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It is generally believed that ionic/zwitterionic surfactants have strong interactions with each other. However, our experiments found that nonionic/ zwitterionic surfactants exhibit strong synergistic effects, which differs from the previous understanding. In this study, a scCO 2 foam system stabilized by the synergistic action of alkyl glucoside (APG-10, nonionic surfactant) and erucamide propyl betaine (EAB, zwitterionic surfactant) was constructed. Through the measurement of drainage and coarsening characteristics, surface properties, bulk rheological properties, dynamic light scattering (DLS) and Cryo-scanning electron microscopy (SEM), the stabilizing mechanism of the synergistic action of APG-10 and EAB was revealed for the first time at the macro and micro levels. The results indicate that APG10 and EAB exhibit mixed adsorption at the interface, resulting in a denser molecular arrangement at the interface and enhancing the foaming ability. At 80℃ and 8 MPa, compared to the EAB foam system, the drainage half-life of the EAB+APG-10 system increased by 276.9 %, and the Ostwald ripening rate decreased by 62.4 %. Rheological studies have shown that the zero shear viscosity of the EAB+APG-10 system is 6837.8 mPa·s, with an increase of 44.4 % compared to the EAB system at 80℃. This is believed to be mainly due to the binding of APG-10 to the micellar structure to enhance the entanglement strength of the micelles, aiding in suppressing the expulsion and deformation of foam bubbles. Additionally, the mixed system exhibits larger G' and E', effectively suppressing the expansion and contraction of the liquid film. Finally, the sizes and distributions of micellar aggregates were measured by DLS and Cryo-SEM, confirming the presence of wormlike micelles and their distribution within the liquid film. The results of this work provide theoretical support for the development and application of the novel VES-scCO 2 foam fracturing fluid system, which contributes to the efficient extraction of unconventional oil and gas. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel system of OAB + AOS is proved to be an outstanding scCO 2 foam stabilizer. • AOS and OAB form pseudo-gemini surfactant under electrostatic interaction, enhancing the foaming ability. • The worm-like micelle structure formed by OAB + AOS ensures foam stabilization at 80 °C, 8 MPa. • Explained the foam stabilization mechanism of OAB synergistic AOS from macro and micro perspectives. • OAB + AOS system have great potential in improving foam stability and constructing ultra-low damage foam fracturing fluids. Worm-like micelles have been recognized as "living polymers," and their outstanding physicochemical properties provide unique advantages in improving foam stability. Recently, the use of pseudo-Gemini surfactants to create worm-like micelles has garnered substantial attention because of their cost-effectiveness and high performance. However, there is a dearth of research on the key properties of scCO 2 foam stabilized by pseudo-Gemini surfactants and the mechanism behind their stabilizing effect. To address this gap, a scCO 2 foam system stabilized by worm-like micelles was constructed using pseudo-Gemini surfactants. Through a combination of surface properties, bulk rheology, dynamic light scattering (DLS), and cryo-scanning electron microscopy (Cryo-SEM) measurements, the synergistic effect and foam stabilization mechanism of oleic amidopropyl betaine (OAB) and alpha-olefin sulfonate sodium (AOS) were investigated. The results confirmed the formation of pseudo-Gemini surfactants in the mixed system (OAB + AOS), with superior surface activity compared with individual surfactants. Furthermore, the assembly parameter P = 0.498, determined through interfacial parameter analysis, indicated the self-assembly of worm-like micelles in the mixed system. The rheological properties of the foam liquid, as well as foam drainage, and coarsening experiments showed that the presence of worm-like micelles increased the elasticity of the liquid film, impeding the contraction and expansion of the foam. This resulted in a 145 % increase in the liquid drainage half-life of the scCO 2 foam and a 19.3 % reduction in the diffusion rate of scCO 2 in the liquid film. Further evidence from DLS and Cryo-SEM confirmed the formation of robust worm-like micelle structures in the OAB + AOS mixed system, enhancing the ability of the liquid film to withstand external disturbances. This study is crucial for advancing the existing understanding of pseudo-Gemini surfactants in terms of improving foam performance and designing effective small-molecule foam fracturing fluids. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel microbial polysaccharide diutan gum is proved to be an outstanding foam stabilizer. • The foam stabilization mechanism of microbial polysaccharides was studied from drainage, coarsening and coalescence. • Diutan gum has a special structure, which makes it have good temperature resistance and water retention at 80 °C. • The mechanism of prolonging the critical drainage time of diutan gum were explained from the microscopic scale. • Microbial polysaccharides have great potential in improving foam stability and constructing green foam systems. Microbial polysaccharides are of great interest in improving foam stability due to their excellent physicochemical properties. At present, there are few reports on the improvement of foam stability by novel microbial polysaccharides diutan gum and welan gum, so that the foam stabilization mechanism of microbial polysaccharides in terms of foam drainage, coarsening and coalescence is lacking. In this paper, the foam stabilization mechanism of three microbial polysaccharides was studied in detail from drainage, coarsening and coalescence, and the superiority of diutan gum in foam stabilization was emphasized. Rheological experiments show that the interaction force between diutan gum molecules is very strong, and 0.08 wt% diutan gum can significantly change the viscosity of the solution. The microscopic characteristics of the foam shows that the diutan foam has obvious advantages in foam