Geothermics最新文献

Thermal hazards in deep mines can be mined synergistically with the deposit as associated geothermal energy resources. Backfill heat exchangers (BFHEs) as a combination of mine backfilling and ground heat exchanger technology are one of the effective methods to realize ore deposit-geothermal energy synergy mining. However, in the complex environment of deep mines, BFHEs will inevitably suffer from fracture damage during long-term heat storage/release cycle operation, which will affect their working performance. To address this problem, this paper established a scaled-down experimental setup for fractured BFHEs units, and used the experimental data to verify the accuracy of the established three-dimensional transient heat transfer mathematical model of the BFHEs unit. Numerical simulation methods were used to investigate the effects of the position, aperture and fractured depth of a single fracture on the heat extraction performance of the BFHEs unit under fully dry or fully saturated working conditions. Significant differences were found in the effect of different fracture positions on the heat extraction capacity of the BFHEs unit. The extraction of geothermal heat by the BFHEs unit was favored when it was in the middle of two tubes, while the opposite was true for other positions. The effect of fracture on the heat extraction capacity of the BFHEs unit increased linearly with fracture aperture. The effect of the fractured depth was relatively greater in the 1/2–3/4 H_b range. The presence of groundwater significantly reduced the effect of the fracture. The effect of the fracture on the heat extraction capacity of the BFHEs unit was reduced by more than 80 % by changing the operating conditions from fully dry to fully saturated condition. The overall effect of a fracture on the heat extraction capacity of the BFHEs unit was not significant, with a 5 mm wide fracture having an effect of no more than 2.5 %, even under the fully dry condition. However, there was a combined effect of multiple fractures, and for a deposit size of 900 × 7 × 80 m, ten 2 mm apertures fractures would results in a maximum reduction of geothermal extraction from BFHEs by 8527 GJ in 10 years, which was equivalent to the heat load of a 13,753 m² building for one winter in the cold climate of China.

Geothermal energy, as a renewable energy source, has been intensively developed recently because its utilization can reduce CO₂ emission, climate warming and sea level rising, and be beneficial to promote sustainable development. However, scaling in the wellbores often happens during the rising of geothermal fluids. At present, the methods to explore the scaling location principally focus on real measurement techniques and numerical simulation method. In comparison with the former, the latter is extremely time- and labor-saving to predict the scaling location based on the coupling model. In this study, the flash point and scaling locations of two geothermal wells in Hebei province are numerically simulated via ECO2N equation of state module in TOUGHREACT software, and the factors affecting the flash point and scaling locations of geothermal wells are explored. At the same time, the mechanism of geothermal fluid flash is analyzed. The prediction results show that the relative errors of scaling locations in geothermal well I and II are 5.43 % and 6.82 % compared to the field measured data; the pressure and temperature of geothermal fluids at well bottom can be calculated based on the related parameters of wellhead fluid; the locations of flash point and scaling can be determined according to the sudden increases of CO₂ partial pressure and scaling amount, respectively. Furthermore, the liquid mass flow rate, CO₂ mass flow rate and wellbore diameter have prominent influence on the locations of flash point and scaling, while wellbore diameter have significant effect on the amount of calcite scaling. The above conclusions provide useful guidelines for the next scaling removal and control in the production wellbore during the utilization of the geothermal energy.

This study provides a comprehensive investigation into the impact of intermittent operation on two borehole designs: coaxial and u-tube heat exchangers. Previous studies have compared borehole heat exchanger (BHE) designs and others have investigated the impact of intermittent parameters; this study provides a systematic analysis of the intersection between intermittency and design. Using detailed numerical models of a coaxial and u-tube heat exchanger, intermittent operating duration, recovery time, and duty cycle were explored. These simulations demonstrated the importance of operating time scales. Findings showed that operating for durations smaller than the transit time (the time needed for fluid to travel the full length of the heat exchanger) provided performance benefits in the u-tube over the coaxial design, due to the limited thermal interaction between inlet and outlet flows in the u-tube. At operating scales above its transit time, however, the coaxial heat exchanger performed up to 12.9% better than the u-tube, with most benefit observed for operation durations close to the transit time. Additionally, the results showed little difference during operation between the two heat exchangers when varying recovery times. Furthermore, an assessment of the duty cycle demonstrated that a high ratio of recovery to operating time at low operating durations provided the greatest performance benefit. These findings highlight the need to model heat exchanger response over all time scales in order to accurately predict performance under intermittent operating conditions.

