Dependency of heat transport on sedimentary heterogeneity: impacts of hierarchy, resolution and upscaling methods in fluvial low-enthalpy geothermal reservoirs

Modeling heat transport in low-enthalpy geothermal reservoirs is essential for optimizing geothermal applications. Fourteen geocellular grids were created to analyze the impacts of numerical grid resolution, permeability upscaling methodology, and modeled scales of sedimentary architecture, employing MODFLOW-2005 and MT3D-USGS to simulate groundwater flow and heat transport for well doublets operating over 35 years. The results reveal complex relationships between reservoir-modeling choices and simulated reservoir behavior. The considered factors have a marked influence on injection pressures but only a modest effect on production temperatures (within 2°C after 35 years across all models); this is likely due, at least in part, to a relative dominance of thermal diffusion over heat advection in the considered scenarios. Simplification of sedimentary architectures through omission of fine-scale features may augment the hydraulic impact of larger flow barriers, such as abandoned-channel mud plugs, whilst removing high-permeability units that may act as thief zones. Permeability upscaling methods also impact dynamic outputs: simulations on permeability fields upscaled using harmonic averaging consistently yield highest near-injector pressures, followed by those based on geometric averaging and arithmetic averaging. The dynamic behavior of grids upscaled via flow-based upscaling approximates closely that of grids upscaled using arithmetic averaging, suggesting that the bulk hydraulic behavior is dominated by the connectivity of high-permeability units. Differences in the impact of the chosen upscaling method decreases significantly for grids with higher resolutions. Simulations of geologic models that incorporate increasingly detailed geologic features predict cold-water plumes with slightly more complex shapes and tortuous fronts, as documented by quantitative shape descriptors. The morphological complexity of the cold-water plume is slightly higher for well doublets oriented at a high angle to the channel-belt axis, but does not increase systematically with the resolution at which fine-scale features are represented.