quality, bubble size and its distribution. For the experimental results of foam drainage, it is found that the critical drainage time of foam is related to the thickening ability of the gum and the liquid drainage half-life of diutan foam is longest. Diutan gum molecule has rod-shaped helical structure and a complex aggregation morphology, which makes diutan gum have good temperature resistance and excellent water retention. These two properties of diutan gum significantly improve its foam stabilization ability. From the analysis of foam coarsening and coalescence, adding microbial polysaccharides can slow down coarsening rate and prevent foam from coalescence. Finally, the morphology of the liquid film at the microscopic scale is observed by SEM, and the thickness of the liquid film of different foams is measured. It is found that the liquid film structure of the foam added with diutan gum is dense and the thickness of the liquid film is the thickest. This study is of great significance to the theoretical exploration of microbial polysaccharides to improve foam performance and the construction of green foam systems. [ABSTRACT FROM AUTHOR]

The development of unconventional resources has brought forth increased requirements for the properties of oilfield chemical materials. Reservoirs with low permeability, high heterogeneity, high temperature, and high salinity conditions challenge the success of the present development methods. Nanocellulose (NC), with many fascinating characteristics, including large specific surface area, high mechanical strength, good chemical modifiability, and good biodegradability, has received considerable research attention as a green additive applied in oil and gas production, especially in enhanced oil recovery (EOR) and hydraulic fracturing (HF). The colloidal properties and interfacial compatibility of NCs are the keys to their superior performance compared to conventional chemicals. By controlling surface charges and hydrophilicity, NC fluids exhibit excellent performance in rheology improvement, oil emulsification, wettability alteration, interfacial tension reduction, and foams and Pickering emulsions stabilization. For various fracturing fluids, the presence of NCs can increase crosslinking efficiency, improve fluid viscoelasticity, reduce liquid filtration and lower frictional resistance. Moreover, NCs show a synergistic effect with surfactants in cellulose network structure enhancement, emulsions and foams stabilization, and wettability alteration. These characteristics highly enhance the oil recovery and the performance of hydraulic fracturing fluids. In this paper, the colloidal and interfacial properties of NCs and their surface modification methods are summarized. Next, recent NCs' applications in the EOR and HF methods are introduced and their performance is compared with conventional chemicals. The mechanism of NCs in EOR and HF efficiency enhancement is revealed. Review of studies has shown that NCs exhibit great technical and economic potentials to overcome the current limitations in oil and gas production. Finally, this review provides theoretical guidance and new research directions for the efficient development of unconventional oil and gas resources. • Surface and colloidal properties of NCs were summarized. • The application of NCs and their derivatives in the oil and gas industry were enumerated. • The recent advances and prospects of NCs in EOR and HF were reviewed. • The mechanism of NCs in efficiency enhancement of EOR and HF was revealed. • The technical and economic feasibility of NCs in oil and gas industry were analyzed. [ABSTRACT FROM AUTHOR]

• A novel microbial polysaccharide diutan gum is proved to be an outstanding foam stabilizer. • Self-generated gas system is analyzed in terms of gas production volume and production efficiency. • The in-situ foam stabilized by diutan gum shows brilliant automatic foaming properties and temperature resistance even at 90℃. • Parallel core flooding experiments prove the distinguished plugging ability and diverting characteristics of the in-situ foam at 150℃. • The in-situ foam has great potential as diverting agent in heterogeneous reservoir stimulation at high temperature. Foam fluid shows good performance as diverting agent for reservoir stimulation, but complex generation procedure and high flow friction restrict its further application especially in offshore fields. To overcome these limitations, the objective of this study is to create stable in-situ foam system and investigate its potential in diverting fluid in heterogeneous reservoirs at high temperature. To prepare in-situ foam system, self-generated gas system was analyzed in terms of gas production volume and production efficiency. Foaming agents and foam stabilizers were optimized at the temperatures ranging from 70 ℃ to 90 ℃. Then in-situ foaming performance of the system combining self-generated gas reactants (sodium nitrite and ammonium chloride), the foaming agent (alpha olefin sulfonate), and the foam stabilizer (diutan gum) without blender was investigated based on foaming volume and bulk stability. Single core and parallel core flooding experiments were applied to investigate the plugging ability and diverting characteristics of in-situ foam. The results showed that the large pore volume was blocked by in-situ foam even at 150 ℃ as shown by the increase of residual resistance factor during subsequent water flooding. Parallel core flooding experiments also proved the diverting ability of in-situ foams as the diversion ratio of low-permeability core was higher than that of high-permeability core during subsequent water flooding. Moreover, the diversion performance of in-situ foam strongly depended on the permeability contrast, environment temperature, permeability of low-permeability core, and injection rate. Although diversion failed when permeability contrast was more than 10, in-situ foam also had good thermal stability and diverting ability at 150 ℃ in the permeability contrast range of 3 to 8. Such stable in-situ foam shows promising potential as diverting agent in heterogeneous formation for reservoir stimulation at high temperature. [ABSTRACT FROM AUTHOR]