Hot Dry Rock Research Articles

Effect of water-cooling shock on fracture initiation and morphology of high-temperature granite: Application of hydraulic fracturing to enhanced geothermal systems

Deep geothermal energy exploitation is primarily accomplished using an enhanced geothermal system (EGS) to extract heat from hot dry rock (HDR). The use of hydraulic fracturing technology to build large-scale fracture networks is essential for improving the EGS heat extraction efficiency. Water-cooling shock on high-temperature rock degrades the mechanical properties of the rock while increasing the fracturing effect of HDR. A thermal–hydraulic–mechanical coupling model is proposed herein to investigate the effect of water-cooling shock on fracture initiation and morphology in HDR hydraulic fracturing. The validity of the coupling model is confirmed using theoretical analytical solutions and physical experimental results, such as hydraulic fracturing experiments on high-temperature granite, as well as analytical solutions for the stress shadow effect and fracture initiation pressure. The numerical simulation is performed using a coupling model to investigate the characteristics of fracture propagation and damage evolution during the HDR hydraulic fracturing process as well as to investigate the fracturing effect of water-cooling shock on high-temperature rock. Finally, the effects of initial rock temperature, in-situ stress, and heat transfer coefficient on fracture morphology are investigated. The results show that the combined effects of tensile strength deterioration water-cooling shock, thermal stress caused by cold shrinkage, and pore water pressure are primarily responsible for the initiation and propagation of hydraulic fractures. The water-cooling shock can considerably reduce the fracture initiation pressure, induce more secondary fractures, and form more complex fracture networks. The initial temperature and heat transfer coefficient of the rock affect the thermal stress and tensile strength deterioration. The formation of an intricate fracture network can be aided by a high initial rock temperature, a large surface heat transfer coefficient, and small in-situ stress differences.

Optimized geothermal energy extraction from hot dry rocks using a horizontal well with different exploitation schemes

In the foreseeable future, the geothermal exploitation from hot dry rocks (HDR) using a horizontal well will bear potential. Thus, in-depth studies should be conducted on the selection of injection-production scheme (IPS) and working fluid, design of reinjection parameters, optimization of wellbore structure and materials, and analysis of geological settings. This paper proposed a fully coupled model to study the above scientific questions. For Model A, the working fluid was injected into the annulus and then flowed out of the thermal insulation pipe (TIP). Its temperature passes through two stages of temperature rise and two stages of temperature decline. But for model B, the working fluid was injected into the TIP and then flowed out of the annulus. Its temperature undergoes five stages, four stages of temperature rise and one stage of temperature decline. The results show that the Model A is the best IPS owing to its high outlet temperature, stable thermal recovery, and low fluid injection volume. In Model A, when the working fluid was supercritical carbon dioxide and the liquid injection volume was 135.73 m3/d, the heat recovery ratio (HRR) was as high as 85.40%, which was 17.85% higher than that of the Model B whose working medium was water, and its liquid injection volume was only 25% of that. Meanwhile, over ten years of continuous production, the outlet temperature decreased by 7.5 °C and 18.38 °C in the latter. The optimal working fluid has a low volume heat capacity and thermal conductivity for any IPS. Sensitivity studies showed that for the area that met the HDR standard, the effect of reinjection temperature on the outlet temperature can be ignored. As for Model A, HRR drops sharply by 6.74–9.32% when TIP goes from completely adiabatic to nonzero thermal conductivity. Meanwhile, the horizontal segment length of the TIP is shorter when Model A obtains the optimal outlet temperature compared with Model B. In addition, the correlation between the outlet temperature and different formations of thermophysical properties was seriously affected by the IPS and exploitation period, which was summarized in detail.

