The effect of ionic strength on oil adhesion in sandstone--the search for the low salinity mechanism

Application of Low-Salinity Waterflooding in Heavy Oil Sandstone Reservoir: Oil Recovery Efficiency and Mechanistic Study

Low-salinity water injection (LSWI) is a recently emerged and promising technique to enhance oil recovery. In addition, it is attractive due to its relatively low-cost, environmental friendliness, and sustainability. However, the underlying mechanisms remain unclear, and very limited research has been conducted on heavy oil. To verify the feasibility of injecting a low-salinity aquifer water (LSAW) to improve the oil recovery of our target offshore heavy oil reservoir, first, a series of experiments on the core scale, including coreflooding and spontaneous imbibition experiments, were carried out. Furthermore, atomic force microscopy (AFM), contact angle, zeta potential measurement, as well as disjoining pressure calculations based on the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory were carried out to explore the underlying governing mechanism at the microscopic scale. The secondary oil recovery factors of the coreflood tests are 67.11, 70.55, and 77.18% for seawater (SW), produced water (PW), and LSAW, respectively. The additional oil recoveries by LSAW when injected in tertiary modes are 6.38% after SW injection and 5.68% after PW injection. These results indicate that compared with SW and PW which have high brine salinity, the low-salinity brine from the subsurface aquifer (LSAW) can improve oil recovery in both secondary and tertiary modes. In addition, the oil recovery factors from the spontaneous imbibition tests (27.52% by LSAW, 17.32% by PW, and 14.00% by SW) and the insignificant variation of IFTs among the three brines lead to the anticipation that the LSAW can alter the rock to a more water-wet condition compared with SW and PW, thereby giving rise to a higher oil recovery factor in the coreflooding tests. By using AFM imaging and contact angle tests, we proved that the polar asphaltene could desorb from the rock surface and consequently reduce the water contact angle substantially when subjected to low-salinity brine. Furthermore, the zeta potential and the disjoining pressure results indicate that a more repulsive force was developed between oil and the rock under the low-salinity environment, which thereby promotes asphaltene desorption and consequent wettability alteration. Our work has paved the way to apply LSWI to the offshore heavy oil sandstone reservoir.

Reuse of polymeric waste for the treatment of marine water polluted by diesel

Accidental diesel spills can occur in marine environments such as harbors, leading to adverse effects on the environmental compartment and humans. This study proposes the surgical mask as an affordable and sustainable adsorbent for the remediation of diesel-contaminated seawater to cope with the polymeric waste generated monthly in hospital facilities. This approach can also be helpful considering a possible future pandemic, alleviating the pressure on the waste management system by avoiding improper mask incineration and landfilling, as instead occurred during the previous COVID-19. Batch adsorption-desorption experiments revealed a complete diesel removal from seawater after 120 min with the intact laceless mask, which showed an adsorption capacity of up to 3.43 g/g. The adsorption curve was better predicted via Weber and Morris's kinetic (R2 = 0.876) and, in general, with Temkin isotherm (R2 = 0.965-0.996) probably due to the occurrence of chemisorption with intraparticle diffusion as one of the rates-determining steps. A hysteresis index of 0.23-0.36 was obtained from the desorption isotherms, suggesting that diesel adsorption onto surgical masks was faster than the desorption mechanism. Also, the effect of pH, ionic strength and temperature on diesel adsorption was examined. The results from the reusability tests indicated that the surgical mask can be regenerated for 5 consecutive cycles while decreasing the adsorption capacity by only approximately 11%.

Exploring carbonate rock wettability across scales: role of (bio)minerals

HYPOTHESIS

The wettability of carbonate rocks is expected to be affected by the organic components of biominerals which are complex, nanostructured organo-mineral assemblages. Elucidating the nanoscale mechanisms driving the wettability of solid surfaces will enable a better understanding of the role of biominerals in the wetting properties of carbonate rocks to control various geological, environmental and industrial processes.

EXPERIMENTS

Using Atomic Force Microscopy and Spectroscopy (AFM/AFS) we probed the wettability properties of carbonate rocks with different amounts of organic material. The adhesion properties of two types of limestones were determined in liquid environments at different length scales (nm to mm) using functionalized tips with different chemical groups to determine the extent of surface hydrophobic and hydrophilic organo-mineral interactions.

FINDINGS

We observed homogeneous hydrophobic areas at length scales below < 5 µm. The origin of this hydrophobicity is linked to the presence of organics, whose amount and spatial distribution depend on the rock composition. Specifically, our results reveal that the biogenic vs non-biogenic origin of the mineral grains is the main rock property controlling the wettability of the solid surface. Overall, our methodology offers a multi-scale approach to unravel the role that organic moieties and biominerals play in controlling the wettability of rock-water interfaces.

