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With the increasing demand persisting for energy extraction from geothermal resources, many scientific research studies have been carried out on the performance of proppants to improve the energy extraction process. Knowledge of the after-effects of proppants exposed to realistic geothermal reservoir conditions is crucial. Therefore, the aim of the experimental study reported here was to investigate the mechanical behaviour of a single proppant under incremental loading conditions, and the mineralogical and microstructural alterations due to exposure to elevated temperatures (100 °C, 200 °C, 300 °C and 400 °C) and different cooling conditions (slow cooling and quenching). Significant mechanical weakening, overall strength reduction of 52.19% and 69.74% and Young’s modulus reduction of 43.64% and 55.45% for the slow cooling and quenching cooling techniques were observed in a single proppant when the temperature increased from room temperature (25 °C) to elevated temperatures. Scanning electron microscopy revealed the occurrence of thermal cracks inside the proppant microstructure, together with the alteration of the mineral structure, and significant changes in zeolite, Na-feldspar and K-feldspar were observed upon exposure to elevated temperatures. The post-failure behaviour of a single proppant was studied conducting 3-D X-ray computed tomography (CT) scanning. Under normal loading conditions, proppants cleave and generate large fragments like a flower, and this happens suddenly and quite violently through the material. Interestingly, post-failure analysis revealed that the failure mechanism of a single proppant consists of three major stress levels, where initially proppant fails at a high stress level and gains some crushing-associated strength at later stages. [ABSTRACT FROM AUTHOR]

Understanding the mechanical behaviour of reservoir rock under different temperatures with different cooling conditions is necessary for safe and effective deep geo-engineering applications, including geothermal energy extraction, deep geological disposal of nuclear waste, deep mining and coal gasification projects. The aim of this study is, therefore, to investigate the effect of increasing temperature (from room temperature to 800 °C) followed by two cooling methods (both rapid and slow) on the mechanical behaviour of Australian Strathbogie granite under uniaxial conditions. Further, a separate experimental program was conducted under continuous heating conditions without cooling the samples to compare the results of cooled samples. In order to investigate the strain developments in granite subjected to heating following slow and rapid cooling, ARAMIS photogrammetry technology was adopted, and the corresponding fracture propagation patterns were investigated using an acoustic emission (AE) system. Optical microscopic imaging technology was used to identify the corresponding micro-structural alterations and crack-formation patterns. According to the results, once the rock mass is subjected to higher thermal stresses, strength and elastic characteristics are significantly reduced, mainly due to thermally-induced damage in terms of both inter-granular and intra-granular cracks. The stress-strain response revealed that the failure mode of granite is changed from brittle to quasi-brittle fracturing with increasing temperature. The following cooling causes the strength and elastic characteristics of the granite to be further decreased through the enhancement of crack density, and the influence of rapid cooling is much greater than that of slow cooling, due to sudden thermal shock. This is evidenced by the AE results, according to which both high pre-heated temperatures and high cooling rates cause much quicker crack initiation and propagation in granite with lesser seismicity in the quasi-brittle region. [ABSTRACT FROM AUTHOR]

Geothermal heat has now been identified as an effective renewable energy source due to severe environmental impacts created by conventional fossil usage on global climatic change. However, its wide application has been limited due to the lack of knowledge, particularly of the geothermal conditions of reservoir rocks at elevated temperatures and pressures. Such high temperatures and pressures possibly alter the mechanical properties of reservoir rocks due to the associated micro-structural and mineralogical alterations of the rock mass, which are an important attribute for wellbore stability and stimulation of geothermal reservoirs for safe and effective geothermal energy extraction. This study therefore investigates the stress-strain behaviour under in-situ stress and temperature conditions by conducting a series of high-pressure, high-temperature tri-axial experiments on Australian Strathbogie granite under four different confining pressures (10, 30, 60, 90 MPa) and four different temperatures (RT, 100, 200, 300 °C). The effect of temperature on the mechanical behaviour of rock specimens was studied under tri-axial conditions and the corresponding fracture propagation behaviour was observed using an advanced acoustic emission (AE) system. The corresponding micro-structure alteration in granite was observed using SEM analysis. According to the findings, increasing temperature leads to an initial increment in reservoir rock strength and shear parameters followed by reduction, and the trend is aligned with the crack formation pattern of the rock mass. This was further confirmed by the SEM analysis, according to which the rock micro-structure is subject to only minor changes at relatively low temperatures and higher temperatures cause micro-cracks to develop along the rock mass grain boundaries. Furthermore, the conventional Mohr-Coulomb criteria failed to model the stress-strain response of rock under geothermal reservoir conditions, and was therefore modified for the corresponding in-situ conditions. [ABSTRACT FROM AUTHOR]