Supercritical flow and heat transfer of SCO2 in geothermal reservoir under non-Darcy's law combined with power generation from hot dry rock

Hot dry rock is one of the huge reserves of clean energy sources, and enhanced geothermal system is the effective utilization. Darcy's law is widely employed to describe the fluid flow in geothermal reservoirs. However, when Darcy's law is applied to describe supercritical flow in geothermal reservoir, there will be the deviation of temperature and pressure because of the high sensitivity of thermal physical properties. To compare Darcy's law with non-Darcy's law, Darcy's law is replaced by the non-Darcy’ law to establish the coupled numerical model. The results show that there is little difference for the temperature distribution of geothermal reservoir between the two assumptions. The pressure drops of geothermal reservoirs with different mass flow rates have a significant impact on power generation through the heat loss in production well. The heat preservation in production well is indispensability when the flow rate is small. Through the comparison between the two assumptions of Richardson number and Prandtl number, the buoyancy effect under non-Darcy's law is more significant than that of Darcy's law. When power generation, net work output, payback period and exergy efficiency are considered, mass flow rate of 50 kg/s is the optimal in EGS under the actual engineering conditions.

Evaluation of the Fault Activation Risk Induced by Hot Dry Rock Reservoir Development Based on Thermal-Hydraulic-Mechanical Coupling.

Due to the nature of hot dry rock resources and the particularity of the development methods, the fault activation induced by injection and production of hot dry rocks involves a complex multifield coupling mechanism. Traditional methods cannot effectively evaluate the fault activation behavior in hot dry rock injection and production. Aiming at the above-mentioned problems, a thermal-hydraulic-mechanical coupling mathematical model of injection and production of hot dry rocks is established and solved by a finite element method. At the same time, the fault slip potential (FSP) is introduced to quantitatively evaluate the risk of fault activation induced by injection and production of hot dry rocks under different injection and production conditions and geological conditions. The results show that under the same geological conditions, the greater the well spacing of injection and production wells, the greater the risk of fault activation induced by injection and production and the greater the injection flow, the greater the risk of fault activation. Under the same geological conditions, the lower the reservoir permeability, the greater the fault activation risk and the higher the initial reservoir temperature, the greater the fault activation risk. Different fault occurrences result in different risks of fault activation. These results provide a certain theoretical reference for the safe and efficient development of hot dry rock reservoirs.

Predicting the Temperature Field of Hot Dry Rocks by the Seismic Inversion Method

Hot dry rocks, as clean and abundant sources of new energy, are crucial in the restructuring of energy. Predicting the temperature field of hot dry rocks is of great significance for trapping the target areas of hot dry rocks. How to use limited logging data to predict the temperature field within a work area is a difficulty faced in hot dry rock exploration. We propose a method to predict the hot dry rock temperature field (using seismic inversion results). The relationship between porosity and transverse wave velocity was established with petrophysical modeling. The difference in porosity calculated from the density and transverse wave velocity was incorporated in the seismic inversion results to find the thermal expansion and predict the temperature field. We applied the method to predict the temperature of hot dry rocks in the Gonghe Basin. The results showed that the temperature in the northeast work area was higher than in the southwest area at the same depth, and a depth of 150 °C of the hot dry rock reservoir was shallower. The thermal storage cover was analyzed from the geological stratigraphic data of the Gonghe Basin. The thermal storage cover in the northeastern part was thicker than in the southwestern part and had better thermal insulation, which is consistent with the prediction of the temperature field.

Geothermal Potential of Hot Dry Rock in South-East Baltic Basin Countries—A Review

