Triaxial Compression Experiments Research Articles

Effects of Dilatant Hardening on Fault Stabilization and Structural Development

AbstractDilatant hardening is one proposed mechanism that causes slow earthquakes along faults. Previous experiments and models show that dilatant hardening can stabilize fault rupture and slip in several lithologies. However, few studies have systematically measured the mechanical behavior across the transition from dynamic to slow rupture or considered how the associated damage varies. To constrain the processes and scales of dilatant hardening, we conducted triaxial compression experiments on cores of Crab Orchard sandstone and structural analyses using micro‐computed tomography imaging and petrographic analysis. Experiments were conducted at an effective confining pressure of ∼10 MPa, while varying confining pressure (10–130 MPa) and pore fluid pressure (1–120 MPa). Above 15 MPa pore fluid pressure, dilatant hardening slows the rate of fault rupture and slip and deformation becomes more distributed amongst multiple faults as microfracturing increases. The resulting increase in fracture energy has the potential to control fault slip behavior.

Influence of pre-existing crack on rockburst characteristics in coal-rock combination: A laboratory investigation

The existing cracks in the roof rock are critical to stability of the coal-rock combination structure during mining activities, which is vital for safe exploitation in coal mines. However, the physical mechanisms by which pre-existing crack in the roof rock induces rockburst in coal mines are still poorly understood. In this study, to investigate the rockburst characteristics in an underground mine, triaxial compressive experiments on coal-rock combinations containing a pre-existing crack in the roof rock were carried out. Both mechanical and failure behaviours of composite specimen containing a pre-existing crack were analysed. The testing results showed that a pre-existing crack in roof significantly affects mechanical properties of combinations. The average peak stress and average elastic modulus of specimen exhibit an M-shaped change trend when dip angle is in a range of 0°–90°. The average peak strain increases before decreasing with increase in dip angle. In addition, the roof rock is more likely to fail when it contains a pre-existing crack. Both the initial failure location and failure pattern of combinations are highly related to angle of pre-existing crack. A novel rockburst model considering in-situ stress and pre-existing crack was proposed to determine the physical mechanism of rockburst. The findings are useful to understand characteristics and physical mechanism of rockburst in mine engineering.

Insight into the elastoplastic behavior of Beishan granite influenced by temperature and hydraulic pressure

Granite is an ideal host rock for high-level radioactive waste (HLW) repositories. In the deep environment of HLW repositories, granite is subjected to variable temperature and hydraulic pressure. In order to investigate the influence of temperature and hydraulic pressure on the elastoplastic behavior of the Beishan granite, the present paper implemented a set of triaxial compression experiments at various temperatures and hydraulic pressures corresponding to the environment of HLW repositories. The deformation processes, damage evolution, elastic coefficients, and strength of the Beishan granite were analyzed, resulting in a limited change versus temperature and hydraulic pressure. The residual strengths of granite are similar, but a higher temperature surpasses about 70 °C degrades the friction strength of granite. Evidence from the acoustic emission (AE) hits shows the damage rate is accelerated by improved temperature and hydraulic pressure. According to the experiment-based mechanism, a modified elastoplastic model was proposed which considered thermal pressurization and changeable compressibility. The results of computation based on the proposed model match well the experiment data.

Experimental and damage model study of layered shale under different moisture contents

To investigate the mechanical properties and damage evolution law of layered shale under varying moisture contents, we conducted triaxial compression experiments on rock samples with different bedding angles and moisture levels. This study analyzed the variations in mechanical properties of layered shale under different conditions, and established a predicted model for elastic modulus based on different bedding angles and moisture content. Additionally, the damage constitutive model of layered shale was improved. The study revealed that shale’s mechanical properties display anisotropy, which is influenced by the bedding angles and moisture contents. The elastic modulus of the rock increases with the rise of bedding angle, exhibiting a ‘U’-shaped change. Conversely, the mechanical properties of rocks deteriorate, and their brittleness weakens with the increase in moisture content. When the confining pressure increased, the overall mechanical properties of shale were enhanced, and the influence of bedding on shale was weakened, but the deteriorating effect of water on rocks was hardly affected. Based on the above experiments, a predicted model of equivalent elastic modulus of shale considering the coupling effect of bedding and different moisture contents was proposed, which could effectively predict the elastic modulus of layered shale with different moisture content under different confining pressures. Furthermore, based on the predicted model of elastic modulus, an improved damage constitutive model of layered shale under triaxial loading was established, and the damage accumulation trend of layered shale was obtained, which showed an “S”-shaped change with strain. Under the coupling effect of bedding and different moisture contents, the damage of shale was advanced, but the accumulation rate of damage slowed down. With the increase of confining pressure, the influence of bedding and moisture content on the damage characteristics of shale decreased, and the damage curves under different conditions gradually tended to isotropy. The developed damage constitutive model for layered shale under different moisture contents provides theoretical support for the study of reservoir fracturing and wellbore stability.