• An elastic damage constitutive model of coal under CO 2 adsorption is established. • High adsorption pressure leads to the increase in the multifractal spectrum width. • More AE spectrum of coal presents intensive distribution under high pressure. • The AE signal caused by microstructure changes results in a high value of Δ α in the compaction stage. Geological sequestration of CO 2 is an effective way to achieve the goal of reducing global warming and carbon neutralization. The complex CO 2 –coal interaction will lead to the change of mechanical properties of coal. Therefore, evaluating the mechanical characteristics of deep coal seam after CO 2 adsorption is of great significance for analyzing the geological characteristics and structural stability of deep strata and storage site selection. In this study, the triaxial compression test and acoustic emission (AE) test were used to evaluate the effect of CO 2 adsorption pressure on the mechanical properties of coal mass. On the basis of the evolution characteristics of AE ring down count (RDC), fractal theory was used to analyze the deformation and fracturing evolution of coal. Besides, an elastic damage constitutive model describing the nonlinear stress-strain relationship of coal under CO 2 adsorption is established. Results show that the growth of adsorption pressure develops the risk of damage and fracturing of coal. With the increase in adsorption pressure, the value of singularity index width Δ α presents a downward trend, indicating the development of internal cracks and the increase in the complexity of microunit fracturing process. When the adsorption pressure is low, the gas in large cracks and pores leads to the generation of small AE signal that occupies the dominant position. High adsorption pressure increases the complexity and nonlinearity of coal deformation, strengthens the proportion of large AE signal, and leads to the increase in the average value of multifractal spectrum width Δ f. With the increase in adsorption pressure, the fracturing degree of coal is higher, a mass of large-size cracks appears, and more AE spectrum presents intensive distribution. In the plastic stage, the load level is close to the strength limit, the microcracks intersect with each other, and the average value of Δ α in this stage is the smallest. [ABSTRACT FROM AUTHOR]

Carbon dioxide (CO 2 ) sequestered in saline aquifers undergoes a variety of chemically-coupled mechanical effects, which may cause CO 2 -induced mechanical changes and time-dependent reservoir deformation. This paper investigates the mineralogical and microstructural changes that occur in reservoir rocks following injection of CO 2 in deep saline aquifers and the manner in which these changes influence the mechanical properties of the reservoir rocks. In this study, cylindrical sandstone specimens, 38 mm in diameter and 76 mm high, obtained from the Gosford basin, were used to perform a series of unconfined compressive strength (UCS) tests. Different saturation conditions: dry, water- and brine-saturated sandstone samples with and without scCO 2 (super-critical carbon dioxide) injection, were considered in the study to obtain a comprehensive understanding of the impact of scCO 2 injection during the CO 2 sequestration process on saline aquifer mechanical properties. An acoustic emission (AE) system was employed to identify the stress threshold values of crack closure, crack initiation and crack damage for each testing condition during the whole deformation process of the specimens. Finally, scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses were performed to evaluate the chemical and mineralogical changes that occur in reservoir rocks during CO 2 injection. From the test results, it is clear that the CO 2 -saturated samples possessed a lower peak strength compared to non-CO 2 saturated samples. According to SEM, XRD and XRF analyses, considerable quartz mineral corrosion and dissolution of calcite and siderite were observed during the interactions of the CO 2 /water/rock and CO 2 /brine/rock systems, which implies that mineralogical and geochemical rock alterations affect rock mechanical properties by accelerating the collapse mechanisms of the pore matrix. AE results also reveal the weakening effect of rock pore structure with CO 2 injection, which suggests a significant effect of CO 2 on failure mechanisms of the reservoir rock, with CO 2 saturation showing a significant influence on crack initiation and crack damage stages. [ABSTRACT FROM AUTHOR]

