Further Investigations into the Nature of Salt Spheres and Inorganic Structures at the Crude Oil−Water Interface

Abstract

A number of factors are known to contribute to and enhance water-in-crude oil emulsion stability. Organic-based and naturally occurring materials (e.g., asphaltenes, resins, naphthenic acids, waxes, etc.) have received a great deal of attention regarding their role in stabilizing and, at times, destabilizing petroleum-based emulsions. While these materials are known to reside at the oil−water interface, inorganic constituents are likewise known to contribute to emulsion stability and have been shown to also reside at the oil−water interface. To study the inorganic components in unresolved oilfield emulsions, numerous field samples were subjected to the American Society for Testing and Materials D4807-88 procedure that involves a hot toluene filtration. This method effectively removes the organic phase and isolates the inorganic components that may have also played a role in emulsion stabilization. The residual inorganic residues were initially characterized by a combination of scanning electron microscopy (SEM) and energy-dispersive spectrometry (EDS). Following up on the initial report where several samples contained highly organized inorganic structures, called salt spheres or salt scaffolds, this study takes a closer look into their compositional makeup using enhanced EDS capability with the addition of digital compositional mapping (DCM). Salts with low solubility-product constants appear to outline once existent water-in-crude oil droplets and, thus, may have played a key role in emulsion stabilization. Sodium chloride crystals often dominate the interior of these structures, and it is this entity that upon further examination was found to be unstable with regard to prolonged exposure to humidity. In comparison to dry or treated sales oils, inorganic solid levels from the oil−water interface of two oilfield separators were found to be considerably higher, with one of the samples containing an abundance of well-defined salt spheres. By use of the DCM technique, the separator sample was shown to contain salt spheres with an outer coating of strontium sulfate.

Characterization of Inorganic Solids Present in Brazilian Crude Oil Emulsions Using Scanning Electron Microscopy (SEM) with Energy-Dispersive X-ray Spectrometry (EDS): Evaluation of the Effect of Solvents

“…It should be noted that Ba, Ca, and Sr sulfates and Ca carbonate have low solubility product constants ( K sp ), which justifies their nonsolubility, even after washing the membrane with hot water. In this context, reports in the literature showed the presence of similar inorganic solids containing Ca, Ba, and Sr sulfates and Ca carbonate in crude oil emulsions …”

“…Iron was identified in the backdrop of the salt scaffolds and was less concentrated on the sphere itself. According to researchers, the combination of ASTM D4807 and SEM–EDS allowed the isolation and identification of inorganic solids present in crude oil emulsions …”

“…In another work, the composition of inorganic structures using SEM with digital compositional mapping was investigated. 23 In this case, the presence of salts with low solubility product constants (such as strontium and barium sulfate and calcium carbonate) was observed on the outer surface of the salt scaffolds. Moreover, Ba, Ca, and Si were present as fine dust throughout the "sphere" and adjacent to it.…”

“…According to researchers, the combination of ASTM D4807 and SEM−EDS allowed the isolation and identification of inorganic solids present in crude oil emulsions. 23 In this work, a new investigation was performed using the ASTM D4807 procedure combined with SEM−EDS, for the identification of inorganic species present in Brazilian crude oils. In addition, to evaluate the influence of the solvent on the sediment content retained on the nylon membrane, three modified methods were evaluated: (i) toluene + isopropanol (50% + 50%, v/v), (ii) toluene + isopropanol + water (50% + 49.5% + 0.5%, v/v), and (iii) use of an additional washing step with hot water of the nylon membrane filter for the unmodified ASTM D4807.…”

Desalting Behavior of Bitumen

“…This could provide a plausible explanation for how metal cations could be retained in bitumen without being present as metal cations coordinated to the bitumen, or metal cations present in connate water. There are several examples reported of salts being present in petroleum as salt spheres and several other solid particle shapes. − In these studies, the structures ranged in size from <1 to ∼100 μm. Of relevance to this work is the fact that salts can be present as solids in petroleum.…”