Investigation of crude oil properties impact on wettability alteration during low salinity water flooding using an improved geochemical model

In low-salinity water flooding (LSWF), modifying the injected brine composition leads to greater oil recovery from carbonate reservoirs. The processes that control improved recovery during LSWF are not totally clear, which could lead to ambiguities in finding optimum brine composition regarding wettability alteration (WA) toward water wetness. One of the methods to determine WA is bound product sum (BPS) calculation using geochemical tools. In the case of wettability improvement, the BPS value of a crude oil-brine-rock (COBR) system should be at its minimum value. In this study, an improved geochemical model is developed, which includes the effects of oil composition (i.e., acid number, base number, and weight percent of nonhydrocarbon components) and physical properties of oil (i.e., density, viscosity, and solution gas-oil ratio) on COBR interactions. The proposed method generates BPS as a function of temperature, pressure, oil and brine composition, and pH for carbonate rocks. The model applicability was validated using several experimental data sets available in the literature. The results of the improved BPS model were in line with the results of contact angle and zeta potential measurements as the major indices of rock wettability. BPS calculations using the available geochemical tools sometimes failed to predict the correct WA trend since they overlooked the impact of oil properties on COBR interactions. The model predictability was also compared with the results of an available geochemical tool, PHREEQC, and the results demonstrate just how important the effect of oil properties and composition inclusion on wettability determination is. The improved BPS approach could be successfully utilized as an optimization tool to optimize the water composition during LSWF for a given COBR system.

Surface Complexation Modelling of Wettability Alteration during Carbonated Water Flooding

CO2 capture and utilization is an effective tool in reducing greenhouse gas emissions and hence, combating global warming. In the present study, surface complexation modeling (SCM) with the geochemistry solver, PHREEQ-C, was utilized to predict the wettability alteration of minerals, sandstone reservoir rocks (SRR), and pseudo-sandstone rocks (PSR) and mineral mixtures during carbonated water (CW) injection. The bond products, which is defined as the product of the mole fraction of oppositely charged mineral and oil surfaces, were calculated to estimate the wettability preferences. For the studied fluid systems, the results from SCM predicted that albite and quartz minerals were strongly water-wet while calcite was strongly oil-wet with formation water (FW). When it came to clay minerals, illite and montmorillonite were more oil-wet than quartz and less oil-wet than calcite. During CW injection (CWI), the wettability preferences of dominant minerals (considering weight and surface area) in SRR (i.e., quartz and calcite) were changed toward more water-wet, while for the clay minerals, the result was the opposite. The results from SCM showed that the wettability preferences of SRR were water-wet in both CW and FW. Moreover, increasing the amount of the water-wet minerals in mineral mixtures increased the rock’s tendency to become more water-wet.

Relationship Between Zeta Potential and Wettability in Porous Media: Insights From a Simple Bundle of Capillary Tubes Model

HYPOTHESIS & MOTIVATION

Experimental data suggest a relationship between the macroscopic zeta potential measured on intact rock samples and the sample wettability. However, there is no pore-scale model to quantify this relationship.

METHODS

We consider the simplest representation of a rock pore space: a bundle of capillary tubes of varying size. Equations describing mass and charge transfer through a single capillary are derived and the macroscopic zeta potential and wettability determined by integrating over capillaries. Model predictions are tested against measured data yielding a good match.

FINDINGS

Mixed- and oil-wet models return a macro-scale zeta potential that is a combination of the micro-scale zeta potential of mineral-brine and oil-brine interfaces and the relationship between macro-scale zeta potential and water saturation exhibits hysteresis. The model predicts a similar relationship between zeta potential and wettability to that observed in experimental data but does not provide a perfect match. Fitting the model to experimental data allows the oil-brine zeta potential to be estimated at conditions where it cannot be measured directly. Results suggest that positive values of the oil-brine zeta potential may be more common than previously thought with implications for surface complexation models and the design of controlled salinity waterflooding of oil reservoirs.

Visualizing and Quantifying Wettability Alteration by Silica Nanofluids

An aqueous suspension of silica nanoparticles or nanofluid can alter the wettability of surfaces, specifically by making them hydrophilic and oil-repellent under water. Wettability alteration by nanofluids has important technological applications, including for enhanced oil recovery and heat transfer processes. A common way to characterize the wettability alteration is by measuring the contact angles of an oil droplet with and without nanoparticles. While easy to perform, contact angle measurements do not fully capture the wettability changes to the surface. Here, we employed several complementary techniques, such as cryo-scanning electron microscopy, confocal fluorescence and reflection interference contrast microscopy, and droplet probe atomic force microscopy (AFM), to visualize and quantify the wettability alterations by fumed silica nanoparticles. We found that nanoparticles adsorbed onto glass surfaces to form a porous layer with hierarchical micro- and nanostructures. The porous layer can trap a thin water film, which reduces contact between the oil droplet and the solid substrate. As a result, even a small addition of nanoparticles (0.1 wt %) lowers the adhesion force for a 20 μm sized oil droplet by more than 400 times from 210 ± 10 to 0.5 ± 0.3 nN as measured by using droplet probe AFM. Finally, we show that silica nanofluids can improve oil recovery rates by 8% in a micromodel with glass channels that resemble a physical rock network.