Most studies on interaction of CO 2 with coal have focused on the permeability and swelling characteristics of coal and fewer investigations have been carried out on its influence on the geomechanical attributes of coal. In this study, bituminous coals from Jharia coalfield in India were investigated for changes in the strength of samples due to saturation with moisture and CO 2 . It was found that the strength of coal samples reduced in each case of gas and moisture saturation. Maximum reduction in strength and elastic modulus of coal was observed in case of CO 2 treatment of moisture saturated samples. The crack initiation occurred early at low stresses in coal while it took place at higher stresses in water saturated samples. The failure behaviour of specimens indicates shearing of coal and CO 2 treated coal while the water saturated specimens collapsed along the fractures and cleats during loading. The level of reduction in strength is different in the two types of coal which highlights site specific study of CO 2 injection on strength of coal. [ABSTRACT FROM AUTHOR]

Highlights: [•] Salinity effects on mechanical properties of deep saline aquifers are investigated. [•] AE counts, SEM and ARAMIS analyses on varying brine saturated sandstone are performed. [•] Crystals develop in the rock’s pore structure, depending on the bine concentration. [•] Increased salinity enhances rock brittleness and reduces crack initiation stress. [ABSTRACT FROM AUTHOR]

Abstract: In any carbon capture and sequestration (CCS) project, well cement plays a vital role as it provides the required zonal isolation and well integrity. Typical wellbore materials including well cement and formation rock will be exposed to a range of saturation mediums such as water and brine with different salinity levels. To date, ordinary Portland cement (OPC)-based well cement has been used. However, its survival has been questioned under CO2 sequestration conditions, as it experiences cement degradation and strength reduction in saline water. Therefore, this experimental work investigates the mechanical characteristics of geopolymer (G) as well cement and sandstone (S) as formation material. The mechanical behaviours of G, S and G–S composite materials in fresh water (W) and two concentrations of brine water (BW), 5% NaCl (5% BW) and 15% NaCl (15% BW), were studied. Based on the results, it was found that G, S and G–S samples experience strength reduction in W and BW. However, the reduction rate of G is almost half of that of OPC-based oil well cement. In addition, the strength reduction rates of G and G–S were less in 15% BW compared to W and 5% BW, due to the lower alkali leaching rates from G in BW compared to W. Therefore, saline aquifers with high NaCl content are always favourable for G well cement. The S samples showed constant strength reduction regardless of the saturation medium, and hence NaCl does not show any significant effect on the mechanical behaviour of quartz-rich sandstone. The crack propagation stress thresholds were higher for G and G–S saturated in 15% BW compared to 5% BW. The S samples did not show major variation in crack propagation stress thresholds in W and BW. The ARAMIS strain measurement results showed that the maximum strain that wellbore materials experience at failure reduces with the introduction of brine. In addition, G and G–S undergo splitting failure, whereas S experiences shear failure in W and BW. [Copyright &y& Elsevier]

Highlights: [•] Water-weakening effect of bedded-sandstone depends on the bedding orientation. [•] Three failure mechanisms of sandstone were observed depending on the bedding orientation. [•] Total energy release is considerably higher at dry state than at saturated state. [•] Total energy release is decreasing with increasing bedding orientation. [•] Energy dissipation is higher at shallow bedding orientation. [ABSTRACT FROM AUTHOR]

Abstract: Acoustic emission (AE) methods are now widely used for damage evaluation. For a better understanding of the damage mechanics of materials such as rocks, AE has been used to monitor stresses which induce crack closure, crack initiation and crack damage. In the present study, an AE system was used to study the damage behaviour of some Australian black coal samples subjected to uniaxial compression. Several samples were left in a container filled with 100% carbon dioxide (CO2) at a certain pressure for 72h prior to testing. The results were compared with samples which had only been exposed to the atmosphere to see if CO2 had any adverse effect on the strength of coal. Strain gauges were installed on the samples and the measured axial and volumetric strains were studied in conjunction with the AE counts. The AE method was successfully used for detecting the onset of crack initiation and the crack damage stress threshold of the black coal samples. Of the coal samples examined, crack initiation and crack closure of the samples subjected to saturation with CO2 occurred at stress corresponding to a higher percentage of the peak strength when compared to the samples which had only been exposed to atmospheric conditions. However, crack damage occurred at a higher percentage of peak strength and the average peak strength showed a higher value for samples in atmospheric condition when compared to CO2 saturated samples. The results show that sorption of CO2 can cause a reduction in strength of the black coal samples when tested under uniaxial compression. As the coal samples were highly inhomogeneous more tests are required in order to be able to confirm whether the adsorption of CO2 will cause strength reduction in coal and to identify the actual underlying mechanisms. [Copyright &y& Elsevier]

