Interfacial Chemistry of Rocks and Soils: 148 (Surfactant Science)

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The Interface and surfactants

Environmental context. Perfluoroalkyl compounds are organic contaminants that exhibit strong resistance to chemical- and microbial-degradation. As partitioning between solid and aqueous phases is expected to control the transport of perfluoroalkyl compounds, we studied the molecular mechanisms of their adsorption—desorption at a representative Fe oxide surface using in situ molecular spectroscopy. The results provide valuable information on the types of bonds formed, and enable a better understanding of the transport and fate of these organic contaminants in natural environments.

Specifically, in addition to electrostatic interactions, PFOA forms inner-sphere Fe—carboxylate complexes by ligand exchange, whereas the PFOS sulfonate group forms outer-sphere complexes and possibly hydrogen-bonds at the mineral surface. The non-ionic or zwitter ionic surfactant will be easier to enter the loose ionic surfactant adsorption layer due to the hydrophobic effect and the possible dipoleion interaction. Thus, the electrostatic repulsion will be weakened and the molecules will arrange closely, then the surface activity will be improved [ 44 , 45 ].

It is reported that zwitterionic-anionic mixed surfactant systems offer synergistic interactions in aqueous medium due to polymorphism of self-assembly structures that have combined properties of surfactants in the mixture. Atrayee et al. Static and dynamic rheological tests indicate lower CMC and superior viscoelastic properties of the later system Figure 1.

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Numerical simulation of mixture surfactants can help us to understand interaction mechanism of different surfactant deeply and comprehensively. AVai et al. The results show that the addition of salt has a significant effect on the structure and size of the micelles. In pure water, the hydrophobic hydrocarbon chain radicals of the surfactant accumulate in the centre of the micelle; while in sodium chloride solution, the polar group will gather around the micelle crown. Meanwhile, the rearrangement is accompanied by micelle shrinkage, and the spherical micelle radius will be reduced from 2.

Wang et aluse molecular dynamics simulation to study the adsorption of cationic surfactant dodecylamine DDA and anionic surfactant sodium oleate NaOL on the interface [ 48 ]. All heads are hydrated in the aqueous phase, with only a small portion of the carbon chain immersed in the water.

Due to the electrostatic interaction between DDA and the NaOL ion-based head group and the hydrophobic interaction between the carbon chains, the mixture display much more compactness and tightness, thus showing stronger surface activity than the pure DDA and NaOL. When the molar ratio of DDA to NaOL is , will demonstrate a stronger interaction and a lower interfacial tension, which corresponds well with the results obtained from experimental results.

Nano fluid are widespread interest in many subsurface engineering applications such as hydrocarbon exploration and production, EOR processes [ 51 - 53 ]. Therefore, Surfactant combination with nanoparticles and polymers is gaining more and more attention. Nanoparticles have a large specific surface area. When added to surfactants solution, nano-emulsions micro-emulsions will form and they have better stability due to small-scale effects and Brownian motion.

Meanwhile, the nanometer-scale droplets are more likely to enter the tiny voids or cracks of the dense rock, and its large specific surface area can make the nano-droplets fully spread on the rock surface, improve the contact efficiency between the nanoemulsion and the reservoir, and improve the overall application effect of the well fluid [ 54 ].

The interfacial tension between NS nanofluids and crude oil was significantly lower than that of surfactant solution without nanoparticles. It is believed that the electrostatic repulsion between the surfactant and the nanoparticles promotes the diffusion of the surfactant to the interface, while the nanoparticles encapsulated by the surfactant carry the surfactant to the interface due to its Brownian motion, and the surfactant and Surfactant-coated nanoparticles are aligned at the interface to reduce interfacial tension synergistically.

Dong et al. As a result, the surface tension of nanofluids decreased firstly and then increased with the increase of titanium dioxide concentration [ 56 ]. It is suggested that when the concentration of titanium dioxide nanoparticles is high enough, the attraction between the particles causes capillary forces, which in turn causes the surface tension to rise.