Analysis of streaming potential flow and electroviscous effect in a shear-driven charged slit microchannel

Investigating the flow behavior in microfluidic systems has become of interest due to the need for precise control of the mass and momentum transport in microfluidic devices. In multilayered-flows, precise control of the flow behavior requires a more thorough understanding as it depends on multiple parameters. The following paper proposes a microfluidic system consisting of an aqueous solution between a moving plate and a stationary wall, where the moving plate mimics a charged oil-water interface. Analytical expressions are derived by solving the nonlinear Poisson-Boltzmann equation along with the simplified Navier-Stokes equation to describe the electrokinetic effects on the shear-driven flow of the aqueous electrolyte solution. The Debye-Huckel approximation is not employed in the derivation extending its compatibility to high interfacial zeta potential. Additionally, a numerical model is developed to predict the streaming potential flow created due to the shear-driven motion of the charged upper wall along with its associated electric double layer effect. The model utilizes the extended Nernst-Planck equations instead of the linearized Poisson-Boltzmann equation to accurately predict the axial variation in ion concentration along the microchannel. Results show that the interfacial zeta potential of the moving interface greatly impacts the velocity profile of the flow and can reverse its overall direction. The numerical results are validated by the analytical expressions, where both models predicted that flow could reverse its overall direction when the interfacial zeta potential of the oil-water is above a certain threshold value. Finally, this paper describes the electroviscous effect as well as the transient development of electrokinetic effects within the microchannel.

Insights into Nanoscale Wettability Effects of Low Salinity and Nanofluid Enhanced Oil Recovery Techniques

In this study, enhanced oil recovery (EOR) techniques—namely low salinity and nanofluid EOR—are probed at the nanometer-scale using an atomic force microscope (AFM). Mica substrates were used as model clay-rich rocks while AFM tips were coated to present alkyl (-CH3), aromatic (-C6H5) and carboxylic acid (-COOH) functional groups, to simulate oil media. We prepared brine formulations to test brine dilution and cation bridging effects while selected concentrations (0 to 1 wt%) of hydrophilic SiO2 nanoparticles dispersed in 1 wt% NaCl were used as nanofluids. Samples were immersed in fluid cells and chemical force mapping was used to measure the adhesion force between polar/non-polar moieties to substrates. Adhesion work was evaluated based on force-displacement curves and compared with theories. Results from AFM studies indicate that low salinity waters and nanoparticle dispersions promote nanoscale wettability alteration by significantly reducing three-phase adhesion force and the reversible thermodynamic work of adhesion, also known as adhesion energy. The maximum reduction in adhesion energy obtained in experiments was in excellent agreement with existing theories. Electrostatic repulsion and reduced non-electrostatic adhesion are prominent surface forces common to both low salinity and nanofluid EOR. Structural forces are complex in nature and may not always decrease total adhesion force and energy at high nanoparticle concentration. Wettability effects also depend on surface chemical groups and the presence of divalent Mg2+ and Ca2+ cations. This study provides fresh insights and fundamental information about low salinity and nanofluid EOR while demonstrating the application of force-distance spectroscopy in investigating EOR techniques.

Effect of Divalent Cations on the Interaction of Carboxylate Self-Assembled Monolayers