Abstract: It is important to have an understanding of the behaviour of concrete under different displacement rates and moisture contents. This is because underground concrete structures are subjected to drying and wetting process and various loading condition including in situ stresses, induced stresses and earthquakes. In this research work, a program of systematic laboratory testing has been undertaken to determine the effect of displacement rates and moisture contents on concrete strength. A total number of 20 concrete specimens were tested. Axial strain, lateral strain and acoustic emission (AE) counts were recorded continuously until the specimens had failed. Findings from those results were used to conduct a detailed analysis of the crack closure, crack initiation, secondary cracking, crack damage and peak failure of dry, partially wet and fully wet concrete specimens at displacement rates corresponding to different loading rates (0.05, 0.10, 0.15, 0.20 and 0.25mm/min). The results showed that when concrete were fully wet, the strength was dependent on displacement rate and the strength was significantly reduced in comparison with the strength of dry specimens at the same displacement rate (e.g. 25% reduction when the loading rate was 0.15mm/min). The relationships between Young’s modulus and Poisson’s ratio with compressive strength were also studied and an increase of both with an increase in strength was observed. [Copyright &y& Elsevier]

• A mixed mode I/III loading test was conducted on thermal damage ENDB granite. • The peak frequency signal of the sample is distributed between 80 kHz and 400 kHz. • A zero-frequency signal have an early warning effect on rock failure. • Proposed an acoustic early warning index based on critical slowing down theory. To explore the fracture morphology and acoustic emission characteristics of thermally damaged granite under mixed mode I/III loading, a series of heat treatment and mixed mode I/III loading tests were conducted on edge-notched disc bend (ENDB) granite samples. First, the thermal damage samples' strength and crack opening displacement were analyzed. Subsequently, based on 3D laser scanning technology, the fracture morphology of the sample was studied, and the fractal dimension was calculated. Then, the samples' acoustic emission characteristics were discussed, including the ringing count, b value, spectrum signature, and AF-RA. Finally, an acoustic early warning index was proposed based on the critical slowing down theory, and the influence of temperature on the early warning effect was analyzed. The results show that under mixed mode I/III loading, the samples formed an antisymmetric curved surface with out-of-plane torsion. As the thermal damage effect is enhanced, the peak strength decreases linearly, and the fractal dimension increases linearly. The failure mode changes from sudden instability to progressive instability. Shear cracks gradually dominate the sample failure. The peak frequency signal of the sample is mainly distributed between 80 kHz and 400 kHz. The appearance of the sudden drop in the b value and a zero-frequency signal has an early warning effect. The quadratic function can describe the relationship between warning time and temperature. As the temperature increases, the deterioration effect of historical loading on the sample is enhanced, the development time of sample failure is prolonged, and the early warning effect is better. [ABSTRACT FROM AUTHOR]

