Screening of Crude Oils for Asphalt Precipitation: Theory, Practice, and the Selection of Inhibitors

Abstract

Summary This paper describes a simple method to screen crude oils for their tendency to precipitate asphalt, which may cause problems during production. The method is based on a thermodynamic model of asphalt solubility, derived earlier by Flory and Huggins. The most important parameters in this model are the Hildebrand solubility parameters for oil and asphaltene, and their molar volumes. The oil parameters can all be correlated with the in-situ density of the crude. The relative change in asphalt solubility in the crude per unit pressure drop is shown to be highest for light crudes that are undersaturated with gas, which usually contain only a small amount of asphaltenes. Hence, the chance that asphalt will precipitate will be greatest for these light crudes. For pressures below the bubblepoint, depressurising of the crude will cause a rapid increase in asphalt solubility, due to changes in liquid composition. It is shown that heavy crudes usually will give fewer problems with asphalt precipitation, despite their higher asphaltene content, certainly if the reservoir pressure is close to bubblepoint pressure. Consequently, the tendency for asphalt precipitation is mainly determined by three parameters: the extent to which the crude is undersaturated with gas, the density of the crude at reservoir conditions, and its saturation with asphalt at downhole conditions. Apart from the simple screening method, more elaborate methods are described to assess the potential for asphalt precipitation more accurately; asphaltene analysis on produced reservoir fluid and tank oil; n-heptane titration of the tank oil; visual inspection of a bottomhole sample in a high-pressure cell during pressure reduction; and dynamic flow tests on tank oil after n-heptane addition. An ultrasonic back scattering technique is described for monitoring the influence of different asphalt precipitation inhibitors. Using above techniques asphalt precipitation inhibitors have been identified that can be applied in the field. Introduction Asphalt is a heavy, highly viscous phase mainly consisting of asphaltenes: the high-molecular weight fraction of a crude that is insoluble in n-heptane. Asphalt precipitation in reservoirs, in wells and facilities has a severe detrimental impact on the oil production economics due to a reduction of well productivity and/or a clogging of the production facilities. Problems with asphalt precipitation during production are difficult to correlate with the asphalt content of the crude. A compilation by Leontaris & Mansoorishowed as extremes the Venezuelan Boscan crude with 17% asphalt that was produced nearly trouble free while the Algerian Hassi-Messaoud with only 0.062% asphalt met with difficulties during production.

Characterisation of Asphaltenes and Modelling of Flocculation – State of the Art

“…All the authors agree that the effect of temperature remains small compared with the effect of pressure. De Boer et al (1995) confirm this results using a solubility model. …”

“…Depending on the authors, this increase is related: -either to the presence of asphaltene aggregates in the medium and corresponds to approaching the flocculation point (El Mohamed et al 1988, De Boer et al 1995, Escobedo and Mansoori, 1995Ali and Islam, 1998). -or to the appearance of a layer of flocculated asphaltenes on the surface of the capillary tube.…”

“…De Boer et al (1995) demonstrated that the properties of the oil can be correlated with its density. The model therefore has two parameters specific to asphaltenes.…”

Investigation of asphaltene deposition under dynamic flow conditions

“…Therefore, it is important to examine the inhibitors in real flow conditions. Besides, a literature study reveals that there are only few reports on asphaltene deposition in real pipe conditions (De Boer et al 1995;Alboudwarej 2003;Wang et al 2004;Zougari et al 2006;Jamialahmadi et al 2009;Vargas et al 2009).…”

Factors Affecting the Stability of Crude Oil Emulsions

“…Then the oil content is progressively increased. At some volume fraction above 60-70%, the emulsion will suddenly invert and produce a W/O emulsion of smaller water droplet sizes than were the oil droplets in the original O/W emulsions (22) .…”