Interactions between organic molecules in aqueous environments, whether in the fluid phase or adsorbed on solids, are often affected by the cations present in the solution. We investigated, at nanometer scale, how surface carboxylate interactions are influenced by dissolved divalent cations: Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺. Self-assembled monolayer (SAM) surfaces with exposed terminations of alkyl, -CH₃, carboxylate, -COO^- , or dicarboxylate, -DiCOO^-, were deposited on gold-coated tips and substrates. We used atomic force microscopy (AFM), in chemical force mapping (CFM) mode, to measure adhesion forces between various combinations of SAMs on the tip and substrate, in solutions of 0.5 M NaCl, that contained 0.012 M of one of the divalent cations. The type of cation, the number of carboxyl groups that interact, and their structure on the SAM influenced adhesion between the surfaces. The effect of the reference solution, which only contains Na⁺ cations, on adhesion force was mainly attributed to van der Waals and hydrophobic forces, explaining the lower force in systems that are more hydrophilic, i.e., -COO^--COO^-, and higher force for more hydrophobic systems. For charged surfaces, i.e., -COO^- and -DiCOO^-, in divalent cation solutions results were consistent with ion bridging. The inclusion of a hydrophobic surface, i.e., the -CH₃-COO^- or -CH₃-DiCOO^- system, decreased the possibility for strong cation bridging with the charged surface, resulting in lower adhesion. For systems including -COO^-, the adhesion force series followed the inverse cation hydrated radius trend (Na⁺ ≈ Mg²⁺ < Sr²⁺ < Ca²⁺ < Ba²⁺) whereas -DiCOO^- was responsible for lower adhesion force and modified trends, depending on the corresponding surface in the system. Differences in force magnitude between the monolayers were correlated with lower charge availability on the -DiCOO^- surface as a result of fewer active sites, probably because of the tendency of exposed malonate surface groups to interact between them, as well as high rigidity, resulting from the molecule structure. The characteristic response of the -DiCOO^- surface in solutions of Sr²⁺ and Ca²⁺ was correlated with possible malonate complexation modes. Comparison with previous studies suggested that the strong response of a -DiCOO^- surface to Sr²⁺ resulted from bidentate chelation, whereas Ca²⁺ response was attributed to alpha-mode association to malonate.

Improved DNA straightening and attachment via optimal Mg2+ ionic bonding under electric field for AFM imaging in liquid phase

In this research, a novel method is proposed to improve DNA straightening under an applied electric field to facilitate imaging in a liquid phase by modifying the substrate with varying Mg²⁺ ion concentrations. A two-dimensional network of DNA structures was successfully stretched on Mg²⁺-modified mica substrates under a DC electric field (1 V, 1 A) and imaged in gaseous and aqueous phases by atomic force microscopy. The results revealed that an optimum concentration of Mg²⁺ ion (4.17 μmol/ml) allowed DNA straightening under an electric field, thus facilitating its imaging in the liquid phase. Furthermore, DNA adhesion under different concentrations of Mg²⁺ was measured and a maximum adhesion force of 76.19 pN was achieved. This vital work has great potential in gene knockout and targeted gene editing.

Competitive effects of interfacial interactions on ion-tuned wettability by atomic simulations

The dependence of wettability on brine ionic composition in organic-brine-mineral systems, which is denoted as ion-tuned wettability in this paper, has important industrious applications but is still not well understood. The dominant mechanisms and their relative importance are still under debate. This paper uses molecular dynamics to study three possible mechanisms of ion-tuned wettability in an oil-brine-quartz system, including electrical double layer (EDL) repulsion, cation bridging, and hydration repulsion. We compare the contact angle and COO^- distribution of the molecular system under different interface charging conditions and the contact angle predicted by EDL repulsion theory. The results indicate the existence of Ca²⁺ bridging and K⁺ bridging, and that medium ionic strength favors the form of K⁺ bridging most. The three mechanisms are all proved to have impact on wettability, of which Ca²⁺ bridging is the strongest, EDL repulsion and hydration repulsion the weaker, K⁺ bridging the weakest. Based on the results, we suggest that all the three mechanisms should be evaluated to predict the ion-tuned wettability, and conclude several possible brine-modifying strategies to make sandstone reservoir more water-wet.

Brine-Dependent Recovery Processes in Carbonate and Sandstone Petroleum Reservoirs: Review of Laboratory-Field Studies, Interfacial Mechanisms and Modeling Attempts

Brine-dependent recovery, which involves injected water ionic composition and strength, has seen much global research efforts in the past two decades because of its benefits over other oil recovery methods. Several studies, ranging from lab coreflood experiments to field trials, indicate the potential of recovering additional oil in sandstone and carbonate reservoirs. Sandstone and carbonate rocks are composed of completely different minerals, with varying degree of complexity and heterogeneity, but wettability alteration has been widely considered as the consequence rather than the cause of brine-dependent recovery. However, the probable cause appears to be as a result of the combination of several proposed mechanisms that relate the wettability changes to the improved recovery. This paper provides a comprehensive review on laboratory and field observations, descriptions of underlying mechanisms and their validity, the complexity of the oil-brine-rock interactions, modeling works, and comparison between sandstone and carbonate rocks. The improvement in oil recovery varies depending on brine content (connate and injected), rock mineralogy, oil type and structure, and temperature. The brine ionic strength and composition modification are the two major frontlines that have been well-exploited, while further areas of investigation are highlighted to speed up the interpretation and prediction of the process efficiency.

