Hot Dry Rock Geothermal System Research Articles

Optimization of Organic Rankine Cycle for Hot Dry Rock Power System: A Stackelberg Game Approach

Due to its simple structure and stable operation, the Organic Rankine Cycle (ORC) has gained significant attention as a primary solution for low-grade thermal power generation. However, the economic challenges associated with development difficulties in hot dry rock (HDR) geothermal power systems have necessitated a better balance between performance and cost effectiveness within ORC systems. This paper establishes a game pattern of the Organic Rankine Cycle with performance as the master layer and economy as the slave layer, based on the Stackelberg game theory. The optimal working fluid for the ORC is identified as R600. At the R600 mass flow rate of 50 kg/s, the net system cycle work is 4186 kW, the generation efficiency is 14.52%, and the levelized cost of energy is 0.0176 USD/kWh. The research establishes an optimization method for the Organic Rankine Cycle based on the Stackelberg game framework, where the network of the system is the primary optimization objective, and the heat transfer areas of the evaporator and condenser serve as the secondary optimization objective. An iterative solving method is utilized to achieve equilibrium between the performance and economy of the ORC system. The proposed method is validated through a case study utilizing hot dry rock data from Qinghai Gonghe, allowing for a thorough analysis of the working fluid and system parameters. The findings indicate that the proposed approach effectively balances ORC performance with economic considerations, thereby enhancing the overall revenue of the HDR power system.

Tectonic-magmatic-hydrothermal interactions in a hot dry rock geothermal system: The role of the transfer and normal faults in the Acoculco caldera (Mexico)

In the Acoculco caldera (hot dry rock system) two geothermal boreholes were planned to intersect fracture systems in the carbonate basement. Even though the caldera is located in an area of active deformation with high rainfall, a scarce permeability was recognized. To shed light on this apparent mismatch, we investigate the area by integrating a structural and kinematic dataset on faults and fractures with the volcanological data. The results highlighted two main coeval fault systems, NW- and NE striking respectively, and two associated minor N-S and E-W striking sets, affecting the area of the Acoculco caldera and controlling its volcanic evolution. The kinematic analysis on fault-slip surfaces demonstrated that the NW-striking faults are characterized by two superposed movements: the first comprises right strike-slip to right lateral oblique-slip, while the second is dominantly normal. In contrast, the NE-striking structures display only dominantly normal movements. We explain the kinematic evolution by two recurring events, determined by the interplay between regional extension and uplift: when a crustal extension was dominant in the region (first kinematic event), the NW- and NE-trending structures played the role of transfer and normal faults, respectively; in contrast, when uplift predominated (second kinematic event) both the pre-existing NW- and NE-striking faults acted with normal movements. The location of eruptive centers is controlled by the permeability along the regional faults and their step-over zones. A closer analysis carried out in the surroundings of the geothermal boreholes suggests that deformation migrated northwards, and that fractures are sealed in this sector of Acoculco, thus strongly reducing permeability. In this view, the most promising areas for potential permeability at depth might occur northwards, beyond of the Acoculco caldera, and along the NW-trending structures. In conclusion, the fault kinematics is influenced by periods of increased partial melting of the lithosphere, the latter determining anomalous heat flux and uplift. This process interacts with the fluid pathways that are in turn controlled by the tectonic segmentation, and this is a new model to explain the tectonic-magmatic and hydrothermal interaction of this region.

Effect of heating–water cooling cycle treatment on the pore structure and shear fracture characteristics of granite

