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In view of enhanced oil recovery, the adsorption behavior of surfactants is usually monitored on smooth model rock surfaces using quartz crystal microbalance with dissipation (QCM-D). However, this is an impractical situation as the effect of the surface roughness of reservoir rocks and its role in surfactant adsorption processes are not yet completely understood. The coupling of electrochemical techniques and QCM-D in one analysis setup (EQCM-D) provides a new methodology to explore complex surfactant adsorption processes. In this work, a uniform, rough, and well-covered model CaCO 3 surface was obtained on gold and platinum sensors to model carbonate rocks. This was achieved by the electrochemically formed hydroxide ions in the presence of bicarbonate and calcium ions, by which the controlled deposition of CaCO 3 resulted in sensor surface coverages in the range 35− 40%. Before using the deposited CaCO 3 surfaces, the adsorption of anionic surfactant alcohol alkoxy sulfate (AAS) on a smooth commercially available CaCO 3 surface was studied with varying CaCl 2 concentrations. For the first time, the structure and characteristics of the formed AAS layer were quantitatively described, indicating the formation of an incomplete bilayer. Compared to the smooth CaCO 3 surface, an increase in the frequency shift from 5 to 15 times was observed in sensors covered with rough CaCO 3 deposit. This observation was primarily attributed to the rougher surfaces that possess more adsorption sites for AAS binding and also to the effect of liquid trapping, inducing additional frequency shifts. The obtained results show that surfactant adsorption on rough surfaces was vastly different from that on smooth surfaces, and they provide a better understanding of the adsorption behavior of surfactants to mineral surfaces.

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Surface roughness of rocks had a significant influence on surfactant adsorption in enhanced oil recovery (EOR), both in terms of the total amount adsorbed as well as of the kinetics of adsorption. Combining electrochemical techniques and quartz crystal microbalance with dissipation monitoring (QCM) into one analysis setup opens up new avenues for depositing model rock surfaces and investigating the adsorption behavior. Using electrochemically assisted deposition, uniform and well-covered metal-CaCO3 sensors were obtained to simulate rough carbonate rocks and characterized by scanning electron microscope with energy dispersive X-ray analysis (SEM-EDX). The deposition process was controlled by the nitrate and oxygen electroreduction reactions in the presence of bicarbonate and calcium ions. The deposited mass of CaCO3 was calculated and the coverages for Au-CaCO3 and Pt-CaCO3 sensors were between 20 - 60%. It is observed that mostly cubic-like CaCO3 crystals were formed with crystal sizes around 20 to 50 µm from the SEM micrographs. The bigger crystals were surrounded by bare regions of Pt surface, suggesting the existence of Ostwald ripening process. Prior to the investigation of the deposited CaCO3 surfaces, the adsorption of anionic surfactant alcohol alkoxy sulfate (AAS) was studied on a smooth commercial CaCO3 surface with varying pH and CaCl2concentrations using QCM. Subsequently, surfactant adsorption was performed on the rough deposited CaCO3 surfaces and their adsorption behavior were compared. On a smooth CaCO3 surface, a fast adsorption of AAS surfactant was observed, whereas the desorption process was characterized as a two-step process. Compared to the smooth CaCO3surface, an increase of the frequency shift of about 5 times was observed on the deposited CaCO3 surfaces. This observation was mainly ascribed to the rougher surfaces, having more adsorption sites for AAS binding, and also the liquid trapping effect, resulting in more frequency shifts. It is suggested that a rough model mineral surface could be a better representation of a rock surface, presenting the implications of the new understanding for surfactant adsorption on different rock surfaces in EOR.