Towards enhanced oil recovery: Effects of ionic valency and pH on the adsorption of hydrolyzed polyacrylamide at model surfaces using QCM-D

Adding polymers to the injection water in water-flooding oil recovery increases the viscosity and thereby enhances recovery factor. However, the beneficial effects are often counteracted by adsorption of polymer to the ambient mineral surface. In here, we used Quartz Crystal Microbalance with Dissipation (QCM-D) measurements to study the adsorption of the frequently used hydrolyzed polyacrylamide (HPAM), a weak polyelectrolyte, to silica and alumina model surfaces representing sandstone reservoirs. At pH 6 and 8 and concentrations from 1 mM to 100 mM of NaCl and CaCl₂, we find that negatively charged HPAM molecules generally adsorb more strongly to positively charged alumina surfaces. For silica surfaces, the presence of Ca²⁺ ions, which form strong complexes with the carboxyl groups on the polymer, as evidenced by titration measurements, strongly enhances HPAM adsorption. This effect is attributed to the formation of Ca²⁺-mediated ion bridges with the substrate. Kinetic measurements suggest a two-step adsorption process consisting of a primary polymer adsorption onto the surface followed – in most cases – by a slower secondary process, which involves rearrangements of previously adsorbed polymer molecules, including polymer-on-polymer adsorption.