Search Results (232)

Shandong Province is rich in geothermal resources, mainly stored in sandstone reservoirs. The setting of reasonable well spacing in the early stage of large-scale recharge has not attracted enough attention. The problem of small well spacing in geothermal engineering is particularly prominent in the sandstone thermal reservoir production area represented by Dezhou. Based on the measured data of temperature, flow, and water level, this paper constructs a typical engineering numerical model by using TOUGH2 software. It is found that when the distance between production and recharge wells is 180 m, the amount of production and recharge is 60 m³/h, and the temperature of reinjection is 30 °C, the temperature of the production well will decrease rapidly after 10 years of production and recharge. In order to solve the problem of thermal breakthrough, three optimization schemes are assumed: reducing the reinjection temperature to reduce the amount of re-injection when the amount of heat is the same, reducing the amount of production and injection when the temperature of production and injection is constant, and stopping production after the temperature of the production well decreases. However, the results show that the three schemes cannot solve the problem of thermal breakthrough or meet production demand. Therefore, it is necessary to set reasonable well spacing. Therefore, based on the strata near the Hydrological Homeland in Decheng District, the reasonable spacing of production and recharge wells is achieved by numerical simulation. Under a volumetric flux scenario ranging from 60 to 80 m³/h, the well spacing should exceed 400 m. For a volumetric flux between 80 and 140 m³/h, it is recommended that the well spacing be greater than 600 m. Full article

As a pivotal technology and infrastructure component for modern power systems, energy storage has experienced significant advancement in recent years. A fundamental prerequisite for designing future energy storage facilities lies in the systematic evaluation of energy conversion capabilities across diverse storage technologies. This study conducted a comparative analysis between pumped hydroelectric storage (PHS) and compressed air energy storage (CAES), defining the concepts of height exergy and temperature exergy. Height exergy is the maximum work capacity of a liquid due to height differences, while temperature exergy is the maximum work capacity of a gas due to temperature differences. The temperature exergy represents innovation in thermodynamic analysis; it is derived from internal exergy and proven through the Maxwell relation and the decoupling method of internal exergy, offering a more efficient method for calculating energy storage capacity in CAES systems. Mathematical models of height exergy and temperature exergy were established based on their respective forms. A unified calculation formula was derived, and their respective characteristics were analyzed. In order to show the meaning of temperature exergy more clearly and intuitively, a height exergy model of temperature exergy was established through analogy analysis, and it was concluded that the shape of the reservoir was a cone when comparing water volume to heat quantity, intuitively showing that the cold source had a higher energy storage density than the heat source. Finally, a typical hybrid PHS–CAES system was proposed, and a mathematical model was established and verified in specific cases based on height exergy and temperature exergy. It was demonstrated that when the polytropic exponent n = 1.2, the theoretical loss accounted for the largest proportion, which was 2.06%. Full article

Granite is the main rock type in hot dry rock reservoirs, and hydraulic fracturing is always required during the process of geothermal production. It is necessary to understand the strength parameters and failure criterion of granite after high-temperature and water-cooling treatment. In this paper, laboratory uniaxial and triaxial compression experiments are carried out on granite samples after high-temperature and water-cooling treatment. Combined with some experimental data collected from pre-existing studies, the variation behaviors of cohesion (c), the internal friction angle ($\phi$) and tensile strength $\left({\sigma }_{\mathrm{t}}\right)$ are systematically studied considering the heating and cooling treatment. It is found that c and $\phi$ generally show two different types of variation behaviors with the increasing heating temperature. Tensile strength decreases in a similar way for the different granite samples with the increasing treatment temperature. Empirical equations are provided to describe these strength parameters. Finally, a modified Mohr–Coulomb failure criterion with a “tension cut-off” is established for the granite samples, considering the effects of high-temperature and water-cooling treatment. This study should be helpful for understanding the mechanical behavior of hot dry rock during hydraulic fracturing in geothermal production, and the proposed failure criterion can be applied for the numerical modeling of reservoirs. Full article

