The Bandaiko hot spring, located in Kusatsu Town, Gunma Prefecture, is characterized by its hyper-acidic nature with a pH level of 1.7 and remarkable flow rate of approximately nine tons per minute. Gunma Prefecture is exploring the possibility of modifying its existing heat exchange system to establish a binary power generation facility utilizing this hyper-acidic hot spring, marking a potential first in the world. However, in the existing heat exchange facility, scale accumulation of the current facility necessitates water washing roughly once a month, but the type of scale and the rate of accumulation rate are still unknown. To address this, hot spring water was sampled and chemically analyzed, and field tests simulating scale adhesion were conducted using this water. For evaluation, quartz glass coupons and a batch-type scale sensor were utilized. The accumulated scale was identified as elemental sulfur. The use of quartz glass coupons did not enable an assessment of the scale deposition rate based on weight changes. By contrast, an 18-d immersion test with batch-type scale sensor indicated a gradual decrease in transmittance over time. The decrease in this sensor's transmittance serves as a proxy indicator for material accumulation under static immersion conditions. As it is not possible to analyze the scale adhering to the titanium heat exchanger during operation, it does not directly indicate the rate of scale deposition. Analysis of deposits on the sensor surface and scale recovered during the water washing of the heat exchanger revealed that the scale was elemental sulfur. Further analysis revealed that the elemental sulfur, derived from the chemical composition of the hot spring water, was undersaturated within the tested temperature range. The hot spring water is transported via pipeline to the heat exchange facility after gas-liquid separation at the source—located 2 km away—yet it is determined that atmospheric air enters the water at the source. This suggests that the precipitation of sulfur is caused either by the oxidation of trace amounts of H2S in the hot spring water by atmospheric oxygen or, alternatively, by bacterial activity. A subsequent bacterial analysis detected sulfur-oxidizing bacteria. These findings suggest that the formation of elemental sulfur scale likely results from the precipitation of sulfur caused by H2S oxidation—driven either by microbial activity or atmospheric oxygen—during the transport of the hot spring water. While installing sterilization equipment is an option for inhibiting bacterial activity, it entails significant costs. Given that the periodic water cleaning currently employed effectively restores heat exchange efficiency, this method remains the most appropriate approach for the time being. However, if the concentration of H2S in the hot spring water increases, reconsideration of a sterilization approach will be necessary. The novelty of this study is to connect observed sulfur scale, water chemistry, field sensor testing, equilibrium modelling, and microbial evidence in an actual utilization setting.
China holds the world's fourth-largest oil shale resource, with deposits concentrated in the Songliao, Ordos, and Junggar basins estimated to contain more than 700 billion tonnes of in-place oil shale, yet surface retorting — the method currently operated at commercial scale in Fushun and Maoming — recovers these hydrocarbons only at prohibitive environmental and economic costs that limit national energy security and conflict with China's dual-carbon neutrality commitments. The Geothermic Fuel Cell (GFC) concept offers a cost-competitive in-situ alternative by placing solid oxide fuel cells underground, converting rejected process heat directly into subsurface kerogen pyrolysis while co-producing saleable electricity that offsets capital and operating expenditure — a profile well suited to China's simultaneous objectives of domestic energy production and carbon intensity reduction. This study presents a techno-economic evaluation of a nine-stack GFC system with three modules coupled to a natural gas reformer and a reactant preheater, grounding the cost and return analysis in control-volume energy and exergy balances derived from 24 h averaged field measurements. The assembly operated continuously underground for nearly 600 h under ambient temperatures from 17.8 to −7.2°C, and at the 65 A near-steady condition, it delivered 29.1 kWth of subsurface heat with 4.4 kWe of exportable electricity, achieving a first-law combined heat and power efficiency of 55% and a second-law exergy efficiency of approximately 28%, with a heat flux of 3.2 kW m−1 over 9 m. The exergy analysis identifies the unreacted reformate stream as both the dominant thermodynamic loss and the principal fuel cost driver, and demonstrates that capturing this stream would raise the combined heat and power efficiency to approximately 66%, substantially improving project economics. These findings confirm that GFC technology holds a clear cost advantage over the grid-powered in-situ heating methods being evaluated for Chinese deposits, establish the electricity co-product as a meaningful revenue stream that supports commercial scalability under Chinese energy market conditions, and identify unreacted fuel recovery and anode exhaust recycling as the highest-return engineering interventions for advancing in-situ oil shale conversion toward full-scale deployment in China.
