Jan Vinogradov

Although CO₂ geological storage (CGS) is thought to be one of the most promising technologies to sequester the anthropogenic CO₂ to mitigate the climate change, implementation of the method is still challenging due to lack of fundamental understanding of controls of wettability, which is responsible for residual trapping of the gas and its flow dynamics. One of the key parameters that controls the wetting state is the zeta potential, ζ, at rock‐water and CO₂‐water interfaces. ζ in systems comprising rocks, carbonated aqueous solutions and immiscible supercritical CO₂ have not been measured prior to this study, where we detail the experimental protocol that enables measuring ζ in such systems, and report novel experimental data on the multi‐phase ζ. We also demonstrate for the first time that ζ of supercritical CO₂‐water interface is negative with a magnitude greater that 14 mV. Moreover, our experimental results suggest that presence of multi‐valent cations in tested solutions causes a shift of wettability toward intermediate‐wet state. We introduce a new parameter that combines multi‐phase ζ and relative permeability endpoints to characterize the wetting state and residual supercritical CO₂ saturation. Based on these results, we demonstrate that ζ measurements could serve as a powerful experimental method for predicting CGS efficiency and/or for designing injection of aqueous solutions with bespoke composition prior to implementing CGS to improve the residual CO₂ trapping in sandstone formations.

Scaling ionic charges has become an alternative to polarizable force fields for representing indirect charge transfer effects in molecular simulations. In our work, we apply molecular dynamics simulations to investigate the properties of NaCl aqueous solutions in homogeneous and confined media. We compare classical integer- and scaled-charge force fields for the ions. In the bulk, we validate the force fields by computing equilibrium and transport properties and comparing them with experimental data. Integer-charge ions overestimate dielectric saturation and ionic association. Both force fields present an excess in ion–ion correlation, which leads to a deviation in the ionic conductivity at higher ionic strengths. Negatively charged quartz is used to simulate the confinement effect. Electrostatic interactions dominate counter-ion adsorption. Full-charge ions have stronger and more defined adsorption planes. We obtain the electroosmotic mobility of the solution by combining the shear plane location from non-equilibrium simulations with the ionic distribution from equilibrium simulations. From the Helmholtz–Smoluchowski equation, the zeta potential and the streaming potential coupling coefficient are computed. From an atomic-scale perspective, our molecular dynamics simulations corroborate the hypothesis of maximum packing of the Stern layer, which results in a stable and non-zero zeta potential at high salinity. The scaled-charge model representation of both properties is in excellent qualitative and quantitative agreement with experimental data. With our work, we demonstrate how useful and precise simple scaled-charge models for electrolytes can be to represent complex systems, such as the electrical double layer.

CO₂ geological storage (CGS) is suggested to be one of the most promising technologies to sequester anthropogenic CO₂ to mitigate climate change. However, implementation of the method is still challenging due to lack of fundamental understanding of controls of wettability, which is responsible for residual trapping of the gas and its flow dynamics. Thermodynamics of wettability is described by a modified Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, which is expanded to include the structural forces. However, due to lack of experimental data the model could not be accurately used for CGS systems, and previously published papers invoked multiple assumptions and used input parameters with a broad range of variation and sometimes contrasting values. In this study, all available data at CGS conditions were collated, the model was adjusted accordingly, a rigorous sensitivity analysis was performed and the produced results were compared against available experimental data. Furthermore, for the first the model was updated using recently published new insights into critical interfacial properties, thus improving our understanding of the CGS process. The modelled results helped in explaining the observations and shed light on physical mechanisms responsible for wettability variation within the range of tested parameters.