Waterflooding is a widely used technique for developing unconventional reservoirs. Long-term waterflooding can lead to the formation of multiple induced fractures. Accurately tracking the location, extent, and dynamic behavior of these induced fractures is crucial. To enhance monitoring capabilities, data obtained from distributed temperature sensors are used to track temperature variations within the wellbore and at the bottom of the well. Combining temperature and pressure inversion techniques helps mitigate the issue of multiple solutions while offering a more comprehensive characterization of multi-induced fractures from various perspectives. In this study, this work presents a model for temperature coupled pressure (TCP) transient analysis. Additionally, this work utilizes COMSOL Multiphysics software to develop and solve a dynamic wellbore temperature (DWT) model. This work proposes a workflow that describes the dynamic behaviour of multi-induced fractures. During model development and solution, this work employs both pressure and temperature transient analysis methods, examining time-dependent non-linear pressure drop and heat transfer variations. Our results reveal that induced fractures exhibit convective heat exchange with the wellbore, resulting in significantly higher temperatures at the induced fracture locations compared to other wellbore locations. By analyzing temperature-wellbore location curves, this work can accurately identify the locations of induced fractures. The TCP model successfully extracts characteristic parameters of induced fractures, while the DWT model allows for monitoring the locations and heights of these fractures. By combining the workflows of both models, this work achieves a comprehensive characterization of the physical properties of induced fractures. In conclusion, this advancement empowers engineers to manage the development of such fractures effectively, thereby averting potential adverse effects on production well performance and preventing the need for well abandonment.

A multidisciplinary framework for the sustainable utilization and efficient use of Patuha geothermal field is proposed here to evaluate the development of 110 MW. The significance of applying a strategic approach is emphasized, taking into account the connections between the analysis of geothermal systems, resources, and power production. A three-dimensional numerical model of the Patuha reservoir for fluid flow and heat transfer was developed using the TOUGH2 simulator based on the analysis of updated integrated geoscience data combined with well data. The model was validated for natural state and production history, then used for reassessment of the geothermal resources using probabilistic resource assessment and experimental design and for reservoir plant optimization. Several case studies were conducted, which concluded that the model could be used as a practical tool to support the Patuha development plan. The forecast indicates the reservoir sustains the additional capacity of 55 MW for Patuha Unit 2, which is at the threshold of 116 MW from a probabilistic resource assessment. According to the base case scenario, it was determined that 18 additional make-up wells are required to maintain 30 years of production.

Temperature alters the rocks’ mechanical behavior and influences the stability and operation of enhanced geothermal wells. The stability of the well is a concern during construction, just after the construction, at the time of production, and injection. Most available analytical and semi-analytical solutions for wellbore stability do not account for rock's temperature-dependent yield behavior. In the present study, we proposed an elastoplastic constitutive model based on the temperature-sensitive yield criterion and a semi-analytical solution for wellbore stability. The applicability of the constitutive model is demonstrated by comparing the experimental results of Comiso limestone with the model output. The model elastic perfectly plastic assumption reasonability captures the stress-strain behavior at elevated temperatures. Temperature-sensitive yield criterion effectively predicts the yield point of Comiso limestone, even the substantial decay in yield stress after the temperature of 400 ^οC. The semi-analytical solution of wellbore stability analysis is validated using FE (Finite element) analysis. An implementation of the semi-analytical solution is showcased for a vertical wellbore, considering changes in in-situ stresses and temperature. A parametric analysis establishes that external factors, like vertical stress increase, do not consistently decrease the size of the plastic zone. Instead, a specific threshold of vertical stress exists at which the plastic zone radius reaches its minimum value. In the presented case, the least radius of the plastic zone is observed for in-situ vertical stress of 80 MPa. Furthermore, the parametric study examines the impact of horizontal stress, support pressure, temperature, and dilation parameter, which followed the generally reported trends.

