Role of Surfactant Structures on Surfactant-Rock Adsorption in Various Rock Types

Abstract

Adsorption of surfactant onto rock surfaces is dependent upon a number of factors, including characteristics of both the adsorbent and surfactant molecules. Considering that surfactant-based unconventional means to improve oil recovery are strongly dependent on the interaction at the liquid/liquid interface between soluble surfactant solution and crude oil, loss of surfactant to liquid/solid interfaces can create a negative effect for some of these applications in terms of performance and economics. This study; therefore, focuses on investigating the adsorption mechanism of surfactants onto sandstone and limestone reservoir media. Besides quantifying how much surfactant is adsorbed, emphasis specifically on the effect of surfactant parameters on the adsorption capacity was evaluated. Although literature well documents that mineralogy, temperature, pH, inclusion of other chemicals, and salinity all play strong roles on the adsorption capacity of surfactants on a solid surface; all of these parameters, with exception of mineralogy, were maintained as constants for this work. Anionic alcohol propoxy sulfate, nonionic alcohol ethoxylate, and ether carboxylate surfactants were studied. Academic focus for this effort was placed on surfactant parameters being evaluated including the structure of the surfactant hydrophilic head group and the surfactant hydrophobic tail. The number of mechanisms involved in surfactant loss from aqueous solutions to assorted porous media adds to the complexity of this phenomenon. Experimental results show that various surfactant parameters affect the adsorption differently based on their interaction with different adsorbents. An increase in hydrophobicity appears to increase surfactant adsorption. This was observed through a number of different mechanisms including increasing percent of propylene oxide (PO) and increasing degree of hydrophobe branching of the surfactants. Conversely, increasing carbon chain length and keeping the percent of PO more constant appeared to show a general decrease in adsorption trend with alcohol propoxy sulfate and a discernible decrease in adsorption in sandstone versus limestone mineralogy. It was also observed that varying ratios of propylene oxide and ethylene oxide extensions to alcohol alkoxy sulfate molecules will have an influence on surfactant adsorption. Surfactant properties provide information on the type and mechanism of interactions involving surfactant molecules at the solid/liquid interface and their efficiency as surface-active agents. The findings from this study can be used to improve understanding on how the role of different surfactant parameters may affect surfactant adsorption. This will help lead to enhancements in designing surfactant molecular structures that in turn minimize adsorption to rock surfaces, while maintaining desired fluid performance for effective oil recovery.