Characterization and Quantitative Assessment of Shale Fracture Characteristics and Fracability Based on a Three-Dimensional Digital Core

At present, assessment techniques for the fracability of shale reservoirs, which rely on the formation of an effective fracture network, are scarce. Hence, in order to assess the fracability, it is critical to establish a quantitative correlation between the pattern of fracture distribution after fracture and fracability. The present investigation utilizes three-dimensional digital core technology and triaxial compression experiments to simulate the fracturing process in typical domestic shale reservoir cores. In addition to utilizing the maximum ball algorithm to extract fracture images, a number of other techniques are employed to compute the spatial quantitative parameters of the fractures, including least squares fitting, image tracking algorithms, and three-dimensional image topology algorithms. The introduction of the notion of three-dimensional fracture complexity serves to delineate the degree of successful fracture network formation subsequent to fracturing. A quantitative fracability characterization model is developed by integrating the constraints of fracture network formation potential and fragmentation potential. The results of this study show that the quantitative characterization of the characteristic parameters of cracks can be achieved by establishing a method for extracting crack information as well as parameters after core compression and completing the construction of a three-dimensional complexity characterization model. Meanwhile, the three-dimensional post-compression fracture image validation shows that the core fracturability index can better reflect the actual fracturing situation, which is in line with the microseismic monitoring results, and significantly improves the accuracy of fracturability characterization, which is an important guideline for the fracturing design of shale gas reservoirs.

Experimental Insights Into Fault Reactivation and Stability of Carrara Marble Across the Brittle–Ductile Transition

AbstractLittle is known about the impact of pressure (P) and temperature (T) on faulting behavior and the transition to fault locking under high P–T conditions. Using a Paterson gas‐medium apparatus, triaxial compression experiments were conducted on Carrara marble (CM) samples containing a saw‐cut interface at ∼40° to the vertical axis at a constant axial strain rate of ∼1 × 10−5 s−1, P = 30–150 MPa and T = 20–600°C. Depending on the P–T conditions, we observed the complete spectrum of deformation behavior, including macroscopic (shear) failure, stable sliding, unstable stick‐slip, and bulk deformation with locked faults. Macroscopic failure and stable sliding were limited to P < 100 MPa and T = 20°C. In contrast, at P ≥ 100 MPa or T ≥ 500°C, faults were locked, and samples with bulk deformation experienced strain hardening at strains ≤8.8%. At T = 100–400°C and P ≤ 100 MPa, we observed unstable stick‐slip behavior, where both fault reactivation stress and subsequent stress drop increased with increasing pressure and temperature, associated with increasing matrix deformation and less fault slip. Microstructures indicate a mixture of microcracking, twinning and dislocation activity (e.g., kinking and undulatory extinction) that depends on P–T conditions and peak stress. The transition from slip to lock‐up with increasing pressure and temperature is induced by an enhanced contribution of crystal plastic deformation. Our results show that fault reactivation and stability in CM are significantly influenced by P–T conditions, probably limiting the nucleation of earthquakes to a depth of a few kilometers in calcite‐dominated faults.

Study on multi-scale damage and failure mechanism of rock fracture penetration: experimental and numerical analysis

In natural rock masses, joints or fractures usually occur at different penetrations. To study the deformation and failure behavior of rock mass with different fracture penetrations, a series of triaxial compression experiments were conducted on rock specimens containing prefabricated fracture at penetrations of 0%, 25%, 50%, 75% and 100%. The results show that the greater the fracture penetration and the smaller the strength of the rock sample, the more obvious the plastic deformation section of the stress–strain curve. Confining pressure can greatly improve the compressive strength of rock samples. For intact rock samples, confining pressure will change the failure mode from tensile failure to shear failure. For fractured rock samples, the confining pressure will aggravate the degree of damage to the rock sample. PFC 3D simulation shows that the failure process of rock samples is accompanied by the transformation of strain energy to damping energy and particle sliding energy, which can quantitatively characterise the damage process of rock. The increasing trend of crack number is slow-steep-slow, which is the ‘S’ type. It began to grow rapidly after reaching peak strength and gradually stabilised after reaching residual strength. It is consistent with the trend of energy change.