In this study, we present a new analytical model for the analysis of transient heat transfer (the model is solved after Laplace transformation) in geothermal monotube heat exchangers (GHE). The model accounts for various factors, including the heat exchanger's geometric parameters, the nature of the fluid (air and water), flow characteristics (laminar and turbulent), and heat transfer boundary conditions. Initially, the model's performance is validated by comparing its results with those obtained from computational fluid dynamics simulations using OpenFOAM. This validation process ensures the reliability and accuracy of the model. Additionally, an analysis of the heat transfer mechanisms and key modeling parameters identified from the comparison is provided. In the subsequent section, the analysis is extended by comparing the proposed model to four existing analytical models available in the literature. These reference models employ distinct approaches to simulate heat transfer to the ground. By examining a range of cases, the performance of the model is systematically evaluated against these conventional methods. This work closes by an analysis of the analytical models and the impact on the accuracy of the results highlighting the advantages of the proposed method in geothermal applications.

Coastal regions undergo dynamic changes due to tidal forces, posing distinct challenges for groundwater management and utilization of ground source heat pump (GSHP) systems. This study examines the impact of tidal fluctuations on groundwater levels and the performance of ground heat exchangers (GHEs), specifically focusing on the suitability of horizontal directional drilling (HDD) technology in high-tidal zones. Itoman City in Okinawa Prefecture, Southern Japan, serves as an ideal location for this study due to its complex coastal hydrogeological conditions and increasing interest in renewable energy solutions. Employing a comprehensive approach involving field experiments, FEFLOW software simulations, and model validation, we aim to elucidate the intricate relationship between tidal dynamics, groundwater behavior, and GHE performance. Furthermore, our investigation assesses the effectiveness of HDD-GHEs installed in highly tidal-affected zones to determine the optimal installation depth for this type of GHE in coastal regions. According to our research, the layer at a depth of 30 m is most affected by tidal influence and fluctuations in groundwater levels in coastal regions. This layer is the most suitable for installing horizontal HDD-GHEs, with an average heat exchange rate of about 89 W/m. This rate is approximately four and two times higher than the depths above and below this layer, respectively, due to groundwater saturation and flow in the formation. Furthermore, we noticed that the heat exchange rate increases from the top to the bottom of this layer, but the extent of this increase diminishes as the depth increases. This is because deeper depths are less affected by tidal fluctuations in groundwater levels.

Hot springs in various granitoid and gneissic complexes in northern Pakistan indicate elevated geothermal gradients, which warrants their evaluation for geothermal applications. Evaluation of such prospects requires extensive knowledge of rock properties and their interaction with reservoir fluids. Our contribution provides first-order information on petrophysical, geochemical, and petrographic descriptions derived from outcrop analogs. About seventy samples of (mostly) granitoids and gneisses from the Nanga Parbat Massif (NPM), the Kohistan-Ladakh Batholith (KLB), and the Karakoram Batholith (KB) were collected. Biotite, K-feldspar, and plagioclase in the granitoid and gneiss show weak to moderate alteration, which increases in shear zones, suggesting fracture-assisted fluid interaction. Geochemical results indicate that the gneisses and granites of the NPM are mostly peraluminous S-type and show enrichment in most LREEs and depletion in HREEs. In addition, depleted Sr and Ba and enriched Rb and U in granites indicate partial melting and high fractionation. On the contrary, granitoids from the KLB are of calc-alkaline I-type and characterized by the depletion of REEs and the enrichment in Ba and Sr. The granitoids of the KB, due to their different magmatic histories, are more diverse, ranging from older I-type calc-alkaline granodiorite, and younger I-type alkaline syenite and S-type calc-alkaline granite. They show an overall enrichment in HREEs along with Ba, Th, Ta, Sr, and Lu. Allanites from syenite of the KB show significant concentrations of Th and U.

Petrophysical measurements reveal low matrix porosities (0.6–3.5 %), with primarily average thermal conductivities (1.48–3.37 W m⁻¹K ⁻¹) and thermal diffusivities (0.68–1.95 ∙10^–6 m² s⁻¹), with minor variation in specific heat capacities (744–767 J kg⁻¹ K⁻¹). The NPM displays higher average thermal conductivity, thermal diffusivity, and lower porosity than the KB, while the petrophysical properties of the KLB range between these two domains.

The geothermal systems in the area operate due to the thickening of a granitoid-dominant crust, which is enriched with radiogenic elements. The fault zones provide channels for meteoric water to access deeply these hot and thermally conductive rocks in the subsurface. After heat exchange, the water is discharged back to the surface as hot springs. The present data set provides a better understanding of regional geothermal regimes from petrological, geochemical, and petrophysical perspectives and assists in numerical modeling for potential geothermal assessment.

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.