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Evaluations of the mechanical properties and failure modes of granite at high temperatures are important issues for underground projects such as enhanced geothermal systems and nuclear waste disposal. This paper presents the results of laboratory experiments that investigated the physico-mechanical failure behavior of granites at high temperatures. The results allowed several important conclusions to be drawn. Both the uniaxial compressive strength (UCS) and tangent modulus decrease with increasing temperature. Specifically, the UCS-temperature curve can be divided into three sections: a section (20-200°C) where UCS shows a slight decrease, a section (200-300°C) where the UCS decreases significantly, and a third section (300-500°C) where the rate of UCS decrease stabilizes. However, in the entire temperature range from 20 to 500°C, the tangent modulus decreases exponentially. The number of acoustic emission (AE) counts decrease and the counts occur less frequently at higher temperatures. Individual grains are surrounded by a large number of microcracks at 200°C and the crack length increased significantly when heating to 300°C. Specifically, the length of micro-cracks in the granite at 300 °C could be 10 times longer okthan that at 200°C. Quenching or injecting cold water into HDRs would further weaken the rock and induce thermal damage to the rock structure. The strength of rock would be further quench-weakened by 10%, 20% and 30% at 200°C, 300°C and 500°C, respectively. Therefore, in Enhanced/Engineered Geothermal Systems (EGS), quenching is much more destructive than normal thermal stress.

Thermally-induced changes in the fracture properties of geological reservoir rocks can influence their stability, transport characteristics, and performance related to various deep subsurface energy projects. The modified maximum tangential stress (MMTS) criterion is a classical theory for predicting the fracture instability of rocks. However, there is a lack of research on the accuracy of MMTS theory when rocks are subjected to different temperatures. In this study, mechanical theoretical analysis and failure and fracture mechanics experiments of granite under the influence of temperatures ranging from 20 °C to 600 °C are carried out. The results showed that the theoretical estimated value of MMTS differs significantly from the experimental data at 20 °C−600 °C. The Keff/KIC ratio is less than the experimental test value due to the critical crack growth radius (rc) estimated by the conventional method being larger than the critical crack growth radius (rce) derived from the experimental data. Varied temperatures affect the fracture process zone size of fine-grained, compact granite, and the MMTS theoretical estimation results. Therefore, it is essential to modify the critical crack growth radius for MMTS theory to accurately predict the fracture characteristics of thermally damaged rocks. In addition, the variation of the rock’s mechanical properties with temperature and its causes are obtained. Between 20 °C and 600 °C, the mode-I, mode-II, and mixed-mode (α = 30° and 45°) fracture toughness and Brazilian splitting strength of the granite decrease by 80% and 73%, respectively. When the rock is heated above 400 °C, its deterioration is mainly caused by a widening of its original cracks.

Hot dry rock (HDR) is considered a new clean and renewable energy source. China is rich in HDR resources, but viable and reliable resource evaluation techniques and exploration methods have not been available yet. This research method, consisting of deep exploration combined with shallow drilling, screens the geothermal anomaly regions using heavy magnetic, electric measurement, seismic and thermal joint interpretation technology, and verifies the geophysical multiplicity with massive oilfield drilling data to locate the favorable HDR target region. The research results show a regionally geothermal abnormal area. We use relative impedance combined with seismic profile to recognize the HDR target region. The geophysical exploration and geological joint interpretation technology are faster and more accurate than the single geophysical exploration technology. The comprehensive analysis believes that the high resistance granite body in the southern Paleo-central uplift zone, with its area 500 km2, its thickness 150–330 m, its static formation temperature reaching 152° in the depth of 3360 m, its high rock thermal conductivity, its good deep fracture, and base fracture development, is a fragmented granite and weathered shell, easy for artificial reservoir re-constructing. There is high preciseness in locating the favorable target region for HDR development.

A new analytical solution for the thermal-hydraulic coupling process is derived with a 1-D steady state conductive heat flow in the body of hot rock with perpendicular water flow in the single fracture and transient heat transfer from rock to water. The heat produced from the hot rock via water flow in the idealized single fracture is demonstrated by arithmetic equations. The applicability of the analytical solution is verified by numerical calculations and is limited to conditions with fast water flow rates or high water flux and long fluid pathways. The lifetime of an EGS reservoir in these reference conditions is 23.2 years and is confined by the produced water temperature of 150 °C for commercial utilization. The heat recovery factor is 12.4%. With a power plant capacity of 5 Mw installed, the total area for extracting recoverable heat within the projected lifetime of a fracture surface of 1.58 × 106 m2 was determined. The total mass flow rate of water injected into the large fracture was 57 kg/s. The discussion shows the ability of the model to estimate heat production and reservoir scale.