Shale brittleness evaluation under high temperature and high confining stress based on energy evolution

Deep and ultra-deep shale gas reservoirs are crucial targets for future natural gas development. Extensive research has shown the shale brittleness is closely associated with the hydraulic fracture network quality. However, the shale brittleness may be altered in deep reservoirs due to the high temperature and high geo-stress. At present, commonly used shale brittleness evaluation methods cannot distinguish the differences in energy evolution caused by different mineral components in shale rock. In order to distinguish this difference, this study innovatively proposes a shale brittleness evaluation method that considers mineral composition based on energy evolution. A high-temperature triaxial compression experiment was carried out on the Longmaxi Formation shale, its fracture mode was analyzed, and the brittleness of the shale was evaluated based on this innovative method. The results indicate that with increasing confining stress, the shale fracture mode transitions from brittle tensile-shear multi-crack penetrating fracture to brittle-plastic shear single-crack fracture, and the brittleness of shale shows a decreasing trend. Under low confining stress, the shale brittleness decreases with temperature. Under high confining stress, temperature is the non-main controlling factor of shale brittleness. This study provides valuable guidance for the design and optimization of hydraulic fracturing in deep shale reservoirs.

Influence of lithology and bedding orientation on failure behavior of “D” shaped tunnel

To explore the influence of lithology on the failure behavior of layered tunnel, true triaxial compression experiments were undertaken on cubical phyllite and yellow sandstone samples containing a “D” shaped hole. The failure progress of the hole sidewalls was monitored and captured using a miniature camera. The results reveal that the initial vertical failure stress of the phyllite samples presents a “U” shaped change as the bedding angle increases. At the bedding angle of 45°, compared with the initial vertical failure stress of the yellow sandstone tunnel, that of the phyllite tunnel is lower, resulting in larger rock fragments and deeper V-shaped grooves. The failure pattern of the phyllite tunnel is primarily manifested as extensive shear sliding failure, and the failure is more severe. The failure of the yellow sandstone tunnel is primarily characterized by the sequential laminar fracturing along the maximum principal stress direction, predominantly manifesting as tensile failure. The primary factors influencing the failure of two types of layered rocks are the significant variations in the clay mineral content within the rocks. For the phyllite, it contains nearly one-third of montmorillonite (a clay mineral). This results in the formation of weak bedding planes within surrounding rocks, which induces shear slip failures along these bedding planes. In contrast, the yellow sandstone has a lower clay mineral content, leading to the absence of distinct weak bedding planes within the surrounding rock. In this case, bedding planes present ignorable effect on the surrounding rock.

Mechanical properties under triaxial compression of coal gangue-fly ash cemented backfill after cured at different temperatures

The triaxial mechanical properties of cemented backfill are the key indicators that affect the stability of cemented backfill in underground mining areas, and the high temperature environment in mines has become one of the main factors affecting the mechanical properties of cemented backfill. Understanding the triaxial mechanical properties of cemented backfill in high temperature environments is crucial for the stability of cemented backfill in mining areas. To investigate the effect of temperature on the mechanical characteristics of cemented backfill under triaxial compression, triaxial compression experiments were done on coal gangue-fly ash cemented backfill that had been cured at three temperatures (20 °C, 35 °C, and 50 °C). The deviatoric stress-strain curve of cemented backfill under triaxial compression was produced, its deformation parameters, dilatancy, and failure characteristics were investigated, and a more appropriate strength criteria was proposed. The research results indicate that, peak strength and dilatancy stress of cemented backfill under the same confining pressure decrease first and then increase as the curing temperature increases. The failure patterns of cemented backfill under triaxial compression will exhibit layered splitting failure except tension and shear failure since the curing temperature has increased. By comparing it with several other criteria, the Mogi-Coulomb strength criteria was found to be a good reflection of the failure strength properties of cemented backfill cured at different temperatures under triaxial compression. With curing temperature rises, internal friction angle of cemented backfill steadily lowers, while the cohesion first decreases and subsequently increases. The alteration in cohesion is consistent with the peak strength change rule, illustrating that the primary factor influencing peak strength of cemented backfill is cohesion. This study provides a good theoretical reference value for guiding the design of deep mine filling and optimizing the mixture ratio of cemented backfill.

