Co-Gelatinization Modification of Iodine–Starch and Its Performance in Drilling Fluid

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Modified starch is used as a drilling fluid treatment agent in oilfields. During the drilling process, modified starch plays a vital role in the drilling fluid system, but its poor temperature resistance limits its application in oilfields. Therefore, this paper studied the performance of several starches in water-based drilling fluids through co-gelatinization modification; studied the effectiveness of several modifiers in gelatinized starch drilling fluids, combined with flow modification performance tests, bentonite linear expansion rate, salt resistance, and other experimental methods to complete the screening of the best modified starch; and systematically compared the temperature resistance, inhibition, compatibility, and salt resistance before and after gelatinization. The possible mechanism of action of modified starch treatment was analyzed and tested by Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), laser particle size analyzer and scanning electron microscopy (SEM). The results showed that the decomposition temperatures of cassava starch (TS), potato starch (PS), and corn starch (CS) were different, and the decomposition temperature of cassava starch (TS) was the highest, at 150 °C. At this temperature, the optimal dosage of gelatinized TS is 2.5%, the maximum shear force is 2.25 Pa, and the filtration loss is only 12.6 mL. TS has obvious performance advantages over other starches. After co-gelatinization with 1.0% iodine and 2.5% TS at 140 °C, it has a good viscosity reduction and filtration loss effect, and the filtration loss is only 5.2 mL, which is 31.6% lower than that of untreated TS drilling fluid. The linear expansion rate at 120 min is 10.85%, indicating that it has a strong inhibitory effect on the hydration and dispersion of bentonite. In addition, iodine cassava starch (ITS) has good compatibility in drilling-fluid formulations and shows good salt resistance when mixed with different concentrations of KCl. The results of this study can be used to improve the temperature resistance of filtration agents and facilitate related research.