The geothermal wells are exposed to thermal loads that can induce thermal degradation of the wellbore cement over time. It is typical for the cement sheath in these wells to undergo mechanical failure due to radial cracks and/or debonding, after which the failure will be followed by long-term strength degradation. Although alternative solutions are in the research phase, API (American Petroleum Institute) cement is still the economically preferred annular casing-formation isolation. When compared to the findings of numerous academic published works, the FEM studies of cement debonding reveal large discrepancies.Typically, casing-cement contact is modeled using the CZM (cohesive zone model) method, and several previous studies have recognized that there is a remaining axial force similar to the friction force between the casing and cement after the shear (Type II) debonding failure. This cannot be explained using the CZM models because there is no normal force existing after the debonding, meaning that the friction should have been zero. The main objective of this study is to explain this key discrepancy in the numerical debonding models. A modified boundary-condition scenario has been modeled to elaborate on this phenomenon. A CZM has been used until the development of debonding, after which the stress conditions have been transferred to a casing-cement friction bonding contact model. The results have shown that the aforementioned remaining friction force can be significant. The study presents the numerical model and the simulation results modeled around a synthetic geothermal well. The observations help justify comparatively limited wellhead growth due to thermal expansion in the geothermal wells. Overall, the methodology recommended in this study can be used to simplify the FEM process for casing-cement debonding and wellhead growth due to steel thermal expansion.
Induced seismicity remains a critical challenge for enhanced geothermal systems (EGS), motivating stimulation strategies that enhance permeability while maintaining fracture stability. Chemical stimulation has been widely studied as a complement to hydraulic stimulation, yet its mechanical effects on fracture friction and slip behavior remain poorly constrained. In particular, chemical stimulation is often implicitly assumed to weaken fractures and promote instability. Here, we investigate the shear-slip behavior of granite fractures chemically treated by a biodegradable chelating agent, N,N-bis(carboxymethyl)-L-glutamic acid tetrasodium salt (GLDA-Na4). A 45° saw-cut fracture in granite was treated in a batch experiment with a 20 wt.% GLDA-Na4 solution (pH 4) at 150 °C, followed by triaxial shear-slip experiments conducted at an axial stress of 55 MPa and a confining pressure of 30 MPa. Static friction coefficients and shear-slip velocities were quantified before and after chemical treatment. Chemical stimulation selectively dissolved biotite, a mineral with a relatively low friction coefficient, and generated dissolution-induced voids on the fracture surface. As a result, the static friction coefficient increased from 0.48 ± 0.03 to 0.62 ± 0.01. Following rapid slip onset, the treated surfaces gradually decelerated into a prolonged lower-apparent-velocity phase. The reported shear displacement and velocity are termed apparent because they were inferred from axial deformation. Microstructural analyses using XCT, μXRF, and SEM suggested that the increase in frictional resistance was associated with low-friction mineral removal and dissolution-induced void formation, possibly accompanied by rim spalling. These results show that chemical treatment does not necessarily reduce fracture frictional resistance. When selective mineral dissolution produces this type of mineralogical and topographical surface alteration, static friction can increase under the present dry, stress-controlled laboratory conditions, providing a basis for future shear-slip experiments involving fluid flow.
Electromagnetic (EM) methods are highly sensitive to subsurface conductivity contrasts associated with clay alteration, fluid circulation, and fracture-controlled hydrothermal systems, and have therefore been increasingly applied to geothermal exploration and reservoir characterization in recent years. However, EM inverse problems are commonly characterized by significant non-uniqueness. Conventional deterministic inversion methods typically produce a single best-fitting model, providing limited ability to quantify model uncertainty or to evaluate the effects of measurement noise and incomplete data coverage on inversion results.To address these limitations, this paper presents a trans-dimensional Bayesian inversion framework for EM inversion problems in geothermal exploration. By sampling an ensemble of models from the posterior distribution, the framework enables quantitative evaluation of model-parameter uncertainty. Specifically, this study implements a two-dimensional reversible-jump Markov chain Monte Carlo (rj-MCMC) algorithm based on wavelet-domain tree-structured parameterization, allowing model complexity to vary adaptively during sampling so that the appropriate level of parameterization is determined by the observed data.The proposed method is first evaluated using a synthetic experiment based on a classical convective geothermal model, with particular emphasis on uncertainty caused by unknown model parameterization. It is then applied to field controlled-source audio-frequency magnetotelluric (CSAMT) data from a geothermal area in southern China. The results demonstrate that the proposed probabilistic inversion method can provide reliable geothermal reservoir characterization while offering quantitative uncertainty information for resource assessment.