Specific Ion Effects on the Interaction of Hydrophobic and Hydrophilic Self-Assembled Monolayers

Interactions between mineral surfaces and organic molecules are fundamental to life processes. The presence of cations in natural environments can change the behavior of organic compounds and thus alter the mineral-organic interfaces. We investigated the influence of Na⁺, Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ on the interaction between two models, self-assembled monolayers, that were tailored to have hydrophobic -CH₃ or hydrophilic -COO(H) terminations. Atomic force microscopy in chemical force mapping mode, where the tips were functionalized with the same terminations, was used to measure adhesion forces between the tip and substrate surfaces, to gather fundamental information about the role of these cations in the behavior of organic compounds and the surfaces where they adsorb. Adhesion force between hydrophobic surfaces in 0.5 M NaCl solutions that contained 0.012 M divalent cations did not change, regardless of the ionic potential, that is, the charge per unit radius, of the cation. For systems where one or the other surface was functionalized with carboxylate, -COO(H), mostly in its deprotonated form, -COO^-, a reproducible change in the adhesion force was observed for each of the ions. The trend of increasing adhesion force followed the pattern: Na⁺ ≈ Mg²⁺ < Sr²⁺ < Ca²⁺ < Ba²⁺, suggesting that ionic potential, thus hydrated radius, controls the interaction. The presence of a -CH₃ surface in the asymmetric system leads to lower adhesion forces than in the hydrophilic system, whereas the ionic trend remains the same. Although specific ion effects are felt in both systems, the lower adhesion force in the asymmetric system, compared with the hydrophilic system, implies that the -CH₃ surface plays an important role.

Controlling wetting with electrolytic solutions: Phase-field simulations of a droplet-conductor system

The wetting properties of immiscible two-phase systems are crucial in applications ranging from laboratory-on-a-chip devices to field-scale oil recovery. It has long been known that effective wetting properties can be altered by the application of an electric field; a phenomenon coined as electrowetting. Here, we consider theoretically and numerically a single droplet sitting on an (insulated) conductor, i.e., within a capacitor. The droplet consists of a pure phase without solutes, while the surrounding fluid contains a symmetric monovalent electrolyte, and the interface between them is impermeable. Using nonlinear Poisson-Boltzmann theory, we present a theoretical prediction of the dependency of the apparent contact angle on the applied electric potential. We then present well-resolved dynamic simulations of electrowetting using a phase-field model, where the entire two-phase electrokinetic problem, including the electric double layers (EDLs), is resolved. The simulations show that, while the contact angle on scales smaller than the EDL is unaffected by the application of an electric field, an apparent contact angle forms on scales beyond the EDL. This contact angle relaxes in time towards a saturated apparent contact angle. The dependency of the contact angle upon applied electric potential is in good agreement with the theoretical prediction. The only phenomenological parameter in the prediction is shown to depend on the permeability ratio between the two phases. Based on the resulting unified description, we obtain an effective expression of the contact angle which can be used in more macroscopic numerical simulations, i.e. where the electrokinetic problem is not fully resolved.

Electrohydrodynamic channeling effects in narrow fractures and pores

In low-permeability rock, fluid and mineral transport occur in pores and fracture apertures at the scale of micrometers and below. At this scale, the presence of surface charge, and a resultant electrical double layer, may considerably alter transport properties. However, due to the inherent nonlinearity of the governing equations, numerical and theoretical studies of the coupling between electric double layers and flow have mostly been limited to two-dimensional or axisymmetric geometries. Here, we present comprehensive three-dimensional simulations of electrohydrodynamic flow in an idealized fracture geometry consisting of a sinusoidally undulated bottom surface and a flat top surface. We investigate the effects of varying the amplitude and the Debye length (relative to the fracture aperture) and quantify their impact on flow channeling. The results indicate that channeling can be significantly increased in the plane of flow. Local flow in the narrow regions can be slowed down by up to 5% compared to the same geometry without charge, for the highest amplitude considered. This indicates that electrohydrodynamics may have consequences for transport phenomena and surface growth in geophysical systems.

Oil Contact Angles in a Water-Decane-Silicon Dioxide System: Effects of Surface Charge

Oil wettability in the water-oil-rock systems is very sensitive to the evolution of surface charges on the rock surfaces induced by the adsorption of ions and other chemical agents in water flooding. Through a set of large-scale molecular dynamics simulations, we reveal the effects of surface charge on the oil contact angles in an ideal water-decane-silicon dioxide system. The results show that the contact angles of oil nano-droplets have a great dependence on the surface charges. As the surface charge density exceeds a critical value of 0.992 e/nm², the contact angle reaches up to 78.8° and the water-wet state is very apparent. The variation of contact angles can be confirmed from the number density distributions of oil molecules. With increasing the surface charge density, the adsorption of oil molecules weakens and the contact areas between nano-droplets and silicon dioxide surface are reduced. In addition, the number density distributions, RDF distributions, and molecular orientations indicate that the oil molecules are adsorbed on the silicon dioxide surface layer-by-layer with an orientation parallel to the surface. However, the layered structure of oil molecules near the silicon dioxide surface becomes more and more obscure at higher surface charge densities.

