Study On Low Molecular Weight Carboxymethyl-hydroxypropyl Guar Gum Gel System And Its Rheological Properties

In this paper, carboxymethyl-hydroxypropyl guar gum (CMHPG) was synthesized from the endosperm of guar gum and the rheological characteristics of aqueous solutions of CMHPG were studied; low molecular weight carboxymethyl-hydroxypropyl guar gum (LMW-CMHPG) was obtained through hydrogen peroxide degradation; the formation process, non-degradable breaking and regeneration of gel systems composed of LMW-CMHPG and borate and the rheological properties were investigated. Main conclusions were obtained as follows:1) CMHPG was synthesized from the endosperm of guar gum through etherification. The influences of reaction temperatures, reaction time, dosage of etherifying agent, etc. on products were illustrated. The proper process for CMHPG were achieved:35%-40%(wt%) ethanol water was used as solvent, the dosage of solvent was 6 times of the guar gum (w/w); alkalization lasted for 45 minutes at ambient temperature while the mass ratio of NaOH to guar gum was 0.3:1. During etherification, the mass ratios of carboxymethylation agent CA, hydroxypropylation agent EP to guar gum were 0.4:1 and 0.3-0.4:1 respectively, reaction temperature was 75-80℃and reaction time was 180-210min. The maximum apparent viscosity of 0.6%(wt%) aqueous solution of the synthesized CMHPG was 140-150mPa.s and water insoluble residue was reduced significantly.2) Aqueous solutions of CMHPG exhibt shear-thinning property and the flow curves coincided with Power Law Model, the zero shear viscosity decreased while the temperature increased and their relation corresponded to Arrhenius equation. Aqueous solutions of CMHPG possessed certain acid-alkali resistance ability. The effects of NaCl and CaCl2 on the viscosity of solutions are significant.3) LMW-CMHPG was obtained through hydrogen peroxide degradation for the first time. The apparent viscosity and intrinsic viscosity of LMW-CMHPG decreased with the increase of hydrogen peroxide dosage. Stable gel system can be formed under certain composition of LMW-CMHPG and borate. The influences of LMW-CMHPG concentration, borate concentration and pH value on gel system were explained. The gels possessed typical shear-thinning property, as well as good viscoelasticity and thixotropy.4) The borate cross-linked LMW-CMHPG gel system was proved to be well regenerated through adjusting pH value; the rheological properties of gels remained basically the same as before regeneration. Non-linear co-rotational Jeffreys model could be applied to describe the flow curve of gel system correctly and the calculated values were in good agreement with the experimental data.The study in this paper could be expected to provide rheological basis for the application of new type of low molecular weight modified guar gum gel fracturing fluid.