In hot dry rock (HDR) geothermal energy extraction systems, jointed rock masses are often subjected to heating–water cooling (H-WC) cycles. In this process, HDR is at risk of failure and instability due to the change in pore structure and fracture characteristics. To identify the shear fracture characteristics of rocks after H-WC cycle treatment. First, shear fracture mechanical tests were carried out on granite specimens treated with H-WC cycles by using the short core compression (SCC) test method. The experimental results show that cyclic H-WC treatment leads to a decrease in fracture toughness values. It is especially noteworthy that the weakening effect of fracture toughness decreases with increasing cycles. Second, the NMR analysis results show that the porosity of granite samples increases with the number of H-WC cycles. At 400℃ and 600℃, the pore size distribution changes under H-WC. The slope of the fitting curve between fracture toughness and porosity indicates that the higher the temperature is, the lower the sensitivity of fracture toughness to porosity. Then, acoustic emission characteristics in the mechanical experimental process were analysed by using the b-value analysis method. The results indicated that the b-value of acoustic emission decreased with increasing cycles at 200 °C, so the crack size and energy release increased; however, at T = 400 °C and 600 °C, the b-value increased with an increasing number of cycles, so the crack size and energy release decreased. Finally, the fracture plane morphology of the failure specimens was obtained by a 3D optical scanner. The calculation results of the fractal dimension DF indicate that the cyclic H-WC treatment enhanced the fracture plane roughness, and the average increase in fractal dimension DF increases with increasing temperature, also indicating that the increasing effect of fracture plane roughness with increasing temperature becomes increasingly significant. At the same time, the fracture toughness under cyclic H-WC action was negatively correlated with the fracture plane roughness of granite.

3-D Inversion of Gravity Data of the Central and Eastern Gonghe Basin for Geothermal Exploration

The Gonghe Basin is one of the most important regions for the exploration and development of hot dry rock geothermal resources in China. However, there is still some controversy about the main heat source of hot dry rock geothermal resources in the Gonghe Basin. Combined with previous research results including three-dimensional magnetotelluric imaging and linear inversion of Rayleigh wave group and phase velocity result, we obtained a high-resolution underground spatial density distribution model of the Gonghe Basin based on satellite gravity data by using 3-D gravity focusing inversion method. According to the results, there are widely distributed low density anomalies relative to surrounding rock in the middle crust of the study area. The low-density layer is speculated to be a low-velocity, high-conductivity partial melting layer in the crust of the Gonghe Basin. The inversion result confirms for the first time the existence of a partial melt layer from the gravity point of view, and this high temperature melt layer may be the main heat source of the hot dry rock geothermal resources in the Gonghe Basin. It can provide a new basis for further research on the genesis of the hot dry rock geothermal system in the Gonghe Basin.

Surface hydrothermal activity controlled by the active structural system in the self-sealing geothermal field of Acoculco (Mexico)

The active volcanic caldera of Acoculco has been explored in the last few years to develop geothermal energy from non-conventional sources. This caldera located in eastern Mexico has been classified as a Hot Dry Rock geothermal system. Our results bring a new understanding to the interaction between two intra-caldera fault systems in terms of their permeability and degassing processes through self-potential, temperature, and CO2 flux surveys. We demonstrate the absence of hydrothermal fluid circulation with convective activity, and we provide new evidence for the origin of cold degassing. These outcomes may have important implications for future exploitation targets in the area.

Energy and exergy analysis of a hot dry rock geothermal resource power generation system based on organic Rankine cycle

Abstract Hot dry rock is an abundant, stable and low-carbon geothermal resource, which has a promising prospect for power generation in China. In this paper, a hot dry rock power generation system model based on conventional organic Rankine cycle was established. The performance of the system was evaluated by thermodynamic analysis, as well as energy and exergy analyses. Four types of organic working fluids were selected to investigate their effects on system energy efficiency. R114 had much higher energy efficiency (17.50%~ 19.00%) than other three organic working fluids (R245fa, R245ca and R601) when water flow rate ranged from 80 t/h to 260 t/h. When R245fa was used as working fluid and power output was set at 4 MW, energy and exergy efficiency of the system were 15.27% and 50.42%, respectively. According to the energy flow analysis, the major energy losses (15.67 MW) in the system were attributed to the discharge temperature of turbine exhaust. However, exergy flow analysis indicated that the energy loss from turbine exhausts was not as serious as expected, since the work potential of that energy was limited. The exergy destruction of each component in the system was calculated, and the results showed that heat exchanger and the vapor separator were the weakness components in the system, and they respectively contributed to 44.8% and 29.8% of the total exergy destruction. The results obtained in this study provide possible strategies to improve the overall performance of the hot dry rock power generation system based on conventional organic Rankine cycle.

