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• Hydroxyl in the spacer intends to promote molecule aggregation and improve the surface activity of the Gemini surfactant. • The tighter double hydration layer helps to improve the salt resistance when wormlike micelles is formed. • More hydroxyl in the spacer leads to better elastic performance. The performance of Gemini cationic viscoelastic surfactant (VES) is affected by the spacer group, which is also the focus for developing the molecular structure design of high -performance clean fracturing fluid. In this study, we synthesized Gemini cationic VESs (VES-O, VES-DO, VES-TO) by introducing different number of hydroxyl groups in the spacer. The effect of the number of hydroxyl groups in the spacer on the performance of Gemini cationic VES was investigated using surface tension measurement, apparent viscosity and viscoelasticity tests in the presence of KCl. Further, the performance of clean fracturing fluids containing VES-O, VES-DO, and VES-TO were evaluated. We discovered that increasing numbers of hydroxyl groups in the spacer improves surfactant performance in fracturing fluid and high-elasticity fracturing fluids with low viscosity. This gives us ideas for future designs of high-performance clean fracturing fluids. [ABSTRACT FROM AUTHOR]

[Display omitted] • A salt-responsive hydrophobically associating polymer (HSRP) was synthesized. • This hydrophobically associating polymer shows an unusual salt-responsive self-assembling transition. • A reasonable core–shell model was proposed for the salt-responsive behavior. • HSRP is expected to find wide applications in saline reservoirs based on its excellent salt-responsive properties. A salt-responsive hydrophobically associating polymer (HSRP) with a molecular weight of 1.21 × 106 g/mol was synthesized and characterized. It has the characteristics of strong association and low molecular weight by choosing monomers and adjusting the polymerization system. HSRP exhibited a high sensitivity to salt ions owing to the peculiarities of its molecular structure. Through various experimental tests, this study further expanded and complemented the salt-thickening effect of hydrophobic association water-soluble polymers. In brine, HSRP exhibited low apparent viscosity and shear stress. In addition, HSRP demonstrated better thixotropic properties and viscoelastic behavior as the salinity increased, which was attributed to a self-assembling transition. The effect of salinity on self-assembling behavior of supermolecules and rheological properties of HSRP was examined through macroscopic properties and microscopic mechanisms. A reasonable core–shell model was proposed for the salt-responsive behavior: The hydrophilic backbones curled up, forming a shell, and the hydrophobic branched chains interassociated to form a core. In brine, HSRP displayed excellent viscoelasticity and low friction. It is expected to find wide applications in tertiary recovery and fracturing in saline reservoirs based on its excellent salt-responsive properties. [ABSTRACT FROM AUTHOR]

[Display omitted] In recent years, cationic surfactants have received extensive attention in cleaning fracturing fluid thickeners due to various unique properties. At the same time, the development of temperature-resistant and salt-resistant clean fracturing fluid is the key to deep well oil and gas resource development. In this study, the rigid-soft combined strategy is adopted for molecular structure designs. N,N -dimethylcyclohexylamine and epichlorohydrin are used as the initial reactants; after the ring-opening reaction occurs, they are quaternized with erucamide propyl dimethyl ammonium to synthesize a novel double-cation viscoelastic surfactant, SY–JS. With double-cationic charges on the head group, water molecules are selectively adsorbed around the cation lattice to increase the hydration layer, and thus, salt tolerance is improved. Cyclohexane with a rigid structure improves temperature resistance. The ultra-long unsaturated hydrophobic tail imparts excellent heat tolerance and dissolubility to the designed viscoelastic surfactant. The clean fracturing fluid prepared by 3 wt% SY–JS with 130,000 ppm NaCl or KCl performs well under a shear rate of 170 s−1 at 140 °C, and the apparent viscosity remains above 30 mPa s−1 after 120 min.. Additionally, the results show that single-tail surfactants exhibit excellent performance at approximately 140 °C. Afterward, the brine produced from Rumaila and Jiyuan oil fields is used to prepare brine-based clean fracturing fluids, and evaluations, including proppant suspension, gel breaking, and permeability damage, were conducted. The results show that the brine-based clean fracturing fluid completely meets the requirements of hydraulic fracturing industrial application. [ABSTRACT FROM AUTHOR]