Acid-induced sludge, a major source of well damage, can vary from a viscous, stable emulsion to the product of a polymeric reaction. This paper examines the nature of sludge and how it is formed.

Sludge particles are often believed to be large, but research shows that the molecular weights of the constituents of sludge particles are relatively low. In size-exclusion chromatography tests, sludge was dissolved in a solvent and analyzed. Results showed that the mass of sludge constituents was only twice that of the highest molecular weight of the oil components. In undiluted pure oils, however, sludge particles were much larger than particles of the constituents; individual sludge particles are even visible by microscopy.

A chemical analysis of sludge shows that hydrochloric acid (HCl) is present in the sludge structure but water is not. Besides HCl, Fe(III) ions in HCl formulations can cause a large amount of sludge. Even though ferric ion could not be detected in the oil phase or in the sludge, it induces sludge mainly because it works as a phase-transport catalyst for HCl.

Sludge is an aggregate of protonated molecules. Aggregation is initiated by protonation of asphaltenes and other compounds, such as resins, that undergo acid-base interactions. Molecules are then formed that have surfactant-like properties that tend to form structures resembling reversed micelles. Stack formation, which occurs in asphaltene aggregation, is another initial mechanism for aggregation of protonated constituents. These initial aggregates eventually become a flocculate, or sludge.

The aggregation theory seems to be supported by the fact that a critical aggregation concentration exists, below this concentration level, sludge will not form; above this concentration level, sludge is formed in increasing amounts.

This paper shows that sludge can be dissolved in pure solvents or in nonsolvents containing surfactants, demonstrating that sludge formation is reversible.

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