A multi-scale methods comparison to provide granitoid rocks thermal conductivity

Granitoid rocks are frequently used in modern construction and restoration, hence the importance of knowing their petrophysical properties. These rocks find several applications in various types of surfaces, from flooring to walls, coupled to radiant devices for indoor air conditioning or as geological target in hot dry rock geothermal systems. In order to support the thermophysical characterization, the thermal conductivity of seven granitoid rocks belonging to five different lithologies (granodiorite, tonalite, granite, gabbro, and syenite) has been analysed, as main property which must to be assessed, by comparison between traditional measurement methods and a new analytical approach. In detail, the thermal conductivity was measured directly on the bulk rock samples, evaluated by applying Quantitative Phase Analysis starting from the Digital Imaging Analysis (DIA) and Rietveld refinement associating thermal parameters to each phase, and analysed by thermal modelling. Thermal simulation obtained by the 2D modelling highlights on the dependence of the thermo-physical properties of considered rocks from the minerals content, according to the abundance of quartz, as well as to the mineral’s grain size and the geometrical grain distribution. This procedure provides a reliable estimation of the rock’s thermal behaviour considering concurrently different mineralogical and textural parameters. The comparison between different approaches to thermal conductivity evaluation of granitoids allowed to consider the validity of the predictive approach, based on thermal modelling.The methodologies here developed and compared considering the thermal properties of granitic rocks can be extended and optimized for other types of rocks.

Numerical simulation of multifracture HDR geothermal reservoirs

This paper presents a three-dimensional (3D) numerical model of multifracture hot dry rock (HDR) systems using coupled thermo-hydro-mechanical (THM) modelling processes. Several numerical models are implemented in the COMSOL Multiphysics Finite Element (FE) solver to analyse the effect of fracture spacing and number on reservoir productivity. The developed multifracture HDR reservoirs employ vertical fracture systems with deviated wellbores to reflect the state of stress in deep geological formations. The models consider the geomechanical effect of fracture aperture, permeability and stiffness changes on each individual fracture during long-term exploitation of 30 years. The crucial parameters investigated concerning reservoir productivity are production temperature, thermal energy and recovery rate. The results show that both fracture number and spacing affect the productivity of HDR geothermal systems. It is clear from the results that variations of geomechanical parameters of fracture aperture, permeability and stiffness can play a crucial role in enhancing the performance of these systems.

Imaging Thermal Anomalies in Hot Dry Rock Geothermal Systems from Near-Surface Geophysical Modelling

Convective hydrothermal systems have been extensively studied using electrical and electromagnetic methods given the strong correlation between low conductivity anomalies associated with hydrothermal brines and high temperature areas. However, studies addressing the application of similar geophysical methods to hot dry rock geothermal systems are very limited in the literature. The Timanfaya volcanic area, located on Lanzarote Island (Canary Islands), comprises one of these hot dry rock systems, where ground temperatures ranging from 250 to 605 °C have been recorded in pyroclastic deposits at shallow (<70 m) depths. With the aim of characterizing the geophysical signature of the high ground temperature areas, three different geophysical techniques (ground penetrating radar, electromagnetic induction and magnetic prospecting) were applied in a well-known geothermal area located inside Timanfaya National Park. The area with the highest ground temperatures was correlated with the location that exhibited strong ground penetrating radar reflections, high resistivity values and low magnetic anomalies. Moreover, the high ground temperature imaging results depicted a shallow, bowl-shaped body that narrowed and deepened vertically to a depth greater than 45 m. The ground penetrating radar survey was repeated three years later and exhibited subtle variations of the signal reflection patterns, or signatures, suggesting a certain temporal variation of the ground temperature. By identifying similar areas with the same geophysical signature, up to four additional geothermal areas were revealed. We conclude that the combined use of ground penetrating radar, electromagnetic induction and magnetic methods constitutes a valuable tool to locate and study both the geometry at depth and seasonal variability of geothermal areas associated with hot dry rock systems.

