Real-time High Temperature Research Articles

Study on the Strength and Failure Behaviors of Sandstone Specimens at Elevated Temperature under Triaxial Compression

Deep rocks are in high geostress and high temperature environment, which makes the exploitation of deep resources face great challenges, so it is of great significance to study the triaxial mechanical behaviour of rocks under high temperature. This paper comprehensively adopts indoor test and numerical simulation methods to analyze the influence of real-time temperature and peripheral pressure on the strength and damage characteristics of sandstone, and reveals the law of rock thermal crack evolution. Firstly, triaxial compression tests on sandstone with different real-time temperatures were conducted and the influences of temperature and confining pressure on the mechanical properties of sandstone were obtained. Secondly, a three-dimensional numerical model of sandstone was constructed using PFC3D, and a set of microscopic parameters reflecting the mechanical behavior. of sandstone were calibrated by comparing with triaxial compression results at room temperature, on the basis of which a numerical simulation on sandstone under realtime high temperature triaxial compression was carried out, and the numerical results were consistent with the laboratory experimental results. The numerical results show that no microcracks are generated in the specimen and the peak strength changes were not obvious when the temperature does not exceed 150℃; when the temperature exceeds 150℃, microcracks begin to sprout in the sandstone specimen. Under low confining pressure conditions, the peak strength of sandstone decreases with increasing temperature. The closure effect of high confining pressure on microcracks reduces the weakening effect of temperature on peak strength of sandstone. The fracture pattern of thermal sandstone is influenced by both temperature and peripheral pressure. The thermal sandstone exhibits axial splitting failure under low confining pressure, and shear failure under high confining pressure.

Experimental study on mechanical damage and creep characteristics of Gonghe granite under real-time high temperature

It is significant to study the mechanical properties of hot dry rock (HDR) for the development of deep geothermal energy. At present, the creep behavior of granite under real-time high temperature is not fully understood. The creep behavior of granite at 25 ∼ 800°C was investigated by real-time high-temperature uniaxial compression and graded load creep tests, and the thermal damage mechanism of granite was studied by scanning electron microscopy (SEM) experiments. The paper systematically analyzes the evolution of mechanical indexes such as uniaxial compressive strength (UCS), elastic modulus, creep deformation, steady creep rate and long-term strength of granite under thermal-force coupling. The results show that the UCS and elastic modulus of granite increase with increasing temperature in the range of 25 ∼ 200 °C, and decrease with increasing temperature in the range of 200 ∼ 800 °C. The damage speed of granite is the fastest in the temperature range of 400 ∼ 600 °C. The steady creep rate of granite increases with the increase of temperature and stress level. The ratio of long-term strength to UCS decreases with increasing temperature, from 93.6% at 25 °C to 73.2% at 800 °C. The research results provide relevant thermal damage mechanical parameters and theoretical basis for the development of HDR.

Study on the Meso-Failure Mechanism of Granite under Real-Time High Temperature by Numerical Simulation

In the development of geothermal resources in hot dry rocks, deep underground rock masses are typically subjected to real-time high-temperature environments. High temperatures alter the physical and mechanical properties of the rocks, directly affecting the safe and efficient utilization of hot dry rock resources. Therefore, a grain-based model (GBM) of particle flow code (PFC) was constructed based on uniaxial compression tests, and the model was verified according to macroscopic mechanical parameters and damage modes, in order to carry out the simulation study of the uniaxial compression of granite and explore the meso-failure mechanism of granite under real-time high temperature. The relationships between stress–strain curves and crack derivation, the evolution of microcracks, and the characteristics of acoustic emission activity and energy changes at different temperatures were investigated in conjunction with the results of laboratory tests. The results show that crack development, acoustic emission activity, and energy evolution during uniaxial compression include four main stages: initial compression, elasticity, plastic strengthening, and post-peak damage. The failure of granite is primarily controlled by mica and feldspar. During loading, intergranular tensile cracks first emerge within the granite, followed by intragranular tensile cracks, with shear cracks appearing last. As the temperature increases, the total number of microcracks continuously rises, the frequency of acoustic emission events increases, and both dissipated energy and boundary energy gradually decrease, showing an upward trend in the energy dissipation ratio, indicating an increase in thermal damage due to high temperatures. At 400 °C, the rate of microcrack formation increases significantly, with intergranular and intragranular cracks starting to coalesce into macroscopic cracks that extend outward. In the post-peak stage, the phenomenon of multiple peaks in acoustic emission events begins to appear. At 600 °C, the rate of microcrack formation reaches its maximum, with cracks extending throughout the sample to form a network of fractures, resulting in the granite exhibiting ductile failure characteristics.

