Summary

A new technique for the determination of the onset of asphaltene flocculation has been developed through accurate viscosity measurements of a crude oil being diluted with a precipitating agent (n-pentane, n-heptane, n-nonane). This detection method is based on experimental observations of an increase in the viscosity of a crude oil-asphaltene-precipitating agent suspension in which asphaltene particle aggregation occurs. The key point in this development is the phenomenon of asphaltene flocculation induced by the addition of a n-paraffin hydrocarbon (i.e. n-heptane, n-pentane, n-nonane) to crude oil. The onset of asphaltene flocculation is detected graphically, and its location is enhanced by comparison of the analyte curve with a reference system. The reference system was developed using polar and non-precipitating solvents (i.e. toluene, benzene, THF).

Introduction and Background

A number of severe problems arise during production, processing, transportation and storage of petroleum fluids due to asphaltene deposition. Therefore, it is crucial to know "when" and "how much" asphaltene will flocculate out of solution under a given set of operating conditions. Accurate determination of the onset of asphaltene flocculation is required in order to test existing theoretical models.1–3 Several methods are available for determining the onset of asphaltene-flocculation and deposition with various degrees of accuracy and difficulty. In general, these may be classified according to the procedure utilized to detect the onset.

Measurements of electrical conductivity4 as a function of precipitating agent have recently been proposed for the detection of the onset and amount of asphaltene precipitation.

The Light Transmission5 method is based on detection of changes in transmitted light intensity as a function of precipitant concentration in the crude oil mixture.

A Gravimetric analysis6 also allows for the detection of the onset of precipitation.

Through Interfacial Tension measurements7 between oil and water upon dilution with a precipitating agent it is also possible to quantify the onset of asphaltene deposition.

The Visual Method8 is based on the utilization of a microscope with light to visually detect the onset of deposition. Visual detection of this point in dark or heavy crude oils may be difficult and inaccurate. One way to overcome some of the difficulties posed by this method is by careful preparation of thin films to be analyzed under the microscope.

In this paper we propose a new method for the determination of the onset of asphaltene flocculation based on viscosity measurements as a function of flocculant added (n-pentane, n-heptane, n-nonane, etc.). This method is based on the fact that for most pure liquids and for many suspensions viscosity is a well- defined quantity, for a given temperature and pressure, which is independent of shear stress and velocity gradient, provided the flow is in the laminar regime (Newtonian fluids). However, for other solutions and suspensions deviations from the Newtonian behavior are observed. The main causes of non-Newtonian viscosities are, in many instances, the inter-particle interactions, assymetry and orientation of the suspended particles. Fluid-particle and particle-particle interactions depend largely on the particle size, shape, concentration and surface characteristics. Particle size determines to a large extent the nature and relative significance of the forces governing suspension rheology and hydrodynamic behavior, while concentration determines the level of interparticle interactions. Non-Newtonian effects are important even at low concentrations for suspensions of rigid nonspherical particles. For the case in which the suspended particles are spherical the non-Newtonian behavior becomes important at higher concentrations when interparticle interactions are more intense.

In general, suspended particles affect the flow characteristics of the host fluid resulting in an increased viscosity. The problem of relating viscosities of colloidal suspensions to the properties of the dispersed particles has been the subject of numerous experimental and theoretical considerations.9–14

Einstein9 concluded that the effect of dispersed particles on the viscosity of a suspension depends only on the total volume they occupy and is independent of their size. However, this is valid only for highly dilute suspensions of rigid, spherical, unisize, and non-interacting particles.

Sherman10 and Rutgers15 found that at moderate concentrations, the disturbances in the regions of flow around the particles are sizable, and they become more important for non-spherical and/or deformable particles. Although these findings are applicable to moderately concentrated suspensions, and account, to some extent, for non-rigidness; they consider only particle-solvent interactions but not particle-particle interactions (i.e. aggregation).

For the case in which particles interact to form aggregates of different sizes and shapes, Gillespie13 studied the effect of particle aggregation and particle size distribution on the viscosity of Newtonian suspensions arriving at the conclusion that it resulted in an increase in the relative viscosity of the suspension.

Crude Oil-Asphaltene-Flocculant Mixtures

Numerous experimental work have revealed the colloidal nature of the asphaltene fraction of a crude oil.16 Kawanaka, et al.3 consider the asphaltenes to exist in crude oil as both dissolved and suspended particles. Dispersed asphaltenes are sterically stabilized by neutral resins; they are electrically charged,17 and have a diameter between 30 and 40 Å.18 The stability of these particles can be disrupted by addition of solvents which cause desorption of the resin molecules from the asphaltene-particle surface (i.e. n-heptane). When two asphaltene particles collide on their empty spaces aggregation occurs, therefore, we may expect that at a particular solvent concentration the aggregation process will be initiated rather vigorously. This point is defined as the onset of asphaltene flocculation. The amount of solvent needed to arrive at the onset depends on the type of crude oil being analyzed, and particularly on the resin content.19,20 It is well understood that asphaltenes in fact show high polydispersity and non-sphericity when they are precipitated from crude oils.21,22 The actual shape of asphaltenes is not known yet, however, Overfield et al.22 proposed a somewhat elongated shape which could well be polydisperse cylinders or ellipsoids. Sheu et al.23 studied the colloidal properties and the self-association character of asphaltenes in polar solvents (i.e. toluene). They concluded that asphaltene/toluene systems are Newtonian.

From the above facts and experimental observations it seems possible that through accurate measurements of viscosity of crude oil upon titration with a precipitating agent the onset of asphaltene flocculation can be determined. Therefore, in order to locate the onset of asphaltene flocculation we must analyze the trends of suspension viscosities as a function of precipitating solvent concentration.

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