Geothermal systems play a crucial role in understanding Earth’s heat dynamics. The Yunkai Uplift in southern China exemplifies a geothermally rich region characterized by ancient lithologies and high heat flow. This study investigates the geochemical characteristics of geothermal waters in the Yunkai Uplift. Both geothermal and non-thermal water samples were collected along the Xinyi–Lianjiang (XL) Fault Zone and the Cenxi–Luchuan (CL) Fault Zone flanking the core of the Yunkai Mountains. Analytical techniques were applied to examine major ions, trace elements, and dissolved CO₂ and H₂, as well as isotopic characteristics of O, H, Sr, C, and He in water samples, allowing for an investigation of geothermal reservoir temperatures, circulation depths, and mixing processes. The findings indicate that most geothermal waters are influenced by water–rock interactions primarily dominated by granites. The region’s diverse lithologies, change from ancient Caledonian granites and medium–high-grade metamorphic rocks in the central hinterland (XL Fault Zone) to low-grade metamorphic rocks and sedimentary rocks in the western margin (CL Fault Zone). The chemical compositions of geothermal waters are influenced through mixing contacts between diverse rocks of varying ages, leading to distinct geochemical characteristics. Notably, δ¹³C_CO2 values reveal that while some samples exhibit significant contributions from metamorphic CO₂ sources, others are characterized by organic CO₂ origins. Regional heat flow results from the upwelling of mantle magma, supplemented by radioactive heat generated from crustal granites. Isotopic evidence from δ²H and δ¹⁸O indicates that the geothermal waters originate from atmospheric sources, recharged by precipitation in the northern Yunkai Mountains. After infiltrating to specific depths, meteoric waters are heated to temperatures ranging from about 76.4 °C to 178.5 °C before ascending through the XL and CL Fault Zones under buoyancy forces. During their upward migration, geothermal waters undergo significant mixing with cold groundwater (54–92%) in shallow strata. As part of the western boundary of the Yunkai Uplift, the CL Fault Zone may extend deeper into the crust or even interact with the upper mantle but exhibits weaker hydrothermal activities than the XL Fault Zone. The XL Fault Zone, however, is enriched with highly heat-generating granites, is subjected more to both the thermal and mechanical influences of upwelling mantle magma, resulting in a higher heat flow and tension effect, and is more conducive to the formation of geothermal waters. Our findings underscore the role of geotectonic processes, lithological variation, and fault zone activity in shaping the genesis and evolution of geothermal waters in the Yunkai Uplift. Full article

Enhanced or engineered geothermal systems (EGSs), or non-hydrothermal resources, are highly notable among sustainable energy resources because of their abundance and cleanness. The EGS concept has received worldwide attention and undergone intensive studies in the last decade in the US and around the world. In comparison, hydrothermal reservoir resources, the ‘low-hanging fruit’ of geothermal energy, are very limited in amount or availability, while EGSs are extensive and have great potential to supply the entire world with the needed energy almost permanently. The EGS, in essence, is an engineered subsurface heat mining concept, where water or another suitable heat exchange fluid is injected into hot formations to extract heat from the hot dry rock (HDR). Specifically, the EGS relies on the principle that injected water, or another working fluid, penetrates deep into reservoirs through fractures or high-permeability channels to absorb large quantities of thermal energy by contact with the host hot rock. Finally, the heated fluid is produced through production wells for electricity generation or other usages. Heat mining from fractured EGS reservoirs is subject to complex interactions within the reservoir rock, involving high-temperature heat exchange, multi-phase flow, rock deformation, and chemical reactions under thermal-hydrological-mechanical (THM) processes or thermal-hydrological-mechanical-chemical (THMC) interactions. In this paper, we will present a THM model and reservoir simulator and its application for simulation of hydrothermal geothermal systems and EGS reservoirs as well as a methodology of coupling thermal, hydrological, and mechanical processes. A numerical approach, based on discretizing the thermo-poro-elastic Navier equation using an integral finite difference method, is discussed. This method provides a rigorous, accurate, and efficient fully coupled methodology for the three (THM) strongly interacted processes. Several programs based on this methodology are demonstrated in the simulation cases of geothermal reservoirs, including fracture aperture enhancement, thermal stress impact, and tracer transport in a field-scale reservoir. Results are displayed to show geomechanics’ impact on fluid and heat flow in geothermal reservoirs. Full article

