| Hydraulic fracturing technology is a necessary method to improve oil recovery for low permeability,ultra low permeability and dense oil and gas reservoirs.With the scale,the number and scope of hydraulic fracturing continue to expand,it not only needs to consume a lot of water resources,while bringing the sewage treatment problems.Fracturing fluid repair and recycling technology can use the thickener and water in the backflow solution,not only meet the demands for water but also can effectively reduce the cost and environmental pollution.Macromolecular polymer and guanidine type fracturing fluid thickening agent will be irreversibly damaged in the process of construction and breaking stage,which cause thickener cannot be reused.The reversible breaking mechanism of clean fracturing fluid is dilution or solubilization,thus it has a good repair recycling prospect.In this thesis,three kinds of N-acylamino acid pH-controlled surfactants were drawn up and synthesized.The optimum synthesis process was determined by orthogonal experiment,and their physical and chemical properties were characterized.The clean fracturing fluid system triggered by pH was constructed through the anti-ion optimization,the main agent type optimization,the main agent chain length optimization,the mixed proportion optimization and the total concentration optimization.The macroscopic performance of the clean fracturing fluid system triggered by pH was systematically evaluated.The viscoelasticity and stable formation mechanism of the clean fracturing fluid system triggered by pH were studied by rheological experiment method and the internal factors that temperature effect on the performance of the cleaning fracturing fluid was discussed.The aggregation behavior of pH-controlled surfactants and counterions in the aqueous solution was studied by coarse-grained molecular simulation.Technical indexes of the cleaning fracturing fluid were systematically evaluated,and the remediation and recycling mechanism was clarified by means of coarse-grained molecular simulation.The results show that the pH-controlled surfactants have the characteristics of environmentally friendly,excellent in performance and easy to operate,which are suitable for use as the main agent of the clean fracturing fluid.When pH≥8,the clean fracturing fluid system triggered by pH for 70℃(0.8%Glu-16+1.2%TSAB+0.5%breaking agent)and 90℃(1.52%Glu-16+2.28%TSAB+1%breaking agent)formation conditions both exhibits excellent macroscopic properties.When pH≤5,the system presents a unique gel breaking phenomenon triggered by pH.Rheological experiments results show that the structure-stable worm-like micelles are formed in the system and the microstructure of the aggregates in the system is observed.The molecular simulation results show that the worm-like micelles are formed in the system,and the formation mechanism is that the TSA~+and Glu~-molecules have high binding energy barrier and dissociation potential barrier,and the ability of shielding water molecules is stronger.The clean fracturing fluid still shows good macro performance after the recycling,so it has a good recycling prospects.In the process of repairing the cleaning fracturing fluid,the self-aggregates of the surfactant undergoes the transition from spherical micelles to worm-like micelles,thus re-importing the macroscopic properties to the cleaning fracturing fluid.The internal mechanism is the surface charge of micelle particles continued to reduce during the repair process,which leads to the decrease of zeta potential,weakening the barrier between micelle particles,and promoting the fusion of micelles and the re-formation of worm-like micelles. |
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