| Xanthan gum(XG)is a kind of anionic polyelectrolyte polysaccharide produced by Xanthomonas campestris and it has an important application in the field of petroleum exploitation as thickener.At present,the production of XG is very large and has been widely used in China.However,there are remain some defects and limitations in the application of XG,for example,lower thixotropy,heat resist performance and salt tolerance,large sample volume required and the thickening effect needs to be improved.Therefore,further study will be needed.In this work,several series of modified xanthan gum products were synthesized and their rheological properties were studied,the application of modified XG as fracturing fluid thickener were also investigated.The results showed that the properties of the modified xanthan gum were significantly improved,and it had a good application prospect in oil exploitation as thickener.The specific works are as follows:Firstly,two water-soluble with low crosslinking density xanthan gum derivatives(BDDEXG and PPGDGE-XG)were prepared by the reaction of 1.4-butanediol diglycidyl ether(BDDE)and polypropylene glycol diglycidyl ether(PPGDGE)with XG respectively.The synthesis conditions were optimized by single-factor method,the structure and morphology of the modified xanthan gum samples were characterized by various characterization methods,and the rheological properties(shear thinning behavior,viscoelasticity and thixotropy),heat resistance and salt tolerance of the modified xanthan gum samples were studied.The Cross model was applied to fit the shear thinning curve,the carrying capacity of the modified sample and the rheological property of the modified sample in artificial seawater were also investigated.The experimental results indicated that the optimal usage of crosslinking agent were both 7wt%,BDDE-XG-7 and PPGDGE-XG-7 were prepared under the optimal conditions.The apparent viscosity of 0.5% BDDE-XG-7 and PPGDGE-XG-7 were 6.9 and 5.1 times higher than that of 0.5% XG solution.After shearing at 80 ℃,the retention viscosities of XG,BDDEXG-7 and PPGDGE-XG-7 were 3.7 and 2.6 times higher than that of XG,respectively.The heat resist performance and carrying capacity of cross-linked modified XG was good.Moreover,the cross-linked samples show good shear thinning behavior and viscoelastic properties in artificial seawater,and the experimental values can be well fitted by cross model.Secondly,several hydrophobic modified xanthan gum(DXG,GXG and BXG)were prepared by the reaction of xanthan gum with C12～14 alkylglycidyl ether(DTGE),epoxy propylphenyl ether(GPE)or n-butyl glycidyl ether(BGE)with different chain lengths and functional groups.The apparent viscosity of DXG1,GXG1.5 and BXG2 at 0.5% concentration were 4.9 times,3.1 times and 2.4 times higher than that of XG samples,respectively.The structure and morphology of hydrophobically modified xanthan gum were characterized.The rheological properties of the sample solutions such as shear thinning behavior,viscoelasticity and thixotropy were tested.The flow curves were well fitted with Cross model and the salt tolerance,temperature resistance and the effects of salt and temperature on apparent viscosity were also investigated.The retained viscosity of DXG1,GXG1.5 and BXG2 after shearing at80 ℃ were 2.8 times,1.7 times and 1.6 times higher than that of XG,respectively.Among all the hydrophobic modified xanthan derivatives,the sample of DXG1 has better temperature and shear resistance and carrying capacity.The DXG1 sample solution had good breaking properties at different temperature and different concentration of breaker.Finally,the cationic etherification modified xanthan gum(CXG)was prepared by etherifying XG with 3-chloro-2-hydroxypropyl-trimethylammonium chloride(CHTA),and the cationic etherification hydrophobic modified material(CDXG)was prepared by DTGE hydrophobic modification of CXG.A Double network structure formed by electrostatic and hydrophobic association was built to improve the rheological properties.The rheological properties were tested in detail.The results show that the modified samples had good rheological properties,heat resistance performance and salt tolerance.The apparent viscosity of 0.5% CXG and CDXG solution were 5.2 times and 6.6 times higher than that of XG solution.The apparent viscosity of CDXG was 32.5% higher than that of DXG1 and 27.8% higher than that of CXG.The heat resistance performance results indicate that the retention viscosities of CXG and CDXG solutions were 2.6 and 3.2 times higher than that of XG solution.The retention viscosity of CDXG was 16.6% higher than that of DXG1 and 22.7% higher than that of CXG.The modified sample solutions have good carrying capacity.Moreover,the gel breaking test was carried out under different temperature and different concentration of breaker.It was found that the sample(CDXG)has a good gel breaking behavior by fitting the gel breaking kinetic equation,which has a good application prospect in fracturing fluid thickener. |
