Enhanced geothermal systems (EGS) are emerging technologies that can recover clean energy from the Earth's crust. However, the short-circulation in geothermal reservoirs can significantly deteriorate their long-term viability as sustainable energy because injected cold fluids could quickly flow through the fractures with large apertures or the fractures directly communicating from injection well to production well without acquiring enough heat from the rock matrix. Therefore, controlling flow regimes outside the wellbore and within the reservoir is vital to mitigate undesirable flow. Polymer gels which have been successfully applied to control the preferential flow in oil and gas reservoirs, could also be adapted to control the preferential flow in geothermal reservoirs. Currently, no hydrogel products are available to meet the harsh conditions of geothermal reservoirs ( greater than 150 oC). Our group has a project funded by the US Department of Energy (DE-EE0009790), which focuses on developing polymer hydrogels that can be stable at 150~250 oC. We have developed several novel high-temperature resistant preformed particle gels (HT-PPG) that can be stable at 200 oC. This work reported the swelling, rheology behavior and thermal stability of one novel HT-PPG. We assessed the effect of variables like temperature, pH, and salinity on swelling behavior, gel strength and long-term thermal stability of this novel HT-PPG. The dried gel particle can swell to over 30 times its original volume, and the elastic modulus of the fully swelled gel can reach over 700 Pa. Additionally, the HT-PPG showed excellent long-term hydrolytic thermal stability. No syneresis was observed during the two months of exposure at 200 oC. HT-PPG described in this work is a promising product for controlling the fluid and heat flow in fractures containing geothermal reservoirs.

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