Reservoir wettability plays an important role in various oil recovery processes. The origin and evolution of reservoir wettability were critically reviewed to better understand the complexity of wettability due to interactions in crude oil-brine-rock system, with introduction of different wetting states and their infl uence on fl uid distribution in pore spaces. The effect of wettability on oil recovery of waterflooding was then summarized from past and recent research to emphasize the importance of wettability in oil displacement by brine. The mechanism of wettability alteration by different surfactants in both carbonate and sandstone reservoirs was analyzed, concerning their distinct surface chemistry, and different interaction patterns of surfactants with components on rock surface. Other concerns such as the combined effect of wettability alteration and interfacial tension (IFT) reduction on the imbibition process was also taken into account. Generally, surfactant induced wettability alteration for enhanced oil recovery is still in the stage of laboratory investigation. The successful application of this technique relies on a comprehensive survey of target reservoir conditions, and could be expected especially in low permeability fractured reservoirs and forced imbibition process.

The adsorption behavior of zwitterionic surfactant dodecyl sulfobetaine (DBS) on a silica/solution interface with Ca(2+), Mg(2+) existing in aqueous solution is explored by atomistic molecular simulations. The interaction energy contribution of van der Waals and electrostatic potentials in the surfactants/water/silica system are respectively calculated, from which the electrical interaction can be found to play a decisive role in the adsorption tendency of DBS on the silica surface with or without inorganic ions, despite different mechanisms. The distinct decrease of energy has been found to be derived from electrical interaction when DBS adsorb on the silica surface covered by Ca(2+) or Mg(2+). Therefore, it can be predicted that the cationic ions combined on the negatively charged silica surface in a mineral water medium might decrease the adsorption trend of DBS on the silica surface, which has been experimentally proven by TOC measurement. Structural information of the close interface layer and the distribution of water molecules are analyzed after the complete molecular dynamics simulation using a ternary model. Ca(2+) and Mg(2+) combined on the silica surface can reduce the adsorption amount of DBS by preventing the direct interaction between DBS and surface, and bringing about the orientation reversal of DBS molecules to break the order of adsorption interface layer. Furthermore, changes in the status of the water spreading on the silica surface caused by the complexation of cations are also an important reason in the adsorption reduction.

Alkyl ether carboxylate is one type of surfactant that can produce ultralow interfacial tension (IFT) under high-salinity and high-temperature conditions. In this paper, the influence of counterions on dynamic IFTs of fatty alcohol polyoxyethylene carboxylate (C12EO3C) against alkanes has been studied. The effect of the temperature on the IFT has been investigated. On the basis of our experimental results, one can find that the NaCl concentration has little effect on the IFT, while divalent ions can reduce the IFT to an ultralow value. With the increasing CaCl2 or MgCl2 concentration, dynamic IFT passes through a minimum at a particular salt concentration (“V” shape). Moreover, the stable value of IFT achieves an ultralow value and also passes through a minimum at the same salt concentration. MgCl2 has a stronger tendency to achieve ultralow IFT than that of CaCl2, while the addition of CaCl2 has a stronger tendency to partition surfactant molecules to the oil phase. Ultralow IFT could also be achieved by improving the temperature because of the enhancement of oil solubility of the surfactant. An interfacial model combining two mechanisms, partitioning the surfactant into the oil phase and decreasing the charge repulsive force between interfacial surfactant molecules, responsible for the effect of the electrolyte on dynamic IFT has been provided. All experimental results above can be explained well. Our studies are of great significance in designing ultralow IFT formulation for the reservoir in a high temperature and with high-salinity formation water.

This present article employs four anionic Gemini surfactants with different spacer groups and investigates their physicochemical and aggregation properties. The critical micelle concentration (CMC), surface tension at CMC (c CMC ) and C 20 of these surfactants have been investigated using the du Nouy ring method. The aggregation number (N) was determined with intrinsic fluorescence quenching method using pyrene as a fluorescence probe and benzophenone as a quencher. Results show that these anionic Gemini surfactants have lower CMC and C 20 values compared with those conventional ones and show higher surface activity. As expected, the spacer plays an important role in the aggregation properties of Gemini surfactants. Under experimental conditions, Gemini B-D with an alkoxylated group as spacer has a lower CMC and a higher aggregation number than Gemini A with methylene as spacer. For Gemini B-D, the CMC and aggregation number values decrease with the increasing flexible spacer length. The micropolarity also affects the aggregation of the present anionic Gemini surfactants. The micropolarity of micelle becomes low when the concentration of surfactants increases. Aggregation numbers of surfactants increase and fluorescence intensities decrease with the increasing concentration of NaCl. These results will help us to understand the relationship between the architectures of Gemini surfactants and their various properties in aqueous solution.

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