The beginning of 2022 was a time of major changes in the perception of energy availability and security in European countries. The aggression of Russia against Ukraine destabilizing the European energy economy, combined with the withdrawal from fossil fuels which has been going on for a dozen years, has strengthened activities to introduce new energy technologies based on renewable energy sources. One of the most promising and stable sources of renewable energy is geothermal energy, in particular enhanced geothermal systems (EGS) in hot dry rocks (HDR). These deposits occur at a great depth in almost every place on Earth, but due to their low permeability, they require hydraulic fracturing, which results in high investment costs. This technology has been developed for several decades. The current situation in Europe seems to confirm that its rapid development to a level that guarantees stable and profitable operation is crucial. This is of particular importance in the case of former member states of the economic zone of the Council for Mutual Economic Assistance, which until recently were heavily dependent on Russian energy. This review, based on the latest available data, covers potential HDR prospective areas in the countries of the south-eastern Baltic basin, including Lithuania, Latvia, Estonia and Poland. It is specific to this region that the original heat flux density is lower as a result of the paleoclimatic effect associated with the youngest ice age; however, thermal conditions do not deviate too much compared to western Europe, especially Rhine Graben, and significantly exceed the conditions of Finland, where an EGS project is currently being operated. In Lithuania, the most prospective area is the ZNI intrusion (south of Klaipeda), characterized by a geothermal gradient of up to 40 °C/1000 m. In addition, the Precambrian batholith south of Liepāja (Latvia) and the Rapakivi granites in the north and center of Estonia are promising EGS sites. Poland has relatively the most explored EGS potential, in both volcanic, crystalline and sedimentary rocks, especially in the area of the Szczecin Trough, Gorzów Block, Moglino-Łódź Trough and Karkonosze Mountains. Unfortunately, local tectonic conditions, in particular the development of faults and natural fracture zones that affect the directions of fracture propagation during hydraulic fracturing, have not been sufficiently recognized, which is one of the main barriers to the expansion of EGS pilot projects in these countries. These issues present challenges for the researchers, especially in terms of petrophysical analyses of rocks in target zones and local stress conditions, which have a key impact on fracturing operations and profitability of the systems. Despite high investment costs on the one hand and a significant slowdown in the global economy in 2022 on the other, it remains hopeful that the authorities of individual countries will decide to accelerate research work, leading to the implementation of pilot projects of EGS installations, and that this technology will be further improved to ensure a stable clean energy supply.

Coupled heat-fluid-solid numerical study on heat extraction potential of hot dry rocks based on discrete fracture network model

Fracture networks within hot dry rock (HDR) geothermal reservoirs are complex, and heat extraction via water injection is thus a coupled process of heat-fluid-solid multifield. In this paper, utilizing the theory of normally distributed random functions, we develop a corresponding pre-processing subprogram to establish a discrete network model of complex fracture distribution in HDR reservoirs; then construct a heat-fluid-solid finite element model for heat extraction via water injection and compare the numerical solution with the analytical solution of the one-dimensional non-isothermal consolidation problem for verification. The numerical simulation results show that the main factors affecting the heat extraction efficiency of HDR reservoirs include fracture width, fracture density, fracture permeability, and matrix permeability. When a HDR reservoir is injected with water for heat extraction, there is a certain threshold value of these influential parameters, beyond which the outlet temperature drops significantly, resulting in an obvious thermal breakthrough. When injecting water for heat extraction, the values of these parameters should be controlled and kept at a reasonable level, otherwise, the HDR reservoir may enter a thermal breakthrough stage in advance, which is not conducive for long-period heat extraction. Influenced by the random distribution of complex fractures, the leading edge of the cold front may present an irregular distribution. During the process of heat extraction, the stress gradually changes from a compressional state to a tensile state, which induces further damage to the HDR reservoir.

Three‐Dimensional Thermoporoelastic Modeling of Hydrofracturing and Fluid Circulation in Hot Dry Rock

AbstractGeothermal energy, featured as a renewable low‐carbon energy resource, exhibits great potential in mitigating global warming. However, efficient mining of geothermal energy from hot dry rock remains challenging due to the lack of a thermoporoelastic modeling approach that allows for integrated simulation of hydrofracturing and fluid circulation, and poor understanding of the mechanisms of hydraulic stimulations. To conquer these challenges, we propose an advanced modeling approach, which enables us to deal with the interplay between fluid transport, heat transfer, rock deformation, and three‐dimensional fractures' growth and interaction. Based on the thermoporoelastic model, a refined investigation is performed to determine the hydrofracturing and fluid circulation processes, using data collected from the EGS Collab Experiment 1. This meso‐scale testbed involves creating major hydraulic fractures, accompanied by numerous secondary hydroshearing fractures, hydraulically connecting two horizontal wellbores to build a fluid circulation system. All wellbores in the testbed were heavily instrumented with monitoring devices, offering high‐quality data to properly constrain the geomechanical model. Our study provides insights into: (a) how the EGS Collab testbed responds to propagating fractures with strong stress shadowing and long‐term injection of chilled water; (b) why the parallel pattern of two hydraulic fractures with a distance much smaller than the fracture dimension forms during multi‐staged hydraulic stimulations; and (c) what the mechanisms are that determine hydrofracturing performance. Overall, this work not only contributes to a novel modeling approach for integrated simulation of hydrofracturing and fluid circulation, but advances our understanding of creating enhanced geothermal systems with interconnected fracture networks.

