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The development of surfactants in clean fracturing fluids
2020-09-07 10:52:44

As a functionally flexible polymer-like material in the aqueous solution,viscoelastic surfactant (VES) has great potential to be a thickener for the fracturing fluid,and it has attracted an increasing attention in recent decades due to its polymer-like behaviors in aqueous solution and stimuli-response properties.Different from the normal polymer applied in the hydraulic fracturing fluid,VES can self-assemble into long and flexible wormlike micelles in the aqueous solution,behaving as a crosslinked polymer. Stimulus conditions such as light, pH, temperature, and electrolyte can induce various responses of VES molecule aggregation, thus the macroscopic nature of viscoelasticity can be adjusted by changing the stimulus conditions. Due to the “magical” properties of the viscoelastic surfactants,they have been welcomed in many industrial applications such as smart optical systems,drug delivery,template synthesis ,biosensors in past decades. Recently, they are introduced into the petroleum industry, applied to enhanced drag reduction,reservoir stimulation and oil recovery.The hydraulic fracturing technique as the most common reservoir stimulation treatment has been applied in low-permeability reservoir stimulation for several decades, for which the fracturing uid must own viscosity and elasticity properties to create an articial fracture and transport proppant into the cracks. Thus, fracturing uids with low damage (clean fracturing uid) draw more and more attention in low-permeability or tight reservoir development. Fortunately, the wormlike micelles assembled by low molecular weight VES and the entangled networks impart very nice viscoelastic properties to the aqueous solution of VES, making the solution present analogous characteristics to polymer solutions. Schlumberger rstly successfully applied a cationic viscoelastic surfactant for fracturing uids in 1997,called polymer-free fracturing uid. The VES fracturing uid showed many advantages, especially for the low-permeability reservoir, including low damage, low drag friction, free of cross-linker and biocides. Unlike polymers or guar gum gel, wormlike micelles collapse into spherical micelles or emulsions when exposed tohydrocarbon in there servoir,which imparts the VES fracturing uid residue free and easy to ow back.The developed viscoelastic surfactants include anionic VES, cationic VES, and zwitterionic VES, among which cationic VES is applied most widely to the VES fracturing uid. Most of these viscoelastic surfactants were single-chain surfactants and deciencies have limited their further application in extreme reservoir conditions. The traditional single-tailed surfactants possess high critical micelle concentrations (CMC) and poor surface activities, and the VES fracturing uid prepared from the single tailed surfactants exhibit poor temperature and shear resistance. Gemini surfactant as a superior type of surfactant was rstly reported in 1971, while not applied in oileld. The special structure of Gemini surfactant is made up of two single-chain surfactants that linked by a spacer group. The structure of the spacer group greatly affects the properties of the Gemini surfactant. Therefore, the performance of the Gemini surfactant can be improved by the modication of the spacer group. Ontheotherhand,thelength of the hydrophobic chain is also crucial to the properties of the Gemini surfactant. The spacer group weakens the electrostatic repulsion between the head groups through strong chemical bonds. Therefore, the Gemini surfactants own superior properties compared to single-tailed surfactants, such as higher surface activity, lower CMC, contrasting self-assembly behavior and better rheological behaviors.

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