Thermal damage mechanisms are crucial in reservoir stimulation for enhanced geothermal system (EGS). This study investigates the thermal damage mechanisms in granite samples from the Gonghe Basin, Qinghai, China. The granite samples were heated to 400 degrees C and then cooled in air, water, or liquid nitrogen. The physical and mechanical properties of the thermally treated granite were evaluated, and microstructural changes were analyzed using a scanning electron microscope (SEM) and computed tomography (CT). The results indicate that cooling with water and liquid nitrogen significantly enhances permeability and brittleness while reducing P-wave velocity, strength, and Young's modulus. Specifically, liquid nitrogen cooling increased granite permeability by a factor of 5.24 compared to the untreated samples, while reducing compressive strength by 13.6%. After thermal treatment, the failure mode of the granite shifted from axial splitting to a combination of shear and tension. Microstructural analysis revealed that liquid nitrogen-cooled samples exhibited greater fracture complexity than those cooled with water or air. Additionally, acoustic emission (AE) monitoring during damage evolution showed that liquid nitrogen cooling led to higher cumulative AE energy and a lower maximum AE energy rate, with numerous AE signals detected during both stable and unstable crack growth. The results suggest that liquid nitrogen induces a stronger thermal shock, leading to more significant thermal damage and promoting the development of a complex fracture network during EGS reservoir stimulation. This enhances both the heat exchange area and the permeability of the deep hot dry rock (HDR) in EGS reservoirs. The insights from this study contribute to a deeper understanding of thermal damage characteristics induced by different cooling media and provide valuable guidance for optimizing deep geothermal energy extraction. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Accurate prediction of hydraulic fracture propagation is vital for Enhanced Geothermal System (EGS) design. We study the first hydraulic fracturing job at the GR1 well in the Gonghe Basin using field data, where the overall direction of hydraulic fractures does not show a delineated shape parallel to the maximum principal stress orientation. A field-scale numerical model based on the distinct element method is set up to carry out a fully coupled hydromechanical simulation, with the explicit representation of natural fractures via the discrete fracture network (DFN) approach. The effects of injection parameters and in situ stress on hydraulic fracture patterns are then quantitatively assessed. The study reveals that shear-induced deformation primarily governs the fracturing morphology in the GR1 well, driven by smaller injection rates and viscosities that promote massive activation of natural fractures, ultimately dominating the direction of hydraulic fracturing. Furthermore, the increase of in situ differential stress may promote shear damage of natural fracture surfaces, with the exact influence pattern depending on the combination of specific discontinuity properties and in situ stress state. Finally, we provide recommendations for EGS fracturing based on the influence characteristics of multiple parameters. This study can serve as an effective basis and reference for the design and optimization of EGS in the Gonghe basin and other sites. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

A new type of thermally controlled subgrade is proposed to mitigate persistent frost heave issues of railway subgrades in seasonally frozen regions. A dedicated ground-source heat pump system collects low-grade geothermal energy from the stable soil layer near the subgrade, converts it into high-grade thermal energy, and transfers it to the frigid subgrade for active heating and temperature control, thereby eliminating the adverse effects of frost heave. A 20-metre-long test of thermally controlled subgrade was constructed in a frost heave of the Junggar-Shenchi Railway in Shanxi Province, China. During the winter spanning 2021 and 2022, the heating temperature of the heat pump, the thermal regime of the test subgrade and the natural subgrade, the frost depth, and the track heave were measured. The results indicate that the heat pump temperature could reach a peak of 59.4 degrees C, with the average daily heating temperature during intermittent operation reaching 25.2 degrees C or higher, indicating an efficient heat source that plays a favourable role. The freezing period of the natural subgrade lasted for 141 days, while the subgrade in the test was 20 days shorter. The maximum frost depths at the track centre, shoulder, and embankment slope toe in the test were 88 cm, 75 cm, and 58 cm, respectively. These depths were 60 cm, 122 cm, and 78 cm less than those of the natural subgrade, effectively controlling the frost depth within the threshold that may cause potential structural damage. Under natural conditions, the track heave reached a peak of 9.4 mm, leading to a harmful frost heave scenario. In contrast, the track deformation in the test was less than 3 mm, which did not exceed the regular maintenance threshold. The thermally controlled subgrade proves to be an effective method for preventing and controlling persistent frost heave damage in critical locations such as low embankments, cut subgrades, turnout areas, and culvert roofs.