The high-temperature response of rock is quite useful for successful exploration and exploitation of various conventional as well as unconventional energy systems such as underground coal gasification, and geothermal energy, where the associated rocks are exposed to various temperature and pressure conditions. In this study, we have investigated the effect of temperature and thermal alteration of an Indian sandstone. An investigation using acoustic emission, digital image correlation through ARAMIS, micro-computed tomography (CT) techniques and thermogravimetric analysis was performed to study the mechanical responses of the sandstone with increasing temperatures. The results indicate that uniaxial compressive strength (UCS), tensile strength and elastic modulus of sandstone reduced significantly and aligned in an overall decreasing trend with increasing temperatures. The maximum and minimum UCS of sandstone were obtained at 400 °C and 700 °C, respectively. Acoustic emission (AE) responses of the sandstone indicated an early crack initiation and crack damage for high-temperature treated specimens. The deformation was further analysed using the ARAMIS digital image correlation technique, and strain levels were visually quantified that exhibited a widespread distribution of higher strain for thermally-treated specimens at 700 °C. Microstructural thermal damage and different pore-characteristics were studied using micro-CT techniques that provided valuable information about the connected and non-connected pores of the sandstone and their evolution with increasing temperatures. The outcomes suggest a heat-induced increment in porosity by nearly 40 percent upon heating the samples from 25 °C to 600 °C, which can be attributed due to the opening of new cracks that stimulated structural damage. Microstructural modification of thermally treated sandstone at 25 °C and 600 °C. Image 1 • Geomechanical investigation of post-high temperatures sandstone is studied up to 900°. • A coupled investigation using AE, ARAMIS, micro-CT has been performed. • Thermal damage has been estimated in terms of UCS, tensile strength and elastic modulus. • Acoustic emission based cracking behaviour of the sandstone has been investigated. • Connected and non-connected pores of the sandstone have been quantified with temperatures. [ABSTRACT FROM AUTHOR]

Knowledge of the effect of carbon dioxide (CO 2 ) on the mechanical properties of low-clay shales is essential to shale gas production and CO 2 sequestration. In this paper, a series of uniaxial compressive strength (UCS) variable-time experiments were performed on low-clay shale samples saturated in sub-/super-critical CO 2 . The crack propagation process and micro scale variations were recorded by acoustic emission (AE) sensors with 3D ARAMIS technology and SEM tests together with EDS analysis. According to the experimental results, sub-/super-critical CO 2 adsorption weakens the strength and increases the ductility of the shale. The UCS and Young's modulus decrease with the increase of saturation time. Compared to samples saturated in sub-critical CO 2 , samples saturated in super-critical CO 2 present lower strength and Young's modulus. AE results show that samples saturated at a longer time in sub-/super-critical CO 2 present a higher number of peak cumulative AE energy. Super-critical CO 2 saturation creates more AE energy than sub-critical CO 2 saturation. Based on the SEM results, sub-/super-critical CO 2 adsorption creates some new pores in shale samples which lead to the strength decreasing. EDS analysis presents that CO 2 adsorption increase the C content of the shale which demonstrates the occurrence of chemical reactions in the shale. [ABSTRACT FROM AUTHOR]

The interactions between drilling/fracking fluids and shale formations have great effect on shale gas exploitation. Previous studies have shown that water/brine saturation will cause clay-rich shale swelling and strength decreasing. However, little research has been done to investigate such effect on resource shales which mainly have low clay contents. This paper presents an experimental study of the impact of water-based fluids on the mechanical properties of a low-clay shale. Uniaxial compressive strength (UCS) tests were performed on shale samples soaked in NaCl, KCl and CaCl 2 solutions with saline concentrations of 0%, 10%, 20% and 25.4%. The crack propagation and the failure mechanism of the shale samples were recorded using acoustic emission (AE) sensors in combination with Digital Image Correlation (DIC) technique. Scanning electron microscopy (SEM) analysis was used to understand the micro scale variation of shale samples after soaking in saline solutions. According to the experimental results, water-saturated samples have the highest swelling increasing and the largest strength reduction. The swelling potential and UCS values decrease with increased saline concentration. Potassium ions have a significant effect on shale strength enhancement when compared to sodium ions and calcium ions. The failure pattern of the samples is mainly splitting breakage. NaCl and CaCl 2 saturated samples present more axial fractures than the KCl saturated samples when failure occurs. The SEM results show that crystals of NaCl, KCl and CaCl 2 were observed on the surface, and the amount of crystals increases with increasing saline concentrations. The AE results also show that saline saturation could increase the cumulative AE energy. [ABSTRACT FROM AUTHOR]

In this study, uniaxial compression tests were conducted on granite specimens containing three non-coplanar holes. The relationships between the stress, acoustic emission (AE) and crack evolution process were analyzed using AE measuring and photographic monitoring techniques. Particle flow code (PFC) was then used to simulate the strength failure behaviors of the specimens with three non-coplanar holes under uniaxially loading. Four typical crack coalescence patterns were identified, i.e., shear, mixed tensile and shear, and tensile. The crack evolution mechanisms around the pre-existing holes in the granite specimens were revealed by an analysis of the force and displacement fields. [ABSTRACT FROM AUTHOR]

