|Publisher||Society of Petroleum Engineers||Language||English|
|Content Type||Conference Paper|
|Title||Acid-Sludge: How Small Particles Can Make a Big Impact|
|Authors||Marcel Rietjens, Marco Nieuwpoort, Halliburton Energy Services, Inc.|
SPE European Formation Damage Conference, 31 May-1 June 1999, The Hague, Netherlands
|Copyright||Copyright 1999,Society of Petroleum Engineers, Inc.|
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.
In some oil fields that experience serious difficulties with sludge, operators have identified acid formulations that prevent sludge formation.1 A fundamental description of the nature of sludge is still needed. Generally, sludge is described either as a polymeric product consisting of asphaltenic compounds created by Friedel-Crafts reactions,2-6 or as a stable emulsion.1 Particles can stabilize emulsions because a mechanical barrier is formed at the interface, which relates to the view of sludge as a stable emulsion. This view, therefore, is a phenomenological description, and we argue that the polymeric hypothesis is not true.
The following three arguments oppose the theory that sludge is a product of polymerization initiated by protonation of a double bond (the first rate-limiting step in the electrophilic addition of acid): (1) HCl and FeCl3 are transferred to the oil phase in very small amounts.7 (2) Acid molecules are bonded (inactivated) by molecules with basic sites. For these reactions, excess catalyst is required as in Friedel-Crafts reactions.8 (3) The double bonds are embedded in large molecules. The steric hindrance, combined with a low concentration of double bonds, reduces reaction rates drastically. Electrophilic addition of HCl and H2O (since water is generally present in crude oils) to double bonds may occur, introducing a Cl or an OH group in the reaction product.8 Similar arguments apply to aromatic electrophilic additions.
In addition to the theoretical arguments mentioned, the following facts do not support the polymerization theory: (1) sludge is readily formed at ambient temperature, and (2) the molecular weights of sludges formed at ambient temperature and at 100°C are similar to those of the untreated pure crude oil components.
|File Size||93 KB||14|