To mitigate the influence of wellbore heat transfer on the physicochemical properties of water-based fracturing fluids in the high-temperature environments of low-permeability shale reservoirs, this study investigates the fluid filtration behavior of water-based fracturing fluids within the wellbore under such reservoir conditions. A wellbore heat-transfer model based on solid–liquid coupling was constructed in order to analyse the effects of different reservoir and wellbore factors on fluid properties (viscosity and filtration volume) in the water-based fracturing fluids. Concurrently, boundary conditions and control equations were established for the numerical model, thereby delineating the heat-transfer conditions extant between the water-based fracturing fluid and the wellbore. Furthermore, molecular dynamics theory and microgrid theory were utilised to elucidate the mechanisms of the alterations of the fluid properties of the water-based fracturing fluids due to wellbore heat transfer in low-permeability shale reservoirs. The findings demonstrated that wellbore heat transfer in low-permeability shale reservoirs exerts a pronounced influence on the fluid viscosity and filtration volume of the water-based fracturing fluids. Parameters such as wellbore wall thickness, heat-transfer coefficient, radius, and pressure differential introduce distinct variation trends in these fluid properties. At the microscopic scale, the disruption of intermolecular hydrogen bonds and the consequent increase in free molecular content induced by thermal effects are the fundamental mechanisms driving the observed changes in viscosity and fluid filtration. These findings may offer theoretical guidance for improving the thermal stability of water-based fracturing fluids under wellbore heat-transfer conditions. Full article

Abandoned mines represent an innovative and under-exploited resource to meet current energy challenges, particularly because of their geothermal potential. Flooded open-pits, such as those located in the Thetford Mines region (Eastern Canada), provide large, thermally stable water reservoirs, ideal for the use of geothermal cooling systems. Thermal short-circuiting that can impact the system performance affected by both free and forced convective heat transfer is hard to evaluate in these large water reservoirs subject to various heat sink and sources. Thus, this study’s objective was to evaluate the impact of natural heat transfer mechanisms on the performance of an open-loop geothermal system that could be installed in a flooded open-pit mine. Energy needs of an industrial plant using water from the flooded Carey Canadian mine were considered to develop a 3D numerical finite element model to evaluate the thermal impact associated with the operation of the system considering free and forced convection in the flooded open-pit, the natural flow of water into the pit, climatic variations at the surface and the terrestrial heat flux. The results indicate that the configuration of the proposed system meets the plant cooling needs over a period of 50 years and can provide a cooling power of approximately 2.3 MW. The simulations also demonstrated the importance of understanding the hydrological and hydrogeological systems impacting the performance of the geothermal operations expected in a flooded open-pit mine. Full article

Taking the coastal area of Binhai County, Jiangsu Province, as an example, this study first investigated the basic natural geography and the regional geological and hydrogeological conditions of the study area, and then carried out in-depth geophysical prospecting, hydrogeological tests, geothermal temperature monitoring, hydrochemistry and isotope analyses, and other studies based on the results to comprehensively and systematically reveal the genesis mechanism of the geothermal water resources of this coastal area from multiple perspectives. The results showed the following: the geothermal water in this area mainly comes from atmospheric precipitation; the deep east–northwest interlaced fracture is the recharge and transportation channel; the Cambrian–Ordovician carbonate rock layer, enriched by the development of cavernous fissures, forms the thermal storage layer; the underground heat mainly comes from the upward heat flow along the deep fracture and the natural warming of the strata; and the thermal reservoir cover comprises Paleozoic and Mesozoic clastic rocks that have a high mud content and form a thick layer. The genesis mode of this area is as follows: the atmospheric precipitation infiltrates and is recharged through the exposed alpine carbonate fissures in the Lianyungang area, and then it is transported to the south along the large deep fracture under the action of a high hydraulic pressure head; meanwhile, it is heated by the heat flow in the deep part of the fracture and water–rock interactions with the strata occur. Geothermal water with a calculated thermal storage temperature of 83.6 °C is formed at a depth of 2.9 km, which is blocked by the intersection of the northeast and northwest fractures to form a stagnant zone in the coastal area. Full article