The article also explains the reason for the decrease in surface tension from the energy point of view. It also shows that the reduction of surface tension is also a dynamic process because the adsorption of nanoparticles on the surface is a dynamic process. Jin et al. The results show that when the surfactant and nanoparticle possess same charge, the electrostatic repulsion will drive more surfactants at the interface, and the two will show synergistic effect, then the surface tension is reduced; While when the surfactant and the nanoparticle possess opposite charge, due to the electrostatic attraction, the nanoparticles will adsorb on the surfactant and agglomerate will form, thereby reducing the effective concentration of the surfactant on the interface, and the surface tension will increased instead.

Luo et al. It is believed that the nanoemulsion has ultra-low surface tension, high solubilisation and small particle size, can effectively enter the pores and fully contact with the solid surface, which can reduce the surface tension and change the surface wetting angle of the solid, then significantly reduce the water absorption of the core, thereby reducing the capillary resistance and effectively improve the fracturing cleanup efficiency. Studies have shown that due to the unique advantages of nanoparticles, the interaction of surfactants and nanoparticles can change the rheological properties and interfacial properties of the system, reducing the surface tension of the system.

The structure of these aggregates described as polymer film will form around surfactant micelles [ 59 ].

Application of Surfactants in Hydraulic Fracturing for Enhanced Oil/Gas Recovery

Tushar et al. The surface tension of nanofluids and the interfacial tension between nanofluids and crude oils are characterized. The results show that the surface tension and oil-water interfacial tension of NP nanofluids and NSP nanofluids are significantly lower than those of traditional oil extraction auxiliaries agent. Furthermore, the surface tension and oilwater interfacial tension of NSP nanofluid are lower than that of NP nanofluid. It is believed that the presence of the surfactant improves the wettability of the nanoparticles, increases the amount of adsorption on the surface, and simultaneously forms the silica nanoparticlespolyacrylamide in the bulk phase due to the Brownian motion of the nanoparticles.

The surfactant agglomerates are carried to the surface and arrange directionally on the surface, reducing the surface tension of the system Figure 2. Molecular dynamics can be used not only for the simulation of surfactant systems, but also for the study the interaction of surfactantpolymer system to obtain detailed information on the kinetic and structural properties that are not possible in the experiment.

Information

Wang et al. The results show that when the surfactant concentration is low, a surfactant-polymer monolayer is formed at the interface, and a multi-molecular layer is formed as the concentration of the surfactant increases. Due to the electrostatic interaction between the negatively charged carboxylate in acrylic acid and the positively charged head group in the surfactant, the oxygen atom of the carboxylate and the hydrogen atom in the water can form a hydrogen bond, which promotes the binding of the surfactant to the polymer chain.

As the concentration of the surfactant increases, due to the hydrophobic interaction between the hydrophobic tails of the surfactant, the layer-layer arrangement is promoted to form a multilayer adsorption structure. Simulation studies have also shown that the polymer-surfactant complex has a dynamic process of ion exchange, consistent with experimental results.

Studies have shown that due to the interaction of surfactants and polymers,the rheological properties and interfacial properties of the system can be improved. In gas condensate reservoirs, well productivity reduction resulting from condensate blockage has been one of the most important factors influencing gas and condensate production rates, which is sharper for the low-permeability reservoirs [ 62 ]. Many investigators have proposed several methods such as gas recycling, hydraulic fracturing and solvent injection, that is to say using surfactants to mobilize the condensate in the region near wellbore, to restore gas and condensate production rates when condensate blockage occurred [ 63 - 65 ].

In the past few years, many researchers have investigated on surfactants to reduce the surface tension and remove the condensate banking [ 66 , 67 ]. However, these chemicals do not alter the wettability of the rock surfaces from liquid to gas. In recent years, wettability alteration, as a new method, has become more attractive for researchers in industry.

Most of the gas condensate reservoirs rocks are naturally liquid-wetting and altering the wettability of the reservoir rock from strongly liquid wetness to preferential gas wetness or intermediatewetting can increase the mobility of condensate and the relative permeability to gas [ 68 ]. Then several researchers develop fluorosurfactants and modify the wettability of rocks from liquid to gas [ 69 - 71 ]. A direct relationship with the increase of roughness and heterogeneity of the surface by means of adsorption of the fluorinated surfactant over the rock surface is discussed.

They conclude that the surfactant adsorption leads to a reduction of the surface free energy due to the formation of a fluorine atoms network [ 72 , 73 ]. Results show that core wettability can be altered from liquid-wetting to intermediate gas-wetting or preferential gas-wetting by the fluorosurfactant at a very low concentration because the core surface free energy sharply decreased after the treatment of surfactants.

