Adsorption of Sodium Alkyl Aryl Sulfonates on Sandstone

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Abstract
Equilibrium adsorption isotherms of commercial alkyl aryl sulfonates (petroleum sulfonates) and pure alkyl aryl sulfonates on disaggregated Berea pure alkyl aryl sulfonates on disaggregated Berea and Benton Tar Springs sandstones were determined. Adsorption isotherms of commercial sulfonates were found to contain maxima, which did not correspond necessarily to the measured critical micelle concentration (CMC). At adsorption maxima, surface coverage corresponded to about one-half the monomolecular layer of sulfonate, but at high surfactant concentrations, coverage sometimes amounted to only about one-tenth of a monolayer. Pure alkyl aryl sulfonates were synthesized and adsorption on sandstone was determined. These materials yielded conventional adsorption isotherms, with adsorption plateaus at about one-half a monolayer of surface coverage. Apparently, adsorption maxima are unique to impure sulfonates. Selectivity of adsorption was studied with respect to molecular weight and structural type. The structure of petroleum sulfonate, the accompanying mineral oil, and also the structures of sulfonate and mineral oil that had been equilibrated with sandstone were determined. A comparison showed no selectivity of adsorption based on carbon number distribution or structural type. However, aggregates relatively rich in mineral oil were found to be adsorbed selectively. Introduction The plan usually used in surfactant-enhanced waterfloods is injection of a slug (less than 1 pore volume of surfactant solution, followed by a viscous drive of polymer-enhanced water. Depending on type, the surfactant slug may itself contain polymer to assure favorable mobility. polymer to assure favorable mobility. The quantity of surfactant lost on rock surfaces and the mechanism of adsorption affect the success of such processes. The magnitude of adsorptive losses determines, in large measure, whether a chemical flood can be profitable. This magnitude also dictates the type of polymer and polymer concentration that can be used for slug mobility control; that is, frontal advance rates of polymer and surfactant must be matched to assure stability. The mechanism of surfactant adsorption influences the effectiveness of the recovery process. Surfactants are often complex mixtures of material that differ in structure and molecular weight. The displacement of oil at a chemical-flood front depends on a delicate balance of properties, including mutual miscibility of slug and oil, and reduction of capillary-trapping forces by lowering oil-water interfacial tension. A chemical-recovery slug with optimum design of surfactant composition can be damaged if adsorption results in chromato-graphic separation of the components. Our goals wereto examine in detail static equilibrium adsorption of selected commercial surfactants on rocks similar to those found in reservoirs and to compare their adsorption with that of pure synthetic surfactants of the same chemical type, andto determine whether static equilibrium adsorption of a realistic oil-recovery surfactant resulted in fractionation of components with different molecular weights and structural types. This study only discusses alkyl aryl sulfonates, the most popular surfactants used in oil recovery in recent years. ADSORPTION OF COMMERCIAL SODIUM DODECYLBENZENE SULFONATE (NaDDBS) ON SANDSTONE Siponate DS-10 was chosen as an example of a commercially pure alkyl aryl benzene sulfonate. It is marketed by American Alcolac and claimed to have 98-percent active ingredients with an average composition of C18H29SO3Na. Siponate DS-10 is soluble in water at room temperature and is available also from the manufacturer as approximately 20-percent aqueous concentrate. Solutions with concentrations in the range studied here are clear and will tolerate considerable increases in ionic strength before the onset of turbidity and eventual precipitation. precipitation. SPEJ P. 75

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