Optical characterization of surface adlayers and their compositional demixing at the nanoscale

Under ambient conditions, the behavior of a solid surface is often dominated by a molecularly thin adsorbed layer (adlayer) of small molecules. Here we develop an optical approach to unveil the nanoscale structure and composition of small-molecule adlayers on glass surfaces through spectrally resolved super-resolution microscopy. By recording the images and emission spectra of millions of individual solvatochromic molecules that turn fluorescent in the adlayer phase, we obtain ~30 nm spatial resolution and achieve concurrent measurement of local polarity. This allows us to establish that the adlayer dimensionality gradually increases through a sequence of 0D (nanodroplets), 1D (nano-lines), and 2D (films) for liquids of increasing polarity. Moreover, we find that in adlayers, a solution of two miscible liquids spontaneously demixes into nanodroplets of different compositions that correlate strongly with droplet size and location. We thus reveal unexpectedly rich structural and compositional behaviors of surface adlayers at the nanoscale.

Characterization of adsorbed molecular layers on surfaces is the key to wide-ranging applications, but elucidating the structure and composition of such adlayers remains challenging. Here the authors develop an approach to unveil the nanoscale structure and composition of adlayers through spectrally resolved super-resolution microscopy.

Observation of Dynamic Surfactant Adsorption Facilitated by Divalent Cation Bridging

Dynamic evidence of the mechanism for surfactant adsorption to surfaces of like charge has been observed. Additionally, removal and retention of surfactant molecules on the surface were observed as a function of time. A decrease in surface charge is observed when metal counterions are introduced and is dependent on charge density as well as valency of the metal ion. When surfactant species are also present with the metals, a dramatic increase in surface charge arises. We observed that the rate and quantity of surfactant adsorption can be controlled by the presence of divalent Ca²⁺. Under isotonic conditions the introduction of Ca²⁺ is also easily distinguishable from that of monovalent Na⁺ and provides dynamic evidence of the divalent "cation bridging" phenomenon. Dynamic changes to surface charge are experimentally determined by utilizing current monitoring to quantify the zeta potential in a microfluidic device.

Chemical Force Microscopy Study on the Interactions of COOH Functional Groups with Kaolinite Surfaces: Implications for Enhanced Oil Recovery

Clay–oil interactions play a critical role in determining the wettability of sandstone oil reservoirs, which, in turn, governs the effectiveness of enhanced oil recovery methods. In this study, we have measured the adhesion between –COOH functional groups and the siloxane and aluminol faces of kaolinite clay minerals by means of chemical force microscopy as a function of pH, salinity (from 0.001 M to 1 M) and cation identity (Na+ vs. Ca2+). Results from measurements on the siloxane face show that Ca2+ displays a reverse low-salinity effect (adhesion decreasing at higher concentrations) at pH 5.5, and a low salinity effect at pH 8. At a constant Ca2+ concentration of 0.001 M, however, an increase in pH leads to larger adhesion. In contrast, a variation in the Na+ concentration showed less effect in varying the adhesion of –COOH groups to the siloxane face. Measurements on the aluminol face showed a reverse low-salinity effect at pH 5.5 in the presence of Ca2+, whereas an increase in pH with constant ion concentration resulted in a decrease in adhesion for both Ca2+ and Na+. Results are explained by looking at the kaolinite’s surface complexation and the protonation state of the functional group, and highlight a more important role of the multicomponent ion exchange mechanism in controlling adhesion than the double layer expansion mechanism.

Anion Layering and Steric Hydration Repulsion on Positively Charged Surfaces in Aqueous Electrolytes

The molecular structure at charged solid/liquid interfaces is vital for many chemical or electrochemical processes, such as adhesion, catalysis, or the stability of colloidal dispersions. How cations influence structural hydration forces and interactions across negatively charged surfaces has been studied in great detail. However, how anions influence structural hydration forces on positively charged surfaces is much less understood. Herein we report force versus distance profiles on freshly cleaved mica using atomic force microscopy with silicon tips. We characterize steric anion hydration forces for a set of common anions (Cl^- , ClO₄^- , NO₃^- , SO₄^2- and PO₄^3- ) in pure acids at pH ≈1, where protons are the co-ions. Solutions containing anions with low hydration energies exhibit repulsive structural hydration forces, indicating significant ion and/or water structuring within the first 1-2 nm on a positively charged surface. We attribute this to specific adsorption effects within the Stern layer. In contrast, ions with high hydration energies show exponentially repulsive hydration forces, indicating a lower degree of structuring within the Stern layer. The presented data demonstrates that anion hydration forces in the inner double layer are comparable to cation hydration forces, and that they qualitatively correlate with hydration free energies. This work contributes to understanding interaction processes in which positive charge is screened by anions within an electrolyte.