Experiment study on shear behavior and properties of granite fractures under real-time high-temperature conditions

Direct shear tests were conducted on granite fractures with similar joint roughness coefficients under real-time high temperatures (up to 400 °C) and constant normal stiffness conditions. In this study, the shearing process was monitored via acoustic emission, and fracture surface morphology was measured with three-dimensional laser scanning. With increasing temperature, peak and residual shear strengths, peak friction coefficient, and maximum normal dilation became higher. In contrast, peak normal displacement and peak shear displacement decreased. The apparent cohesion and internal friction angle exhibited opposite trends as temperature increased, primarily due to competition between thermal hardening and thermal cracking. The elastic strain energy of stick-slip events increased with temperature. From 100 °C to 400 °C, the degradation of fracture roughness and the volume of sheared-off asperities grew. Real-time temperature influences shear properties and is mainly attributed to physical and geometrical changes. Acoustic emission parameters, including count, energy, and hit, show segmented variation during fracture shearing. Based on parameter analyses of the average frequency and rise angle values, during the shearing process, the proportion of shear cracks at each stage varied with temperature. Shear failure was the predominant failure mode. Results from this study provide insight into the shear behavior of granite fractures under real-time high temperatures.

Thermal stimulation mechanical response and shear dilatation predictive model improvement of granite fractures in an enhanced geothermal system

The exploitation of shear dilatation behavior exhibited by natural rock fractures can serve as an effective strategy for stimulating reservoirs in the enhanced geothermal systems (EGS). However, few studies have considered the shear dilatation characteristics of rock fractures treated by real-time high temperature. Firstly, the self-developed Mechanical(M)-Thermal(T) coupling shear test cell and the MTS816.01 rock shear test system were used to carry out the direct shear tests of granite fractures. The M-T coupling effect on granite's physical properties such as cohesion, the mechanical properties such as shear strength, and strain energy characteristics such as dissipated strain energies were analyzed. On these Base, the shear dilatation mechanism of granite fractures treated by real-time high temperature is further revealed. Subsequently, based on the dynamic changes in the area degradation degree of granite fractures, the shear dilatation model of rock fractures treated by real-time high temperature were improved and verified. The results reveal that the M-T coupling effect can considerably impair the shear dilatation behavior of granite fractures. The temperature undermines the shear dilatation behavior of granite fractures by negatively affecting their physical and mechanical properties, while the normal stress deteriorates their shear dilatation behavior by constraining the normal displacement of the rock fractures. The modified shear dilatation model of rock fractures exhibits high accuracy and can accurately represent the shear dilatation behavior of rock fractures under M-T coupling effects. The novel insights provided in this paper contribute to a more comprehensive understanding of the wellbore stability and permeability improvement mechanism of EGS.

Study on the Periodic Collapse of Suspended Sandstone Interlayer under Coupled Thermo–Hydro–Mechanical Environment

In the process of in situ heat injection mining of coal seams, a circular combustion chamber will gradually be formed. However, for the interlayer coal seam, the interlayer will gradually suspend in the combustion chamber. When the critical distance is reached, the initial collapse and periodic collapse will occur, which will affect the flow field distribution and extraction efficiency of combustion gas. In order to study the initial and periodic collapse steps of interlayer in real-time high temperature and high-pressure thermo–hydro–mechanical (THM) coupling environments, sandstone specimens were collected from the field, and the macroscopic mechanical parameters of sandstone interlayer were tested using a self-developed thermo–hydro–mechanical coupling tester. The mechanical parameters (e.g., triaxial compressive strength, elastic modulus, Poisson’s ratio) of sandstone under different stress conditions were obtained. Then, based on the elastic thin plate theory, the mechanical modeling of the annular sandstone interlayer is carried out and the preceding mechanical parameters are brought into the generated analysis equation. Thus, the periodic collapse step of the suspended circular sandstone interlayer is obtained. The study results show that: (1) the macroscopic physicomechanical parameters of the sandstone interlayer do not vary monotonically with temperature—a slight increase in macroscopic parameters occurs at 400°C; and (2) the periodic caving pace of the sandstone interlayer decreases with the increase in the number of collapses. With the increase of temperature, the periodic caving pace of the sandstone interlayer decreases first, then increases, and then decreases. It gradually decreases with the increase of surrounding pressure. The research results of this paper can provide theoretical guidance and technical support for in situ heat injection mining technology.

