Calcium carbonate (CaCO₃) deposition plays a significant role in processes such as scale formation in power plants and in oil or gas production wells. The development of appropriate methods based on well suitable in situ sensors is important to evaluate and predict the deposition process. In this study, a combination of electrochemical techniques and quartz crystal microbalance with dissipation monitoring (QCM-D) in one analysis setup (EQCM-D) was used for the first time to monitor the CaCO₃ deposition in real time and provide kinetic details of the CaCO₃ deposition process. Through recording the frequency change of quartz crystal sensors, it allows us to perform a quantitative analysis of the morphology, coverage, deposition rate, and mass changes with nanogram sensitivity. By varying the applied voltage, it was found that a lower applied voltage resulted in more deposition of CaCO₃ mass and increase of the thickness of the deposited layer. Under the absence of flow, the CaCO₃ growth rate switched from accelerating to decelerating and this point is characterized by an inflection point (IP). A lower applied voltage resulted in a lower IP. Increasing Ca²⁺ and HCO₃⁻ concentrations, both the deposited amount of CaCO₃ mass and coating thickness increased correspondingly. With the addition of 50 mM Mg²⁺, a reduction in the deposition rate of CaCO₃ as high as 73% was achieved. The higher the Mg²⁺ concentration, the larger the deposition rate reduction, which was attributed to the incorporation of Mg²⁺ into the growing CaCO₃ mineral, resulting in the reduction of growth sites (inhibiting effect). The obtained results contribute to a better understanding of electrochemically induced CaCO₃ deposition and provide valuable insights into the determination of optimal precipitation parameters, with the aim to optimize industry scaling and anti-scaling processes.

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 CaCO3 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 CaCO3 resulted in sensor surface coverages in the range 35-40%. Before using the deposited CaCO3 surfaces, the adsorption of anionic surfactant alcohol alkoxy sulfate (AAS) on a smooth commercially available CaCO3 surface was studied with varying CaCl2 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 CaCO3 surface, an increase in the frequency shift from 5 to 15 times was observed in sensors covered with rough CaCO3 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.

Surfactant losses by adsorption to rock surfaces make surfactant-based enhanced oil recovery economically less feasible. We investigated polyacrylate (PA) as a sacrificial agent in the reduction of anionic surfactant adsorption with focus on calcite surfaces by using quartz crystal microbalance with dissipation monitoring. It was found that the adsorption of the anionic surfactant alcohol alkoxy sulfate (AAS) followed a Langmuir adsorption isotherm, and the adsorbed amount reached saturation above its critical micellar concentration. Adsorption of PA was a much slower process compared to AAS adsorption. Increasing the calcium ion concentration also increased the amount of AAS adsorbed as well as the mass increase rate of PA adsorption. Experimental results combined with density functional theory calculations indicated that calcium cation bridging was important for anionic surfactant AAS and PA adsorption to calcite surfaces. To effectively reduce the amount of surfactant adsorption, it was needed to preflush with PA, rather than by a simultaneous injection. Preflushing with 30 ppm of PA gave a reduction of AAS adsorption of 30% under high salinity (HS, 31,800 ppm) conditions, compared to 8% reduction under low salinity (LS, 3180 ppm) conditions. In the absence of PA, the amount of adsorbed AAS was reduced by already 50% upon changing from HS to LS conditions. Lower calcium ion concentrations, as under LS conditions, contributed to this observation. On different mineral surfaces, PA reduced the AAS adsorption in the order of alumina > calcite > silica. These results offer important insights into mitigating surfactant adsorption using PA polyelectrolyte as sacrificial agent and contribute to improved flooding strategies with reduced surfactant loss.

We have investigated the conditions of colloidal stability of silica nanoparticles smaller than 100 nm for their applications in enhanced oil recovery (EOR), especially pertaining to chemical flooding processes. Using zeta sizer and dynamic light scattering techniques, the stability of silica nanoparticle (SNP) dispersions has been investigated by variation of the pH, composition of salt solutions, addition of surfactants and polyelectrolytes. Such conditions can be encountered in oil reservoirs. It was found that changing pH from 5 to 10 had a negligible effect on the size of SNPs, whereas its zeta potential increased with increasing pH. Aggregation of SNPs is a partially reversible process for low degrees of aggregation in 500 mM NaCl, whereas observed strong aggregation in 1000 mM NaCl was irreversible. A critical aggregation concentration (CAC) was defined for the different salts investigated, above which the SNP dispersion became unstable at a fixed pH of 9.5. The CAC for NaCl was approximately 200 times higher than for CaCl₂ and MgCl₂. Our observations could not be explained completely by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Therefore, we have included non-DLVO interactions such as cation bridging, hydration forces, and steric effects. The additional presence of anionic alcohol alkoxy sulfate (AAS) surfactant slightly destabilized the SNP solution, but by the addition of polyacrylate (PA) was effectively stabilized. With increasing PA concentration, the CAC for both CaCl₂ and MgCl₂ increased. Upon addition of 100 ppm PA, the CAC increased by a factor of five compared to the situation in the absence of PA. Reducing the solution pH below 8.5, SNP can be stabilized in higher salinity in the presence of PA. The obtained results contribute to a better fundamental understanding of the SNP stability mechanism and a guide to optimize the SNP injection process with EOR chemicals.