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The modification of the surface wetting characteristics in fractured oil-wet carbonate reservoirs, by reversing wettability from oil-wet to water-wet, leads to improved oil recovery. However, in order to obtain a successful oil recovery process, it is crucial to understand the active mechanisms of wettability alteration. This study looks at the effect of sulfate ions as one of the most promising wettability influencing ions on the wetting properties of oil-wet calcite; the effect is studied both with and without the presence of cationic surfactant and possible mechanisms of wettability alteration are explored. A number of analytical techniques were utilized to analyze the mineral surface before and after treatment. The study presents a thorough discussion of the influence of sulfate ions in displacing adsorbed carboxylate from the oil-wet surface, both with and without the presence of cationic surfactant are discussed thoroughly. The interaction between sulfate ions and the calcium ions attached to carboxylate groups on the surface is believed to be the main active mechanism of wettability alteration at high concentration of sulfate ions. Ion-exchange between the hydroxide group and the adsorbed stearate ion on the calcite surface is shown to act as a supplementary mechanism that desorbs stearate ions from the surface. In the treatment of an aged calcite surface with sulfate ions, a combination of these two mechanisms resulted in a more water-wet surface. The copresence of sulfate ions and cationic surfactant resulted in a further reduction in the amount of adsorbed carboxylate on the surface. This can be attributed to the release of adsorbed carboxylate groups on the surface through ion pair formation with the cationic surfactant. The desorption of negatively charged carboxylate groups from the surface facilitates the approach of negatively charged sulfate ions to the aged calcite surface. It can therefore be concluded that sulfate ions accompanied by cationic surfactant molecules can alter the wetting properties toward water-wet state more effectively than sulfate ions alone.

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Oil displacement and recovery efficiency during gas injection depends on the competition between driving forces and capillary resistance that is governed by gas−oil interfacial behavior. Detailed study of the interfacial forces during gas injection is the main objective of this research work. The effects of injecting gas composition and the possibility of asphaltene precipitation in a wide pressure range were determined through comprehensive experimental study. This was performed by measurement of interfacial tension of a highly asphaltenic Iranian crude oil in three surrounding gas mediums. The results showed that as pressure increases, the rate to reach miscibility reduces in the vicinity of the asphaltene precipitation onset. As the surface coverage of the asphaltene at the gas−oil interface exceeded a threshold value the rate was reduced furthermore. Component extractions, noncondensable gas film formation, asphaltene precipitation, and asphaltene accumulation at the interface are found to be the main parameters affecting the miscibility. The observations showed that miscible displacement is practically impossible for this asphaltenic crude oil. Dimensional analysis proved that pressure increase in N 2 and flue gas injection is not effective in improving oil recovery; however, CO 2 tests revealed the presence of optimum pressure range with highest gravity drainage potential and minimum capillary resistance.