Zeta potential is an important interfacial property that controls electrostatic interactions between mineral, water, and non-aqueous phase fluids. These interactions play an important role in defining the wetting state of reservoir rocks and transport of ionic species through porous media. The zeta potential is shown to be an efficient means for a broad range of applications including monitoring of single- and multi-phase flows in subsurface settings, characterization of fracture networks, efficiency of CO2 sequestration, hydrogen underground storage and enhanced oil recovery. It is widely agreed that the zeta potential in carbonate rocks is controlled by the concentration of potential determining ions (PDI), but the understanding of the underlying mechanisms is still limited as there are little experimental data on quantitative characterization of the dependence of the zeta potential on concentration of negative potential determining ions (PDI) such as SO42-, CO32-, HCO3-, especially when their concentration is high and exceeds that of the positive PDIs.In this study, the streaming potential method is used to investigate the zeta potential of natural carbonate rock samples in contact with natural aqueous solutions of low-to-high ionic strength and with varying concentration of sulphate (SO42-) and carbon (C4) related (HCO3-, CO32-) ions. In each set of experiments the total ionic strength was kept constant to eliminate the impact of concentration on the zeta potential so that the increasing/decreasing concentration of negative PDI was adjusted by decreasing/increasing concentration of indifferent Cl- ions. The study probed the concentration of negative PDIs that has never been reported before, with their respective lowest concentration consistent with previously reported equilibrium values, and the highest concentration being equal to the maximum achievable through stripping the tested solutions of Cl-.Our results demonstrate for the first time that magnitude of the negative zeta potential increases linearly with log10 of C4 concentration, however its dependence on the log10 of SO42- concentration is non-linear suggesting varying mechanisms of this PDI’s specific adsorption. Moreover, the results demonstrate that the zeta potential strongly depends on the total ionic strength, interpreted from slopes of the linear regressions for each negative PDI in different background solution. This observation suggests that equilibrium constants of negative PDI specific adsorption may be affected by the total ionic strength. Our findings improve the current understanding of the complex physicochemical processes that take place at calcite-water interface and provide important experimental data for surface complexation modelling of carbonate-brine systems.

Measurements of the zeta potential using streaming potential method are frequently used to characterise flows in subsurface settings owing to a broad range of applications of this petrophysical property; examples include CO2 geological storage, hydrocarbon reservoirs, geothermal sources and freshwater aquifers. Many experimental studies of the zeta potential have been carried out covering a wide range of parameters including different rock mineralogy, brine concentration and composition, and temperature to understand the impact of each parameter. The capability of the streaming potential method to be used on intact rock samples, single-/ and multi-phase flows, wide range of salinity, pressure and temperature makes the method suitable for representation of typical subsurface conditions. However, none of previous studies reported high multi-phase measurements at high pressure conditions typical for deep reservoirs. To adequately represent subsurface conditions of carbon geological storage sites, the minimum experimental pressure of 7.38 MPa and minimum temperature of 31 °C, consistent with the supercritical-CO2 (scCO2), need to be used. Obtaining stable measurements of the streaming potential under these conditions is extremely challenging. We report a detailed design of a high-pressure experimental system and experimental protocol for multi-phase streaming potential measurements that were carried out on scCO2-sandstone-brine systems at temperature of 40 °C, pressures ≤10 MPa and with a variety of aqueous solutions.The obtained results demonstrate for the first time that the multi-phase zeta potential correlates with the measured scCO2 residual saturation and rock’s wetting state interpreted from other parameters. Moreover, our results unambiguously identify for the first time the polarity and likely magnitude of the scCO2-brine interfacial zeta potential. Our findings improve the current understanding of the complex wetting behaviour of scCO2 and provide important experimental data for numerical (surface complexation, molecular dynamics), analytical (DLVO) or combined models.

The zeta potential is a measure of electric potential at the mineral‐electrolyte interfaces. The zeta potential of natural sandstones depends on mineralogy, electrolyte pH, concentration, composition, amount of dissolved CO₂, and temperature. We report for the first time the zeta potential measured on clayey sandstone comprising quartz, kaolinite, illite, albite and microcline saturated with NaCl solutions at supercritical CO₂ conditions. Our results demonstrate that zeta potentials in clayey sandstone samples at supercritical CO₂ conditions are significantly different from similar measurements conducted under ambient conditions and from those obtained with clean sandstones. Supercritical CO₂ zeta potential remains negative but is influenced by clays and feldspars due to their significant presence and exposure to large pores, which yields less negative zeta potential compared to quartz, under identical conditions. Our results have significant implications to natural subsurface systems such as CO₂ geo‐sequestration sites, aquifers, geothermal sources and hydrocarbon reservoirs.