Variation of Oil-solvent Mixture Viscosity In Relation to the Onset of Asphaltene Flocculation And Deposition
Authors: Turta A. , Fisher D. , Singhal A.K. , Najman J. 
Published: March 2010

High pressure experiments with three oils and propane as an asphaltene flocculant were performed. A capillary tube was used for the viscosity measurements. Viscosities of oil mixed with a gaseous solvent under miscible conditions were measured and compared with data on onset of asphaltene flocculation (OAF). For these oil-propane systems the OAF values were previously determined by a photometric method. Sharp increases in viscosity of the oil-solvent mixture, at increasing amounts of solvent (beyond the point of asphaltene flocculation), were detected.

Viscosity increases caused by adding hydrocarbon solvent gases at high pressure were very significant in contrast to the expected decreasing trend in oil viscosities upon adding increasing amounts of a non-flocculating solvent with a lower viscosity. Upon asphaltene flocculation/ deposition, a minimum in mixture viscosity followed by a sharp increase was noted. This was interpreted as the onset of asphaltene deposition. The values of measured viscosity were compared with corresponding values obtained from diferent correlations. It was seen that these correlations resulted in values substantially lower than the real values, even at low solvent concentrations (lower than the OAF), confirming that the flocculation/ deposition mechanisms are much more complex than are generally perceived.

Introduction

The oil industry has faced the problem of asphaltene deposition for many years. Asphaltene deposition in the reservoir, well bores, well head equipment, pipelines and other downstream equipment can lead to serious problems and can increase the cost of producing oil.

Asphaltene deposition increases whenever anything disrupts or destabilizes the asphaltene-resin micelles equilibrium. For example, the addition of low-molecular weight hydrocarbons-such as propane, n-pentane, petroleum naphtha or gasoline-will result in a flocculation of asphaltenes by solubilizing resins in the bulk oil phase. In this case the viscosity of the oil-solvent mixture would also heavily depend on asphaltene content.

There are two different types of asphaltene deposition:Asphaltene deposition during primary recovery of oil, which is mainly due to the changes in pressure and temperature.Asphaltene deposition during miscible flooding involving rich hydrocarbon gases and carbon dioxide.

For the latter, the most important problems are to determine:the viscosity of the oil-solvent mixture,the OAF (which is the minimum amount of solvent that would trigger the flocculation process, at the prevailing pressure) andthe onset of asphaltene deposition (OAD).

The amount of asphaltene deposited is also of interest. In addition, presently, there is no reliable method to estimate the viscosity of oil-solvent mixtures for the purposes of calculating upwards flow within oil wells or, in the surface pipelines or, in flow through porous media; all the existing viscosity correlations are valid only when asphaltene flocculation does not occur. The objective of this paper is to fill this gap and to propose a method to determine the OAD.

Determination of Onset of Asphaltene Flocculation for the Oil-Propane Systems

As mentioned, accurate determination of onset of asphaltene flocculation (OAF) and onset of asphaltene deposition (OAD) are very important for the choice of methods for preventing/combating asphaltene deposition.