In enhanced geothermal systems, thermally altered fractures may experience cyclic normal stress perturbations during injection, production, and seismic disturbance. However, the frictional response of thermally treated granite joints under oscillatory normal stress remains insufficiently constrained. To address this issue, this study employed a large DJZ-500 direct shear apparatus to conduct laboratory research on the granite joint that had experienced different temperature heat treatments (23–500 °C) under constant normal load (CNL) and dynamic normal load (DNL). The results revealed that the joint shear strength exhibited a nonlinear evolution process dependent on temperature. Both static and dynamic shear strengths showed initial thermal strengthening (23–200 °C), followed by severe thermal degradation (200–500 °C). Moreover, when the heat treatment exceeded 300 °C, the shear strength dropped below the pre-treatment strength. With increasing dynamic amplitude, dynamic shear strength changes from being higher than to lower than the corresponding static strength. We refer to this phenomenon as frictional strengthening-to-weakening transition. The degree of this frictional strengthening or weakening depends on the temperature. Through three-dimensional surface morphology and results of uniaxial compressive test, we propose a mechanism indicating that the non-linear variation of shear strengths depends on the coupled effects of joint roughness and matrix strength. These findings provide some mechanistic insights into the thermodynamic stability of hot dry rock faults and offer important references for optimizing geothermal energy extraction in deep hot dry rock faults.
Thermal fracturing in limestone is relevant to geothermal operations, thermal stimulation of low-permeability reservoirs, underground energy storage, and the assessment of structural integrity under elevated temperatures. However, the thermo-hydro-mechanical (THM) behavior of fluid-saturated limestones under undrained conditions remains poorly understood, particularly regarding the interaction between saline pore fluids and thermally induced microstructural damage. This study systematically investigated the effects of heating from 100 to 800 °C under undrained conditions, using two NaCl concentrations (10 and 20 wt%) and deionized water, on the petrophysical properties and microstructure of Pink Desert limestone, a monomineralic limestone composed of >99.9% calcite. Saturated samples were subjected to confined heating and subsequently characterized by bulk density, effective porosity, permeability, X-ray diffraction (XRD), thermogravimetric and differential scanning calorimetry analyses (TG/DSC), and scanning electron microscopy (SEM). The rock exhibited high thermal stability up to approximately 625 °C, with minimal variations in density, porosity, and permeability. More pronounced changes were observed only at 800 °C, where permeability increased by up to approximately 4.2 times relative to its initial value (observed for a single specimen), whereas porosity exhibited only a modest increase (∼1.1 times). These results suggest the existence of a threshold for accelerated thermal damage development somewhere between 625 and 800 °C. The FWHM analyses confirm the preservation of calcite crystallinity throughout the thermal interval, indicating that damage occurs predominantly at the intergranular scale. Within the resolution of the analytical techniques employed and under the experimental conditions investigated, no systematic influence of salinity on the evaluated properties was detected. In summary, Pink Desert limestone exhibits high thermal resistance, with significant degradation of its transport properties occurring only as temperatures approach the initial decomposition interval of calcite. The observed permeability increase is associated with the development of a more interconnected network of thermally induced microfractures. These findings provide relevant parameters for evaluating the performance of fluid-saturated limestones in applications involving intense thermal gradients and highlight temperature as the primary controlling factor governing thermal degradation in monomineralic calcitic limestones under undrained conditions.
