English abstract
The average oil recovery by conventional water flooding from carbonate reservoirs is often less than 30%. As most of these reservoirs are fractured and oil-wet, injected water displaces the oil in the fractures, however the oil in the matrix blocks remains un-swept. Negative capillary pressure prevents water from penetrating through the oil-wet matrix. An effective approach to overcome the capillary pressure barrier to enhance oil recovery is altering wettability of the rock towards preferentially water-wet state. Removal of adsorbed acidic components and/or asphaltenes from the carbonate surface results in
wettability alteration towards water-wet state. In order to remove the adsorbed carboxylate and/or asphaltenes from the rock surface effectively and efficiently during enhanced oil recovery (EOR) processes, a clear understanding of the adsorption mechanism as well as the contribution of different crude oil compounds on the surface wettability is crucial.
Long chain fatty acids, long chain naphthenic acids and asphaltene are the major components in crude oil which are responsible for wettability alteration in carbonate reservoirs. In this study, different types of carboxylic acids and asphaltene were used as polar organic compounds in crude oil. Different analytical tools (contact angle, zeta potential measurement and thermal analysis) were utilized to study the effect of carboxylic acids and asphaltene on calcite surface wettability. The results indicated that polar organic
compounds act in different ways according to their structure and properties. Presence of double bond could make the acid molecule more polar, reactive and less hydrophobic. In addition, it can affect the spatial arrangement of molecule and hence its orientation on the surface. Presence of benzene ring increases the reactivity and polarity of carboxylic group. Moreover, it enhances hydrophilic properties of the acid due to electrons of it double bonds that can resonate over aromatic carbon atoms. The results revealed that, in the presence of water film, combination of acid dissociation in water, surface activity of acid and water solubility controls the acid adsorption and hence wettability alteration of the surface during aging process. Chemisorption of carboxylic acids/ asphaltene changes the wettability of calcite surface. In addition to chemical adsorption, some acids adsorb physically on the surface. Physical adsorption of polar compounds may happen via hydrogen bonding between carboxylic groups and carbonate ions on the surface. Although physical adsorption is reversible and forms weak bonds between adsorbate and adsorbent, it may partially contribute to wettability alteration