Обмеженість викопних ресурсів, що витрачаються на теплових і атомних станціях, викликає тривогу. До того ж їх використання істотно погіршує екологічну ситуацію. Використання відновлюваних джерел енергії в якості первинного палива є виключно перспективним напрямком в технологіях виробництва електроенергії і тепла. В останні роки велика увага приділяється геотермії, тобто тепловим процесам, що відбуваються в надрах Землі, для виробництва не тільки тепла, але і електроенергії. Значний внесок у створення ефективних геотермальних установок такого типу вніс допитливий інженер і талановитий вчений Олександр Каліна. Ним створено цикл, що носить його ім'я, у якому в якості робочого тіла використовується водоаміачний розчин. Особливість установки, що реалізує цикл Каліни, полягає в тому, що в її основних елементах передбачені такі зміни концентрацій розчину, які обумовлюють істотне зростання термічного ККД. Розглянуто цикли і схеми установок, що використовують водоаміачний розчин. Підтверджена їх висока ефективність. Показано, що при переході від води до розчину вода-аміак може спостерігатися помітне збільшення питомої роботи. Відзначається, що на початковому етапі геотермальні станції споруджувалися в зонах високої вулканічної активності, гарячих джерел і гейзерів. Повідомляється, що можна при будівництві станцій з циклом Каліни орієнтуватися на технологію «Hot Dry Rock», що дозволяє розміщувати їх практично в будь-якому місці нашої планети. Відзначається, що пом'якшення вимог до температури верхнього джерела тепла в циклі Калини дозволяє розробляти підземні пласти, які раніше визнавалися неперспективними. Проаналізовано можливості більш ефективного вироблення електроенергії за допомогою циклу Каліни, який використовує природну різницю температур між нагрітою поверхнею океану і його студеними глибинами. Відмічається, що альтернативну енергетику, побудовану на геотермії, циклі Каліни і технології HDR, чекає успішне майбутнє

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.

Hot dry rock (HDR) geothermal energy, as a clean and renewable energy, has potential value in meeting the rapid demand of the social economy. Predicting the temperature distribution of a subsurface target zone is a fundamental issue for the exploration and evaluation of hot dry rock. Numerical finite–element simulation is currently the mainstream method used to study the variation in underground temperature fields. However, it has difficulty in dealing with multiple geological elements of deep and complex hot dry rock models. A Unity networking for hot dry rock temperature (HDRT–UNet) is proposed in this study that incorporates the matrix rock temperature field equation for relating the three parameters of density, specific heat capacity and thermal conductivity. According to the numerical geological structures and rock parameters of cap rocks, faults and magma intrusions, a new dataset simulated by the finite element method was created for training the HDRT–UNet. The temperature simulation results in the Gonghe basin show that the predicted temperatures within faults and granites were higher than their surrounding rocks, while a lower thermal conductivity of the cap rocks caused the temperature of overlying strata to be smaller than their surrounding temperature field. The simulation results also prove that our proposed HDRT–UNet can provide a certain evolutionary knowledge for the prediction and development of geothermal reserves.

In the Enhanced Geothermal System (EGS), the flow state of the fracture of the thermal reservoir (granite) is important for heat transfer. In this paper, the effects of reservoir temperature, flow rate and fracture shape on the thermal convection that occurs in the fracture of rough granite are studied based on the fractal dimension theory. The rough fracture is manufactured regularly by carving technology. The shear flow experiments were carried out at confining pressures of 10, 15 and 20 MPa to analyze the flow state. The convective heat transfer experiments were conducted at 60 and 70 °C to simulate different heat reservoir conditions. The results indicated that the outlet temperature and overall heat transfer coefficient (OHTC) increased with the increase in the rock temperature. A higher flow rate can enhance the thermal performance. The complexity of the fracture surface is also an important factor that affects the convective heat transfer process. The larger fractal dimension of the fracture surface can more easily excite turbulence, affecting the thickness of the boundary layer. The OHTC will increase as the fractal dimension increases.

This paper aims to fill the research gap on the effect of steam quality and non-condensable gas on heat-carrying fluid productivity, system performance and optimization. First, the effect of the temperature and quality of the heat-carrying fluid and non-condensable gas (NCG) content on the production parameters was evaluated. After that, three energy conversion systems which included a single flash (SF) system, an organic Rankine cycle (ORC) system and a single flash combined ORC (SFORC) system were constructed in this paper to utilize the heat-carrying two-phase flow with non-condensable gas. Finally, based on thermodynamic modeling, the effects of the temperature and quality of the heat-carrying fluid and non-condensable gas content on the performance and optimization of the three power conversion systems were investigated. The results show that single-phase heat-carrying fluids are more productive than two-phase heat-carrying fluids. NCG is always detrimental. The heat-carrying fluid temperature and quality are positively correlated with system efficiency and negatively correlated with the net power output. In the comparison of comprehensive performances, the SFORC system is the better, and the ORC system and the SF system are the worse. The optimal net power output of the SF system, the ORC system and the SFORC system is 4883 kW, 6557 kW and 7251 kW, respectively.

