Strength Of Shale Research Articles

Rock mechanics and wellbore stability of deep shale during drilling and completion processes

The seepage coupling effect between the borehole rock mass and borehole fluid and the effective stress field distribution around the wellbore are obviously different under different working conditions of drilling, fracturing and completion, which affect the stability of horizontal borehole of shale gas reservoir. The physical, chemical, and mechanical properties of the shale in Longmaxi formations in a block of China immersed with different working fluids were tested experimentally. Combined with the experimental data, a theoretical model was established to evaluate and analyze the wellbore stability of Longmaxi formations shale under different working conditions of drilling, fracturing and completion. The results show that the shale of the Longmaxi Formation is a typical hard and brittle shale, the bedrock is dense, with high mechanical strength and weak hydration and expansion ability. The immersion effect of different working fluids has little influence on the expansion and mechanical properties of shale bedrock. However, the mechanical strength of shale with relatively developed bedding fractures is low, and the immersion effect of different drilling fluid further weakens the mechanical strength, resulting in obvious anisotropy of mechanical strength of underground rock. The coupling effect of seepage between wellbore and formation has obvious influence on the dynamic distribution of borehole pore pressure and wellbore stability under different working conditions. The effective fluid column pressure at the bottom of the hole is the highest during the fracturing process, which leads to the increase of pore pressure near the wellbore, and then to the increase of collapse pressure around the borehole wall. The equivalent density of collapse pressure rises to the maximum of 1.93 g/cm3, and the wellbore stability is the worst in the fracturing process. The wellbore stability is secondary in the drilling process, and the wellbore stability is good when drilling along the direction of the maximum horizontal principal stress, and the equivalent density of collapse pressure is 1.69 g/cm3. During the completion process, the formation fluid flows into the wellbore, the pore pressure in the borehole is relieved, and the equivalent density of the collapse pressure is reduced to approximately 1.35 g/cm3, the wellbore stability is the best and open hole completion can be attempted in the horizontal well section. The results provide a certain theoretical reference for the selection of the drilling and completion schemes in shale formation.

The experimental investigation of effect of supercritical CO2 immersion on mechanical properties and pore structure of shale

The interactions between supercritical carbon dioxide (ScCO2) and shale significantly affect shale reservoir exploitation. This study investigates the effect of ScCO2 immersion on the mechanical behavior and microstructure of shale with inclined layering. The orientations α of the bedding plane with respect to the horizontal direction are 0°, 30°, 45°, 60°, and 90°. Mechanical experiment results indicated that the compressive strength of shale decreased, while the elastic moduli increased in ScCO2 immersion. The compressive strength exhibited the same variation tendency, while the elastic modulus exhibited the opposite change trend with increasing α before and in ScCO2 immersion. ScCO2 alters the mechanical properties of shale through pressure, adsorption, and dissolution; the dominant factor enhancing the elastic modulus is adsorption. X-ray diffraction and low-pressure nitrogen adsorption experiments showed that the influence of ScCO2 on the microstructure of the layering was more significant than the influence on the matrix; ScCO2 mainly broadened the micropores in the matrix, while transforming the mesopores and macropores in the bedding plane. This study provides the theoretical basis for the development of shale reservoirs using ScCO2.

Experimental of the influence of mineral composition and normal stress on the frictional slip of shale

Natural fractures are widely distributed in shale reservoirs. During the hydraulic fracturing process, frictional slip occurring on natural fractures can increase the reservoir permeability and is of great significance to improve the efficiency of reservoir stimulation. Shale contains a large amount of clay and organic matter, and its frictional behavior is different from that of other previously studied lithologies. In this paper, the frictional behavior of shale is analyzed, and the results show that the frictional behavior is controlled by the content of clay and organic matter. As the content of clay and organic matter increases, the micro support type transforms from the particle support mode by hard quartz mineral to matrix support mode by plastic clay and organic matter. Accordingly, the shear strength and friction coefficient of shale both decrease, and the shear type transforms from brittle to plastic. When the content of clay and organic matter is low, the asperity of friction surfaces will break in a brittle manner and the wear degree of surfaces is low. Therefore, fractures are still featured by moderate apertures after friction. The lower the content of clay and organic matter is, the easier the asperity of crack surfaces supports themselves, and the higher the fracture residual permeability is. Thus, promoting shear slip is the main measure of reservoir stimulation. However, when the content of clay and organic matter is high, the remaining post-slip fracture aperture is small. It is difficult to increase reservoir permeability through the frictional slip of natural fractures, and in this situation, the proppant support efficiency needs to be improved.

The temperature and mechanical damage investigation of shale with various dielectric properties under microwave irradiation

Microwave stimulation technology is a potential effective method to improve shale gas recovery. It is very important to study the influence factors of microwave heating and evaluate the effect of microwave heating on promoting shale microstructure development. Infrared thermal imaging technology was used to study the influence of shale permittivity, saturation and microwave irradiation (MI) parameters on shale heating. The results show that the larger the permittivity is, the faster the heating rate of the sample is and the higher the final temperature is. Meanwhile, it was further confirmed that the contents of pyrite and siderite are positively correlated with the dielectric constant of the samples. The shale with higher saturation has better heating effect under the same microwave heating condition. In addition, when the microwave is used to heat the samples, there is a threshold effect of microwave power. When the microwave power is larger than the threshold, the samples will be heated rapidly. However, when the microwave power increased to a certain extent, better heating effect and higher microwave energy utilization efficiency can be obtained by extending the heating time rather than increasing the microwave power. The SEM images analysis shows that when the thermal stress are greater than the cementation strength between the minerals, micro-fractures will be produced. By analyzing the stress-strain curves of shale samples with different saturation, it is found that the peak stress and elastic modulus of the samples all decreased in different extent. In conclusion, the experimental results show that microwave heating can effectively improve the temperature of shale reservoir, weaken the shale strength, and induce effective fractures. Therefore, this technology has certain application potential in shale gas recovery.