The work aims to give a preliminary investigation of the geothermal potentiality of the hot dry granitic rocks in the Hail area, Northern KSA. The Hail area is characterized by a massive exposed belt of radioactive granitic rocks in the southern part, while the northern part is covered by a sedimentary section. A comprehensive methodology utilizing different categories of mineralogical petrographic, geochemical, geophysical well logging and, radiometry datasets, was used to assess the radiogenic heat production capacity of this granite. The measured data are integrated and interpreted to quantify the potential geothermal capacity of the granite and estimate its possible power production. The radioactivity and radiogenic heat production (RHP) of the Hail granites are among the highest recorded values in Saudi Arabia. Land measurements indicate uranium, thorium, potassium, and RHP values of 17.80 ppm, 90.0 ppm, 5.20%, and 11.93 µW/m³, respectively. The results indicated the presence of a reasonable subsurface geothermal reservoir condition with heat flow up to 99.87 mW/M² and a reservoir temperature of 200 °C (5 km depth). Scenarios for energy production through injecting water and high-pressure CO₂ in the naturally/induced fractured rock are demonstrated. Reserve estimate revealed that at a 2% heat recovery level, the Hail granites could generate about 3.15 × 10¹⁶ MWe, contributing to an average figure of 3.43 × 10¹² kWh/y, for annual energy per capita Saudi share. The results of this study emphasized the potential contribution of the Hail granite in the future of the energy mix of KSA, as a new renewable and sustainable resource. It is recommended to enhance the surface geophysical survey in conjunction with a detailed thermo-mechanical laboratory investigation to delineate the subsurface orientation and geometry of the granite and understand its behavior under different temperature and pressure conditions. Full article

A thorough understanding of the chemistry involved in reinjecting heat-depleted geothermal water into poorly consolidated sandstone is vital for the effective design of treatments targeting subsurface rock formations. The intricate chemical interactions occurring within sandstone systems can result in the dissolution of certain minerals and the subsequent precipitation of others, which may significantly contribute to damage within the formation. This process can alter the physical properties of the rock, potentially leading to reduced permeability and other challenges in resource extraction. Thus, it is imperative to monitor not only the concentration of various chemical species present in the geothermal water and sandstone, but also the spatial distribution of these geochemical reactions. By doing so, we can better predict and mitigate their potential adverse effects on rock formations, ensuring the long-term success and efficiency of geothermal energy extraction and other subsurface activities. In this study, we conducted laboratory experiments using both model and natural formation waters, as well as rock samples, to investigate water–rock interactions in a sandstone reservoir in the Szentes area of Hungary. Geochemical models were run with two different thermodynamic databases to simulate laboratory experiments, predict the effects of heat-depleted geothermal water reinjection into the reservoir, and assess predictions of different geochemical databases. Our study shows that calcite dissolves while quartz, kaolinite, and dolomite form. Other mineral reactions, however, remain less certain. The PHREEQC database indicates chlorite dissolution along with the formation of small amounts of feldspars and hematite, whereas the Thermoddem database predicts montmorillonite dissolution and chlorite precipitation. The reservoir porosity and permeability are expected to change over time as a result of mineral reactions. Modeling results, however, indicate negligible porosity changes as the reservoir reaches equilibrium state. The general concept proposed here, which focuses on the geochemical properties of the reinjected water and reservoir, provides a framework for detailed analysis of the geothermal system—a critical step for ensuring sustainable geothermal operations. Full article

