Surfactant flooding is one of the mechanisms used to enhance oil recovery from oil reservoirs. Surfactants mainly reduce interfacial tension, increase oil mobility thus allowing better displacement of oil by injected water. One of the main challenges in this process is the loss of surfactant due to adsorption on reservoir rocks. The first adsorbed layer of surfactant is mostly governed by electrostatic attraction. Once this first adsorbed patch forms on the surface, a second layer of surfactant may be adsorbed by surfactant tail-tail hydrophobic interactions at high surfactant concentrations. Both these interactions would be affected by the salt type, salt concertation and pH of the injected solution. In this study, adsorption of anionic surfactants is investigated on negatively charged silica and positively charged calcite and alumina surfaces under various brine conditions. Further, we also check the efficiency of a polyelectrolyte solution as sacrificial agent to minimise the surfactant adsorption. We use QCM and surface tension measurements to study the same.
Both QCM and surface tension measurements reveal the extent of surfactant adsorption. In case of QCM, the change in frequency of quartz crystal gives information about adsorbed mass while in case of surface tension (γ) measurements, change in γ tells about the concentration change of surfactant in the bulk and hence indirectly about the surfactant adsorption. In case of negatively charged silica surface, the anionic surfactants adsorb readily in presence of divalent ions, while there is very slight or negligible adsorption in presence of only monovalent ions. Hence the adsorption is mediated via cation bridging mechanism. In case of alumina (positively charged below pH 9), we see significant adsorption even in absence of divalent ions. This occurs due to direct electrostatic attraction between the negative polar head group of surfactants and the positive alumina surface. The surfactant adsorption can be reduced by a pre-flush of anionic polyelectrolyte which has stronger binding affinity for the surface compared to surfactants. The strongly adsorbed poly-electrolyte repels similarly charged surfactant molecules and hence reduces surfactant adsorption. Similar trends are also observed with the surface tension measurements. The pre-flush of anionic polyelectrolyte reduces the surfactant adsorption on the sandstone powder, which leads to higher bulk concentration of surfactant and hence a smaller value of surface tension. We discuss our findings in the context of improving the efficiency of the alkaline surfactant polymer flooding process.
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