Enhanced geothermal systems (EGS) are emerging as a promising carbon-neutral energy source. However, the short-circulation in some geothermal reservoirs diminishes heat extraction efficiency and production lifespan. Natural and hydraulic fractures introduce significant heterogeneity in the reservoir, making injected cold fluid to direct quick flow through super-K channels or fractures, thus limiting heat-extraction efficiency. Therefore, plugging unexpected fractures is vital to mitigate heterogeneity and increase heat recovery. Polymer gel treatment is a common method to be applied in oil and gas reservoirs, which can also reduce undesirable flow in geothermal reservoirs. Current particle gels can remain stable in hydrothermal conditions (greater than 150 ℃) for several months but at a rapid swelling rate, confining its transport ability gel. Supported by DOE, we are developing a novel swelling-delayed high-temperature preformed particle gel (HT-PPG) and evaluating the effect of temperature on swelling behavior, mechanical strength, and long-term hydrothermal stability. The dried gel can swell over 60 times its original weight, and the elastic modulus of the fully swelled gel can reach over 4000 Pa at room temperature at a swelling ratio of 22; after 3 months of aging at 150℃, the swelling-delayed HT-PPG(HT-DPPG) still has the gel strength of 340 Pa at a swelling ratio of 60. HT-DPPG offers a promising approach to control swelling rates, enabling it for preferential fluid flow control in geothermal reservoirs.

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