Energy shallow foundations represent an innovative technology that can simultaneously support structural loads and harvest geothermal energy. During geothermal operations, the underlying soils are subjected to structural loads and temperature fluctuations. Despite the potential, knowledge regarding the thermo-hydro-mechanical behavior of the multilayered soils beneath the energy foundations remains scarce. This study proposed an analytical approach to investigate the thermo-hydro-mechanical response of soft fine-grained soils beneath energy shallow foundations. The analysis focused on the evolutions of the temperature, pore water pressure, and vertical displacement of the underlying soils. The results indicate that the generation and development of the thermally induced excess pore pressure are controlled by thermal transfer processes and soil hydraulic properties. Furthermore, the mechanical load-induced ground settlement decreases upon heating and increases upon cooling, primarily due to the development of thermally induced pore pressure and the thermal volume changes of the soil skeleton. Under the considered conditions, ignoring the thermally induced mechanical effects could result in a settlement prediction error of nearly 120%. Therefore, the thermo-hydro-mechanical interactions within the soils should be appropriately considered in the analysis and prediction of the displacement behavior of the energy foundations.

Fluid flow in fractures controls subsurface heat and mass transport, which is essential for developing enhanced geothermal systems and radioactive waste disposal. Fracture permeability is controlled by fracture microstructure (e.g. aperture, roughness, and tortuosity), but in situ values and their anisotropy have not yet been estimated. Recent advances in geophysical techniques allow the detection of changes in electrical conductivity due to changes in crustal stress and these techniques can be used to predict subsurface fluid flow. However, the paucity of data on fractured rocks hinders the quantitative interpretation of geophysical monitoring data in the field. Therefore, considering different shear displacements and chemical erosions, an investigation was conducted into the hydraulic-electric relationship as an elevated stress change in fractures. The simulation of fracture flows was achieved using the lattice Boltzmann method, while the electrical properties were calculated through the finite element method, based on synthetic faults incorporating elastic-plastic deformation. Numerical results show that the hydraulic and electrical properties depend on the rock's geometric properties (i.e. fracture length, roughness, and shear displacement). The permeability anisotropy in the direction parallel or perpendicular to the shear displacement is also notable in high stress conditions. Conversely, the permeability -conductivity (i.e., formation factor) relationship is unique under all conditions and follows a linear trend in logarithmic coordinates. However, both matrix porosity and fracture spacing alter this relationship. Both increase the slope of the linear trend, thereby changing the sensitivity of electrical observations to permeability changes. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

Shallow geothermal energy systems (SGES) are a promising technology for contributing to the decarbonization of the energy sector. Soil thermal conductivity (lambda) governs the heat transfer process in ground under a steady state; thereby, it is a key parameter for SGES performance. Soil mixing technology has been successful in enhancing the shear strength of soils, but is adopted in this paper for the first time to improve soils as a geothermal energy conductive medium for SGES applications. First, the thermal conductivity of six types of soils was systematically investigated and the key parameters analyzed. Next, graphite-based conductive cement grout was developed and mixed with the six soils in a controlled laboratory setting to demonstrate the significant increase in soil thermal conductivity. For example, the thermal conductivity of a very silty dry sand increased from 0.19 to 2.62 W/m.K (a remarkable 14-fold increase) when mixed with the conductive grout at a soil-to-grout ratio of 6: 1. In addition, the mechanical properties of the cement grouts and cement-mixed soils were examined along with the microstructural analysis, revealing the mechanism behind the thermal conductivity improvement. (c) 2024 American Society of Civil Engineers.

Reservoir fracturing stimulation is the key to constructing an enhanced geothermal system (EGS) for geothermal development in hot dry rock (HDR) reservoir. To clarify the crack propagation law of HDR fracturing, a 3D thermo-hydro-mechanical coupling simulation model of fracture propagation is produced based on the continuum-discontinuum element method (CDEM-THM3D). The correctness of the CDEM-THM3D model is validated by the theoretical solution of the nonisothermal soil consolidation model and Penny fracture model. Then, hydraulic fracturing numerical simulations are performed to analyse the influence of controlling variables on fracture propagation. The results indicate that the thermal tensile stress induced by injecting cold water can decrease reservoir fracture pressure and fracture extension pressure, causing an increasement in fracture width and a reduction in fracture length. Increasing thermal expansion coefficient and temperature difference enhances the effect of thermal stresses and even creates new branch fractures. A large elastic modulus favours an increase in fracture length, while large rock tensile strength and minimum horizontal stress lead to a decrease in fracture length. With increasing injection flow rate and fracturing fluid viscosity, the reservoir fracture pressure and the fracture width rise significantly, and the fracture easily breaks through the barrier of the high-stress compartment.

