Adsorption Of Surfactants For EOR At Daqing Sandstone/Water Interface

The adsorption of a series of alkylcarboxylbetaines and fatty amide ethoxylates at negatively charged Daqing sandstone/water interface at 45°C were studied and compared with that of typical anionic and cationic surfactants. To illustrate the effects of molecular structure the alkylcarboxylbetaines contain those with single long alkyl of different chain length (C₁₂B, C₁₄B, C₁₆B, and C₁₈B) and that with double long alkyls (diC₁₂B), whereas the fatty amide ethoxylates have similar hydrophobic chains but different EO number: coconut monoethanolamide (CMEA), dodecyl monoethanolamide polyethylene (1) ether (C₁₂MEAEO₁), and coconut monoethanolamide polyethylene (2) ether (CMEAEO₂). Moreover the mixed adsorption of some binary surfactant mixtures was also examined.The adsorption at solid/water interface can be well characterized by a layer number, n, defined as the ratio of the saturated adsorption at solid/water interface to that at air/water interface. In general the saturated adsorption of different species follows an order of anionic (n<0.5) < nonionic (0.5<n<1) < zwitterionic (n>1) < cationic surfactants (n>2), and for homologues the adsorption is strongly structure dependent. The adsorption of the alkylcarboxylbetaines is initially driven by the electrostatic interaction between the opposite charges in surfactant molecules and on surface followed by the chain-chain interaction which inducing admicelle formation on solid surface. C₁₂B, C₁₄B, C₁₆B and C₁₈B all have similar saturated adsorption with n between 1.60 and 1.76, but diC₁₂B displays a much high saturated adsorption with n≈5.83, which is even higher than that of cationic surfactant with similar structure, probably as a result of the partially positive ionization of the molecules in pure water. Although the adsorption of the fatty amide ethyoxilates is in general low, CMEA has a high adsorption with n=3.06 due to its high hydrophobicity and compact packing in the admicelles. For mixed adsorption systems nearly ideal mixing and weak synergism are observed for C₁₂B/diC₁₂B (homologous mixture) and C₁₂MEAEO₁/diC₁₂B (nonionic/zwitterionic) mixtures, but strong synergism is observed for SDS/C₁₂B and SDS/C₁₆B (anionic/zwiterionic) mixtures. Thus for SP flooding the achievement of low adsorption retention depends strongly on the species selected if mixed surfactants are necessary.