Given the high proportion of global fossil energy consumption, the Ordovician karst water in the North China-type coalfield, as a green energy source that harnesses both water and heat, holds significant potential for mitigating environmental issues associated with fossil fuels. In this work, we collected geothermal water samples and conducted borehole temperature measurements at the Xinhu Coal Mine in the Huaibei Coalfield, analyzed the chemical composition of regional geothermal water, elucidated the characteristics of thermal storage, and explored the influence of regional structure on the karst geothermal system in the northern region. The results indicate that the geothermal water chemistry at the Xinhu Coal Mine is of the Na-K-Cl-SO₄ type, with its chemical composition primarily controlled by evaporation and concentration processes. The average temperature of the Ordovician limestone thermal reservoir is 48.2 °C, and the average water circulation depth is 1153 m, suggesting karst geothermal water undergoing deep circulation. The geothermal gradient at the Xinhu Coal Mine ranges from 22 to 33 °C/km, which falls within the normal range for ground-temperature gradients. A notable jump in the geothermal gradient at well G1 suggests a strong hydraulic connection between deep strata within the mine. The heat-accumulation model of the hydrothermal mine geothermal system is influenced by strata, lithology, and fault structures. The distribution of high ground-temperature gradients in the northern region is a result of the combined effects of heat conduction from deep strata and convection of geothermal water. The Ordovician limestone and extensional faults provide a geological foundation for the abundant water and efficient heat conduction of the thermal reservoirs. Full article

Calcite in hydrocarbon reservoirs records abundant information about diagenetic fluids and environments. Understanding the formation mechanisms of calcite is crucial for predicting reservoir characteristics and hydrocarbon migration. This study identifies the types of authigenic calcite present in the Lower Paleozoic carbonate reservoirs of the Bohai Bay Basin through petrographic analysis, cathodoluminescence, and other experimental methods. By integrating electron probe microanalysis, in situ isotopic analysis, and fluid inclusion studies, we further constrain the source of the diagenetic fluids responsible for the authigenic calcite. The results show that there are at least three types of authigenic calcite in the Lower Paleozoic carbonate reservoirs of the Bohai Sea. Calcite cemented in the syn-depositional-to-early-diagenetic stage displays very weak cathodoluminescence, with δ¹³C and δ¹⁸O and paleo-salinity distributions similar to those of micritic calcite. These features suggest that the calcite was formed during burial heating by sedimentary fluids. Calcite filling fractures shows heterogeneous cathodoluminescence intensity, ranging from weak to strong, indicating multiple stages of cementation. The broad elemental variation and multiple cementation events suggest that the diagenetic fluid sources were diverse. Isotopic data show that samples with carbon isotope values greater than −2.9‰ likely formed through water–rock interaction with fluids retained within the strata, whereas samples exhibiting more negative δ¹³C were formed from a mixed-source supply of strata and mantle-derived fluids. Calcite that fills karst collapse pores exhibits alternating bright and dark cathodoluminescence, strong negative δ¹⁸O shifts, and variability in trace elements such as Mn, Fe, and Co. These characteristics indicate a mixed origin of diagenetic fluids derived from both meteoric freshwater and carbonate-dissolving fluids. Full article

