Mechanistic study of surfactant/polymer adsorption and its effect on surface morphology and wettability

The need for a better understanding of surfactant/polymer adsorption, its quantification, and its influence on the physiochemical properties of reservoir rock can be crucial in the application and optimization of ASP flooding for EOR. This study is aimed to quantify surfactant/polymer adsorption onto Berea sandstone and reservoir rock. Surfactant and polymer adsorption onto crushed core samples and their consequent influence on surface morphology and wettability alteration were investigated. In addition, the effect of the presence of a pre-adsorbed polymer or surfactant layer on the subsequent surfactant or polymer adsorption was investigated. The chemicals used were a partially hydrolyzed polyacrylamide and an Internal Olefin Sulfonate surfactant. Polymer and surfactant concentrations in solutions were measured using total organic carbon (TOC) analyzer and potentiometric titration, respectively. Crushed core samples were analyzed for clay content and minerals by X-ray powder diffraction (XRD). Contact angle was measured on glass slides that has been incubated in brine, polymer and surfactant solutions for brine/hexadecane and brine/crude-oil systems. Amott tests were performed to quantify the wettability alteration due to brine, polymer and surfactant solutions using reservoir oil and core plugs. Results show that polymer and surfactant adsorption follows Langmuir adsorption isotherms. The polymer adsorbed amount on crushed reservoir core was around 450 mg/100g at the plateau region using the synthetic brine (salinity of 1% and pH of 8). Surfactant adsorption on crushed Berea core and crushed reservoir core at the plateau region were 700 mg/100g and 400 mg/100g, respectively. The higher surfactant adsorption on Berea cores is attributed to its higher clay and calcite content compared to the reservoir core. Contact angle (CA) on glass slides increased against both hexadecane and crude oil after surfactant adsorption onto the glass surface, which indicates that wettability-altering effect of surfactant on the surface to be more oil-wet. However, the CA remained nearly unchanged after polymer adsorption. Glass slides treated with polymer adsorption followed by surfactant adsorption ‘PS’ and surfactant adsorption followed by polymer adsorption ‘S-P’ changed towards more oil-wet indicating higher influence of surfactant adsorption on wettability alteration. The FESEM images showed different adsorption pattern for surfactant treated slides. The Amott index of original brine/oil system was altered from 0.87 to 0.67 and 0.50 for polymer/oil and surfactant/oil systems, respectively. Hence, surfactant shows a pronounced influence on altering the wettability of the original reservoir rock to be less water-wet.