A modified HJC model for geological materials subjected to blasting loadings

In this paper, a modified HJC model is proposed to address the problem that the HJC model cannot simulate tensile damage cracks. The modified HJC model modifies the strength surface, strain rate effect and damage model of the original HJC model, respectively. The modified model is embedded in LS-DYNA software for single and multiple finite element validation. The effects of different parameters on the modified HJC model stress–strain curves are analyzed in the single finite element validation. In multiple finite element verification, uniaxial and triaxial compression experiments, Brazilian disc splitting experiments, SHPB dynamic compression experiments, and SHPB dynamic splitting experiments are performed. The results of single and multiple finite element numerical validation illustrate that the modified HJC model can well simulate the pre-peak and post-peak nonlinear mechanical behaviors of geological materials such as rocks and concrete. Compared with the HJC model, the damage pattern of the modified HJC model is closer to the real damage pattern. For the strain rate effect, the modified HJC model can reflect the real compressive strain rate effect and improve the sensitivity of tensile strength to strain rate. The results of the numerical simulation of the blasting funnel illustrate that: the modified HJC model can simulate explosive tensile cracking well and can explain the formation mechanism of blasting funnel due to the distinction between tensile and compressive meridian surfaces and the addition of tensile damage. The modified HJC model is more applicable to the numerical simulation of engineering blasting.

Study on the mechanical properties of sandstone-shale composite continental shale gas based on the discrete element method

China has abundant continental shale oil resources, and multiple shale oil exploration and development demonstration zones have been established, making it a key field for oil and gas exploration and development. However, drilling and fracturing treatment often encounter vertically interlaced and superimposed sandstone and shale reservoirs, which significantly affect the stability of the reservoir and the propagation of fractures. Therefore, it is of great significance to study the rock mechanics characteristics of sandstone-shale composite. Using the orthogonal experimental method for microscale parameter calibration, this study established the relationship between the macroscopic and microscopic parameters of sandstone-shale in the parallel bond model. The results of the orthogonal experimental scheme were analyzed using multi-factor variance analysis and linear regression analysis to determine the influence and ranking of each microscale parameter on the macroscopic parameters. Based on the results of uniaxial and triaxial compression experiments, the particle flow simulation of sandstone-shale composite rock samples was studied using the discrete element method. The study investigated the influence of the thickness ratio, combination form, and confining pressure of sandstone-shale on the mechanical properties of composite rock samples. The results showed that: (1) Under uniaxial loading conditions, for the combination form of sandstone on top and shale on the bottom (M-S type), the peak stress of the composite rock sample increased first and then decreased and then increased again with the increase of the thickness of sandstone; for the combination form of shale on top and sandstone on the bottom (S-M type), the peak stress of the composite sample decreased first and then increased and then decreased again with the increase of the thickness of shale. (2) After applying confining pressure, the compressive strength of the M-S type composite sample decreased with the increase of the thickness ratio of sandstone, while the compressive strength of the S-M type composite sample increased with the increase of the thickness ratio of shale as well as the compressive strength of the composite sample was lower than that of any complete rock sample. (3) Under uniaxial loading, the fracture mode of the composite sample was related to the combination form and the thickness ratio of different lithologies. For the M-S type sample, when the thickness ratio of sandstone was <30%, the sample was mainly fractured by tension, and when the thickness ratio of sandstone was >30%, it was mainly fractured by shear mode. For the S-M type sample, when the thickness ratio of shale was <70%, the sample had the most internal shear cracks, and when the thickness ratio of shale was >70%, it had the most tension cracks.

Deformation evolves from shear to extensile in rocks due to energy optimization

Determining how fracture network development leads to macroscopic failure in heterogeneous materials may help estimate the timing of failure in rocks in the upper crust as well as in engineered structures. The proportion of extensile and shear deformation produced by fracture development indicates the appropriate failure criteria to apply, and thus is a key constraint in such an effort. Here, we measure the volume proportion of extensile and shear fractures using the orientation of the fractures that develop in triaxial compression experiments in which fractures are identified using dynamic in situ synchrotron X-ray imaging. The fracture orientations transition from shear to extensile approaching macroscopic, system-size failure. Numerical models suggest that this transition occurs because the fracture networks evolve in order to optimize the total mechanical efficiency of the system. Our results provide a physical interpretation of the empirical internal friction coefficient in rocks.