Calcium-Mediated Adhesion of Nanomaterials in Reservoir Fluids

Globally, a small percentage of oil is recovered from reservoirs using primary and secondary recovery mechanisms, and thus a major focus of the oil industry is toward developing new technologies to increase recovery. Many new technologies utilize surfactants, macromolecules, and even nanoparticles, which are difficult to deploy in harsh reservoir conditions and where failures cause material aggregation and sticking to rock surfaces. To combat these issues, typically material properties are adjusted, but recent studies show that adjusting the dispersing fluid chemistry could have significant impact on material survivability. Herein, the effect of injection fluid salinity and composition on nanomaterial fate is explored using atomic force microscopy (AFM). The results show that the calcium content in reservoir fluids affects the interactions of an AFM tip with a calcite surface, as surrogates for nanomaterials interacting with carbonate reservoir rock. The extreme force sensitivity of AFM provides the ability to elucidate small differences in adhesion at the pico-Newton (pN) level and provides direct information about material survivability. Increasing the calcium content mitigates adhesion at the pN-scale, a possible means to increase nanomaterial survivability in oil reservoirs or to control nanomaterial fate in other aqueous environments.

Organic-Silica Interactions in Saline: Elucidating the Structural Influence of Calcium in Low-Salinity Enhanced Oil Recovery

Enhanced oil recovery using low-salinity solutions to sweep sandstone reservoirs is a widely-practiced strategy. The mechanisms governing this remain unresolved. Here, we elucidate the role of Ca²⁺ by combining chemical force microscopy (CFM) and molecular dynamics (MD) simulations. We probe the influence of electrolyte composition and concentration on the adsorption of a representative molecule, positively-charged alkylammonium, at the aqueous electrolyte/silica interface, for four electrolytes: NaCl, KCl, MgCl₂, and CaCl₂. CFM reveals stronger adhesion on silica in CaCl₂ compared with the other electrolytes, and shows a concentration-dependent adhesion not observed for the other electrolytes. Using MD simulations, we model the electrolytes at a negatively-charged amorphous silica substrate and predict the adsorption of methylammonium. Our simulations reveal four classes of surface adsorption site, where the prevalence of these sites depends only on CaCl₂ concentration. The sites relevant to strong adhesion feature the O⁻ silica site and Ca²⁺ in the presence of associated Cl⁻, which gain prevalence at higher CaCl₂ concentration. Our simulations also predict the adhesion force profile to be distinct for CaCl₂ compared with the other electrolytes. Together, these analyses explain our experimental data. Our findings indicate in general how silica wettability may be manipulated by electrolyte concentration.

Acid-Base Polymeric Foams for the Adsorption of Micro-oil Droplets from Industrial Effluents

Separation of toxic organic pollutants from industrial effluents is a great environmental challenge. Herein, an acid-base engineered foam is employed for separation of micro-oil droplets from an aqueous solution. In acidic or basic environments, acid-base polymers acquire surface charge due to protonation or dissociation of surface active functional groups. This property is invoked to adsorb crude oil microdroplets from water using polyester polyurethane (PESPU) foam. The physicochemical surface properties of the foam were characterized using X-ray photoelectron spectroscopy, inverse gas chromatography, electrokinetic analysis, and micro-computed tomography. Using the surface charge of the foam and oil droplets, the solution pH (5.6) for maximum separation efficacy was predicted. This optimal pH was verified through underwater wetting behavior and adsorption experiments. The droplet adsorption onto the foam was governed by physisorption, and the driving forces were attributed to electrostatic attraction and Lifshitz-van der Waals forces. The foam was regenerated and reused multiple times by simple compression. The lowest trace oil content in the retentate was 3.6 mg L^-1, and all oil droplets larger than 140 nm were removed. This work lays the foundation for the development of a new class of engineered foam adsorbents with the potential to revolutionize water treatment technologies.

