding 40 volumes in excess of low boiling point paraffin hydrocarbons can produce a fraction which is contaminated with a significant amount of waxes (Chouparova and Philp 1998;Thanh et al 1999;Liao et al 2006;Acevedo et al 2009;Coto et al 2011). The presence of paraffin hydrocarbons in asphaltenes can be explained by their coprecipitation as a result of low solubility in low molecular weight n-alkanes (Thanh et al 1999;Coto et al 2011), by interaction between the alkyl lateral chains (C 7 -C 10 ) of asphaltenes with high molecular weight n-alkanes (Garcia and Carbognani 2001;Stachowiak et al 2005;Ganeeva et al 2014), and by their adsorption and/or occlusion inside asphaltene aggregates (Liao et al 2005(Liao et al , 2006Acevedo et al 2009;Gray et al 2011). Now, it is well-known that the asphaltenes exist as three-dimensional aggregates due to the strong intermolecular forces such as hydrogen bonding and p-bonding (Murgich 2002) and even at very low concentration as 50-150 mg/L in a good solvent such as toluene (Mullins et al 2012).…”

Geochemical quantification of mixed marine oils from Tazhong area of Tarim Basin, NW China

“…Recently, the characteristics of compounds adsorbed and occluded inside asphaltenes were used to identify mixed oils (Liao et al, 2006a, b;Zhao et al, 2010). Since asphaltenes can adsorb and even occlude hydrocarbons inside their specific molecular structures (Liao et al, 2005(Liao et al, , 2006c. Liao et al (2006a) employed a mild oxidation system where for asphaltenes of crude oils, the aliphatic components chemically bonded to the asphaltene structures were cleaved as aliphatic acids by the oxidative reagents.…”