Investigation of Rheological Behavior of Silica-Based Nanofluids for Enhanced Geothermal Systems

Abstract

Geothermal energy is becoming a critical component in the global transition toward sustainable energy, providing a reliable and environmentally friendly alternative to fossil fuels. Geothermal resources span a broad spectrum, from easily accessible hydrothermal reservoirs to deeper, untapped heat sources. Among these, Enhanced Geothermal Systems (EGS) represent a significant opportunity for energy extraction through advanced heat-mining techniques. EGS technology, known for its moderate to high enthalpy, shows promise for efficient heat extraction and power conversion. However, one of the key challenges in EGS is thermal short-circuiting, where the working fluid flows preferentially through high-permeability fractures, limiting heat recovery from less conductive areas and reducing overall system efficiency. This study investigates the potential of nanofluids as an innovative approach to mitigating thermal short-circuiting and optimizing fluid flow in EGS reservoirs. We propose the use of stimulus-responsive, heat-transfer nanofluids to regulate permeability and improve flow distribution beyond the near-wellbore region, enhancing heat extraction efficiency. In our work, a series of nanofluids were developed by dispersing fumed silica (SiO2) nanoparticles in a polyethylene glycol (PEG) carrier fluid. The rheological properties of the nanofluids were evaluated using an ARES-G2 rheometer, with a frequency sweep test conducted in bob-and-cup geometry. Key rheological parameters investigated include viscosity, storage modulus, loss modulus, and shear stress. Additionally, thermal conductivity was measured using the transient hot-wire method to assess the thermal performance of the nanofluids. The results indicate that the rheological behavior of the silica-based nanofluids is highly dependent on both the concentration of fumed silica and the molecular weight of the PEG. At low shear rates, the nanofluids exhibited shear-thinning behavior, while beyond critical shear rates, the viscosity profile transitioned to shear-thickening. The increase in viscosity of the nanofluids at high shear rates impacts the fluid flow and permeability of the reservoir. Furthermore, the inclusion of nanoparticles significantly enhanced the thermal properties of the fluid, improving heat extraction potential in EGS reservoirs. This study highlights the novelty of using nanofluids as a pioneering solution for enhancing heat recovery in EGS. The shear-thickening properties of the nanofluid allow for the effective regulation of permeability in fractured reservoirs, redirecting flow toward high-temperature zones through hydrocluster formation, while simultaneously improving the thermal conductivity of the working fluid.