Understanding the thermomechanical behaviors of geomaterials is crucial for ensuring the successful design and long-term performance of energy geostructures. This study investigated the response of an undisturbed, low-plasticity weathered granite soil (WGS) from Gwangju, Korea, a saprolitic soil derived from regional granite that behaves as a transitional soil between sands and clays, under cyclic thermal loading. A modified temperature-controlled oedometer was used to subject both normally consolidated (NC) and overconsolidated (OC) states to multiple-cycle drained heating (60 °C) and cooling (20 °C) tests at different preconsolidation pressures. The experimental results indicated that the NC-WGS exhibited irreversible thermal consolidation under a single thermal cycle, whereas the heavily OC-WGS primarily exhibited reversible thermo-elastic expansion. In addition, the magnitude of NC-WGS thermal consolidation was strongly stress-dependent, with the maximum volumetric strain during the first heating phase decreasing from 0.404 % at 240 kPa to 0.134 % at 640 kPa. Multiple thermal cyclic loadings produced accumulated consolidation in NC with rapidly diminishing incremental strain (thermal hardening-type stabilization), reaching 0.915 % and 0.825 % after three cycles at 240 kPa and 320 kPa, respectively, and 1.015 % after eight cycles at 640 kPa. In contrast, OC responses became nearly repeatable from the second cycle onward with negligible additional plastic strain. Even under repeated cycling, the low-plasticity WGS (plasticity index of 2.6 %) exhibits markedly lower thermally induced volumetric strain than high-plasticity clays, supporting long-term serviceability and structural safety throughout cyclic thermal operation in energy geostructures.
Lithium (Li+) and cesium (Cs+) ions show rapid reversible adsorption and desorption on geothermal reservoir rocks, making them potential candidates for sorbing tracer tests to estimate the heat exchange surface area of the geothermal reservoir. However, water geochemistry data from the Frontier Observatory for Research in Geothermal Energy (Utah FORGE) project show significant background concentrations of Li+ and Cs+, which obscure the tracer recovery response. To address this problem, we extended the semi-analytical Reactive Transport Laplace Transform Inversion model (RELAP) to account for non-zero background concentration and to simulate positive, negative, and multiple tracer pulse tests within a unified background-aware framework. The modified model was first history matched to a previous conservative tracer test at Utah FORGE to estimate the governing flow parameters and provide a data-informed basis for subsequent design calculations. Results indicate that transport between the injection and production well at the Utah FORGE is better represented by a two-component upscaled transport structure than by a single equivalent pathway. Using the history-matched model, the analysis showed that the Li+ or Cs+ tracer mass required to produce detectable positive concentration perturbations is economically prohibitive. In contrast, a negative pulse design based on freshwater injection produces detectable negative perturbations in the tracer recovery response. Expressed in normalized concentration form, the breakthrough curves remain directly comparable to those of a positive tracer pulse and preserve the standard retardation-based interpretation. These results establish a methodological and design-oriented framework for background-aware tracer testing in geothermal reservoirs.
Mount Parakasak in Banten hosts promising yet underexplored geothermal resources. This study integrates two-dimensional magnetotelluric (MT) and time-domain electromagnetic (TDEM) surveys to delineate subsurface resistivity structures and characterize the geothermal system. Thirty-six MT soundings, corrected for static shift using TDEM data, were inverted to generate 2D resistivity sections. These sections were subsequently interpolated to construct a three-dimensional resistivity visualization constrained by geological and geochemical datasets. The results identify a conductive clay cap (∼100 Ω·m) overlying an upflow zone near the Kaipohan manifestation, corresponding to estimated reservoir temperatures of 240–260 °C. The Batukuwung hot spring defines an outflow zone with temperatures of 52–75 °C. The integrated interpretation indicates an active geothermal system and identifies promising drilling targets at approximately 1500 m depth. These findings provide a robust framework for geothermal exploration and resource assessment, supporting future geothermal development in Banten Province.