And then, oil displacement efficiency, relative permeability, and gas flow in gas—oil systems can be effectively improved by fluorosurfactant treatment due to alteration of wettability. Hassanajili et al. Fahimpour et al. Then a new approach of combining anionic and nonionic chemical agents is proposed. However, Because of the amphiphilic character of surfactant, they tend to form micelle at certain concentration, which could not penetrate all pores, especially in the tight reservoirs [ 77 ].

Therefore, it is urgent to develop chemicals with appropriate size which can diffuse into deeper pore throats in low permeability rocks to alter their surfaces from liquid-wet to gas-wet state. Nanotechnology has recently emerged as an attractive topic of research in the oil and gas industry due to its exceptional characteristics that allow nanoparticles to travel smoothly through porous media without additional risks of pore blockage due to their small size, which can be used to avoid formation damage.

At the nano-scale, exceptional properties can be obtained, such as a high surface-area-to-volume ratio and dispensability, in addition to high thermal, chemical stability and dispersibility [ 78 ]. Hence, the use of nanofluids based on the interaction of fluorosurfactants and nanoparticles with the appropriate particle size could be a promising alternative for enhancing wettability alteration to gas-wet systems. The results showed that the wettability of the core could be altered to intermediate gas-wetting or super gas-wetting after nanofluid treatment.

They conclude that the dehydration-condensation reactions of silicon dioxide nanoparticles and fluorocarbon surfactant forms a compact layer which can remarkbly alter the wettability from water-wet to gas-wet Figure 3. Figure 3: Sketch of surface modification of a silica nanoparticle and its gas-wetting alteration application.

Maribel et al. The results of different concentration of nanoparticles and surfactant are supported by contact angle and imbibition tests on oil-wet and water-wet sandstone samples for the wettability alteration, then the optimum concentration is achieved and performed under tight gas-condensate reservoir temperature and pressure conditions.

The result indicate that the synthesized nanofluid can alter the wettability of the system from a strongly liquid-wet to a gas-wet condition, then the formation damage caused by the condensate banking can be reduced,the production of oil and gas can be considerably improved.

At present, there is only few research concerning the wettability alteration resulted from interaction of nanoparticle and surfactant. Nanofluid combining the benefit of nanoparticle and surfactant may be an effective way to change the wettability from water-wet to gas-wet, which can be a research hotspot in the near future.

In the development of shale gas reservoirs, the addition of surfactants can reduce the viscosity and vortex formation in the shear flow of the wellbore, thus reducing the flow resistance of fracturing fluid [ 82 ]. At present, the drag reducer added in the fracturing fluid is mainly linear vegetable gum and poly acrylamide, Whose shear resistance is poor at higher shear rate, and residue content is high, which may causes serious formation damage [ 83 ].

Surfactant is a new type of drag reducer different from polymer, the appearance and disappearance of drag reduction is reversible, and shear degradation does not occur, so it has stable drag reduction effect, especially for large-scale and high shear rate fracturing, as well as the recirculation of fracturing fluid [ 84 ]. Studies have shown that when the surfactant drag reducing solution reaches the critical micelle concentration CMC , spherical micelles are first formed in the solution; with the concentration further increased to the transition concentration CMCII , the spherical micelles in the solution will transform into rod micelles.

When the fluid flows, the rod micelles gradually assemble into a shear-inducing structure and a spatial network structure under shear, then the viscosity of the solution increases and becomes a viscoelastic fluid [85]. Yu et al. Since the latter is stronger than the former, the surfactant exhibits drag reducing effect when added. A single surfactant has a limited temperature range and needs large amount in the drag reduction process, which greatly limits its use for drag reduction. Adding surfactants of different alkyl chains together is an effective method to expand the effective temperature range and reduce the amount of surfactants, because it is more favourable to form a stable linear micelle network structure after mixing [ 44 ].

The addition of counter ions and salts to the surfactant solution can promote the formation of rod micelles. Tuan et al. Subsequently, a network structure appear, which exhibit significant viscoelasticity, known as worm-like micelles. Peng et al. The electrostatic energy between the micelles is reduced, so that the electrostatic stability of entire rod shape micelles in the aqueous solution is enhanced.

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