The long-term integrity of CO 2 storage in deep saline aquifers has become uncertain due to the unsteady character of surrounding factors, and the time-dependent nature of the aquifer's overburden load (the vertical stress imposed on the aquifer by the weight of overlying materials (rock/soil layers), which may vary over time as a result of natural incidents such landslides and earthquakes) is critical. The aim of this study is to identify the influence of overburden load variations on the long-term integrity of the CO 2 storage process in deep saline aquifers. High-pressure tri-axial strength and permeability tests, along with acoustic emission (AE) and scanning electron microscopy (SEM) analyses, were conducted on Hawkesbury sandstone obtained from the Gosford basin. According to the results, the injection of CO 2 into the Hawkesbury formation may dissolve aquifer rock minerals, enhancing aquifer flow performance and reducing aquifer strength. Increasing the stress applied on the aquifer causes aquifer flow ability to reduce to some extent due to pore matrix compaction. Further increase of the overburden pressure may accelerate the aquifer's flow performance due to dilation-induced pore opening. This permeability transition point occurs earlier at greater CO 2 injection pressures and overlaps with the crack formation point of the aquifer rock mass. Therefore, weakening of the rock mass after the transition point can be expected. Importantly, this permeability transition point occurs at lower overburden loads after longer interaction of CO 2 with the saline aquifer. This exhibits the long-term risk associated with CO 2 sequestration in saline aquifers. Permeability enhancement after the transition point may also produce environmental disasters, such as sudden leakages of injected CO 2 from the reservoir to surrounding fresh water aquifers (Evans et al. 2004; Little and Robert 2010), exceeding the specific rates proposed by many regulatory frameworks. Therefore, it is essential to study the long-term integrity of the sequestration process in order to develop a regulatory structure to meet the demands of deep saline sequestration projects. [ABSTRACT FROM AUTHOR]

As an environmentally friendly anhydrous fracturing technology, liquid nitrogen (LN 2)-assisted refracturing can effectively improve fracture complexity and build Enhanced geothermal systems by repeatedly cooling the hot dry rock. In this study, multiple heating and LN 2 -cooling cyclic tests were conducted on granite samples to evaluate the impact of LN 2 cooling on the mechanical properties of granite. Subsequently, we performed uniaxial compression tests and acoustic emission (AE) detection to investigate the evolution of microcracks and fracture mechanisms in granite. The results show that heating and LN 2 -cooling cyclic stimulation aggravates the damage of granite, i.e., its physical and mechanical properties degrade in different degrees. After cyclic stimulation, there are regular changes in the types and sizes of microcracks in granite. The distribution range of the average frequency (AF) value of AE data decreases from 20–350 kHz to 20–250 kHz, while the rise time/amplitude (RA) value increases from 0–2 ms/v to 0–10 ms/v. Heating and LN 2 cooling lead to an increased likelihood of mineral particles in granite undergoing shear deformation and generating shear microcracks. After three cycles of heating and LN 2 -cooling, the proportion of shear microcracks begins to exceed that of tensile microcracks, which are 65.3% and 34.7%, while the failure pattern of granite samples changes from tensile to shear fracturing. As the number of cycles increases, the proportion of AE counts with peak frequency exceeding 700 kHz generally show an upward trend. The appearance of high-frequency AE signals indicates that the fracture scale of granite is further increased. The research results provide a scientific foundation for the refracturing of high-temperature granite under LN 2 cooling. [ABSTRACT FROM AUTHOR]

