Screening of Crude Oils for Asphalt Precipitation: Theory, Practice, and the Selection of Inhibitors
- R.B. de Boer (Koninklijke/Shell E&P Laboratorium) | Klaas Leerlooyer (Koninklijke/Shell E&P Laboratorium) | M.R.P. Eigner (Shell Intl. Petroleum Mij. B.V.) | A.R.D. van Bergen (Koninklijke/Shell E&P Laboratorium)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Facilities
- Publication Date
- February 1995
- Document Type
- Journal Paper
- 55 - 61
- 1995. Society of Petroleum Engineers
- 5.5 Reservoir Simulation, 5.2.1 Phase Behavior and PVT Measurements, 5.2 Reservoir Fluid Dynamics, 1.8 Formation Damage, 4.3.3 Aspaltenes
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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.
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.
|File Size||2 MB||Number of Pages||7|