An Innovative Method to Analyze the Hydraulic Fracture Reopening Pressure of Hot Dry Rock.

This paper focuses on a new test method and theoretical model for measuring and evaluating the reopening pressure during hot dry rock hydraulic fracturing. Firstly, rock blocks of four lithologies were collected from the hot dry rock strata. Hydraulic fracturing tests at high temperatures in real-time were conducted using drilled cubic specimens and drilled cubic specimens with a pre-crack. Breakdown pressure, reopening pressure, and fracture toughness were measured, respectively. In addition, Brazilian splitting tests at high temperatures in real-time were performed using Brazilian disc specimens to measure tensile strength. Secondly, an empirical equation for evaluating the reopening pressure during hot dry rock secondary fracturing was developed based on fracture mechanics and hydraulic fracturing theory. Third, the values calculated by the new equation, considering breakdown pressure, fracture toughness, and tensile strength, were compared to the values determined by the classical equation and to measurement results. It was found that the new equation predicted closer reopening pressure to the measurement results, regardless of the lithology of the hot dry rock. Moreover, with increasing temperature in the specimens, the error between the value calculated by the new equation and the measurement value remained low. In contrast, the difference between the classical equation predictions and the measurement results was widened. In addition, the reopening pressure was positively correlated with tensile strength and fracture toughness. Variations in lithology and temperature affected tensile strength and fracture toughness, which then changed the hot dry rock reopening pressure.

Discrete element modeling for the multistage hydraulic stimulation of a horizontal well in hot dry rock

Hydraulic fracturing aimed at building a network of fractures in hot dry rock (HDR) often induces strong seismic risks. However, few studies have focused on seismic risks, and methods for increasing the stimulated reservoir volume (SRV) while reducing seismic risks have not been adequately studied. Therefore, based on the full hydro-mechanical coupling, a seismic-fracturing model with a discrete fracture network (DFN) was developed, and the methods for increasing the SRV while reducing the seismic risk of horizontal wells at the FORGE site in Utah were discussed. The results showed that the fluid channeling effect between each segment is the main reason for the uneven fracture growth in the horizontal well. Additionally, the spatiotemporal evolution of fracturing-induced seismicity shows that the two ends (L1 and L3) of the horizontal well tend to form high stress concentrations and induce larger seismic events than the middle (L2). To solve these problems, a novel hydraulic fracturing method with a differentiated-stage, variable-injection volume suitable for the multi-fracturing of horizontal wells in HDR was proposed. With the application of the method, the SRV of the horizontal well was improved effectively, and the average magnitude induced by hydraulic fracturing was also decreased by 15.4%.

Thermo-economic optimization of an enhanced geothermal system (EGS) based on machine learning and differential evolution algorithms

Hot dry rock (HDR) is considered as a promising low-carbon alternative to fossil fuels, but the remaining economic challenges are leading to its unsuccessful exploitation. Therefore, incorporating economic indicators into consideration is essential to optimize an enhanced geothermal system (EGS). However, the conventional optimization approaches based on numerical simulations are time-consuming and a global optimal operation strategy is hard to determine. In this study, an optimization framework based on Artificial Neural Network (ANN) and Differential Evolution (DE) is proposed with considering a levelized cost of electricity (LCOE) being an economic performance indicator to optimize a three-horizontal-well EGS in the Qiabuqia field. Specifically, four different ANN models are constructed to predict different geothermal productivities to substitute numerical models. Based on these ANN models, a DE optimization process is conducted to determine an optimal LCOE under two field operating constraints, followed by a performance comparison between the resulting optimal geothermal system and 2,150 randomly created cases using a numerical simulator. The results show that these ANN models all achieve a coefficient of determination R2 higher than 0.996, demonstrating their predictive abilities and potential as surrogate models. The determined optimal parameters configuration brings a promising LCOE of 0.0376 $/kWh which is around 50 % of a local electricity cost, and this is the lowest LCOE among all random cases. Importantly, the proposed framework can significantly save operation time by 36,000 times compared with the numerical simulation method. The proposed method provides a valuable reference for the geothermal system studied, and it can also be effectively applied to other energy systems, thereby facilitating their optimal development.