This study presents the development of a multifunctional epoxy/polytetrafluoroethylene (PTFE)/carbon nanotube (CNT) hybrid nanocomposite coating designed to simultaneously mitigate localized corrosion and silica scaling in geothermal environments. Morphological and surface property evaluations identified an optimal formulation (Epoxy/PTFE40/CNT0.8) that successfully balances surface hydrophobicity (water contact angle of 139°) with enhanced adhesion strength (8.68 MPa), avoiding the nanoparticle agglomeration observed at excessive filler loadings. Thermal analysis demonstrated that the integrated CNT network acts as a physical diffusion barrier, modifying the matrix degradation into a stabilized two-stage process capable of withstanding continuous exposure up to 150 °C. Electrochemically, the optimized composite established a highly tortuous barrier against corrosive Cl⁻ ions, achieving a charge transfer resistance of 882.9 Ω·cm² (47% above neat epoxy), a corrosion protection efficiency of 94.05%, and a minimal corrosion rate of 0.0098 mm/year. Furthermore, the tailored micro/nano-topography formed by PTFE particles and CNT networks successfully prevented the mechanical interlocking of mineral deposits, delivering peak silica scaling reductions of 65.08% under Upstream (near-wellhead) brine conditions and maintaining a 50.92% mitigation efficiency under Downstream (near-pond) supersaturation conditions. Ultimately, this synergistic hybrid coating offers an effective and scalable strategy for simultaneously combating the dual threats of scaling and corrosion, extending the operational lifespan of geothermal infrastructure.
Understanding transient heat transfer in heterogeneous volcanic formations improves the design of shallow geothermal energy systems (SGES). However, the thermal behaviour of stratified volcanic successions remains poorly constrained due to lithological variability, porosity contrasts, and groundwater driven heat transport.This study investigates the thermal diffusivity and thermal response of the Sabatini volcanic succession (Central Italy) using laboratory measurements, in situ thermal tests and Distributed Temperature Sensing (DTS) monitoring at the ENEA-Casaccia shallow geothermal field. Thermal conductivity (λ), bulk density (ρ), and specific heat capacity (cp) were determined along a borehole and used to derive thermal diffusivity (α). DTS monitoring enabled characterised transient thermal evolution along the borehole. Thermal diffusivity values range from 4.2 × 10⁻⁷ to 7.1 × 10⁻⁷ m²/s. Highest values are associated with lava and sandy tuffs, whereas the lowest occur in fine-grained, clay-rich deposits. DTS monitoring during a 12 h thermal stimulation test revealed depth-dependent heterogeneity and contrasting cooling behaviours among lithologies. Saturated lava units showed rapid heat uptake and dissipation, while fine-grained tuffs and sandy silts retained thermal anomalies for up to 48 h, indicating higher thermal inertia and heat storage capacity. Results demonstrate that lithology, porosity and saturation control transient thermal behaviour in volcanic successions.
The study estimates the Curie Point Depth (CPD) of the Colombian territory using magnetic anomaly data and two inversion methods, providing insights into crustal thermal structure. The availability of new magnetic information from the Geological Survey of Colombia and EMAG2V3 satellite data updates the Curie isotherm for northwestern South America. This development represents a significant asset for geothermal energy engineering. The calculation uses the centroid and fractal nonlinear curve adjustment methods, with results compared to tectonic features, ANH geothermal gradients, and element concentrations. The average CPD value was determined to 65.7 km ± 37.8 km (centroid method) and 21.0 ± 9.1 km (nonlinear adjustment method). Following ANH bottom hole temperature data integration, cokriging interpolation refines the nonlinear fractal results and heat flow estimates. This allows direct comparison between three thermal indicators: heat flow derived from CPD centroid calculations, cokriging-based estimates, and direct bottom hole temperature measurements. Shallow CPD values (18–23 km) are observed in the Southern Andes, aligning with high volcanic density and geothermal anomaly areas including Las Animas-Chiles, Cerro Bravo-Cerro Machín, San Diego, and Huila-Sucubun. Deeper values (>30 km) are found in the Northern Andean sector, particularly in the Magdalena Basin, Caldas Tear zone, Bucaramanga's seismic nest, Paipa-Iza regime, and aged geologic bodies. These correlate with the convergence of the Nazca, South American, and Caribbean plates. CPD results show a NE-SW trend controlled by the orogenic system of the three Andean Cordilleras. Moderate correlations exist for calcium (Ca) and potassium (K) in active sediments, but not for iron (Fe), titanium (Ti), and magnesium (Mg).
