Sludge is formed during acid stimulation operations and clogs pore throats. It is generally assumed that sludge is asphaltenic in nature. However, crude oils that contain little or no asphaltenes can produce sludge too. These types of sludges are referred to as non-asphaltenic sludge. In this paper, the results of a fundamental approach are described to reveal the cause and nature of sludge formed during the contact of crude oil with acid. In the past, several experimental observations have been misinterpreted. This paper gives a new interpretation of these observations. Because much confusion exists about the sludging effects of various organic acids, the sludging tendencies of several organic and inorganic acids were examined. The results of the examination showed that all Bronsted acids (HCl, acetic, etc.) have a similar effect on sludge, the amount of sludge being a function of the acidity function Ho only. (The concept of pH cannot be applied in concentrated acidic media.) For each crude oil, a threshold Ho value can be identified above which no sludge was observed. At decreasing Ho values (more acidic), progressively more sludge is observed. It is generally assumed that Friedel-Crafts-type of reactions is the cause of sludge. Although this assumption is reasonable, no explanation has been given about why only Fe(III) ions sludge as opposed to Fe(II) and especially Al(III), which is known as an extremely powerful Friedel-Crafts catalyst in synthetic organic chemistry. The activity of Lewis acids (e.g., Fe(III) ions), depends on whether a particular ion can be transported to the oil phase. At acidising conditions, only Fe(III) chloride complexes are sufficiently oil soluble while aluminum fluoride or chloride complexes are practically oil insoluble. Aluminum ions can be made to sludge by adding ligands, which produces oil soluble aluminum complexes such as thiocyanate.
Slower breaking emulsions produce more sludge because of the longer exposure time with a larger contact area. Once formed, the sludge is very fine and can pass through a 100-mesh screen, which suggests little or no sludge is present.
Problems with sludge have long been recognised and several papers with regard to this matter have been published. Because sludge is insoluble in either oil or acid, it will precipitate and plug formation pores. Oilfield researchers have accepted two assumptions about how sludge is formed. Higher HCl acid strengths, the presence of Fe(III) ions, and additives such as corrosion inhibitors promote sludge formation. Sludge is formed as a result of Friedel-Crafts-type reactions and that the nature of sludge is asphaltenic in character.
This paper shows that in addition to asphaltenes other compounds exist in crude oils that produce sludge when these compounds are exposed to acid. Moreover, a number of observations have been made by various researchers that are either not in agreement with each other, or cannot be explained rationally. For example, HCl/HF acid of comparable strength seemed to sludge more than HCl. Diluted acetic acid caused sludge in one case whereas glacial acetic acid was trouble free. Another study showed that diluted acetic acid did induce sludge. A confusing issue is the observation that slower emulsion breaktimes reduce sludge. Claims about the positive effects of ligands that result in oil soluble iron complexes (iron extractants) have caused even more confusion since these claims contradicts the theory of sludge formation.
As mentioned above, oilfield researchers have generally accepted that sludge is formed as a result of Friedel-Crafts-type reactions. A number of heterocyclic compounds, present in oil, are prone to polymerise under acidic conditions.