• The NSCB and BD tests combined with AE and DIC were conducted to investigate the anisotropy of mode-I fracture toughness and tensile strength of shale. • The universality of the extended critical plane approach validated and confirmed for both mode-I fracture toughness and tensile strength prediction. • The variation of mode-I fracture toughness and tensile strength versus loading angle was discussed. • The relationship between mode-I fracture toughness and tensile strength was discussed. • The typical failure modes of both NSCB and BD specimens and the fracture initiation angle of layered shales are discussed comprehensively. Mode-I fracture toughness (K IC) and tensile strength (σ t) are significant parameters in hydraulic fracturing. Owing to the unique mineral composition and sedimentary environment of shale, the mechanical properties of shale exhibit anisotropy. In this study, the notched semi-circular bend (NSCB) and Brazilian disc (BD) tests were combined with acoustic emission (AE) and digital image correlation (DIC) technology to investigate the anisotropic fracture and tensile behaviors of layered shale. The critical plane approach (CPA) criterion is extended and validated, and the universality of the extended CPA criterion is confirmed for both K IC and σ t prediction. In addition, the variation of K IC and σ t versus loading angle, the relationship between K IC and σ t , the typical failure modes of both NSCB and BD specimens, and the fracture initiation angle of layered shales are discussed comprehensively. The following conclusions were drawn: (1) K IC and σ t increased with increasing in the loading angle (β), which indicated significant anisotropy of layered shale, and the AE responses for different loading angles also differed significantly. (2) The variation of anisotropic K IC and σ t of different shales could be predicted by adjusting the spatial distribution parameter (Ω 0), so that the extended CPA criterion could predict K IC and σ t of different shales accurately, which confirmed the universality of the extended CPA criterion for both K IC and σ t prediction. (3) There exists an approximately linear or power relationship between K IC and σ t , and the power relationship can match both experimental and theoretical results much better. (4) The data from DIC and failure modes confirmed that when β was low, the failure modes were primarily affected by the bedding plane. In contrast, when β was great, the failure modes were primarily affected by the matrix. The NSCB specimens exhibited tensile failure, whereas the BD specimens exhibited more complex failure modes, which were a combination of tensile and shear failure. The results of this study can be used for guiding the design and construction of hydraulic fracturing and rock engineering. [ABSTRACT FROM AUTHOR]

Study of rocks under high temperature and strain-rate condition can serve as vital information in the restoration process of fire damaged buildings. In this study, the mechanical response of thermally treated fine-grained Dholpur sandstone was observed under increasing strain rates. Dholpur sandstone, a popular construction material, has been used to build some of the iconic monuments in India, a list which includes the Parliament, House of Indian President and the Buddhist Stupas of Sanchi. Thermal treatment spanned across ten days which included heating of the samples (200, 400, 600, 800 and 1000 °C) for five days followed by cooling, in room condition, for the exact same duration. The samples were then tested to failure at three different strain rates (2 × 10 −5 , 1 × 10 −4 and 2 × 10 −4  S −1 ). The elastic modulus was measured using non-contact laser extensometer. Acoustic emission (AE) technique was used to observe the microcrack development under compressive loading. The results from AE were used to demarcate the various stress thresholds and the stress regimes. The result of the experimental analysis suggests that the strength and elastic properties of rocks tend to increase till 400–600 °C followed by a fall in the mechanical characteristics. Onset of plasticity can be observed at high temperature. Strength of the rocks decrease at high strain rates due to the influence of strain rate dependency. The threshold values follow the behaviour of the mechanical properties under the influence of temperature and strain rate. [ABSTRACT FROM AUTHOR]

• The effects of supercritical CO 2 and water immersion on the mechanical properties of shale under different dynamic pressures are investigated. • The strength and Young's modulus of shale are decreased more significantly after water immersion compared to supercritical CO 2 immersion. • Compared with the constant pressure imbibition, the dynamic pressure imbibition changes the microstructure of shale and weakens its mechanical properties more significantly. During exploitation of shale gas and geological sequestration of CO 2 in shale gas reservoirs, the temperature and pressure of shale are increased with depth, whereas the reservoir pressure is gradually decreased with depth as exploitation proceeds. Therefore, it is important to investigate the effects of CO 2 and water on the mechanical properties of shale under dynamic pressure conditions. In this study, we have investigated the effects of supercritical CO 2 and water immersion on the mechanical properties of shale under different dynamic pressures (pressure 1: 20–8 MPa; pressure 2: 40–28 MPa), respectively. The test results indicate that after soaking in supercritical CO 2 and water, the uniaxial compressive strength (UCS) of shale is decreased by 51.05% and 58.36% (pressure 1), and by 35.98% and 36.84% (pressure 2), respectively. The strength and Young's modulus of shale are decreased more significantly after water immersion compared to supercritical CO 2 immersion. Due to the matrix compression effects, the mechanical properties of shale are changed more significant under lower imbibition pressures. Supercritical CO 2 immersion leads to increase in the Poisson's ratio along with more complex fracture patterns, whereas water immersion results in slight decrease in the Poisson's ratio along with shear fractures only. The acoustic emission (AE) signals have obvious stage characteristics during the compressional deformation of samples, and the AE energy is mainly generated in the unstable crack propagation stage. Supercritical CO 2 immersion plays an important role in crack generation, whereas water immersion is dominated by the alteration of pore structure. Compared with the constant pressure imbibition, the dynamic pressure imbibition changes the microstructure of shale and weakens its mechanical properties more significantly. The results of this study may help to better evaluate the effects of CO 2 and water on the mechanical properties of shale during exploitation of shale gas. [ABSTRACT FROM AUTHOR]

