In this work, a series of new sulfonate Gemini surfactants (abbreviated as m-3-m) were studied as an attempt to develop a single surfactant that can effectively reduce the oil-water interfacial tension to ultralow values. The novel surfactants were synthesized and characterized by ¹HNMR and ¹³CNMR spectroscopies. The surface activeness of those surfactants at the air-water interface and thermodynamic properties in the bulk aqueous phase were investigated. The oil-water interfacial tension, viscous and viscoelastic properties of solutions were studied. The results showed that the novel surfactants exhibit fascinating surface activeness and low values of CMC; spontaneously formed micelles and adsorbed at the air-water interface. They can substantially reduce the oil-water interfacial tension to ultralow values ( ${10}^{-3}\mathrm{ }\mathrm{mN}/\mathrm{m}$) when the surfactant concentration is less than 0.5 wt%. The surfactants possess attractive viscous and viscoelastic properties towards EOR applications through flooding methods. In accordance with the experimental studies, the surfactants are promising in formulating fluid systems with ultralow interfacial tension. Graphical Abstract

In order to optimize the viscoelastic properties and temperature-resistance for the sodium 2,2'-(propane-1,3-diylbis(stearoylazanediyl))diethanesulfonate Gemini viscoelastic surfactant, the effects caused by the addition of nanoparticles were investigated. The viscous and rheological behaviors and microstructures of surfactant solutions with/without magnesium oxide (MgO) or lipophilic silicon dioxide (SiO₂) nanoparticles were studied using rheometer and field emission scanning microscopy electron microscopy (FESEM). The experimental results showed that the addition of optimal nanoparticle mass fractions significantly increases the viscoelastic properties for the viscoelastic micelle solutions at 90 °C. The enhancements in viscoelastic properties are ascribed to the formation of network structures due to the addition of nanoparticles. The use of nanoparticles showed promising improvements to be applied for fracturing fluids at high temperatures.

Hydraulic fracturing is one of the important methods to improve oil and gas production. The performance of the fracturing fluid directly affects the success of hydraulic fracturing. The traditional cross-linked polymer fracturing fluid can cause secondary damage to oil and gas reservoirs due to the poor flow-back of the fracturing fluid, and existing conventional cleaning fracturing fluids have poor performance in high temperature. Therefore, this paper has carried out research on novel sulfonate Gemini surfactant cleaning fracturing fluids. The rheological properties of a series of sulfonate Gemini surfactant (DSm-s-m) solutions at different temperatures and constant shear rate (170 s⁻¹) were tested for optimizing the temperature-resistance and thickening properties of anionic Gemini surfactants in clean fracturing fluid. At the same time, the microstructures of solutions were investigated by scanning electron microscope (SEM). The experimental results showed that the viscosity of the sulfonate Gemini surfactant solution varied with the spacer group and the hydrophobic chain at 65 °C and 170 s⁻¹, wherein DS18-3-18 had excellent viscosity-increasing properties. Furthermore, the microstructure of 4 wt.% DS18-3-18 solution demonstrated that DS18-3-18 self-assembled into dense layered micelles, and the micelles intertwined with each other to form the network structure, promoting the increase in solution viscosity. Adding nano-MgO can increase the temperature-resistance of 4 wt.% DS18-3-18 solution, which indicated that the rod-like and close-packed layered micelles were beneficial to the improvement of the temperature-resistance and thickening performances of the DS18-3-18 solution. DS18-3-18 was not only easy to formulate, but also stable in all aspects. Due to its low molecular weight, the damage to the formation was close to zero and the insoluble residue was almost zero because of the absence of breaker, so it could be used as a thickener for clean fracturing fluids in tight reservoirs.

A quantity of recoverable barite exists in high-density waste drilling fluid. Based on the inefficiencies and complexities of existing recycling methods, a novel pre-treatment approach which includes clean-breaking, high-speed washing, ultrasonic dispersion, and microwave heating and a new depressant (Gellan Gum) was proposed. The floatability, separation efficiency and mechanism were discussed by SEM, adsorption capacity, zeta potential measurements and contact angle tests. The results of reverse flotation experiments results indicated that secondary water washing proves highly effective in enriching a significant quantity of barite solid phase. Subsequent microwave-ultrasonic and flotation can obtain barite of high quality with recovery and density reaching 81.5% and 4.238 g/cm³, respectively. It can be utilized directly in the preparation of drilling fluid. Mechanism studies shown that the per-treatments substantially enhances the barite grade while effectively eliminating low-density solid phases adhering to the barite surface, thus exposing additional contact points between the constituents so as to improve flotation separation. This new recovery scheme has environmental advantages and great reference value for the separation of barite within high-density waste drilling fluids.