Microscopic mechanisms of inorganic salts affecting the performance of aqueous foams with sodium dodecyl sulfate: View from the gas–liquid interface

Highlights

• Effects of inorganic salts on aqueous foam with SDS were studied at multiple scales.

• NaCl improved liquid carrying capacity of SDS foam and MgCl₂ enhanced its stability.

• Pathway of salts changing surface tension at gas–liquid interface with SDS was found.

• A method was proposed to simulate the passage of CH₄ molecule through liquid film.

Abstract

Inorganic salts contained in wellbore accumulations have a significant impact on the suitability and effectiveness of foam draining technology in gas wells. Despite many studies on the effects of such salts on the stability of water-based foams generated by surfactant, the findings are not conclusive due to the differences in surfactant structures and research methods used. In order to gain a better understanding of the influence of inorganic salt types and concentrations on foam systems containing sodium dodecyl sulfate (SDS), a combined experimental and molecular simulation-based approach was used. The experimental results showed that MgCl₂ significantly improves the stability of the SDS foam system, while NaCl greatly improves its liquid carrying capacity. Distinguishing from the traditional classification including foam generation, gas coarsening and bubble rupture, the process of foam system from generation to rupture was divided into three new stages, as manifest in foam generation, liquid drainage and gas transfer. In conjunction with molecular dynamics simulations, the effect of MgCl₂ and NaCl on these three stages was explored. In addition, the microscopic mechanisms by which MgCl₂ and NaCl influenced the stability and liquid-carrying properties of the SDS foam system were clarified. It was found that MgCl₂ improved the foam stability by reducing the gas–liquid interfacial tension and enhancing the strength of the interface containing SDS, while NaCl improved the liquid-carrying capacity of the SDS foam system through hydration. The understanding of this study is of great importance for the application of aqueous foams with SDS in upstream natural gas industry.

Introduction

Foams are widely used industrially, in applications as diverse as petrochemicals [1], mineral flotation [2], food processing [3], and fire protection [4]. In the production of natural gas, foam drainage technology (in which aqueous foams play a vital role) has been successfully applied to the elimination of liquid accumulation in gas wells [5], [6], and aims at reducing bottom-hole effusion, improving the well production efficiency, and preventing the occurrence of water flooding accidents. In addition, the use of aqueous foams to remove liquid from gas gathering pipelines is a current focus of research interest [7], [8]. The performance of foams is of paramount importance to their effectiveness in carrying liquids [9], [10].

Current methods of evaluating foam performance are mainly focused on experimental studies aided by foam generating devices which reflect the foaming capacity of systems and the stability of foam with changes in the volume [11], [12]. Langevin [13] found that the presence of a mixed layer of flexible polymers and surfactants with opposite charges enhanced the stability of foams generated from the solution. Wang and Yoon [14] showed experimentally that NaCl promoted the adsorption of SDS surfactant at the air–water interface, and that the hydrophobicity decreased with increasing SDS and NaCl concentrations. In addition, the factors affecting foam stability in pipelines were investigated by Govindu et al. [15], and it was found that the presence of an inclination enhanced the drainage of foam systems and shortened the foam half-life.

However, it is known that foam undergoes three main processes during stabilization— namely, free drainage, bubble coarsening, and bubble bursting [16], [17]. These processes persist once foam has been created, and vary to different extents with time [18]. Further, external substances, such as surfactants [19], [20], inorganic salts [21], temperature [22], condensate oil [23] and nanoparticles [24], [25], all have an impact on the foam stability to varying degrees. Angarska et al. [26], [27] experimentally found that liquid films formed in the presence of divalent magnesium ions were more stable than those formed in solutions containing monovalent sodium ions. Ramanathan et al. [28] found that different types of gases permeate foam membranes at different rates, with the speed of permeation being inversely correlated with the molecular diameter of the gas. Chen et al. [29] synthesized a photoresponsive surfactant, 4-octoxy-4′-[(trimethylamino)ethoxy]-azobenzene (OTAEAzo), which could be used to tune the stability of foam systems through irradiation with different types of light. Obisesan et al. [30] investigated the effect of salt on the drainage performance of aqueous foams and showed that the drainage of foams containing NaCl decreased with increasing foam quality, while CaCl₂ have a significant effect on the system. Hence, by itself, the half-life of foam is not sufficient to characterize the stability performance of a foam system. In the face of such complex foam stabilization processes, it is only through clarifying the underlying microscopic mechanisms of the factors affecting foam stabilization that a comprehensive system for the evaluation of foam stability can be established.