Sustainable groundwater management requires accurate delineation of a protection zone (PZ) around wells. While PZ has been widely studied for solute protection zone (SPZ) of drinking water wells, the growing use of geothermal applications requires also the delineation of both solute and thermal protection zone (TPZ). In this study, the effects of reservoir heterogeneity on SPZ and TPZ size were investigated using numerical simulations around a deep geothermal reinjection well, where reservoir heterogeneity was studied by stochastic permeability distributions. In the first part of the study, synthetic simulations were performed to highlight the importance of the scientific question, where the role of the permeability heterogeneity scale, and the thermal retardation were examined. The SPZ and TPZ size was characterized by monitoring the evolving position of the solute and thermal front. In the main part, the theoretical findings were applied to a real geothermal project in Hungary, proposing a novel workflow where the heterogeneity scale was derived from seismic attributes and geophysical logs. The results show that the SPZ and TPZ size increases with the heterogeneity scale, indicating that a larger protection zone is needed, although the effect of heterogeneity decreases over time. Heterogeneity has significant effect on the TPZ extension especially on small time scale, so its investigation in heat transport cannot be neglected despite greater heat diffusion. The probability maps of SPZ and TPZ areas illustrate well the uncertainty caused by reservoir heterogeneity. Finally, results highlight that TPZ cannot be directly rescaled from SPZ because of the time-varying difference between apparent and effective thermal retardation.
Mid-low temperature hydrothermal systems are widely distributed in continental intraplate basins, and accurately characterizing their deep thermal structures is crucial for understanding regional geothermal genesis. As a typical intraplate field, the Xiong'an Area in North China possesses significant deep potential. However, previous studies focused on localized assessments of isolated shallow fields, lacking regional 3D simulations based on fine structural constraints, which restricts the understanding of deep heat transfer mechanisms.In this study, we integrated three deep seismic reflection profiles, one magnetotelluric (MT) profile, regional geological, and well-log data to construct a high-resolution 3D geological-geophysical model. Constrained by petrophysical data, we employed a Multi-source data integration approach to solve the 3D steady-state heat conduction equation, achieving high-precision geothermal simulation down to 5 km depth, validated by measured well temperatures.Our results reveal significant spatial differentiation in the regional thermal structure, primarily controlled by varying "caprock-reservoir" configurations. The northern region (e.g., Rongcheng and Niutuozhen fields) exhibits a "thin caprock-shallow reservoir" pattern (< 3 km), whereas the southern region (e.g., Gaoyang field) features a "thick caprock-deep reservoir" pattern (> 3 km). The 3D model indicates the southern deep system possesses a significantly higher geothermal gradient, forming a promising target for deep exploration. Furthermore, we identify a concealed deep geothermal prospect near the Raoyang depression. This study delineates reliable exploration targets and provides a universally applicable framework for modeling Multi-source data integration in intraplate geothermal systems.
This study explores the potential of multi-lateral slim-hole drilling to enhance reservoir access and connectivity in fractured geothermal reservoirs. At the core of this work is Directional Steel Shot Drilling (DSSD), a technology that combines conventional mechanical drilling with the erosive action of pressure-accelerated steel shots. This approach enables the cost-effective construction of multi-directional boreholes from a single wellbore.The DSSD technology was piloted at the VersuchsStollen Hagerbach underground research facility in Switzerland. The test site provides access to fractured hard rock formations under realistic stress and pore-fluid pressure conditions. Two horizontal wellbores, each 125 m long, were drilled. Rates of penetration (ROP) increased up to 2–5 times depending on the steel shot concentration in the drilling mud. Controlled building rates under steering (∼ 3.8–6.6°/30 m) and stable trajectories under non-steering conditions (∼ 0.3° /30 m) were achieved. The pilot demonstrated several key aspects: straightforward retrofitting of DSSD on conventional drill rigs, effectiveness of the steering concept, efficient transportation of steel shots in the mud, and the resulting enhanced bit performance in hard rock.To assess field-scale implications, reservoir simulations and a techno-economic assessment were performed using a fractured carbonate reservoir case. A set of 1000 geological realizations was generated to capture uncertainty in permeability distribution. These models were used to evaluate thermal energy production for 20 years, employing single-, dual-, and triple-lateral well architectures. Median results indicated that adding a second lateral boosts production by 45 – 50%, while a third lateral leads to an additional 25 – 30% gain. The techno-economic evaluation shows corresponding reductions in the levelized cost of heat by ∼20% when moving from single- to dual-lateral configurations, and by a further ∼25% when a third lateral is added. Similarly, the specific capital cost declines by ∼24% from single to dual laterals and by an additional 16% for triple laterals. Results remain conditional on assumptions of geology, well design, economics, and scalability to greater depth.
