Abstract

The work described in this paper has been conducted over several years under both laboratory and field conditions. Several field case histories and supporting laboratory observations are presented to substantiate the conclusions expressed.

Asphaltenes are often invoked to describe the nature of organic damage and obstructions found when atypical forms of organic deposits appear. In fact a majority of problems studied by this laboratory have been misidentified as asphaltene when the true problem was due to paraffin wax deposition. Asphaltenes are poorly understood, consequently methods aimed at dealing with deposit mechanisms and chemical prevention present a doubly difficult challenge. Asphaltene deposition mechanisms and the chemical structures responsible are less well characterized than those of emulsion breaking, corrosion protection, and paraffin deposit control. Since most asphaltene deposits are associated with paraffin and there exists many proven methods of paraffin control, in many cases it makes better sense to attack organic deposit problems with paraffin control products than some unproven "asphaltene inhibitor".

This paper suggests that adequate methods exist today for the removal and control of paraffin wax damage, and that methods for the control of asphaltenes are still in a relatively embryonic state of development. Until the day comes that asphaltenes are well understood chemically, as are waxes, emulsions, corrosion, and scale, the hope of economically treating them will remain a problem for research.

Introduction

Asphaltenes are basically defined as comprising an organic fraction of crude oil that is insoluble when that oil is placed in fifty times its volume in pentane.1 This definition is extremely broad and non-informative, and very little information about the chemical structure of these pentane insoluble fractions is gained by this procedure. In fact if the crude oil is not properly pretreated to remove water and solids, confusion can arise about the true quantity of "asphaltenes" that are actually present. This operational definition of asphaltenes results from historical observations made by oil industry personnel, and was never intended to offer a rigorous description of these substances. One outstanding feature of this method of isolating asphaltenes is its clear tendency to exclude ionic and/or polar species possessing strong intermolecular interactions from solvation by pentane. Hidden within this feature is the fact that higher molecular weight normal paraffins tend to aggregate by inductive charge association through London dispersion forces (Fig.1). Unless the pentane solution is heated to disrupt these forces of aggregation the higher normal paraffins will also be included as part of the precipitated "asphaltenes".

Many organic molecules can be classified as ionic and/or polar. Carboxylic acids and their salts are examples of naturally occurring organic molecules that can be and are often found in crude oil (Fig. 2). Additionally, many amines and anionic amine salts are known to be present in crude oils (Fig. 3). Although the concentration of these molecules is usually low in the majority of crude oils produced, they are often associated with problems in its production. "Asphaltenes", as they are operationally defined, can thus be thought of as including these molecules. Further, as mentioned above, normal paraffins can also be included as part of the isolated "asphaltenes" when no heat has been applied to the pentane oil mixture. Although researchers attempting to elucidate structural features of "asphaltenes" have expended considerable efforts, the basic lack of a clear definition of these materials hinders progress.2 Vague descriptions of "resins and maltenes" are given to describe the components thought to be responsible for the suspension of "asphaltenes" (Fig. 4). Thus, confusion on confusion results and speculation replaces fact when dealing with the ill-defined nature of asphaltenes.

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