Asphaltenes precipitation

In oil bearing formations, the presence of polar chemical functions of asphaltenes probably makes the rock wettable to hydrocarbons and limits their production. It also happens that during production, asphaltenes precipitate, blocking the tubing. The asphaltenes are partly responsible for the high viscosity and specific gravity of heavy crudes, leading to transport problems. 

Table I. Elemental compositions of asphaltenes precipitated by different flocculants from various sources (16)...

Asphaltene precipitation, in many instances, carries from the well tubing to the flow lines, production separators, and other downstream equipment. It has also been reported (19) that asphaltic bitumen granules occured in the oil and gas separator with oil being produced from certain oil fields. 

An additional mechanism affects the deposits formation from the H-Oil reactor, rejection of vanadium and nickel sulfides from the catalyst. In the vacuum tower, asphaltene precipitation was found to be the prevalent fouling mechanism. In asphaltene... 

Visbreaking severity is monitored to help minimize cracking and alteration of the nature of asphaltenes within the visbreaker feed. Paraffinic side chain cracking or destruction of the asphaltene-resin complex may occur during visbreaking operations and may result in precipitation of asphaltenes from solution. Asphaltene precipitation has been seen especially when visbroken material is blended with lighter-viscosity paraffinic fuels. 

Kueter, Klaus E. Asphaltene precipitation Cause and effects within Oxy-Peru jungle operation. Bakersfield, Calif. Occidental Oil and Gas Corporation. 

The amount of asphaltene precipitate from one crude oil is dependent on the carbon number of the alkane solvent. A decrease in the solvent carbon number results in an increase in the asphaltene precipitate. This observation would suggest that asphaltenes and resins are not greatly different materials. Rather, a continuum exists in the solubility behavior. An increase in boiling point in heavy oil is generally accompanied by an increase in both aromaticity and in the concentration of heteroelements or polar aromatic molecules (Corbett, 1969). Similarly, there is a gradual... 

The effect of temperature on the association of vanadium compounds in asphaltenes was investigated by Tynan and Yen (1969). Using electron spin resonance (ESR), they observed both anisotropic and isotropic hyperfine structures of vanadium, interpreted as bound or associated and free vanadium, from asphaltenes precipitated for a Venezuelan petroleum and reintroduced to various solvents. Higher temperatures and more polar solvents resulted in a transition from bound to free vanadium, as shown in Fig. 12. At 282°C, only 1% of the anisotropic spectrum was observed. An activation energy of 14.3 kcal/mole was observed for the transition. 

This is termed the a rent miscibility pressure because there is a considerable amount of evidence that asphaltenes, which are colloidally dispersed in most crude oils, will aggregate under appropriate dilution conditions and precipitate, so that the physicochemical definition of miscibility (single-phase for all proportions of the fluids in question) is not realized. There are also conditions, particularly found in oil reservoirs at temperatures below about 135 F, at which two liquid phases, or two liquid phases plus a gas phase, appear in addition to an asphaltene precipitate (11-13). In general, however, this does not prevent the attainment of 95 % oil recovery or more in slim-tube tests at the same pressures at which these multiple phases appear. Hence, the process is deemed "miscible," for all practical purposes. 

Either the asphaltene precipitation or the multiple phase behavior of the process under "miscible" conditions appears to cause a certain extent of reduction of the mobility of the carbon dioxide, as compared to the mobility which would be calculated from the relative permeability of the carbon dioxide divided its viscosity (14-16). However, it is usually considered that this is not sufficient to resultinafavorable mobility ratio (a ratio of carbon dioxide mobility to crude oil mobility less than one). 

Many questions concerning the nature of petroleum asphaltenes remain unresolved (1) What is the chemical composition of petroleum asphaltenes (2) What are the molecular weights of asphaltene components (3) Why are asphaltenes precipitated from solution in petroleum by the addition of a hydrocarbon solvent such as n-pentane In this chapter we attempt to answer these questions. In addition, we suggest that asphaltene formation is a general phenomenon that is pertinent to the chemistry of coals, tar sand bitumens, shale oil, and other complex solutions of organic compounds. 

Analyses of deposit and stream samples from a commercial H-Oil unit indicate that several mechanisms influence fouling. The rejection of "vanadium- and nickel sulfides" from the catalyst and the precipitation of polycyclic aromatics appear to contribute to fouling in the reactor, reactor outlet line, and high pressure separator. "Asphaltene precipitation" is the prevalent fouling mechanism in the H-Oil vacuum tower. 

GC/MS analyses give evidence that some heavy ends material entering the vacuum tower rises through entrainment and impacts fouling on the upper trays. X-ray and combustion analyses reveal the deposit to be primarily organic the LOI value is 96 % S and N contents are low at 3.1 % and 1.8 %, respectively V and Ni values are only 0.9 % and 0.5 % and the Fe content is negligible. These data are typical for deposits caused by asphaltene precipitation. 

Oil properties that increase with depth include the C7+ mole fraction and dewpoint, whereas a decrease with depth is noted for the Ci mole fraction, gas/oil ratio and bubble point. End-member compositional trends include asphaltene precipitation and tar mat formation at the base of the oil column, and concomitant formation of a volatile-rich liquid phase at the top. The evolution of the oil column from a mass balance perspective is accompanied by distinct molecular fractionations that can be used to diagnose the cause for the oil column disequilibrium. 

The thickness measurements for the heptane-diluted bitumen films are presented in Fig. 16 (curve 1). Below the onset of asphaltene precipitation at a heptane itumen ratio of about 1 1, the film drained to an equilibrium gray film of about 27 nm thickness. Above the precipitation onset at a heptane/bitumen ratio of 2 1, the black film reached a thickness of about 28 nm. The film thickness then decreased with increasing diluent bitumen ratio to about lOnm at a ratio of 20 1. The thickness then remained constant, indicating that a bilayer film was probably reached. At lower diluent ratios (< 20 1), the greater thickness of heptane itumen films may be caused by the presence of unprecipitated asphaltenes. 

KA Ferwom. Thermodynamic and Kinetic Modelling of Asphaltene Precipitation from Heavy Oils and Bitumens. PhD thesis. University of Calgary, Alberta, 1995. 

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