Mechanical responses and failure characteristics of longmaxi formation shale under real-time high temperature and high-stress coupling

For the safe and effective extraction of deep shale gas, it is essential to analyze the mechanical responses and failure characteristics of shale under real-time high temperature and high-stress coupling. Herein, triaxial compression experiments on Longmaxi Formation shale were carried out with various bedding angles under different confining pressures and real-time temperatures. The experimental results demonstrate that high confining pressure can transform the mechanical performance of shale from brittle to plasticity. Real-time high temperatures not only can aggravate this transition process but also cause discontinuous micro-rupture before shale failure. The acoustic emission (AE) evolution of shale under various confining pressures and real-time temperatures is generally similar and can correspond with the division of stress-strain stages. However, changes in confining pressure and real-time temperature have opposite effects on AE activity rate and associated parameters. In the deep environment, the real-time high temperature can also drive additional pores and microcracks, which change the microstructure of shale samples. Under high confining pressure, the impact of real-time high temperature on macroscopic failure modes diminishes, resulting in shale samples typically exhibiting the single shear failure mode. Furthermore, high confining pressure and real-time high temperature have different micro-mechanisms for improving shale ductility, as evidenced by their distinct effects on AE evolution.

Shear mechanical properties and surface damage characteristics of granite fractures treated by real-time high temperature

Shear mechanical properties and surface damage characteristics of granite fractures treated by real-time high temperature

Shear Strength and Energy Evolution of Granite under Real-Time Temperature

The shear mechanical properties of rock under high temperature and high pressure are key issues in geothermal energy development. In order to explore the variation in shear mechanical properties of rock under high temperature and high pressure, the shear experiments of granite under real-time temperatures and normal stresses were carried out using the servo-controlled true triaxial experimental system for rock shearing testing and acoustic emission technology. The results show the following trends: (1) the peak shear strength of granite increases slightly first and then decreases sharply with real-time temperature, with 200 °C considered as the threshold temperature for the peak shear strength of granite. When the temperature is constant, the shear strength of granite increases linearly with the increase in normal stress. (2) Before 200 °C, the shear modulus of granite decreases slowly with the increase in temperature and decreases rapidly after 200 °C. The shear modulus always increases linearly with the increase in normal stress. (3) Under the coupling effect of real-time high temperature and normal stress, the cumulative acoustic emission energy released during the shear deformation of granite gradually decreases with temperature, and the main failure mode of granite gradually changes from tensile failure to shear failure.

Hydraulic fracturing of granite under real-time high temperature and true triaxial stress

Hydraulic fracturing of granite under real-time high temperature and true triaxial stress

Real-time mode-I fracture toughness and fracture characteristics of granite from 20 °C to 600 °C

The mode-I fracture toughness of semicircular bend granite specimens at high temperature was tested in real time, and the failure specimens were examined using X-ray computed tomography. The results showed that with increasing temperature, the fracture toughness increased from room temperature (20 °C) to 200 °C, then decreased beyond 200 °C. Quasibrittle failure was observed up to 500 °C, followed by plastic characteristics at 600 °C. The cracks tended to propagate along the boundaries of mineral crystals. With increasing temperature, the main cracks became rougher and tended to propagate perpendicular to the tensile stress, and more branch cracks formed. The increase of temperature within a certain range had toughening effect on a variety of rocks and temperature-treatment method affected the mode-I fracture toughness of granite. Compared with the mode-I fracture toughness after cooling to room temperature, the temperature threshold of mode-I fracture toughness of granite at real-time high temperature was higher. The results provide guidance to determine fracture toughness and stability for high-temperature rock engineering.

