Abstract

The viscosity of bitumens/heavy oils may be related to the tendency of the asphaltenes to form aggregates. This paper concentrates on the effects of asphaltenes on bitumen/heavy oil viscosity. The macro and micro structural arrangements of heavy oils/biturnens are described and their effects on viscosity discussed. Experimental evidence suggesting viscosity to be not only a function of asphaltene concentration but more importantly, to be a function of the nature of asphaltene aggregates is shown. A new equation quantifying the influence of asphaltenes on viscosity is presented and compared with experimental data.

Introduction

In many petroleum reservoirs around the world reservoir fluid composition has been found to vary with location and depth (1–3). In almost all of the cases reservoir fluid density increases with depth of the reservoir1. Patel found the viscosity of Athabasca, Peace River, Wabasca and Cold Lake bitumens to vary with depth of the formation. In most of the cases, samples obtained from higher depths showed greater viscosity. Schulze (3) studied compositional variations within a hydrocarbon column due to gravity and found the gravitational forces to be responsible for the variation in composition in thick reservoirs. He found the extent of this variation to be higher with larger aromatic fraction in the hydrocarbon fluid. Hirschberg (5) in his analysis of the role of asphaltenes in compositional variation of a reservoir fluid column concluded that the heavy polar omponents play a key role in this regard and in particular identified asphaltene segregation to have a dominant effect.

One of the principal effect of compositional variation is the variation of viscosity. For a reservoir oil sample from a North African field Hirschberg found the viscosity to increase by a factor of 4 (from 9 to 36 mPa.s) when asphalt content increased from 10 to 16%. Indeed the effect of asphaltene concentration on the viscosity of an oil has been known for long time. Mack6, in 1932, presented viscosity data on asphaltene and oil mixtures obtained from a Mexican asphalt which clearly showed an increase in viscosity of the mixture with increasing asphaltene concentration. Waxman et al.(7) as well as Kitzan and Parsons(8) found viscosity of Peace River bitumen sample to vary with asphaltene concentration. Dealy(9) studied the effect of asphaltene concentration on the viscosity of Athabasca bitumen by adding 5 wt% of additional asphaltene to a bitumen sample which originally contained 16 wt% of asphaltene. Bitumen viscosity was found to increase from about 300 Pa.s to 1,000 Pa.s. When high shear was applied the viscosity of the mixture was found to decrease to the original level. This indicated formation of asphaltene aggregates which caused the rise in viscosity and eventual desegregation of them with the application of high shear. Datta(10) presented viscosity data of various asphaltene blends of a California oil which also show an increase of viscosity with asphaltene concentration.

Allgeit and Harle(11) also studied the effect of asphaltenes on asphalt viscosity. They found asphaltenes to form aggregates in solution, the degree of which was found to depend on structure, molecular weight and concentration of the asphaltenes and the solvent power.

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