Elasticity and Characteristic Stress Thresholds of Shale under Deep In Situ Geological Conditions.

The mechanical properties of shale are generally influenced by in situ geological conditions. However, the understanding of the effects of in situ geological conditions on the mechanical properties of shale is still immature. To address this problem, this paper provides insight into the elasticity and characteristic stress thresholds (i.e., the crack closure stress σcc, crack initiation stress σci, and crack damage stress σcd) of shales with differently oriented bedding planes under deep in situ geological conditions. To accurately determine the elastic parameters and crack closure and initiation thresholds, a new method-i.e., the bidirectional iterative approximation (BIA) method-which iteratively approaches the upper and lower limit stresses of the linear elastic stress-strain regime, was proposed. Several triaxial compression experiments were performed on Longmaxi shale samples under coupled in situ stress and temperature conditions reflecting depths of 2000 and 4000 m in the study area. The results showed that the peak deviatoric stress (σp) of shale samples with the same bedding plane orientation increases as depth increases from 2000 m to 4000 m. In addition, the elastic modulus of the shale studied is more influenced by bedding plane orientation than by burial depth. However, the Poisson's ratios of the studied shale samples are very similar, indicating that for the studied depth conditions, the Poisson's ratio is not influenced by the geological conditions and bedding plane orientation. For the shale samples with the two typical bedding plane orientations tested (i.e., perpendicular and parallel to the axial loading direction) under 2000 and 4000 m geological conditions, the ratio of crack closure stress to peak deviatoric stress (σcc/σp) ranges from 24.83% to 25.16%, and the ratio of crack initiation stress to peak deviatoric stress (σci/σp) ranges from 34.78% to 38.23%, indicating that the σcc/σp and σci/σp ratios do not change much, and are less affected by the bedding plane orientation and depth conditions studied. Furthermore, as the in situ depth increases from 2000 m to 4000 m, the increase in σcd is significantly greater than that of σcc and σci, indicating that σcd is more sensitive to changes in depth, and that the increase in depth has an obvious inhibitory effect on crack extension. The expected experimental results will provide the background for further constitutive modeling and numerical analysis of the shale gas reservoirs.

Study on Brittleness Characteristics and Fracturing Crack Propagation Law of Deep Thin-Layer Tight Sandstone in Longdong, Changqing

Tight-sandstone oil and gas resources are the key areas of unconventional oil and gas resources exploration and development. Because tight-sandstone reservoirs usually have the characteristics of a low porosity and ultralow permeability, large-scale hydraulic fracturing is often required to form artificial fractures with a high conductivity to achieve efficient development. The brittleness of rock is the key mechanical factor for whether fracturing can form a complex fracture network. Previous scholars have carried out a lot of research on the brittleness characteristics of conglomerate and shale reservoirs, but there are few studies on the brittleness characteristics of sandstone with different types and different coring angles in tight-sandstone reservoirs and the fracture propagation law of sandstone with different brittleness characteristics. Based on this, this paper carried out a systematic triaxial compression and hydraulic fracturing experiment on the tight sandstone of Shan 1 and He 8 in the Longdong area of the Changqing oilfield. Combined with CT scanning cracks, the brittleness characteristics and fracturing crack propagation law of different types and different coring angles of sandstone under formation-confining pressure were clarified. The results show that there are great differences between different types of sandstone in the yield stage and the failure stage. The sandstone with a quartz content of 100% has the highest peak strength and a strong brittleness. Sandstones with a high content of natural fractures and dolomite have a lower peak strength and a weaker brittleness. There are also differences in the peak strength and fracture morphology of sandstone with different coring angles due to geological heterogeneity. The sandstone with a comprehensive brittleness index of 70.30 produces a more complex fracture network during triaxial compression and hydraulic fracturing than the sandstone with a comprehensive brittleness index of 14.15. The research results have important guiding significance for on-site fracturing construction of tight-sandstone reservoirs.