Mechanical behavior and constitutive model of shale under real-time high temperature and high stress conditions

The high temperature and high in-situ stress geological environment can significantly affect the mechanical properties, failure modes, and deformation characteristics of deep shale reservoirs. In this study, real-time high temperature triaxial compressive tests simulating the deep shale formation environment (temperature: 25–150 °C, confining pressure: 0–100 MPa) are carried out. The GSI-strength degradation and constitutive models are derived based on the Hoek–Brown criterion. The results show that in low confining pressure conditions, the mechanical behavior of shale is greatly influenced by temperature. Compared with shale at 25 °C, the compressive strength of shale at 150 °C decreases by up to 13.7%, and the elastic modulus decreases by up to 36.9%. The peak strain was increased by a factor of up to 1.4, and the yield stress level was advanced by as much as 7.4%. However, in high confining pressure conditions, the shale plasticity characteristics are significantly enhanced and the failure mode is relatively single. The GSI-strength degradation model can well characterize the variation law of shale strength with confining pressure under high temperature conditions. The statistical damage constitutive model matches the actual stress–strain curve very well, and it can fully reflect the deformation and failure characteristics of deep shale. The findings of this study can help us better understand the variation of mechanical properties of deep shale.

Effect of real-time high temperature and loading rate on mode I fracture toughness of granite

An in-depth understanding of the effect of real-time high temperature and loading rate on the fracture toughness of rocks is highly important for understanding the fracture mechanism of Hot Dry Rock (HDR). Three-point bending tests on notched semi-circular bending (NSCB) samples at the real-time temperatures (25, 100, 200, 300, 400 and 500 ℃) and different loading rates (0.1, 0.01 and 0.001 mm/min) were performed to characterize the temperature and rate dependence of the mode I fracture toughness. Besides, the characteristic of the fracture surface morphology was investigated by scanning electron microscope (SEM) and crack deviation distance analysis. Results show that the temperature has a significant effect on the development of intergranular and transgranular cracks. The fracture toughness and peak load are similarly influenced by temperature (i.e., they both decrease with increasing temperature). At the loading rates of 0.1 mm/min and 0.01 mm/min, from 25 to 400 °C, the fracture toughness decreases slightly with decreasing loading rates. However, at a loading rate of 0.001 mm/min, the fracture toughness values above 200 °C are very similar, and the fracture toughness does not strictly follow the law of decreasing with decreasing loading rate. Especially at 500 °C, fracture toughness and loading rate are negatively correlated. Our study also indicates that the effect of loading rate on macroscopic crack propagation path at real-time high temperature is not obvious. This study could provide an important basis for evaluating the safety and stability of geothermal engineering.

Effects of Real-Time High Temperature and Loading Rate on Deformation and Strength Behavior of Granite

Knowledge of deformation and strength behavior of rocks under high in situ temperature is highly important for the control of geological disasters in exploration of hot dry rock and mining in deep formation. In this study, uniaxial compression tests were carried out on granite under different real-time high-temperature conditions (25, 200, 300, 400, 500, 600, and 700°C) and loading rates (0.01, 0.1, and 0.5 mm/min). The effects of real-time high temperature and loading rate on the uniaxial compressive strength and elastic modulus of granite were studied, and the microscopic morphology of the fracture surface was analyzed. The results show that the uniaxial compressive strength and elastic modulus of granite increase first and then decrease with the increase of temperature. The uniaxial compressive strength clearly increases at 200°C and decreases gradually when the temperature exceeds 300°C. Under the same temperature conditions, the uniaxial compressive strength of granite decreases and the elastic modulus increases with increasing loading rate. When the temperature reaches 600°C, the effect of the loading rate on the uniaxial compressive strength and elastic modulus of granite decreases significantly. The test results are compared with the results of work performed on quenched granite. Under real-time high-temperature conditions, the thermal crack effect has a significant influence on the uniaxial compressive strength and elastic modulus of granite, without the thermal hardening effect of quenched granite. During hydraulic fracturing, the rock skeleton near the injection well is cooled and shrunk, as is the thermal hardening effect caused by high-temperature quenching. The formation of thermal equilibrium leads to the large-scale extension of fracture cracks along the weak plane structure, such as the effect of thermal cracks on granite under real-time high-temperature conditions.

