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"GEOTHERMAL DEVELOPMENT"
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Study on the Accuracy of Fracture Criteria in Predicting Fracture Characteristics of Granite with Different Occurrence Depths
The fracture network of a deep geothermal reservoir forms the place for heat exchange between injected fluid and rock mass with high temperature. The fracture resistance ability of reservoir rocks will affect the formation of fracture-network structure, heat exchange and transmission characteristics, and reservoir mechanical stability. However, there are few reports on the fracture toughness and trajectory prediction of geothermal reservoirs with different depths. In this paper, the modified maximum tangential stress criterion (MMTS) is analyzed. The results show that the experimental data are significantly different from the theoretical estimate of MMTS under the influence of different occurrence depths. It is found that the fracture process zone (FPZ) seriously affects the accuracy of predicting fracture initiation angle and mixed-mode (I+II) fracture toughness by MMTS. The FPZ value, considering the influence of different occurrence depths, is modified, and the accuracy of MMTS in predicting the fracture mechanical characteristics of granite is improved. In addition, the mechanical test results show that the Brazilian splitting strength (σt) of granite fluctuates increase with the increase in temperature. With the increase in deviatoric stress, the Brazilian splitting strength and the Brazilian splitting modulus of rock show a trend of first increasing, then decreasing, and then increasing.
Journal Article
Effect of Grain Size on the Mechanical Behaviour of Granite Under High Temperature and Triaxial Stresses
The optimal development of hot dry rock (HDR) geothermal is deep HDR geothermal. Because of different diagenesis environments, the mineral composition and micro-structure of deep granite are quite different than those of shallow granite. To reveal the characteristics of deep granite and guide HDR geothermal development, the difference in thermal and mechanical properties between the granite in Luya Mountain, Shanxi Province, China (coarse-grained granite) and the granite in Shandong Province, China (fine-grained granite) under high-temperature (100–400 ℃) triaxial stress was studied. The results show that the thermal expansion coefficient of the coarse-grained granite increases linearly with increasing temperature, and the thermal expansion coefficient of the coarse-grained granite is 1.52 times that of the fine-grained granite on average, and the difference reaches a maximum at 400 ℃. The elastic modulus of the coarse-grained granite increases slowly first and then decreases sharply with increasing temperature, and its threshold temperature varies with temperature at approximately 300 ℃. The elastic modulus of the fine-grained granite is 1.4–2.6 times that of the coarse-grained granite, and the difference increases with increasing temperature and confining pressure. According to the failure test under triaxial stress (confining pressure (σc) = 25 MPa) and at 400 ℃, for the coarse-grained granite, the peak strength, elastic modulus and its threshold temperature change with temperature are smaller, the peak strain is larger and the elasto-plastic transition occurs easier than those for the fine-grained granite. Micro-observation shows that the larger crystal particles and the extreme heterogeneity of the coarse-grained granite lead to larger thermal deformation and greater deterioration of the mechanical properties, compared with those of the fine-grained granite, further leading to higher permeability of the coarse-grained granite under high temperature and high pressure. The existence of coarse-grained granite provides a good geological foundation for high-efficiency, low-cost and large-scale construction of artificial reservoirs in the process of HDR geothermal development.
