Study On Properties And Structure And Chemical Modification Of Xanthan Gum

Xanthan gum (XG) polymer flooding widely used at present in oil field had been researched in this article. According to the required capability of polymer flooding and the shortcomings that existed in the process of applications, such as the high dosage and the limit of temperature resistance, we would change the molecule structure and chemically modified the xanthan gum. Hydrophobic association with the property of the modified xanthan gum were synthesized through the etherification of xanthan gum and chlorinated hydrocarbons (ArCl), introduced hydrophobic groups in the polymer chains. The influence of reaction conditions such as reaction temperature, reaction pH, polymer concentration, the dosage of modifier and reaction time on the tackification performance of the modified polymer solution were studied, and the suitable reaction parameters of etherification were obtained respectively. The polymer of modified xanthan gum were characterized by ultraviolet absorption spectra(UV), Fourier transform infrared (FT-IR) spectroscopy and proton nuclear magnetic resonance (1H-NMR); and proceeded quantitative analysis for the modifier content of modified xanthan gum using ultraviolet absorption spectra. The results showed that the polymer MXG was prepared for the target product. Following, solution properties and microstructure of hydrophobically associating were in-depth systematic studied. The study results indicated, compared with the xanthan gum, modified xanthan gum exhibited the hydrophobic association behavior in water and brine solution, and good solution properties such as thickening properties, heat resistance, salt resistance, shear resistance; but in the strong acid and alkali solution, as the original molecule structure of polymer MXG and XG was destroyed, and its performance was poor compared with pure water solution, salt solution. The polymer MXG exhibited good thickening properties in pure water and NaCl, CaCl2 solution. For the aqueous solution and NaCl solution with polymer concentration of 2 g/L, the apparent viscosities were 354 mPa·s and 314 mPa·s, respectively. In the range of 0～60 g/L NaCl concentration, the hydrophobic association could be formed by the hydrophobic interaction of the hydrophobic group; and the C-O bonds of chains could be complex with the Na+. Therefore, the MXG brine solutions exhibited the salt-thickening behavior twice and good salt-resistant performance. Their apparent viscosity with the polymer concentrations of 1.5 g/L and 2 g/L were up to 174 mPa·s and 340 mPa·s at high NaCl concentration of 60 g/L, respectively, and at the same salt concentration, XG also exhibited the salt-thickening behavior, but its apparent viscosity had only 140 mPa·s and 239 mPa·s compared with the corresponding concentration of MXG. The variation law of apparent viscosity of polymer in CaCl2 solution was similar with that in NaCl solution, but the salt effect of Ca²⁺ is stronger. All the viscosity above had been determined under conditions for 30 oC and 7.34 s^-1.In the range of 30⁷5 oC, 1.5 g/L and 2 g/L MXG showed feeble heat-thickening in 5 g/L NaCl solutions because of intermolecular association and the apparent viscosity were the highest at 35 oC, and were 188 mPa·s and 318 mPa·s, respectively. The solution viscosity of the two concentrations were still up to 86 mPa·s and 190 mPa·s at 75 oC, respectively, which exhibited good heat resistance of the polymers. At this temperature, the apparent viscosity of XG solution corresponding to the concentration was only 38 mPa·s and 76 mPa·s. The apparent viscosity of the 1.5 g/L MXG and XG solution at 70 oC after the aging of 60 days was 111 mPa·s and 65 mPa·s, respectively, and the viscosity retention rates were 60% and 54%, separately. From the above that the anti-aging property of MXG Polymer solution was better than XG. The apparent viscosity decreased sharply with increasing shear rate as MXG associative structure at low shear rate was destroyed; and the apparent viscosity decreased gradually at high shear rate as macromolecular chain oriented in the flow field. The consecutive shear cycles measurement indicated that the MXG exhibited thixotropy behavior. The results also indicated that the Molecular chain structure did not been destroyed upon consecutive shear. When MXG polymer was under the shear rate of 7.34 s^-1 for a long time, both in pure water or brine solutions, the apparent viscosity of the solution decreased with the increase of shearing time only in the initial shear, and the solution apparent viscosity was no longer decreased when the shearing time was more than 2 min, which indicated that the MXG polymer displayed excellent anti-shearing properties.The UV by pyrene as a probe was used for investigating the formation of hydrophobic microenvironment for the MXG polymer. The number of intermolecular hydrophobic microdomains structures increased and absorbance of the solution enhanced with increasing of polymer concentrations in pure water and 5 g/L NaCl solutions. Meanwhile, the association of hydrophobic groups was stronger and associative structures were more closely with the adding of NaCl, which leaded to the increase of UV absorbance in brine solutions. In 5 g/L of NaCl salt solution, the solution absorbance curve occurred a turning point in the polymer concentration of 1.5 g/L, which indicated that the polymer MXG had a critical aggregation concentration of 1.5 g/L in 5 g/L NaCl solution. This performance was consistent with the results determined in the tackifying solution. When the concentration of NaCl raised to 3 g/L, the association of hydrophobic groups was reinforced and the non-polarity of associative structures was enhanced as polymer chains produced shielding effect with introducing the Na+, resulting in more pyrene molecules in the structure of association, so 2 g/L polymer MXG first occurrence of the maximum absorbance, was 1.39; when the concentration of NaCl was still up to 20 g/L the intermolecular association was reinforced due to the C-O bonds in polymer chains and the Na+ complexation in solution, then absorbance of the solution appeared a second maximum, 1.62. For the 1.5 g/L MXG polymer solution, the variation regulation of solution absorbance with NaCl concentration was similar with and 2 g/L MXG polymer.The study of the polarity of hydrophobic microdomains of MXG at different temperatures displayed that elevating temperature contributed to the association of hydrophobic groups to increase the solution of the non-polarity of hydrophobic microdomains. When the temperature was raised from 30 oC to 60 oC, the gradual rising of temperature resulted in the strengthenment of the intermolecular hydrophobic association, the enhancement of the non-polarity of hydrophobic microdomains and the increase of absorbance of 1.5g/L MXG polymer in pure water. As the temperature was still up to 65 oC, the associative structures became loose and the non-polarity of the hydrophobic microdomains abated and the absorbance decreased. The intermolecular association of polymer was too strong, the tightness of associative structures further increased, and the absorbance enhanced with further increasing the temperature. The variation laws of absorbance with temperature of 1.5 g/L, 2 g/L polymer in brine solution and 2 g/L MXG in pure water were similar with 1.5 g/L MXG polymer in pure solution.