The performance of geothermal heat extraction in shallow aquifers depends on both Borehole Heat Exchanger (BHE) and soil or aquifer properties. In this work, an analysis of the thermal yield of a shallow geothermal reservoir was made numerically with the finite element method used to simulate heat and mass transfer in the three-dimensional reservoir. The main parameters for analysis which have been considered are the geometry and physical parameters of the BHE and grout, as well as aquifer matrix and groundwater fluid. Physical parameters are thermal conductivity, flow conductivity, expansion coefficient, porosity, volumetric heat capacity, anisotropy and dispersivity. The numerical tests have been performed in single BHE line source configuration representing numerically modelled thermal response test for the estimation of sustainable heat extraction. The domain size was a 100x100 meter rectangle with a depth of 200 meters. Three main lithological configurations have been modelled: gravel aquifer with low and high convection of groundwater fluid, as well as a shallow geothermal reservoir dominated by clay material without convection. For selected cases, the analysis for temporal and spatial discretization was also made. Three-dimensional transient modelling was made in FEFLOW (R) software with pre- and post-processing done in user-defined Python scripts. The results show the most influential parameters to be considered when setting up the real case simulation of geothermal heating and cooling, as well as optimal temporal and spatial discretization set-up with respect to expected thermal gradients in the reservoir.

Temperature in 2 km deep borehole Litomeice, drilled in 2007, was repeatedly logged down to 1700 m in the period 2007 - 2020. We were able to monitor a return of the temperature to the equilibrium temperature-depth profile undisturbed by drilling. The uppermost part of the profile contains signal of the recent warming manifested by a negative temperature gradient close to the surface and a temperature minimum at a depth of about 40 m. The minimum has been migrating downward at a rate of 1.5 - 2 m per year in the period 2015 - 2020. A detailed knowledge of temperature gradient together with thermal conductivity, diffusivity and heat production measurements on the drill-core samples of mica-schist that occurs below 900 m depth enabled us to analyze the heat flow vertical variations in the lithologically homogeneous depth 900 - 1700 m. We came to the conclusion that temperature-depth profile in this contains a robust climate signal of the last glacial cycle. The reconstructed ground surface temperature history indicates the magnitude of the last glacial - Holocene warming 13 -15 K and existence of a minimum 15 - 20 ka. The long-term mean ground surface temperature +1 - +2 degrees C suggests that the borehole site was permafrost free for most of the glacial cycle. Existence of about 100 m deep permafrost is possible in the coldest part of the last glacial. The steady-state surface heat flow has been estimated at 88 mW/m2. The reconstructed ground surface temperature history used as a surface forcing function in a numerical solution of the transient heat conduction equation provided an estimate of the present-day heat flow in the well. The estimate is practically independent from the poorly constrained conductivity of the 900 m thick sedimentary cover. According to it the present-day heat flow is lower than the steady-state one by 20 - 30 mW/m2 in the first hundreds of meters below the surface and still by about 10 mW/m(2) at a depth of 1 km.

Significant climate warming, as observed over the past decades and projected by global climate models, would inevitably cause permafrost degradation in the Arctic regions. Several studies have been conducted to assess geothermal response to climate change in natural conditions; no study, however, has been observed yet to examine the potential response of the permafrost geothermal regime in a building environment. This paper presents a methodology and the results of a case study in the community of Inuvik. Canada of the spatio-temporal dynamics simulation of the geothermal regime under climate change scenarios in a building environment. A process-based, surface-coupled, 3-dimensional geothermal model was used for the simulation. The results suggest that the permafrost under the study would deteriorate under all the three climate change scenarios assessed, and the rate of the deterioration would depend on geotechnical properties of subsurface materials and climate change scenarios. Two patterns of the geothermal dynamics were revealed from the simulation results: spatially, there are significant differences in the rate of increase in active layer thickness underneath vs. around a building; and temporally, there is an abrupt rise in the active layer thickness around the middle of this century. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.