ABSTRACT: Geothermal energy is a precious resource, which is widely distributed, varied, and abundant. China has entered a period of rapid development of geothermal energy since 2010. As shallow geothermal energy promoting, the depth of hydrothermal geothermal exploration is increasing. The quality of Hot Dry Rock (HDR) and related exploratory technologies are better developed and utilized. On the basis of geothermal development, this paper reviews the geothermal progress during the “12th Five-Year Plan”, and summarizes the achievements of hydrothermal geothermal and hot dry rocks from geothermal survey and evaluation aspects. Finally, the authors predict the development trend of the future geothermal research to benefit geothermal and hot dry rock research.

The paper describes application of the cross-impact method in the process of selecting locations and technologies used in a geothermal system based on energy accumulated in a dry rock formation, where CO2 is used as the working medium. The survey is based on the opinion of a group of 20 experts representing different fields of earth and technical sciences. They represent Norway and Poland, where the location of such a system is considered. Based on experts’ experience and opinions, all factors that seem to be significant were classified into the following groups: targets, key factors, results, determiners, motor and brakes, regulating factors, external factors, auxiliary factors, and autonomous factors. Direct influences between variables were indicated. Due to major differences in geological conditions in Poland and Norway, the factor of on- or offshore technology was pointed out as the primary determiner. Among key factors, the system operation’s long-term safety and level of technological readiness were indicated. As a target factor, an interest of local authority was pointed out. Among the variables that are important when selecting locations for this type of system, nine are essential: (1) Formal constraints related to local nature protection areas—this variable is essential in the case of an onshore system; (2) Availability of CO2 sources; (3) Level of geological recognition; (4) The distance of the CO2-EGS from a thermal energy user and electricity grid; (5) Existing wells and other infrastructure; (6) Depth of the EGS system; (7) Water depth if offshore, this variable is only important when offshore systems are involved; (8) Physical parameters of reservoir rocks; (9) Reservoir temperature.

Hydraulic fracture (HF) propagation behavior is significant when building an enhanced geothermal system for hot dry rock (HDR) and evaluating the simulated reservoir volume (SRV) for shale gas reservoirs. The HF propagation behaviors are closely related to geologically structural discontinuities (SDs), which differ significantly between HDR and shale. Granite, one of the most common HDRs, mainly possesses natural fractures (NFs), quartz veins (QVs), and lithological interfaces (LIs). The SDs in the shale are mainly bedding planes (BPs) and NFs. According to the physical and mechanical property differences regarding the rock matrix, SDs can be divided into discontinuous planes and discontinuous rocks. The physical simulation experiment of hydraulic fracturing is an effective way to assess the geometry and propagation behaviors of HFs. However, the HF propagation behaviors are not generally well understood, especially the influence of multiple SDs on the HF geometry. To clarify this further, a comparative study of hydraulic fracturing on granite and shale was conducted to investigate the intersection mechanism between HFs and different SDs. The results show that HF propagation behaviors are characterized by six basic patterns: along the SD, crossing without dilation, crossing and dilation, captured by the SD, branching, and deflection. The intersection behaviors are closely associated with the cementing strength of the SD and differences in the fracture toughness of discontinuous rocks. Moreover, the fluctuation degree of the pressure-time curve and complexity of the HFs seem positively correlated. HF propagation in a rock matrix or cross interference with multiple SDs would induce a higher injection pressure and frequent fluctuations. The acoustic emission (AE) energy in granite was higher than that observed in shale. In addition, the generation of new HFs in the rock matrix can induce more AE events than those created simply by propagating along SDs. Experimental investigations can provide a theoretical basis to optimize the engineering parameters of field fracturing. [ABSTRACT FROM AUTHOR]