• The cracking process and localized deformation are investigated using 3D-DIC. • Different crack stress thresholds are distinguished from insight of AE technique. • There is a strong correlation between complexity of cracks and fractal dimension. • The stress distribution induced by infilling is revealed based on PFC modeling. The infilled flaws within rocks significantly influence the mechanical behavior of non-through flawed rocks. To investigate the influence of infilling on strain localization in the pre-cracking, cracking and cracking coalesce stages, experimental and numerical investigations were conducted on sandstone containing two gypsum-infilled parallel flaws, but of different flaw geometric configurations. The full-field strain evolution and progressive cracking process are analyzed using the acoustic-optical-mechanical methods, while the stress evolution and stress transfer by infilling were studied by the particle flow code modeling. The results show that the major principal strain of flawed sandstone with infilling changes from the diffused ellipse to highly localized linear strip with the increase of loading level, while the shear strain keeps a diffused ellipse shape invariable. The shear strain localization of infillings with a large flaw inclination initiates at a lower loading level than that of small one. The peak strength and Young's modulus of flawed sandstone show a first decrease, then increase and finally decrease trend with the ligament angle increasing from 0° to 150°, and both achieve their minimums at 60°. Ten types of cracks were observed, and four patterns of crack coalescence were identified. By comparison with sandstone of open flaws, the flaw infilling simplifies the geometric pattern of produced crack networks, which is quantitatively characterized by fractal dimension analysis. The numerical modeling shows that the infilling enlarges the compressive bond force zone and decreases the tensile bond force zone in the surrounding areas of pre-existing flaw, thus increasing the reinforcement coefficient. Moreover, the flaw geometric configuration has a significant influence on the reinforcement coefficient. It is also found that the increment of reinforcement coefficient defined by crack initiation stress is greater than those defined by either crack coalescence or peak stresses. [ABSTRACT FROM AUTHOR]

• Progressive fracturing process of rock under biaxial compression is investigated by DIC. • The crack coalescence mode is greatly influenced by the lateral stress. • There is a strong correlation between fractal dimension and lateral stress. • Different micro cracks were distinguished from insights of AE waveform. Discontinuities widely exist in natural rocks. To investigate the progressive micro-cracking process and failure mechanism of fissured rocks, a series of biaxial compression tests were conducted on sandstone specimens containing two parallel filled flaws using acoustic emission (AE) analysis synchronized with digital image correlation (DIC) monitoring. Experimental results show that the peak strength and elastic modulus of sandstone decrease first and then increase with the change in the ligament angle from 0° to 150°, achieving a minimum at 60°. The flaws remarkably facilitate crack coalescence under low lateral stress, such as at 2.5 MPa and 5 MPa. However, with an increase in lateral stress to 10 MPa, the crack coalescence is less influenced by the presence of pre-existing flaws. The AE events produced by flawed sandstone during the loading process conform to the Hurst statistical law. Fractal analysis shows that the lateral confinement reduces the irregularity of ultimate fracture geometry. Based on the AE dominant frequency features, the micro-tensile cracks, micro-shear cracks and micro-tensile-shear cracks are distinguished. The results show that with an increase in lateral stress, the percentage of micro-tensile cracks are constrained, but the number of micro-shear and mixed tensile-shear cracks increases. In addition, the micro-shear cracks preferentially appear in flawed sandstone specimens under high lateral stress as compared with specimens subjected to low stress. [ABSTRACT FROM AUTHOR]