Salinity-Dependent Contact Angle Alteration in Oil/Brine/Silicate Systems: the Critical Role of Divalent Cations

The effectiveness of water flooding oil recovery depends to an important extent on the competitive wetting of oil and water on the solid rock matrix. Here, we use macroscopic contact angle goniometry in highly idealized model systems to evaluate how brine salinity affects the balance of wetting forces and to infer the microscopic origin of the resultant contact angle alteration. We focus, in particular, on two competing mechanisms debated in the literature, namely, double-layer expansion and divalent cation bridging. Our experiments involve aqueous droplets with a variable content of chloride salts of Na⁺, K⁺, Ca²⁺, and Mg²⁺, wetting surfaces of muscovite and amorphous silica, and an environment of ambient decane containing small amounts of fatty acids to represent polar oil components. By diluting the salt content in various manners, we demonstrate that the water contact angle on muscovite, not on silica, decreases by up to 25° as the divalent cation concentration is reduced from typical concentrations in seawater to zero. Decreasing the ionic strength at a constant divalent ion concentration, however, has a negligible effect on the contact angle. We discuss the consequences for the interpretation of core flooding experiments and the identification of a microscopic mechanism of low salinity water flooding, an increasingly popular, inexpensive, and environment-friendly technique for enhanced oil recovery.

Zeta potential in oil-water-carbonate systems and its impact on oil recovery during controlled salinity water-flooding

Laboratory experiments and field trials have shown that oil recovery from carbonate reservoirs can be increased by modifying the brine composition injected during recovery in a process termed controlled salinity water-flooding (CSW). However, CSW remains poorly understood and there is no method to predict the optimum CSW composition. This work demonstrates for the first time that improved oil recovery (IOR) during CSW is strongly correlated to changes in zeta potential at both the mineral-water and oil-water interfaces. We report experiments in which IOR during CSW occurs only when the change in brine composition induces a repulsive electrostatic force between the oil-brine and mineral-brine interfaces. The polarity of the zeta potential at both interfaces must be determined when designing the optimum CSW composition. A new experimental method is presented that allows this. Results also show for the first time that the zeta potential at the oil-water interface may be positive at conditions relevant to carbonate reservoirs. A key challenge for any model of CSW is to explain why IOR is not always observed. Here we suggest that failures using the conventional (dilution) approach to CSW may have been caused by a positively charged oil-water interface that had not been identified.

Understanding the role of brine ionic composition on oil recovery by assessment of wettability from colloidal forces

The impact of injection brine salinity and ionic composition on oil recovery has been an active area of research for the past 25years. Evidence from laboratory studies and field tests suggests that implementing certain modifications to the ionic composition of the injection brine leads to greater oil recovery. The role of salinity modification is attributed to its ability to shift wettability of a rock surface toward water wetness. The amount of trapped oil released depends on the nature of rock, oil, and brine surface interactions. Reservoir rocks exhibit different affinities to fluids. Carbonates show stronger adsorption of oil films as opposed to the strongly water-wet and mixed-wet sandstones. The concentration of divalent ions and total salinity of the injection brine are other important factors to consider. Accordingly, this paper provides a review of laboratory and field studies of the role of brine composition on oil recovery from carbonaceous rock as well as rationalization of results using DLVO (Derjaguin, Landau, Verwey and Overbeek) theory of surface forces. DLVO evaluates the contribution of each component of the oil/brine/rock system to the wettability. Measuring zeta potential of each pair of surfaces by a charged particle suspension method is used to estimate double layer forces, disjoining pressure, and contact-angle. We demonstrate the applicability of the DLVO approach by showing a comprehensive experimental study that investigates the effect of divalent ions in carbonates, and uses disjoining pressure results to rationalize observations from core flooding and direct contact-angle measurements.

Molecular Dynamics Simulation of Atomic Force Microscopy at the Water-Muscovite Interface: Hydration Layer Structure and Force Analysis

With the development of atomic force microscopy (AFM), it is now possible to detect the buried liquid-solid interfacial structure in three dimensions at the atomic scale. One of the model surfaces used for AFM is the muscovite surface because it is atomically flat after cleavage along the basal plane. Although it is considered that force profiles obtained by AFM reflect the interfacial structures (e.g., muscovite surface and water structure), the force profiles are not straightforward because of the lack of a quantitative relationship between the force and the interfacial structure. In the present study, molecular dynamics simulations were performed to investigate the relationship between the muscovite-water interfacial structure and the measured AFM force using a capped carbon nanotube (CNT) AFM tip. We provide divided force profiles, where the force contributions from each water layer at the interface are shown. They reveal that the first hydration layer is dominant in the total force from water even after destruction of the layer. Moreover, the lateral structure of the first hydration layer transcribes the muscovite surface structure. It resembles the experimentally resolved surface structure of muscovite in previous AFM studies. The local density profile of water between the tip and the surface provides further insight into the relationship between the water structure and the detected force structure. The detected force structure reflects the basic features of the atomic structure for the local hydration layers. However, details including the peak-peak distance in the force profile (force-distance curve) differ from those in the density profile (density-distance curve) because of disturbance by the tip.