Permeability evolution in fine‐grained Aji granite during triaxial compression experiments

AbstractTriaxial compression experiments were carried out on samples of fine‐grained Aji granite to measure the evolution of permeability during deformation prior to failure under confining pressures of 20 and 40 MPa. During the initial stages of deformation, a small decrease in permeability was observed, due to the closure of pre‐existing microcracks; permeability then increased with increasing differential stress. During deformation, permeability varied by up to two orders of magnitude, and we observed a small pressure dependence, with a larger variation observed at 20 MPa than at 40 MPa. This suggests that more cracks developed during brittle deformation under the lower confining pressure. The observed increase in permeability during our experiments was approximately proportional to inelastic volumetric strain, which corresponded to the volume of dilatant cracks. On the other hand, prior to brittle failure, we observed a further increase in permeability that was greater than the inelastic volumetric strain, suggesting crack aperture opening accelerated at this stress level (&gt;∼80%). The permeability enhancement resulting from the crack dilation affects the gas‐sealing capacity of the fluid‐saturated caprock. Our experimental findings would be beneficial for the safety assessment in applications such as the long‐term storage of various gaseous wastes.

The Influence of Confining Pressure and Preexisting Damage on Strain Localization in Fluid‐Saturated Crystalline Rocks in the Upper Crust

AbstractThe spatial organization of deformation may provide key information about the timing of catastrophic failure in the brittle regime. In an ideal homogenous system, deformation may continually localize toward macroscopic failure, and so increasing localization unambiguously signals approaching failure. However, recent analyses demonstrate that deformation, including low‐magnitude seismicity, and fractures and strain in triaxial compression experiments, experience temporary phases of delocalization superposed on an overall trend of localization toward large failure events. To constrain the conditions that promote delocalization, we perform a series of X‐ray tomography experiments at varying confining pressures (5–20 MPa) and fluid pressures (0–10 MPa) on Westerly granite cores with varying amounts of preexisting damage. We track the spatial distribution of the strain events with the highest magnitudes of the population within a given time step. The results show that larger confining pressure promotes more dilation, and promotes greater localization of the high strain events approaching macroscopic failure. In contrast, greater amounts of preexisting damage promote delocalization. Importantly, the dilative strain experiences more systematic localization than the shear strain, and so may provide more reliable information about the timing of catastrophic failure than the shear strain.

Mechanical data corrections for triaxial compression testing at high pressures and high temperatures with a Paterson gas-medium mechanical testing apparatus

Although restricted by a limited range of strain, the triaxial compression test is a mature and common technique for investigating the rheological properties of rock materials at high pressures and high temperatures, especially when establishing the constitutive equations for various flow laws. The Paterson gas-medium high-pressure and high-temperature mechanical testing apparatus (Paterson apparatus) is the best apparatus for triaxial compression testing due to its high stress resolution. However, to derive accurate mechanical information from the raw data recorded by the Paterson apparatus, some technical issues should be addressed, including the simultaneous distortion of the apparatus, the load force supported by the jacketing tube, and the change in the cross-sectional area of the specimen. In this paper, we introduce correction methods corresponding to these three technical issues for triaxial compression on a Paterson apparatus equipped with an internal load cell to significantly reduce experimental errors so that high-precision mechanical data for establishing the constitutive equations of flow laws, such as differential stress, strain, and strain rate, can be obtained. To facilitate corrections for the distortion of the apparatus and the load force supported by the jacketing tube, we determine the distortion of the Paterson apparatus as a function of axial load force by deforming tungsten steel specimens with a known Young’s modulus and the high-temperature flow laws of two common jacketing materials, iron and copper, by triaxial compression experiments at confining pressures of 200–300 MPa. Previous flow laws of iron and copper established by Frost and Ashby (1982) using ambient mechanical data are carefully compared with the flow laws obtained in this study to evaluate their effectiveness for correcting jacket tube strength. Finally, the errors eliminated by each correction step are analyzed and discussed to better understand the necessity of mechanical data corrections.

Large-scale heterogeneities can alter the characteristics of compressive failure and accelerated seismic release.

Externally stressed brittle rocks fail once the stress is sufficiently high. This failure is typically preceded by a pronounced increase in the total energy of acoustic emission (AE) events, the so-called accelerated seismic release. Yet, other characteristics of approaching the failure point such as the presence or absence of variations in the AE size distribution and, similarly, whether the failure point can be interpreted as a critical point in a statistical physics sense differs across experiments. Here, we show that large-scale stress heterogeneities induced by a notch fundamentally change the characteristics of the failure point in triaxial compression experiments under a constant displacement rate on Westerly granite samples. Specifically, we observe accelerated seismic release without a critical point and no change in power-law exponent ε of the AE size distribution. This is in contrast to intact samples, which exhibit a significant decrease in ε before failure. Our findings imply that the presence or absence of large-scale heterogeneities play a significant role in our ability to predict compressive failure in rock.

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.