It is imperative to understand the shear mechanical properties and pore evolution of granite under thermal shock to assess the fracturing of hot dry rock reservoirs. In this study, variable-angle shear tests were performed on coarse- and fine-grained granite samples following liquid nitrogen (LN₂) cooling under different high-temperature conditions. The effect of thermal treatment temperature, particle type, and cooling method on the shear strength, cohesion, and angle of internal friction of granite was then analyzed. To this end, low field nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM) were used to investigate the pore size distribution and microstructural evolution of granite. The experimental results indicate that both the shear strength and cohesion of granite initially increase and then decrease with the rise in thermal treatment temperature. The maximum increases in shear strength and cohesion are 38.0% and 36.7%, respectively, while the maximum decreases reach 43.7% and 42.4%. Notably, the most pronounced thermal hardening effect is observed at 200 °C. In contrast, the internal friction angle exhibits a decreasing-then-increasing trend as the temperature rises, with a maximum reduction of 5.4% and a maximum increase of 14.5%. In addition, fine-grained granite exhibits superior shear strength and a more pronounced thermal hardening effect compared to coarse-grained granite. Furthermore, the damage effect caused by thermal shock increases with increasing heat treatment temperature, with the damage effect induced by liquid nitrogen cooling being particularly significant compared to water cooling. Furthermore, for both types of granite at the same shear angle, an increase in the heat treatment temperature results in a corresponding increase in the total fracture area, with the fracture area after liquid nitrogen cooling being more significant. The macroscopic failure mode changes from a mixed compression–shear failure mode to a direct shear failure mode with increasing shear angle. NMR testing shows that liquid nitrogen cooling can effectively increase the proportion of medium pores and large pores in the granite and increase the connectivity of internal pores; specifically, in coarse-grained granite, medium pores and large pores collectively increased by 10.5%, while in fine-grained granite, the total increase in medium pores reached 51%. As the heat treatment temperature increases, the type of crack that develops in granite changes from intragranular to transgranular. In addition, the fracture surface of granite is more prone to form micropores and small pores when cooled with liquid nitrogen, increasing the connectivity of the crack network. The results of this research will be useful for fracturing hot dry rock reservoirs. Full article

Horizontal and multilateral well EGSs can potentially achieve the high-heat extraction of HDR. Herein, the performance of a multilateral horizontal well system in geothermal exploitation was investigated based on the geothermal and geological conditions of Qiabuqia geothermal field. The target reservoir is a granitic basement at 2650–3650 m depth, with initial temperatures of 151–190 °C. The simulation results indicate that the initial production temperature and electric power reached 178.4 °C and 2.90 MW and decreased by 8.3% and 17.9% over 20 years of production under the scheme of injecting 60 °C cold water at 50 kg/s, respectively. The reduction in total greenhouse gas (GHG) emissions amounts to 0.15–0.51 Mt relative to a fossil fuel plant. The multilateral horizontal well EGS outperforms conventional double vertical wells in terms of production temperature and electric power generation. The heat production efficiency of the multilateral horizontal well is significantly influenced by well depth and the arrangement of branch wells. Reservoir stimulation can effectively reduce flow impedance and improve the system economy. The multilateral horizontal well system is a promising development scheme for geothermal exploitation in Qiabuqia geothermal field. Full article

Traditional conservation strategies often prioritize minimizing water use; nevertheless, water can also enhance thermal comfort by incorporating a water-to-air heat exchanger (WAHE) alongside non-direct evaporative and radiant cooling techniques. A WAHE can be installed in features such as ponds, water tanks, or rainwater cisterns. This article assesses the cooling potential of two prototypes of roof ponds and a green roof connected to a WAHE, and the results are compared to a baseline unit featuring a roof that meets California’s energy code standards. Several testing units, measuring 1.35 × 1.35 × 1.35 m, with identical heat characteristics, excluding the roof, were constructed and tested. In the first system, the heat that the green roof could not absorb was transferred to a water reservoir and then dissipated to the outside. The first roof pond prototype features a 0.35 m deep water pond topped with a 0.03 m thick insulating panel and a spray system. The second roof pond variant has an aluminum sheet with a 0.10 m air gap above a 0.25 m deep water pond. The results suggest that combining a WAHE with different roof configurations offers promising benefits while keeping water consumption limited. Notably, when the WAHE is operating, the green roof increase its performance by 47%, the insulated roof pond by 22%, and the roof pond with an aluminum sheet by 13%. Full article