Evaluating the environmental and economic impacts of one China’s HDR geothermal energy based heating system in a life cycle framework

PurposeIn recent years, fast urban expansion in China has stimulated rapid energy consumption growth and increased environmental pollution. Therefore, it is important to utilize clean and renewable energy in district heating for the sustainable urban development. This study aimed to investigate the environmental and economic impacts of one hot dry rock (HDR) geothermal energy-based heating system in a life cycle framework.Design/methodology/approachBy using the input–output-based life cycle analysis model, the energy consumption, CO2 emission and other pollutants of the HDR-based heating system were evaluated and then compared with those of other four heating systems based on burning coal or natural gas. The life cycle costs of the HDR-based heating system were also analyzed.FindingsThe results showed that using HDR geothermal energy for heating can significantly reduce fossil fuel consumption, CO2 emission as well as environmental pollution, and its life cycle costs are also competitive.Originality/valueThis study not only evaluated the environmental and economic impacts of the HDR-based heating system in a life cycle framework but also provided a methodological life cycle assessment framework that can estimate both economic and environmental benefits, which can be used in policy making for China’s urban development.

Experimental investigation on thermal cracking, permeability under HTHP and application for geothermal mining of HDR

Thermal cracking behavior of granite at high temperature and high pressure (HTHP) is the key to the performance of Hot Dry Rock (HDR) geothermal energy extraction system. In this study, permeability tests accompanying acoustic emission (AE) tests in granites are first conducted under HTHP by 600 °C 20MN servo control rock triaxial testing machine. The test results show that granites, nearly impermeable rocks, can show a striking increase of permeability by heating from the critical temperature. The growth curve of granite permeability shows two phases because of the multi-period of thermal-cracking in the heating process from room temperature to 500 °C. The coupled effect of temperature and pressure shows that critical temperature of permeability change decreases with increasing confining pressure. Then, a detailed characterization of the sample microstructure is presented using Micro-CT method. It is discovered that thermal cracking mainly occurs at grain boundaries in forms of inter-granular microcracks along apparent weaknesses, and develops with increasing temperature. Meanwhile intra-granular cracks are observed when heating to 500 °C, indicating that thermal cracking in granite under HTHP is induced by both intra-granular and inter-granular thermal stress. At last, experimental stimulation and application for geothermal mining of HDR are discussed.

Performance Analysis on a Hot Dry Rock Geothermal Resource Power Generation System Based on Kalina Cycle

Abstract Based on the conventional Kalina cycle, a hot dry rock geothermal resource power generation system is recommended in this paper. To predict the system performance, the corresponding thermal calculation model is established. A high pressure condenser and a low pressure condenser are used to condense the working fluid(Ammonia-water mixture) and the basic fluid in the recommended system, respectively, and a regenerator is adopted to recover part of exhaust heat of the turbine, at the same time to provide energy for the separation of ammonia-water mixture. The parameter performance analyses are carried out on the system. Results show that both the thermal efficiency and dynamic power recovery increase with elevation of heat source temperature, the dynamic recovery efficiency varies in the range of 8.5-18 percent, in the heat source temperature range of 150-220°C, and the geothermal recovery efficiency varies in the range of 86 to 88 percent. A relative low basic solution concentration and a high working fluid concentration is helpful to improve the system efficiency under the satisfied separation condition. The minor variation of the system efficiency with variation of circulating ratio indicates that the vary of circulating ratio due to the environmental elements will not cause greet effect on system performance.

Fluid geochemistry and soil gas fluxes (CO2–CH4–H2S) at a promissory Hot Dry Rock Geothermal System: The Acoculco caldera, Mexico

