Effects Of Betaine Surfactants For EOR On The Wettability Of The Negatively Charged Surfaces

Didodecylmethylcarboxyl betaine (diC12B) has been found to be a good surfactantapplicable in alkali-free SP flooding. However, its adsorption loss on the negatively chargedsandstone and thereby the effects on the wettability of the sandstone, which is very importantfor the success of SP flooding, have rarely been reported. In this thesis the adsorptionisotherm of diC12B at negatively charged Daqing sandstone/water interface at45℃wasstudied, and the wettability alteration of the sandstone due to the adsorption of diC12B wasevaluated by using model systems: measuring contact angles of pure water drops onnegatively charged glass slide pre-adsorbed surfactants, and the contact angles of n-decanedrops captured by the glass slide immersed in surfactant solutions at different concentrationat25℃. In addition, the effects of diC12B on the wettability of negatively charged particleswere also evaluated by examining the in situ surface activation of the surfactant to thenegatively charged SiO2nanoparticles and the dispersibility of the nanoparticles in theaqueous surfactant solutions. As a comparison, typical anionic, cantionic and nonionicsurfactants were also evaluated and examined.The results showed that the adsorption isotherm of individual diC12B at Daqing oilsand/water interface at45℃follows apparently the Langmuir isotherm, with the saturatedadsorption being around0.03mmol/g, significantly higher than that of anionic and nonionicsurfactants and closing to that of cationic surfactants. On the glass slides pre-adsorptedwith diC12B, the contact angles of water drops were found to increase initially, reaching amaximum, and then decrease with increasing surfactant concentration. The maximumcontact angle, ca.65o, appeared at a concentration in the vicinity of cmc. The contact anglesof water phase on the slide immersed in surfactant solutions, obtained by measureing thecontact angles of the n-decane drops captured by the slide, increase too, with increasingsurfactant concentration and reach a plateau, about170o, at concentration near cmc. It issuggested that at low concentration the diC12B molecules adsorbed at solid/water interfacewith positively charged headgroups toward to the solid surface and the hydropbobic tailstoward water and thus increasing the hydrophobicity of the solid surface; and at highconcentration, double layer adsorption via chain-chain interaction of the diC12B moleculesadsorbed and in solution may be formed, which returns the solid surface hydrophilic. Similarmolecular configuration happes at silica/water interface, which makes the nanoparticle insitu surface activated and thus become surface active nanoparticles to be able to stabilize O/W emulsions, as did by using cationic surfactants. When dispersed in diC12B aqueoussolution, the particles were observed to disperse-flocculate-redisperse, also similar todispersed in cationic surfactants. As a comparison the adsorption of anionic surfactant SDSand nonionic surfactant lauryl diethylene glycol amide is much lower and they do not makethe solid surface hydrophobic. Obviously, the SP flooding with alkylbetaines as mainsurfactants, once diluted, may tend the sandstone surface to be hydrophobic or oil-wetable,which is not beneficial to the oil recovery, although the interfacial tension between crude oiland water can be reduced to ultralow.