Seismic Diffraction Attribute Fusion for Geological Discontinuities in Hot Dry Rock Resources

For the safe development and utilization of hot dry rock resources, it is essential to understand the distribution characteristics of underground faults. However, the commonly used reflection attribute analysis method has an insufficient resolution, and the diffraction attribute analysis method is affected by multiple solutions. Moreover, both are highly dependent on the interpreters’ experience and take a long time. Therefore, based on the classical U-Net model, a diffraction attribute fusion model (DAF-U-Net) with 27-layer convolution is proposed. The DAF-U-Net network takes four-channel diffracted attributes as an input and underground fracture distribution as an output. The new network adds a spatial attention and channel attention mechanism to improve the positioning and extraction ability of the U-Net model for the attribute characteristics of diffractions. After optimizing the diffraction attributes of hot dry rock slices in the Gonghe basin, Qinghai, the slices are input into the network to train the model. According to the prediction and identification results of the network model, the DAF-U-Net network has a high reliability in predicting fracture distributions. It has a specific reference role in the subsequent exploitation of hot dry rock.

Mechanical Response of Stress Wave Attenuator With Layered Structure Under Impact Load: Experimental and Numerical Studies

Abstract The fluidic down-the-hole (DTH) hammer drilling is generally regarded as one of the best means for hot dry rock (HDR) drilling. The fluid oscillator, as the core part of the fluidic DTH hammer, is prone to fracture when subjected to impact loads due to its brittle characteristics. Therefore, a steel-layered structure with different contact areas is developed as a stress wave attenuator to protect the fluid oscillator for the DTH hammer under high-temperature drilling conditions. In this paper, the stress wave attenuation performance with different steel-layered structures is analyzed based on the split Hopkinson pressure bar (SHPB) technique. The effects of contact area ratio, the orientation of contact surfaces, and number of layers on the stress wave attenuation are investigated by numerical simulations and laboratory tests. It is found that the attenuation ratios in stress amplitude and impact energy gradually increased with the increase of the contact area ratio. Besides, the orientation of contact surfaces has a significant influence on the attenuation effect. For the layered structure with a two-layer object part, the maximum attenuation ratios of stress amplitude and impact energy are 62.4% and 79.6%, respectively, when the included angle between the two convex structures is increased to 90 deg. Additionally, the attenuation ratio of the layered structure can be improved by increasing the number of layers. The results demonstrate that the stress wave attenuator with layered structures has great potential for brittle materials protection against impact loads.

Review of deep fluids in sedimentary basins and their influence on resources, with a focus on oil and geothermal exploitation

Deep fluid activity is widespread in large oil-gas basins around the world. Deep fluids, as the links between internal and external factors of a basin, run in the way of organic-inorganic interactions through the oil-gas formation and aggregation. Herein, the identification characteristics of deep fluids in sedimentary basins as well as their influence on oil-gas reservoir formation and geothermal resource are summarized. The deep fluids of sedimentary basins are identified from three aspects, including mineral composition, fluid inclusions, and geochemical characteristics. The effects of deep fluid activities on oil-gas reservoir formation are manifested in two key aspects of matter and energy. As for the matter effects, deep fluids can improve the primary productivity of sedimentary basins and carry abundant inorganic hydrogen, which contributes to improving the hydrocarbon productivity through hydrogenation. As for the energy effects, the heat energy of deep fluids can promote the mature evolution from organic matter to oil and gas. During this process, the heating of deep fluids will cause the oil-generation window depth of the hydrocarbon source rocks to become thinner, and it will also generate very high pressure, which will promote the discharge of abundant hydrocarbons formed by the hydrocarbon source rocks. Furthermore, deep fluids can directly form volcanic rock oil-gas reservoirs. And another manifestation of deep fluid energy is geothermal. And the thermal energy of deep fluids can directly form hot dry rocks, which is the most important existing form of geothermal resources. The geological exploration of hot dry rocks should be supported by further geochemical and geophysical research.