Study on Dynamic Constitutive Model of Polypropylene Concrete under Real-Time High-Temperature Conditions

Polypropylene (PP) concrete, a kind of high-performance fiber-reinforced concrete, is widely used in large concrete structures. Studies on the dynamic mechanical properties of polypropylene concrete under temperature–impact load can provide a theoretical basis for research on the structural stability of concrete structures during fires, explosions, and other disasters. The purpose of this paper was to study the dynamic mechanical properties of polypropylene concrete under real-time high-temperature conditions and to establish a dynamic damage constitutive model for polypropylene concrete under real-time high-temperature conditions. In this paper, Split Hopkinson Pressure Bar (SHPB) equipment was used to test the dynamic mechanical properties of polypropylene concrete with different high strain rates under different real-time high temperatures (room temperature, 100 °C, 200 °C, 300 °C, 400 °C, 500 °C, 600 °C, 700 °C, and 800 °C). A modified “Z-W-T” model was used to determine the recursion of the dynamic damage constitutive model of polypropylene concrete under different temperature–impact loads, and the model was compared with the experimental data. The results show that the thermal conditions influenced the chemical composition and microstructure of the polypropylene fiber concrete, which was why the high temperatures had a strong influence on the dynamic mechanical properties of polypropylene concrete. When the heating temperature exceeded 300 °C, although the polypropylene concrete specimen was still able to maintain a certain strength, the dynamic mechanical properties showed a deterioration trend as the temperature increased. The comparation between the experimental data and the fitting curve of the dynamic damage constitutive model showed that the dynamic stress–strain curves could be well matched with the fitting curves of the dynamic damage constitutive model, meaning that this model could describe the dynamic mechanical properties of polypropylene concrete under different real-time high temperatures well.

Study on Dynamic Mechanical Properties and Failure Mechanism of Sandstones under Real-Time High Temperature

The research on dynamic mechanical properties of rocks under high temperature is the basis for safe and efficient implementation of deep coal mining and underground coal gasification engineering. In this paper, the split Hopkinson bar (SHPB) with real-time high-temperature function was adopted to systematically study dynamic mechanical properties of sandstones. The research showed that under the condition of a fixed temperature, with the increase of strain rate, the dynamic compressive strength and dynamic peak strain of sandstone increased gradually, and the variation of dynamic elastic modulus with strain rate was not obvious. With the increase of temperature, the dynamic compressive strength of sandstone increased first and then decreased, the dynamic peak strain increased gradually, and the dynamic elastic modulus decreased overall. The variation law of macroscopic failure mode and energy dissipation density with temperature was revealed, and the change mechanism was explained considering the influence of high temperature on the internal structure of sandstone. Based on the principle of component combination and the theory of micro-element strength distribution, the dynamic statistical damage constitutive model was established, and its parameters had certain physical significance. Compared with the experimental results, the established model can well describe the dynamic stress-strain relationship of sandstone under real-time high temperature.

Improved test method for convection heat transfer characteristics of carbonate fractures after acidizing etching

Understanding of convection heat transfer characteristics of fractures is of great significance to improve the heat extraction efficiency of carbonate reservoir. Previous studies on convection heat transfer of fluid flowing through rock fractures are on either granite or sandstone. Limited experimental research has been performed on carbonate fractures after acidizing etching. In this work, an improved test method is developed to analyze the convection heat transfer characteristics of carbonate fractures after acidizing etching under real-time high temperature and high confining pressure. In this method, the traditional test method of convection heat transfer coefficient is improved by monitoring the temperature of inner fracture surface and flowing water. Two thermocouples are especially arranged inside the sample to monitor the temperature of inner fracture surface along the flow direction, and two other thermocouples for the inlet and outlet water temperatures. The results show that the temperature differences between the fracture surface and the flowing water are significantly dependent on confining pressure, fracture roughness and flow rate, and the maximum temperature difference could be reached 2.2 ◦C, which leads to a significant difference in the convection heat transfer coefficient between the traditional and improved test methods. A larger number of pores, caves, and micro-fractures caused by acid etching are observed on the fracture surface by scanning electron microscopy. The special fracture morphology of carbonate is totally different from those of granite and sandstone in previous studies, and can increase the convection channel and increase the contact area with flowing fluid and results in the inapplicability of the hypothesis to carbonate fractures after acidizing etching. The present work could improve the knowledge of convection heat transfer characteristics of carbonate fractures. Cited as: Zhan, H., Dong, W., Chen, S., Hu, D., Zhou, H., Luo, J. Improved test method for convection heat transfer characteristics of carbonate fractures after acidizing etching. Advances in Geo-Energy Research, 2021, 5(4): 376-385, doi: 10.46690/ager.2021.04.04