Molecular dynamics (MD) simulations have emerged as a powerful research tool in recent years and are widely used to study systems with different phase interfaces. Xu et al. [31] and Shi et al. [32] investigated the ability of various surfactants to reduce surface tension at oil–water interface. Zhong et al. [33] investigated the adsorption behavior of crude oil on wet silica surfaces, with the results showing that the pre-adsorption of polar components in crude oil would promote the adsorption of non-polar components on the surface. Zhang et al. [34] investigated the effect of methane on the oil–water interface and found that an increase in the amount of methane molecules led to both a reduction in the interfacial tension and an increase in the interfacial roughness and thickness, which promoted the compatibility between the two phases. R. Ahmed and W. Alshahrani [35] investigated the adsorption and corrosion inhibition properties of ionic liquids at metal surface.

To date, investigations on the effects of different variables on the performance of foam films have been carried out using MD simulations. Hu et al. [36] showed that high temperatures promoted the entry of CH₄ molecules into the foam film, and that both high pressures and high temperatures negatively affected the ordered arrangement of the hydrophobic tail chains of surfactants. Pang et al. [37] found that the use of dimethylsiloxane ethoxylate-propoxylate (DSEP) enhanced the orderliness and densification of SDS monomolecular films. Zhang et al. [38]’s findings indicated that the structure of fluorocarbon surfactants was more ordered than that of hydrocarbon surfactants at gas–water interface. Sun et al. [39] prepared an aqueous foam based on the different responses of surfactants to CO₂ and N₂, which enabled the foam stability to be controlled by the two gases. Xu et al. [40] showed that polyvinyl alcohol (PVA) significantly improved the stability of sodium dodecyl ether sulphate (SDES) interfacial films, which contributed to the foaming capacity of such solutions. In addition to the abovementioned factors, the effects of inorganic salts on foam properties are more complex and hence less predictable, due to the complexity of salt ions with different valence states. Zhao et al. [41] found that the distribution of oppositely charged ions near the gas–water interface could shield the electrostatic repulsive forces between surfactant head groups. Yang et al. [42] studied the effect of calcium ions on surfactants, with the results showing that the presence of these ions led to a significant reduction in the membrane stability of SDS systems. Yan et al. [43] showed that the presence of calcium ions caused surfactants to form tighter aggregates through salt bridges, and that the ion-pair free energy barrier between the sodium dodecyl sulfonate (SDSn) head group and calcium ions was higher than that of the sodium dodecyl carboxylate (SDC) system. Combining previous experimental results with theoretical derivations, Katsiavria et al. [44] discovered that the addition of salt has a strong stabilizing effect on foam, as it promotes surface adsorption by weakening the electrostatic repulsion, thus achieving significant surface tension reductions in low-concentration surfactant systems. However, despite plenty of efforts to analyze the effect of inorganic salts on the adsorption behavior of surfactants at the gas–liquid interface via MD simulations, such studies are mostly restricted to qualitative analysis of microscopic mechanisms, and have failed to support their claims with experimental data.

Against this backdrop, in order to enrich the research system on the influence of inorganic salts on foam systems and their microscopic mechanisms of action, the present study aims to comprehensively investigate the effect of various salts on the stability of different foam systems using a combined experimental and simulation approach. To this end, the stability of SDS aqueous foams containing different inorganic salts was systematically evaluated through MD simulations and multi-scale experiments. In addition, the micro-mechanisms of the foam stabilization process at different stages were analyzed to promote the application of foam drainage technology with mineralized water in gas wells and gathering pipelines.