| Hydraulic fracturing technique is a necessary measure to increase oil and gas seepage capacity,improve oil and gas production capacity.While fracturing fluid is an indispensable injectant and affects the results of the fracturing.Traditional fracturing fluids often cause fluid blockage and residue in reservoirs,which reduce the conductivity of fractures,affect oil and gas production,even cause permanent damage to reservoirs.By contrast,clean fracturing fluid automatically breaks and has the advantages of no residue and complete backflow,which makes up for the defects of traditional fracturing fluid and has important significance and application prospects in oil and gas development.The development of clean fracturing fluid is a key aspect of the hydraulic fracturing process.For more than two decades,clean fracturing fluids based viscoelastic surfactant fluids have been utilized in increasing the rate of hydrocarbon production of low-permeability reservoir formations.These fluids have been widely used due to their many advantages.However,most conventional viscoelastic surfactants have poor viscoelasticity and microstructural properties,poor resistance towards elevated temperature and shear.Constructions of viscoelastic fluids with great performances consume the high concentration of surfactants that elevates development costs.The current study intends to synergistically enhance the viscoelasticity and microstructural aggregations properties of low concentrations carboxybetaine solutions.The viscoelastic surfactant systems were developed using oleyl amidopropyl betaine(OAPB)/dodecyl sulfate(SDS),OAPB/ortho phthalic acid(OPA),and erucamidopropyl betaine(EDAB)/hydroxybenzoate salts.The optimal concentration and conditions were established based on the system performance.The viscoelasticity of the systems was analyzed by rheometer.The dynamic light scattering was used to examine the distribution sizes of micelles in the solution,which can clarify the process of aggregate transformation.The microstructural morphology in the solution was investigated by transmission electron microscopy,which can reveal the mechanism of phase transformation.The effects of p H value,concentration and temperature on the viscoelasticity of the system were studied by rheometer,to reveal the viscosity control mechanism of the system.To examine the stability of clean fracturing fluids towards high temperature and shear the rheometer was used.The sand carrying ability and stability of studied systems was measured by the static sand settling method.The certain volume ratios of kerosene were mixed with a viscoelastic fluid to evaluate how quickly the gel can break.Finally,a new type of clean fracturing fluid system is obtained through the concentration and component optimization,and its feasibility and application conditions in the oilfield are comprehensively evaluated.The experimental results revealed that the addition of SDS to OAPB can significantly enhance the viscoelasticity and microstructural properties of the system.The low viscosity(62m Pa?s)of conventional 47 m M OAPB was amplified to 3.5×10~6 m Pa?s when 8.7 m M SDS was added to the system.Apart from hydrophobic interactions between surfactant chains,the improved properties in the OADS solution are due to electrostatic attractions between head groups of these surfactants that increased micellar contour length and micellar network entanglements.To investigate the effect of aromatic acid on OAPB,the 30 m M OAPB/20 m M OPA(OAPA)system was formulated.Similar to SDS addition in the OAPB solution,the OPA has also improved the microstructural properties of the OAPB solution.Compared to OADS the concentration of the OAPA system has been reduced by 10.23%.The improved results of OAPA were influenced by the existence of electrostatic attraction and hydrogen bonds that are essential for reducing the repulsive forces between the hydrophilic groups of the surfactant molecules by decreasing the effective head group area and increasing the surfactant packaging parameter which is a positive effect for the formation of strong and stable wormlike micellar networks.The effect of isomers of aromatic salts;sodium 2-hydroxybenzoate(o Ben),sodium3-hydroxybenzoate(m Ben),sodium 4-hydroxybenzoate(p Ben)on the viscoelasticity of erucamidopropyl betaine(EDAB)solution at elevated temperature were studied.Compared to conventional EDAB solution at p H=7,the EDAB-hydroxybenzoates showed high viscoelasticity and viscosity,EDAB-o Ben(Eo Ben)solution being great of the rest.The designed systems were responsive towards p H,the viscosity was tuned between high and low at certain p H for three cycles without deterioration of their properties.It means the systems can be reused to save chemical costs.It is found that p H value,temperature and concentration can control the viscoelasticity of the systems.The formation and destruction of wormlike micelles can be controlled by p H,while concentration and temperature can control the solution performance by controlling the growth process of wormlike micelles and the destruction degree of the system structure.The performance evaluation results of the designed clean fracturing fluid showed that the sand settling velocity rate,gel breaking ability,core permeability damage,and residual content performances of these systems were within the acceptable standards of fracturing fluids,Eo Ben being best of all.In this study,the effects of additives on the performance of viscoelastic surfactant fracturing system under different conditions were studied,and the problems of VES fracturing fluid with poor viscoelasticity,good performance and high concentration utilization were proposed.This study revealed the effect of additives on the properties of responsive wormlike micelles,revealed the methods to weaken and enhance the solubilization effect of additives,and proposed its mechanism.The current study unlocks the significant prospect of designing smart fluids for various industrial applications. |
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