Accurate prediction of heat transport in porous media is important for understanding geoscience processes and properties and to design applications, for example geothermal energy systems. While heat transport is generally modelled assuming of local thermal equilibrium (LTE), i.e., instantaneous heat transfer between the fluid and solid phases, previous studies have demonstrated presence of local thermal non-equilibrium (LTNE), i.e., delayed heat transfer, in natural porous materials. However, factors that influence the rate of heat transfer between the phases and their significance for inherently heterogeneous natural systems remain unknown and untested. We develop an open-source fully coupled, finite-element application to numerically simulate heat transfer between the fluid and solid phases. This is based on the Multiphysics Object-Oriented Simulation Environment (MOOSE) and allows massively parallel modelling of heat transport including customized transfer rates. We verify our model using an analytical solution considering LTNE and illustrate several applications. The model can be used to investigate processes that affect heat transport such as heat transfer mechanisms and their dependence on different hydrogeological conditions.
<p>Surface temperature variations have been well shown to transfer their thermal signature into the subsurface. This continuous heat transfer manifests in altered thermal conditions in the subsurface where temperature variations over a long lapse of time are more pronounced than shorter ones. Hence, repeated temperature depth profiles allow to investigate the effects of recent climate change on the subsurface. In this study we present recent temperature trends in more than 40 observation wells in Bavaria, Germany. Temperature depth profiles have been quarterly measured for one year between 1992-1994 and measurements have been repeated two times in 2019. The quarterly measurements reveal that the periodic seasonal temperature signal dampens to around 0.1 K at a depth of 15 m below ground surface. This implies that temperature variations below this depth can be used as climate archives as they store the temperature history of multiple years. The measurements span a time period of almost 30 years which is the most common period of reference for deriving climate normals according to the World Meteorological Organization. Therefore, the findings of recent subsurface temperature variations are assessed versus and complemented by 22 air temperature stations. Preliminary results show, that the linear regression of the annual mean air temperature since 1990 yields a slope of 0.35 &#177; 0.11 K 10a<sup>-1</sup>. In the subsurface, median temperature differences of the respective baselines from 1992-94 period and 2019 are 0.26, 0.13 and 0.07 K 10a<sup>-1 </sup>at 20, 40 and 60 m depth below surface, accordingly. Despite the common magnitude and continuous downward decrease, subsurface temperature differences exhibit a much higher variance compared to air temperature changes. This is due to local effects, such as varying thermal conductivities of the subsurface, latent heat transport caused by evapotranspiration, lateral and vertical groundwater flow, and anthropogenic influences. Our contribution will feature a comparison of this temperature change in response to recent atmospheric climate change in Bavaria and link these results with perceptions gained by similar investigations on local scale in other European regions.</p>
Aquifers contain the largest store of unfrozen freshwater, making groundwater critical for life on Earth. Groundwater temperatures infl uence stream thermal regimes, groundwater-dependent ecosystems, aquatic biogeochemical processes, water quality, and the geothermal potential. Yet little is known about how groundwater responds to surface warming across spatial and temporal scales. We simulate current and projected groundwater temperatures at the global scale and show that groundwater at the depth of the water table is projected to warm on average by 3.3 ° C between 2000 and 2099 (RCP 8.5). However, regional groundwater warming patterns vary substantially due to spatial variability in climate and water table depth. The highest warming rates are projected in Central Russia, Northern China, and parts of North America and the Amazon rainforest. Results also show that by 2099, 234 million people are projected to live in areas where groundwater exceeds the highest threshold for drinking water temperatures set by any country.
Unmanaged heat extraction, as well as the adjacency of multiple borehole heat exchangers (BHEs) in a field, can lead to undesirable thermal conditions in the ground. The failure to properly control induced thermal anomalies is perceived as a severe risk to closed-loop geothermal systems, as the detrimental effects on the ground can substantially deteriorate performance or nullify the compatibility of an operating system with regulatory mandates. This paper presents a flexible framework for the combined simulation-optimization of BHE fields during the entire lifespan. The proposed method accounts for the uncertainties in subsurface characteristics and energy consumption in order to minimize the temperature change caused by the heat extraction during the operation. The descriptive uncertainty is introduced as a deviation of the monitored temperature from the simulated temperature change, whereas the variation of the energy demand appears as over- or under-consumption against the scheduled demand. The presented new sequential procedure, by updating the thermal conditions of the ground with temperature measurements, continuously executes the optimization during the operation period and enables the generation of revised load distributions. In this study, two fields with five and 26 BHEs are considered to demonstrate the performance of the proposed method. Sequential optimization outperforms single-step optimization by providing the basis for more strategic load-balancing patterns and yielding lower temperature anomalies of about 2.9 K and 8.9 K in each BHE configuration, respectively, over 15 operational years.
Abstract CO 2 injected into storage formations may escape to the overlying permeable layers. Mixed‐phase diffusivity, namely the ratio of hydraulic conductivity and specific storage of the phase mixture, declines with increasing CO 2 saturation. Thus, it can be an indicator of CO 2 leakage. In this study, we perform interference brine or CO 2 injection tests in a synthetic model, including a storage reservoir, an above aquifer, and a caprock. Pressure transients derived from an observation well are utilized for a travel‐time based inversion technique. Variations of diffusivity are resolved by inverting early travel time diagnostics, providing an insight of plume development. Results demonstrate that the evolution of CO 2 leakage in the above aquifer can be inferred by interpreting and comparing the pressure observations, travel times, and diffusivity tomograms from different times. The extent of the plume in reservoir and upper aquifer are reconstructed by clustering the time‐lapse data sets of the mixed‐phase diffusivity, as the diffusivity cannot be exactly reproduced by the inversion. Furthermore, this approach can be used to address different leaky cases, especially for leakage occurring during the injection.
Despite the global interest in green energy alternatives, little attention has focused on the large-scale viability of recycling the ground heat accumulated due to urbanization, industrialization and climate change. Here we show this theoretical heat potential at a multi-continental scale by first leveraging datasets of groundwater temperature and lithology to assess the distribution of subsurface thermal pollution. We then evaluate subsurface heat recycling for three scenarios: a status quo scenario representing present-day accumulated heat, a recycled scenario with ground temperatures returned to background values, and a climate change scenario representing projected warming impacts. Our analyses reveal that over 50% of sites show recyclable underground heat pollution in the status quo, 25% of locations would be feasible for long-term heat recycling for the recycled scenario, and at least 83% for the climate change scenario. Results highlight that subsurface heat recycling warrants consideration in the move to a low-carbon economy in a warmer world.