The Acoculco caldera has been recognized by the Mexican Federal Electricity Company (CFE) as a Hot Dry Rock Geothermal System (HDR) and could be a potential candidate for developing an Enhanced Geothermal System (EGS). Apart from hydrothermally altered rocks, geothermal manifestations within the Acoculco caldera are scarce. Close to ambient temperature bubbling springs and soil degassing are reported inside the caldera while a few springs discharge warm water on the periphery of the caldera. In this study, we infer the origin of fluids and we characterize for the first time the soil degassing dynamic. Chemical and isotopic (δ18O–δD) analyses of spring waters indicate a meteoric origin and the dissolution of CO2 and H2S gases, while gas chemical and isotopic compositions (N2/He, 3He/4He, 13C, 15N) reveal a magmatic contribution with both MORB- and arc-type signatures which could be explained by an extension regime created by local and regional fault systems. Gas geothermometry results are in agreement with temperature measured during well drilling (260°C–300°C). Absence of well-developed water reservoir at depth impedes re-equilibration of gases upon surface. A multi-gas flux survey including CO2, CH4 and H2S measurements was performed within the caldera. Using the graphical statistical analysis (GSA) approach, CO2 flux measurements were classified in two populations. Population A, representing 95% of measured fluxes is characterized by low values (mean: 18gm−2day−1) while the remaining 5% fluxes belonging to Population B are much higher (mean: 5543gm−2day−1). This low degassing rate probably reflects the low permeability of the system, a consequence of the intense hydrothermal alteration observed in the upper 800m of volcanic rocks. An attempt to interpret the origin and transport mechanism of these fluxes is proposed by means of flux ratios as well as by numerical modeling. Measurements with CO2/CH4 and CO2/H2S flux ratios similar to mass ratios of sampled gases were considered as reflecting advective transport. A numerical model of CO2 migration in the subsoil system under fully water and gas saturated conditions was performed using the TOUGH2 code in order to reproduce semi-quantitatively field measurements. The main results show that high flux values produced by advective geothermal degassing can be very localized and that low and heterogeneous permeability conditions can induce low advective CO2 flux values. Therefore, in this case the populations discriminated by the GSA method should not be interpreted in terms of origin and/or transport mechanism but rather in terms of permeability conditions.

フルオレセインの熱分解挙動に関する一次反応速度式による解析

Sodium fluorescein (uranine) has been used frequently in tracer tests that carried out in order to monitor recirculating fluids utilized for heat exchange with artificial geothermal reservoir such as hot dry rock geothermal system. It is known that fluorescein decomposes in high temperature condition. In the case which tracer solution contained fluorescein is diluted to around 0.1 mg/l of fluorescein concentration by mixing with existing natural geothermal brine in the underground, fluorescein may be strongly affected by thermal effect. In spite of being relatively short residence time, the recirculating fluid utilized in hot dry rock geothermal system is used under the condition in which the temperature dramatically changes. Therefore it is important to understand the detailed thermal decomposition characteristic of fluorescein.In this study, the experiments for thermal decomposition of fluorescein were carried out using flow-type reactor, and the effects of the initial concentration of fluorescein and temperature on the thermal decomposition behavior of fluorescein were examined experimentally and kinetically. It was found that the proportion of decomposed fluorescein to the starting became higher as the initial fluorescein concentration became lower. The kinetics of the thermal decomposition of fluorescein was approximately regarded as first-order from the results in our experiments. On the other hand, the rate equation for thermal decomposition of fluorescein should contain the term of the dissolved oxygen concentration, because the oxidation reaction is greatly related to the decomposition of fluorescein. Therefore, the effect of the dissolved oxygen concentration on thermal decomposition of fluorescein was introduced as the term of the abundance ratio of dissolved oxygen for fluorescein in the rate equation. That also means that the influence of the initial fluorescein concentration on the thermal decomposition of fluorescein is represented as the initial abundance ratio of fluorescein for dissolved oxygen. Then the new kinetics on thermal decomposition of fluorescein was suggested considering the initial concentrations of fluorescein and dissolved oxygen.

Fractal concept in numerical simulation of hydraulic fracturing of the hot dry rock geothermal reservoir

AbstractHydraulic fracturing or hydraulic stimulation is one of the most effective methods of enhancing hot dry rock (HDR) geothermal system productivity. The 3D structure of the fractured rock is approximated by the network models of ‘fractal geometry’. The models of fracture networks are generated by distributing fractures randomly in space and assuming the fractal correlation Nr = Cr−D that incorporates the number of fractures Nr, fractal length r, fractal dimension D and fracture density within the rock mass C. This procedure makes it possible to characterize the geothermal reservoirs using parameters measured from field data. Based on this approach, a mathematical model of the hydraulic rock fracturing is proposed. The model incorporates approximations of the fracture mechanical behaviour drawn from the rock mechanics literature, a very simplified analysis of the operative physical processes and mapping of the connectivity of fracture network to a cubic regular grid. Along with the fractal‐type distribution of the fracture lengths, the fracture surfaces are also assumed to follow fractal geometry. The latter allows numerical simulation of the natural rock fracture dilation caused by fracture shear offset. The other problem that can be resolved by fracture surface modelling is the apparent limitation on the number of fractures that can be analysed experimentally. In this respect, the suggested mathematical model can be used to simulate fractal surfaces identical to fractures found in the natural rocks. Taken together, these tools permit the approximate engineering resolution of the multi‐parametric, highly complex mechanical problem. The reliability of the developed model was validated by comparisons with the experimentally determined data for the Hijiori deep reservoir. This implicitly justifies the numerical results and conclusions drawn in the present research. In particular, a series of computations indicate that the connectivity of the fracture network is greatly affected by the fractal dimension of the fracture network. The strong effect of the fractal dimension on the reservoir's size is also observed. The numerical results illustrate the controlling effect of the pressure and flow rate in the stimulating well for reservoir growth. Copyright © 2003 John Wiley & Sons, Ltd.

