International audienceThe morphology and growth of gas hydrate at the interface between an aqueous solution and gaseous mixtures of CO2 and CH4 are observed by means of a simple experimental procedure, in which hydrate formation is triggered at the top of a sessile water drop by contact with another piece of gas hydrate and the ensuing hydrate growth is video-monitored. The aqueous solution is either pure water or a solution of a nonionic or anionic surfactant at low concentration (in the 100-1000ppmw range). In agreement with previously published data, hydrates formed from pure water and aqueous solutions of non-ionic surfactant grow rapidly as a low-permeable polycrystalline crust along the water/gas interface, which then inhibits further growth in a direction perpendicular to the interface. Lateral growth rates increase strongly with subcooling and CO2 content in the gas mixture. Similar lateral growth rates, but varying morphologies, are observed with the non-ionic surfactants tested. In contrast, the two anionic surfactants tested, sodium dodecyl sulfate (SDS) and dioctyl sodium sulfosuccinate (AOT), promote in the presence of CH4 (but not in the presence of CO2) a rapid and full conversion of the water drop into hydrate through a 'capillary-driven' growth process. Insights are given into this process, which is observed with AOT for an unprecedented low concentration of 100ppmw
Résumé -Déstabilisation des émulsions eau dans pétrole brut par des polymères siliconés désémulsionnants -Des émulsions stables de type eau dans huile se forment lors de la production de pétrole brut. Les asphaltènes, molécules tensioactives naturellement présentes dans le pétrole, créent un film viscoélastique qui empêche les gouttelettes d'eau de coalescer. La séparation de l'eau par l'ajout de tensioactifs désémulsionnants est indispensable avant l'étape du raffinage. Récemment, des formulations à base de copolymères polyéther siliconés ont prouvé leur efficacité et leur caractérisation aux interfaces eau/air et eau/pétrole a été effectuée. Différents mécanismes de désémulsification peuvent être envisagés : (i) dissolution des agrégats d'asphaltènes ou (ii) déplacement de ceux-ci dû à l'adsorption des polysiloxanes à l'interface eau/pétrole. Afin d'élucider le type de mécanisme, diverses techniques ont été utilisées pour étudier les interactions entre asphaltènes et copolymère : rayons X aux petits angles permettant d'étudier une éventuelle influence du copolymère sur la taille des agrégats d'asphaltènes, capacité du copolymère à chasser les agrégats d'asphaltènes préalablement adsorbés sur des billes de silice (modélisant les gouttelettes d'eau), microscopie à force atomique utilisée pour observer l'action du copolymère sur la structure des films asphalténiques.
Abstract -Destabilisation of Water-in-Crude Oil Emulsions by Silicone Copolymer Demulsifiers -Asphaltene aggregates are known to form viscoelastic film preventing the coalescence of droplets in water-in-oil emulsions formed during crude oil exploitation. Since phase separation is necessary for oil refining process, demulsifying additives are used. It was found that formulations based on polysiloxane copolymers promote separation of water from crude oil even at very low concentration (few tens of ppm). Two alternative scenarios of emulsion destabilisation can be envisaged: (i) dissolution of asphaltene aggregates or (ii) displacement of the asphaltene network by adsorption of the more surface active copolymer into void sites at the oil/water interface. In order to reveal the mechanism of destabilisation, interactions between asphaltene aggregates and copolymer were explored. For that purpose various techniques have been employed: small angle X-ray scattering allowing the determination of the influence of copolymer on the size of asphaltene aggregate; capacity of copolymer to displace asphaltene aggregates initially adsorbed on silica particles (which simulate water droplets); Atomic Force Microscopy (AFM) was used to observe the influence of copolymer on the interfacial structure of asphaltene films spread on water surface.
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