Formation, Effect and Prevention of Asphaltene Sludges During Stimulation Treatments
Authors: Moore E.W. , Crowe C.W. , Hendrickson A.R. 
Published: December 2007

Abstract
Formation of asphaltic sludge during acid stimulation has been a serious problem in many areas for several years. Recent studies have shown that sludge may also affect results in many areas where it has not yet been recognized as such. These studies indicate that:
sludge is a precipitate of colloidal materials present in crude oil; the precipitates occur due to changes in environmental conditions of the crude by addition of materials such as, acid, once formed, sludge is insoluble in most treating chemicals; and sludge can be prevented or controlled by use of stabilizing agents in treating fluid or by use of certain solvents as the outer phase of acid-in- oil emulsions.
The purpose of this paper will be to show how and why sludge is formed and how it can be prevented or controlled. Simple laboratory tests to determine the probability of sludge formation prior to treatment are discussed.
Introduction
Formation of sludges by crude oil on contact with acid has been recognized as a serious problem in isolated areas for some time. The problem apparently was first observed in certain California wells. Following acidizing treatments, the wells were very slow to clean up, and often a great deal of asphalt-like material was returned with the treating fluids. In some cases, complete or partial plugging of the well resulted from the treatment. A study of this problem revealed that the crude oils produced from these wells actually formed solid precipitates upon contact with acid. These precipitates were mainly asphaltenes, resins, paraffin waxes, and other high-molecular-weight hydrocarbons. These materials were apparently precipitated from the crude by the reduction in pH as a result of acid contact. Recently, it was found that the formation of crude oil sludge during acidizing is also a serious problem in many other formations throughout the United States. A comprehensive study recently undertaken indicates that these sludges may also be a problem in many areas where it has not been recognized as such. A survey of crude oils from many fields and formations in all the oil- producing areas of the United States and Canada showed that a substantial percentage (28 to 35 per cent) of all naturally occurring oils tested produced precipitates upon contact with acid.
Nature of Asphaltic Material in Crude Oil
The colloidally dispersed asphaltic and related substances in crude oils which are precipitated upon contact with acid are of such a complex nature that they are classified chiefly on the basis of their physical properties. The most common classification is as follows.
Neutral Resins
These substances are high-molecular-weight aromatic hydrocarbons, which are insoluble in alkalies and acids and completely miscible with petroleum oils, including light fractions (C fraction).
Asphaltenes
These materials are similar to the neutral resins, but insoluble in light gasolines and petroleum ether. In contrast to the neutral resins, the asphaltenes are precipitated in the presence of an excess of petroleum ether. Both asphaltenes and neutral resins are completely soluble in benzene, chloroform and carbon disulfide.
Asphaltogenic Acids
These substances are soluble in alkaline solutions and in such solvents as benzene. Since these are present in petroleum in insignificant quantities, the neutral resins and asphaltenes are the most important asphaltic compounds of petroleum. The asphaltic material present in crude oil is in the form of colloidal particles. Detailed analytical ultracentrifuge studies of several crudes have shown that the asphaltic particles range from 35 to 45 angstroms (A) in diameter. The colloidal range is generally considered to be from 10 to 5,000 A. Other studies have indicated that colloidal particles in crude oil are composed of asphaltic material surrounded by absorbed peptizing materials. These studies have proposed the concept that asphaltenes form the center of the micelles, wth neutral resins absorbed on the surface of the asphaltene particles.
JPT
P. 1023ˆ