Economic Analysis of Heat Mining

The extraction of heat or thermal energy from the Earth—heat mining—has the potential to play a major role as an energy supply technology for the 21st century. This paper looks specifically at the potential for hot dry rock (HDR) geothermal energy in the United States. A generalized multiparameter economic model was developed for optimizing the design and performance of HDR geothermal systems. Assuming reservoir productivity comparable to existing hydrothermal systems, high-grade HDR systems (∇T > 60 °C / km) produce electricity in the 6-7c/ kWhe range. For low-grade systems (∇T < 40 ° C / km) to become commercially attractive, much higher reservoir productivity levels and / or substantially lower drilling costs are required.

室内水圧破砕実験により造成されたき裂の幅の推定

An artificial fracture made by hydraulic fracturing plays an important role as a heat exchanger in Hot Dry Rock geothermal system. An estimation on distribution of fracture width before hydraulic fracturing is necessary in designing HDR system. A laboratory test was carried out to estimate the width of an artificial fracture. A cubic rock specimen, Inada granite, was set into a rigid steel frame, and hydraulic pressure was applied to make a fracture. After hydraulic fracturing, water was injected. A flow rate of injected water was increased step by step. An injection pressure, flow rate and displacements of free surfaces were measured during these injection tests. Unlikely to similar experiments done before, the displacements of the free surfaces which was parallel to fracture were measured. Based on the experiment result, the distribution of fracture width and pressure along the fracture was estimated by using combine analysis of analytical and Finite Element Method. By the analysis proposed in this paper, the distribution of fracture width and pressure, which had been hard to be measured, could be estimated successfully.

Parametric study of the energy extraction from hot dry rock based on fractal fracture network model

Models of two-dimensional and three-dimensional subsurface fracture networks are proposed on the basis of “fractal geometry” to describe geothermal water flow. Using this fracture network model, as well as the single fracture model and the permeable zone model, a parameter study of the performance of Hot Dry Rock geothermal energy extraction systems, such as the changes in output water temperature and thermal output with time, is performed. The results show a strong dependence of the system performance on geothermal reservoir characteristics, such as the fracture density of the rock mass. The fracture network model proposed here can describe the performance of reservoirs which vary between the single fracture model and the permeable zone model, and is expected to bridge the gap between the two models. It is confirmed that the performance of geothermal energy extraction systems can be predicted as a function of the fracture density of the rock mass by using the fracture network model.

Prospects for universal geothermal energy from heat mining

The extraction of heat or thermal energy from the Earth—heat mining—has the potential to play a major role as an energy supply technology for the 21st century. However, even if reservoir productivity goals are achieved, the role of heat mining with today's energy prices and development costs is limited to only a small fraction of the Earth's surface. A generalized multi‐parameter economic model was developed for optimizing the design and performance of hot dry rock (HDR) geothermal systems. Key technical and institutional obstacles to universal heat mining are discussed in a more general context. Advanced concepts in drilling technology are reviewed in light of their potential impact on overcoming some of these obstacles to universal heat mining

Heat transfer by water flow in rock fractures and the application to hot dry rock geothermal systems : J. Zhao & C. P. Tso, International Journal of Rock Mechanics & Mining Sciences, 30(6), 1993, pp 633–641

Heat transfer by water flow in rock fractures and the application to hot dry rock geothermal systems : J. Zhao & C. P. Tso, International Journal of Rock Mechanics & Mining Sciences, 30(6), 1993, pp 633–641