Abstract

The production and processing problems associated with the presence of asphaltenes in crude oil is well known. This is especially true in bitumen production from tar sands when the use of light hydrocarbon solvents is being considered as an alternative to the conventional steam injection and in miscible flooding of production reservoirs with carbon dioxide and other hydrocarbon injection fluids. Since it is well known that asphaltenes readily precipitate from crude oils with the addition of light saturated hydrocarbons, there are serious concerns about the potential plugging problem associated with the solvent injection technique. The present study is undertaken to determine the effect of saturated hydrocarbon olvents such as n-heptane and n-hexadecane on the recipitated asphaltene particle size and size distribution as it relates to potential production problem. An opticalmicroscopy/computer image analysis approach is being used. It was found that the mean particle size of the precipitated asphaltenes from Cold Lake bitumen is a function of its concentration, being larger at higher bitumen concentration and decrease to some limiting value as asphaltene concentration is decreased. In addition, some preliminary results on determining the time dependent asphaltene particle size distribution will also be reported. This could have significant implications on the use of miscible flooding in both bitumen production and in secondary and tertiary ecoveries.

Introduction

Asphaltenes are present in all erode oils in varying degrees, The amount varies from less than 1% in some light erodes such as those from Judy Creek and Norman Wells to ore than 15% as the case with Cold Lake bitumen. Irrespective of the amount, the asphaltene fraction in crude oil always poses a challenge to both recovery and refining rocesses. In the refinery, it is usually responsible for coke formation and catalyst activity degradation(1), In heavy erode roduction and in secondary and tertiary recoveries using iscible flooding by carbon dioxide and light hydrocarbons, asphaltenes precipitation could potentially cause plugging of the reservoir(2,3). An improved understanding of the asphaltenes precipitation process, especially in regard to their quilibrium particle size would aid in identifying methods to lleviate these problems.

Characteristics of asphaltenes

Asphaltenes is generally referred to as the fraction of crude oil that is insoluble in light saturated hydrocarbons. Depending on the chain length of the saturated hydrocarbon olvents used, this fraction can be further subdivided into a resin fraction and an asphaltene fraction. Numerous attempts ave been made to define the chemical composition of asphaltene(4,5). However, due to variability of crude origin, a pure compositional definition is clearly impossible. Some understanding on the size of asphaltenes in various solventscomes from studies of molecular association via molecular weight determination in various solvents and at different temperatures using techniques that include x-ray and neutrondiffraction and scattering(6–8). Two models for asphaltenes structure in solution have been proposed by Pfeiffer and Saal(9) and Yen(10,11). The accumulated experimental results seem to favour the Yen's model over the Pfeiffer and Saal model(12). Irrespective of the choice of model, the asphaltene will, in the presence of a light saturated hydrocarbon such as -heptane, precipitate and eventually settle out of solution.

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