Abstract

The precipitation and deposition of asphaltenes can cause serious problems during petroleum production that range from formation damage to total blockage of producing wells. A knowledge of the aggregation, precipitation and deposition mechanisms is fundamental to the development of preventive and curative measures for this potential problem. Recent findings on the question of reversibility of asphaltene precipitation in petroleum fluids indicate the dissolution of precipitated asphaltenes with addition of oil and evaporation of flocculant The addition of dispersants such as toluene appear to cause changes in asphaltene particle viscoelasticity, changing its relaxation behavior and reducing the efficiency of the coagulation. The surfactants ethoxylated nonylphenols and hexadecyltrimethylammonium bromide proved to be the best of the various inhibitors of asphaltene deposition tested in this study. The results point out to the importance of recognizing the nature of asphaltenes in the development of viable inhibitors for asphaltene deposition. Surface tension measurements in n-pentane insolubles/model solvent systems confirm the occurrence of an aggregation phenomenon, possibly a micelle formation with increase in asphaltene concentration.

Introduction

It is widely known that asphaltenes and resins are the main constituents in heavy and polar fractions of petroleum fluids. These constituents have a condensed polyaromatic structure containing alkyl chains and heteroatoms (such as O, S and N) and some metals. Asphaltene deposition is responsible for serious problems during production, transportation and processing of petroleum fluids. These problems include reservoir permeability reduction (phenomenon known as formation damage), mobility reversal, production column diameter reduction and plugging of production and processing pumps, flow tubing and other equipment causing loss of efficiency and increase of production and maintenance costs.

The molecular nature of asphaltenes the target of many studies over the past decades, is still elusive. Asphaltenes and resins are characterized according to the solubility behavior of crude oils in n-alkanes. While various physical and mathematical models for asphaltene precipitation have been proposed (among others), the true nature of asphaltene aggregation and flocculation is still inconclusive and constitutes an active area of research.

While the solubility parameter concept has significantly contributed to the description of the phase behavior of asphalts and asphaltenes it is not sufficient to explain the complete destabilization mechanism of asphaltenes in petroleum crudes. It is recognized that besides the fact that asphaltenes are formed by aromatic compounds with interactions, there are also acid-base interactions among the varIous acidic and basic group components. Current tendencies point to the following mechanism for the behavior of asphaltenes in crude oil:

dissolved monomer - micelle - particulate - precipitate

Micelles can be formed by monomer association and large particles formed by association of micelles. The few experimental observations presented in the literature, through calorimetric and surface tension measurements, point to the existence of a critical micelle concentration. Results of infrared spectroscopy revealed the importance of hydrogen bonding in asphaltenes association and suggested the occurrence of asphaltene self-association in organic solvents at very low concentrations. In the majority of studies encountered in the literature, asphaltene concentrations in crude oil or model solvents are substantially higher than the critical micelle concentration.

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