Hot dry rock (HDR) geothermal energy, as a clean and renewable energy, has potential value in meeting the rapid demand of the social economy. Predicting the temperature distribution of a subsurface target zone is a fundamental issue for the exploration and evaluation of hot dry rock. Numerical finite–element simulation is currently the mainstream method used to study the variation in underground temperature fields. However, it has difficulty in dealing with multiple geological elements of deep and complex hot dry rock models. A Unity networking for hot dry rock temperature (HDRT–UNet) is proposed in this study that incorporates the matrix rock temperature field equation for relating the three parameters of density, specific heat capacity and thermal conductivity. According to the numerical geological structures and rock parameters of cap rocks, faults and magma intrusions, a new dataset simulated by the finite element method was created for training the HDRT–UNet. The temperature simulation results in the Gonghe basin show that the predicted temperatures within faults and granites were higher than their surrounding rocks, while a lower thermal conductivity of the cap rocks caused the temperature of overlying strata to be smaller than their surrounding temperature field. The simulation results also prove that our proposed HDRT–UNet can provide a certain evolutionary knowledge for the prediction and development of geothermal reserves. [ABSTRACT FROM AUTHOR]

The exploration of Hot Dry Rock (HDR) geothermal energy is essential to fulfill the energy demands of the increasing population. Investigating the physical and mechanical properties of heated rock under different cooling methods has significant implications for the exploitation of HDR. In this study, ultrasonic testing, uniaxial strength compression experiments, Brazilian splitting tests, nuclear magnetic resonance (NMR), and scanning electron microscope (SEM) were conducted on heated granite after different cooling methods, including cooling in air, cooling in water, cooling in liquid nitrogen, and cycle cooling in liquid nitrogen. The results demonstrated that the density, P-wave velocity (Vp), uniaxial compressive strength (UCS), tensile strength (σt), and elastic modulus (E) of heated granite tend to decrease as the cooling rate increases. Notably, heated granite subjected to cyclic liquid nitrogen cooling exhibits a more pronounced decline in physical and mechanical properties and a higher degree of damage. Furthermore, the cooling treatments also lead to an increase in rock pore size and porosity. At a faster cooling rate, the fracture surfaces of the granite transition from smooth to rough, suggesting enhanced fracture propagation and complexity. These findings provide critical theoretical insights into optimizing stimulation performance strategies for HDR exploitation. [ABSTRACT FROM AUTHOR]

Characterizing the evolution of mechanical properties of hot dry rock (HDR) after supercritical CO2 (CO2(sc)) injection is crucial for assessing the heat extraction rate and reservoir security of CO2 based enhanced geothermal systems. This study designed the experiments of triaxial seepage and mechanical properties considering no CO2(sc) injection, CO2(sc) injection, and alternating injection of water-CO2(sc) (AIWC) in granite at 150–300 ℃. The experiments can reveal the mechanical properties of HDR in single-phase CO2 zone, CO2-water two-phase zone and dissolved CO2 liquid phase zone in HDR reservoir. The results indicate that the failure mode of the rock samples primarily exhibits sudden instability after no CO2(sc) injection and AIWC, whereas it predominantly manifests progressive instability after CO2(sc) injection. Compared with 25 ℃, the uniaxial compressive strength (UCS) after no CO2(sc) injection at 150–300 ℃ decreased by 13.86%–32.92%. After CO2(sc) injection, the UCS decreased by 40.79%–59.60%. After AIWC, the UCS decreased by 27.74–40.48%. This shows that the strength of rock mass in the single-phase CO2 zone is lower than that in the other two zones, and this weakening phenomenon increases with the increase of temperature difference. At the same temperature, the elasticity modulus after AIWC was greater than that after no CO2(sc) injection and CO2(sc) injection. With no CO2(sc) injection, when the temperature was increased to 200 ℃ and 300 ℃, intergranular cracks and transgranular appeared respectively. After AIWC, mineral crystals such as calcite were precipitated on the surfaces of the connected large cracks, accompanied by kaolinite clay minerals. This increases the frictional contact of the mineral particles and enhances the stability of the HDR reservoir. [ABSTRACT FROM AUTHOR]

In the geothermal development of hot dry rock (HDR), both the drilling of the wellbore and the heat exchange of the heat reservoir involve the effects of different cold and hot conditions on the high-temperature rock mass. The testing machine for rock mechanics was used to conduct a uniaxial compression test and carry out micro testing on the treated samples; furthermore, with the help of scanning electron microscopy the fracture mechanism of granite subjected to different temperatures and cooling methods was studied. The results show: (1) With the gradual increase in temperature, the compressive strength of granite under the two cooling methods gradually decreases. (2) The failure modes of the samples under the two cooling methods are mainly shear failure of the "Y" type. The degree of damage of the sample under water cooling is significantly greater than that under natural cooling. Electron micrographs could confirm these results. (3) It can be obtained by testing the mineral composition and element changes of granite at different temperatures. When the temperature reaches 600℃, its change is more pronounced. The results of this study can provide a theoretical reference for the failure of the wellbore and the degree of fracture of the thermal reservoir rock mass during geothermal development. [ABSTRACT FROM AUTHOR]