Fig. 1(a). Dynamic contact angle values expressed a function of time for 0.10% 14-6-14 GS system; (b) Sessile drop profiles of surfactant fluid at different time intervals for sandstone rock at 303 K.
Surfactant adsorption behavior
Surfactant adsorption is a major problem encountered during chemical EOR owing to material losses observed within reservoir pore-throats. Adsorption profile, obtained from UV spectroscopic analysis as a function of 14-6-14 GS concentration for sandstone, is presented in Fig. 2. At low 14-6-14 GS concentrations (> CMC), surfactant molecules begin to form aggregates/micelles and are subsequently attracted to dispersed rock particle charge as electrical double layer [48,49]. This ultimately leads to a sharp increase in adsorbed surfactant. With further addition, 14-6-14 GS dimer molecules start to occupy active “adsorption” sites; and repel previously adsorbed micelles or aggregates [48]. Consequently, rock adsorption for 14-6-14 GS increased gradually with increasing concentration. At this stage, solid surface shows a very slow increasing trend for 14-6-14 GS adsorption density profiles owing to nearly complete saturation of rock-liquid interfaces with surfactant dimer molecules. The main influencing factor responsible for 14-6-14 GS adsorption is the formation of an ionic or electrostatic pairing bond between cationic head-groups of surfactant molecules and negatively charged rock surface [49]. However, attractive forces such as electrostatic pairing between surfactant head and charged rock surface, dispersive forces, hydrophobic forces among adsorbed and free surfactant molecules, hydrogen bonding and covalent interactions also aid in enhancing the adsorption behavior of surfactant species [48,49].
Fig. 2. Adsorption density onto sand surface for 14-6-14 GS at 303 K.
Experimental data for rock adsorption was investigated by Langmuir and Freundlich isotherms. The model parameters obtained by fitting experimental data with adsorption theories are depicted in Table 2. Though both isotherms have their respective behavioural traits and applications, a careful evaluation of adsorption density versus surfactant concentration plots with these theories may help in understanding the molecular arrangement of adsorbed 14-6-14 GSs onto sandstone rock. Langmuir model assumes that surfactant molecules form a single layer onto adsorbent rock surfaces, wherein Freundlich model suggests a multiple layer stacking arrangement of adsorbed surfactant species [50]. Langmuir model exhibited better results as compared to Freundlich adsorption model, which was evident from larger values of coefficient of determination (R2). Furthermore, smaller values of chi-square statistic (χ2) were obtained during Langmuir isotherm fitting, which further corroborated our inclination to the Langmuir isotherm model to predict surfactant adsorption data. Therefore, monomolecular layer of adsorbed 14-6-14 GS molecules is proposed with no stacking behavior during rock adsorption studies.
Table 2. Langmuir and Freundlich model parameters obtained by fitting experimental adsorption data