Journal Article
Experimental Study of Temperature Distribution and Evolution Law of Fractured Rock Mass During Heat Transfer Process
To investigate the hindering effect of fracture on heat transfer within rock mass, real-time temperature monitoring was conducted on fractured granite heated by different constant-temperature heat sources. The heat transfer characteristics were analyzed based on the temperature field distribution, temperature variation pattern, heating rate, temperature difference, and temperature gradient evolution within the fractured granite. Additionally, the difference in temperature field distributions of both intact and fractured granite under different heat source temperatures were discussed. The study reveals that fracture exerts significant control over temperature field distribution in granite, with heat transfer governed by the combined effects of rock heterogeneity and fracture presence. During heating, fractured granite exhibits three distinct temperature response stages: rapid heating, slow heating, and temperature stabilization. Steady temperatures decrease nonlinearly and linearly with increasing distance from the heat source on the left and right side of the fracture. All monitoring points display unimodal heating rate trends (rise–peak–decline–stabilization), with peak rates of 20~120 °C/h above the horizontal monitoring line F-F versus 10~20 °C/h below it. The hindering effect of fracture on heat transfer shows spatial heterogeneity; under the condition of the 80 °C heat source, the temperature difference in the central region is the largest (ΔTmax = 10.5 °C), and the top is the smallest (ΔTmin = 4.5 °C). Concentrated isothermal gradient contours form around fractures, with the time-to-peak gradient correlating positively with heat source distance. Maximum and minimum temperature gradients within the fracture reached 825 °C/m and 345 °C/m, respectively, at 90 °C source temperature. The research results can provide a theoretical basis and technical support for HDR geothermal development.
Journal Article
Study on Enhanced Heat Transfer and Stability Characteristics of Al2O3–SiO2/Water Hybrid Nanofluids
Hybrid nanofluids have better thermal conductivity and heat transfer properties than common fluids and currently have good prospects for applications in enhanced geothermal development. In this paper, a convective heat transfer experimental platform was built independently to study the convective heat transfer capability of Al
2
O
3
–SiO
2
/water hybrid nanofluid. The effect of different parameters on the enhanced heat transfer capability of the hybrid nanofluid was investigated, and the stability of the hybrid nanofluid under flow heat transfer conditions was evaluated. The following research results were obtained. Compared to water and single nanofluid, Al
2
O
3
–SiO
2
/water hybrid nanofluid has better enhanced heat transfer capability. The coaxial casing with \"outside in and inside out\" is more energy-efficient compared with the U-shaped heat exchanger tube. For Al
2
O
3
–SiO
2
/water hybrid nanofluid, there was an optimal particle concentration and ratio to optimize the convective heat transfer capability: a particle concentration of 0.10 wt % and a particle ratio of 5:5. The heat extraction of the fluid was negatively correlated with the inlet temperature and positively correlated with the flow rate. However, after the flow rate was higher than a certain extent, the heat exchange energy efficiency dropped significantly. The stability of Al
2
O
3
–SiO
2
/water hybrid nanofluid was well maintained in the flow heat exchange process. And the higher the flow rate, the more stable the particles are. However, the enhancement effect of macroscopic flow on the particle suspension stability would be weakened by too high fluid temperature.
Journal Article
Cross-Scale Linkages of Centralized Electricity Generation: Geothermal Development and Investor–Community Relations in Kenya
Based on a study of Kenya’s geothermal-energy development in Baringo-Silali, we explore how and with whom government actors and local communities in rural and peripheral areas interact when planning and implementing large-scale power plants. Starting from a comparison of decentralized and centralized energy systems, we demonstrate that the development of this large-scale infrastructure project and the associated investor-community relations are governed by various cross-scale linkages. To this end, we adapt the concept of cross-scale linkages from the literature on natural-resource governance to explore actors, rules, and practices at local, regional, national, and international levels.
Journal Article
Artificial Intelligence-Centric Low-Enthalpy Geothermal Field Development Planning
Low-enthalpy geothermal energy can make a major contribution towards reducing CO2 emissions. However, the development of geothermal reservoirs is costly and time intensive. In particular, high capital expenditures, data acquisition costs, and long periods of time from identifying a geothermal resource to geothermal heat extraction make geothermal field developments challenging. Conventional geothermal field development planning follows a linear approach starting with numerical model calibrations of the existing subsurface data, simulations of forecasts for geothermal heat production, and cost estimations. Next, data acquisition actions are evaluated and performed, and then the models are changed by integrating the new data before being finally used for forecasting and economics. There are several challenges when using this approach and the duration of model rebuilding with the availability of new data is time consuming. Furthermore, the approach does not address sequential decision making under uncertainty as it focuses on individual data acquisition actions. An artificial intelligence (AI)-centric approach to field development planning substantially improves cycle times and the expected rewards from geothermal projects. The reason for this is that various methods such as machine learning in data conditioning and distance-based generalized sensitivity analysis assess the uncertainty and quantify its potential impact on the final value. The use of AI for sequential decision making under uncertainty results in an optimized data acquisition strategy, a recommendation of a specific development scenario, or advice against further investment. This approach is illustrated by applying AI-centric geothermal field development planning to an Austrian low-enthalpy geothermal case. The results show an increase in the expected value of over 27% and a reduction in data acquisition costs by more than 35% when compared with conventional field development planning strategies. Furthermore, the results are used in systematic trade-off assessments of various key performance indicators.