Evolution of permeability and microscopic pore structure of sandstone and its weakening mechanism under coupled thermo-hydro-mechanical environment subjected to real-time high temperature

Thermal damage mechanisms and the characteristics of performance deterioration of a repository host rock is critical for evaluating the potential of an underground coal gasification project. Previously, many studies have primarily focused on the microstructural evolution of various rock types after high-temperature treatment only. However, relatively little is known regarding the coupled thermo-hydro-mechanical (THM) behavior of rock, which is frequently encountered in geo-engineering underground regions with high-temperatures. In this study, using a self-developed universal tester with the ability of THM coupling of high temperature and high pressure, permeability evolution in sandstone under real-time high temperature (20–700 °C) and triaxial stress (hydrostatic pressure = 25 MPa) were observed. The microphysical parameters of these specimens subjected to the THM environment were then measured, and the corresponding morphological evolution processes were also assessed using micro-computed tomography (MCT). Further, for the purpose of contrast, we have also determined the microphysical parameters for the sandstone after heat treatment only. The results show that the evolution of the microscopic structures within the sandstone under the coupled THM condition is vastly different from those subjected to heat treatment only. The experimental results in this study can provide theoretical guidance for the stability of surrounding rock of a combustion cavity during in-situ coal gasification.

Mechanical properties of granite under real-time high temperature and three-dimensional stress

Knowledge of mechanical properties of rocks under real-time high temperature and three-dimensional stress is critically important for stability analysis in Hot Dry Rock. In this context, the effects of temperature and horizontal stress on the mechanical behaviors of granite were investigated. A series of triaxial compression tests were performed on granite samples under real-time high temperature (up to 400 °C) by a self-developed real-time high temperature true triaxial test system. The test results showed that the mechanical properties of studied granite first increase before some temperature threshold, and then decrease with increasing temperature. The brittle characteristics of studied granite observed under low horizontal stress and room temperature are attenuated with increasing stress and temperature. Furthermore, observation of the fracture morphology of the samples indicates that the biotite particles undergo toughness enhancement at elevated temperature, which is the main reason for the reduction of rock brittleness and reinforcement in mechanical properties with increasing temperature. Finally, the effects of real-time high temperature on cohesion and internal friction angle were analyzed and compared with the previous test results of cohesion and internal friction angle of the samples subjected to pre-treatment of high temperature. A sound difference in cohesion and little difference in the internal friction angle were observed between the two cases. The test results could improve our understanding of the impact of temperature and stress on the mechanical properties of granite.

Mode-I fracture toughness and mechanisms of Salt-Rock gypsum interlayers under real-time high-temperature conditions

In this study, an attempt was made to investigate the influence of temperature on the stability and compactness of gypsum rock. The fracture toughness was tested using a real-time online high temperature system, and the sample was characterized by scanning electron microscopy and X-ray diffraction in order to further investigate the fracture characteristics and mechanisms. The results showed that (1) The change in gypsum fracture toughness (KIC) with temperature can be divided into four stages: a slow decrease with increasing temperature between 20 and 100 °C, a sharp decrease from 100 to 300 °C, a slow decrease from 300 to 600 °C, and a rapid decrease from 600 to 700 °C; (2) When gypsum is at the low-temperature stage (20–300 °C), the fracture surfaces are smooth and flat. The main failure mode is trans-granular fracture. In the high-temperature stage (400–700 °C), the fracture surface is rough. The fracture morphology is complex and diverse, with fibrous stripes and secondary cracks being observed on the fracture surface. Intergranular fracture is the main fracture mode. (3) When the temperature of gypsum is between 100 and 300 °C, weakening occurs by thermal dehydration and calcium sulfate dihydrate is dehydrated to β-calcium sulfate and γ-calcium sulfate. At temperatures ranging from 300 to 700 °C, the weakening mechanisms are thermal fracturing and crystal distortion of calcium sulfate. The results of this study provide theoretical guidance for evaluating the stability and compactness of nuclear waste storage.