Constitutive and numerical modeling for the coupled thermal-hydro-mechanical processes in dual-porosity geothermal reservoir

Enhanced geothermal system (EGS) is viewed as one of the most practical methods to explore geothermal energy in hot dry rock (HDR), which can contribute to zero carbon emissions and provide reliable and renewable energy. Modelling the performance and long-term environmental effects of the EGS remains a challenge as the rock is often fractured and experiences complicated multi-physics coupling behaviour. Despite studies on constitutive or numerical modelling of coupled behaviour in deformable dual-porosity media, those developed models often ignore the fully coupled processes in heat transfer and are highly empirical. Based on a non-equilibrium thermodynamics approach, the Mixture-Coupling Theory, this research derives the fully coupled Thermo-Hydro-Mechanical (THM) governing equations for dual-porosity geothermal reservoirs, addressing the interaction between strain, pore/fracture pressure and temperature. The constitutive model is obtained by the analysis of Helmholtz free energy evolution in dual-porosity reservoirs. The proposed model determines the fully coupled evolution of solid stress, both pore and fracture porosity, and solid entropy density. The governing equations can predict the fully coupled THM effect in dual-porosity geothermal reservoirs. Numerical modelling is then performed to study the production performance and coupled THM response in an EGS. The modelling results show that the production temperature is determined by the coupled THM effects. Porosity change is mainly determined by temperature change (accounting for over 87% of the total porosity change) during the extraction. The porosity change accounts for 1.03% and 1.23% of the initial porosity for pore and fracture, separately.

Modeling wellbore instability in hot dry rock under various temperature conditions

Hot Dry Rock (HDR) is a renewable energy source that has garnered attention due to its abundant reserves, widespread distribution, and consistent energy supply. However, the injection of cold water during the drilling and production process of HDR can alter the temperature of the rock in the HDR reservoir, leading to variations in its physical and mechanical properties near the wellbore. These changes can impact the stability and safety of the HDR wellbore. This study investigated the alterations in the physical and mechanical properties of HDR under different temperature conditions. The results revealed that there were negligible changes in the physical and mechanical properties when the temperature rose from 25 °C to 400 °C. However, noticeable changes occurred as the temperature increased from 400 °C to 800 °C, establishing 400 °C as the threshold for physical and mechanical property variations in the granite. Building upon these experimental findings, a segmented wellbore instability model was developed and validated. The model demonstrated that an increased temperature difference between the drilling fluid and the borehole corresponded to an expanded range of borehole failure. Furthermore, higher wellbore temperatures led to more pronounced disparities between the maximum and minimum principal stress of the borehole, rendering it more susceptible to instability. The research also uncovered that the optimal drilling position was influenced by temperature.These research outcomes hold significant importance for understanding the mechanisms of wellbore instability in HDR with high-temperature.

NUMERICAL SIMULATION ON THE HEAT EXTRACTION OF ENHANCED GEOTHERMAL SYSTEM WITH DIFFERENT FRACTURE NETWORKS BY THERMAL-HYDRAULIC-MECHANICAL MODEL

Hot dry rock (HDR), as a kind of geothermal energy resource, has attracted much attention due to its wide distribution and huge reserves. This paper presents a numerical study on energy mining in the HDR by the enhanced geothermal system (EGS). The thermal-hydraulic-mechanical coupling model is employed in these simulations. The multi-field evolution process and the influence of the fracture parameters on the heat recovery performance of an EGS are analyzed comprehensively. The results show that the fractures parallel to the connecting line of the injection-production well can lead to an early thermal breakthrough, resulting in a thermal recovery performance decrease, while fractures perpendicular to the connecting line between the production and injection wells can enhance the production temperature of an EGS, when compared with the reservoir without fractures. The production temperature drop rate of the EGS with percolated fracture network is much quicker than that of the EGS with an isolated fracture network. Additionally, short fractures can lessen the potential for working fluid preferred flow channels to emerge; therefore, an EGS with short fractures may operate better than an EGS with lengthy fractures.