Journal Article
New geothermal law and its implications for geothermal development in Indonesia
Purpose
This paper aims to investigate the new geothermal law and its implications for geothermal development in Indonesia.
Design/methodology/approach
This paper investigates new geothermal law and its implications for geothermal development in Indonesia by using comparable law article to discuss this development. All the data are obtained from literature studies based on the history and background, ownership and access, investment and risks, electricity market and re-policies, environmental regulations and community participation around the geothermal resources.
Findings
Geothermal industries in Indonesia need assurance and clear legal basis to be developed. Geothermal investors, whether foreign or local, need policy assurance and good business structures that can create a secure investment environment. Furthermore, a good relationship with international cooperation agency needs to be improved to establish knowledge transfer regarding technologies and information about exploration, production and geothermal development. This is essential to further develop the geothermal utilization and prepare Indonesia to be the country with the biggest reduction in CO2 emission in 2025. New geothermal regulation (Law No.21 Year 2014) has facilitated the geothermal activities. After the issuance of this regulation, the activities can be done at conserved, production and conservation forest. Moreover, the public service agency (BLU) or state-owned enterprise (BUMN) assignment scheme can boost the geothermal industries development.
Originality/value
Indonesia has huge geothermal resources because of its geological condition that is located at volcanoes path, also known as “Ring of Fire”. According to 2017 ESDM data, Indonesia’s geothermal potentials are almost 30 GW and are distributed to 331 potential points. However, only 5 per cent of those potentials (1,643.5 MW) have been utilized to date. The huge potentials can be utilized optimally to support the Government’s plan in achieving the energy security. Therefore, policies that regulate geothermal utilization is strategically essential for Indonesia’s future.
Journal Article
Energy Loss Assessment for Geothermal Development of High-Temperature Rock Mass in Ocean Plate Based on Particle Swarm Optimization and Interior Point Hybrid Optimization
Zhang, Y., 2019. Energy loss assessment for geothermal development of high-temperature rock mass in ocean plate based on particle swarm optimization and interior point hybrid optimization. In: Hoang, A.T. and Aqeel Ashraf, M. (eds.), Research, Monitoring, and Engineering of Coastal, Port, and Marine Systems. Journal of Coastal Research, Special Issue No. 97, pp. 170–176. In the process of geothermal development of high-temperature rock mass in ocean plate, there are three coupling effects of seepage field, stress field and temperature field. Combining with the thermal fracture, temperature field variation and heat convection exchange law of high-temperature rock mass around reservoir, a geothermal-fluid-solid coupling damage model of high-temperature rock mass is established, and the energy loss in the coupling process is calculated by using the model. A mathematical model of energy loss in the process of heat energy acquisition is constructed by studying the internal structure of collector. Based on the study of geothermal development and application process of high temperature rock mass and the above model, an energy loss assessment model for geothermal development of high temperature rock mass in Ocean plate is constructed, and optimized by particle swarm optimization and interior point hybrid optimization algorithm. Particle swarm optimization and interior point penalty constraints are used to reduce the computational complexity of the assessment process and improve the accuracy of energy loss assessment results. The feasibility of the proposed method is verified by comparing the experimental results with the current methods and expert assessment results.
Journal Article