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Summary Polymer gels are widely used in sealing operations in the oil and gas industry. Gel with high strength is strongly needed to perform such operations as fluid loss control, temporary plugging, casing integrity treatment, and water shutoff, etc. In this paper, a simple method is proposed to obtain ultrahigh strength gel (USGel), which is called multistage reinforcement method (MRM) and implies the philosophy of noncompetitive crosslinking, multistage reinforcement of molecular chains, and increased entanglement. By this method, acrylamide (AM), xanthan (XC), and two crosslinkers [N,N'-Methylenebisacrylamide (MBA), chromium acetate (Cr)] are selected to develop an USGel with potential for medium to ultralow temperature reservoir applications. The thermal stability and sealing performance of USGel are evaluated. The effect of component concentration, gelant pH, and temperature on the strength and gelation time of USGel are investigated. Counterintuitively, adding solid materials even reduces USGel strength. The strength of USGel exceeds other gels by comparison with reported stress-strain data from similar literature. After data fitting analysis, without considering the XC concentration, 1% Cr can increase the strength of the gel by 78.32 times, while other gels with 1% reinforcement material (e.g., solid materials) can only increase by 1.09 to 1.35 times. At low concentrations (0.4% XC and 0.2% Cr), USGel strength can also reach 84.59–383.21 kPa at 15°C to 105°C when strain = 60%. Environmental scanning electron microscope(ESEM) and gelation data are used to further reveal the multistage reinforcement mechanism of USGel and the reason for strength reduction owing to adding solid materials. USGel is promising to be applied in sealing operations in medium to ultralow temperature reservoirs.
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