Capacity Allocation of Hybrid Power System with Hot Dry Rock Geothermal Energy, Thermal Storage, and PV Based on Game Approaches

This study utilizes hot dry rock (HDR) geothermal energy, which is not affected by climate, to address the capacity allocation of photovoltaic (PV) -storage hybrid power systems (HPSs) in frigid plateau regions. The study replaces the conventional electrochemical energy storage system with a stable HDR plant assisted by a flexible thermal storage (TS) plant. An HPS consisting of an HDR plant, a TS plant, and a PV plant is proposed. Game approaches are introduced to establish the game pattern model of the proposed HPS as the players. The annualized income of each player is used as the payoff function. Furthermore, non-cooperative game and cooperative game approaches for capacity allocation are proposed according to the interests of each player in the proposed HPS. Finally, the proposed model and approaches are validated by performing calculations for an HPS in the Gonghe Basin, Qinghai, China as a case study. The results show that in the proposed non-cooperative game approach, the players focus only on the individual payoff and neglect the overall system optimality. The proposed cooperative game approach for capacity allocation improves the flexibility of the HPS as well as the payoff of each game player. Thereby, the HPS can better satisfy the power fluctuation rate requirements of the grid and increase the equivalent firm capacity (EFC) of PV plants, which in turn indirectly guarantees the reliability of grid operation.

Potential of fixed-plane-perforation fracturing technique in enhanced geothermal system

Fixed-plane-perforation fracturing (PFFP) has been employed to construct complex fracture networks to stimulate unconventional reservoirs. Few studies have been reported on its application potential in geothermal energy extraction. Here we first introduced the PFFP to the enhanced geothermal system (EGS) and numerically examined its heat extraction performance by comparing to those of differing stimulation strategies. We found that much longer fractures and significantly larger damage area are induced by the PFFP than the single-perforation fracturing (SPF), and the heat extraction of the reservoir stimulated by the FPPF is approximately 60% higher than that by the SPF, suggesting the great potential of the PFFP in deep geothermal energy harvest. We further improved our previously-proposed sensitivity indicator to assess the influence degree of critical factors on the PFFP where fracture networks were characterized by the pore pressure of hot dry rock. We found that a high in-situ stress difference reduces the spacing between hydraulic fractures and increases the stress shadowing, causing an increase in heat extraction. The injection rate affects strongest on fracture network area whereas the initial fracture angle is the least. When the injection rate increases from 0.0005 m 3 /s to 0.0020 m 3 /s, the internal fracture length grows by 61.3%, the fracture network area expands by 74.7% of the total area, and the heat extraction increases by 54 GJ. Our findings benefit hydraulic stimulation strategy selection and heat extraction optimization over EGS construction. • We assess the potential of fixed-plane-perforation fracturing (FPPF) in EGS. • Geothermal mining models are optimized by considering pore pressure distribution. • PFFP creates a favorable fracture network and benefits geothermal mining. • Injection rate affects strongest on fracture network area and heat extraction.

Research on physical-mechanical properties and energy evolution characteristics of granite after high temperature treatment

Granite is the main carrier of geothermal energy of hot dry rock. Therefore, the study of influence of different temperatures on the physical-mechanical properties and energy evolution characteristics of granite after high temperature treatment is an important way to guide the successful exploitation of hot dry rock geothermal resources. Physical tests and uniaxial compression tests were carried out on granite after high temperature treatment ranging from 25-350?C. The results show that change rates in volume, mass, density, P-wave velocity and porosity of granite increase with increasing temperature. The research results can provide basic data support for the safe and efficient development of hot dry rock in China, and also develop the theory of high temperature rock mechanics.