Experimental Investigation of the Anisotropic Evolution of Tensile Strength of Oil Shale Under Real-Time High-Temperature Conditions

Under natural conditions, oil shale with abundant kerogen and bedding planes exhibits obvious mechanical anisotropy. High temperature affects significantly the tensile strength and microstructure of oil shale; therefore, studying the tensile strength of oil shale at different temperatures is necessary. In this study, the tensile strengths σt-par, σt-per, and σt-orth in the temperature range 20–600 °C were obtained by loading parallel to bedding, perpendicular to bedding, and orthogonal to bedding, respectively, and the changes in the morphology of the oil shale at the same position were obtained as well. The results show that before the temperature reached 400 °C, the σt-par, σt-per, and σt-orth decreased rapidly, except in the temperature range 100–200 °C; however, the σt-par did not change much and the values of σt-per and σt-orth increased. When temperature exceeded 400 °C, the values of σt-par and σt-orth decreased slowly, while that of σt-per increased rapidly. The change in surface morphology indicates that thermal action led to the closure of primary pores and fractures and the formation of new pores at less than 300 °C. When temperature exceeded 300 °C, several cracks appeared, and the form of thermal cracking was inter-granular fracture. The thermal effect also affected the fracture types of the oil shale after failure. In the temperature range 20–300 °C, the fracture type was a multiple fracture type, in which the main crack was a through central fracture. When temperature exceeded 300 °C, the central crack changed into a non-central crack with increasing temperature. These results are important to determine the fracturing parameters in the process of in situ oil shale exploitation.

Observations of breakage for transversely isotropic shale using acoustic emission and X-ray computed tomography: Effect of bedding orientation, pre-existing weaknesses, and pore water

Shale is typically an anisotropic material for which the mechanical characteristics can vary significantly along different orientations with regard to its bedding planes. In underground formations, the strength of shale shows strong dependence on the pre-existing weaknesses and the water content. This study is concerned with the mechanical behavior of Marcellus shale specimens under three-point bending where loading is applied in different directions with respect to its bedding plane orientation under both dry and saturated conditions. Acoustic Emission (AE) is recorded in order to investigate the nucleation of microfracturing via AE locations and Moment Tensor Analysis (MTA). Complimentary to the AE detection and analysis, X-ray Computed Tomography (CT) is utilized to image internal damage sustained due to loading. Experimental results show lower nominal tensile strength when bending loads are applied parallel to bedding in dry samples. Further comparisons between tests under different conditions point to the strong impact of the pore water and pre-existing weaknesses on the samples. Detailed results from MTA show tensile microfracturing is the dominant crack type. Surprisingly, event locations show a large number of AE events occur in areas of the beam that are nominally subjected to compressive stress, implying that fracturing can be strongly impacted by local stress fluctuations which may result in opening of the interfaces between bedding layers even when the nominal stress in an area is compressive. In several cases, AE associate with visible fractures in the CT scan which are not connected to the main rupture surface through the specimen. Additionally, visualization of the internal fracture paths using image segmentation on the CT results show more complex fracture systems in parallel loading cases, which brings together localized fractures generated by different mechanisms.

Study on the Microscopic Fracture Process and Acoustic Emission of Shale Based on Digital Image

The Niutitang Formation shale is often filled with calcite minerals, which significantly affects the physical and mechanical properties of shale reservoirs. To correctly understand the microscale fracture characteristics of the Niutitang Formation shale and the evolution of acoustic emission signals, this paper uses digital image processing technology to characterize the geometric characteristics and nonuniform distribution of calcite minerals in the shale at the microscale and then maps it to finite elements; uniaxial compression tests of different calcite vein inclination angles are carried out on a microscale. The results show that under the microscale structure, the changes in compressive strength and brittleness index of the Niutitang Formation shale with different calcite vein dip angles are all N-shaped. The calcite veins affect the distribution of the stress field, leading to significant differences in the shale fracture process and fracture mode. The shale fracture process can be divided into two types. The first type (0°, 15°, 30°, 45°) is that the shale matrix is destroyed first, and then, the calcite veins are destroyed; the second type (60°, 75°, 90°) is that the calcite veins are destroyed first, and then, the shale matrix is destroyed. Shale fracture modes can be divided into w-type, v-type, inverted v-type, and inverted z-type. The inclination angle of calcite veins has a significant influence on the AE evolution characteristics of the Niutitang Formation shale. According to the characteristics of the AE active period, it can be divided into two types: surge type and step type. The surge type has a short active period, the number of AE count surges is small, the AE peak is large, and the failure mode is relatively simple. The step type has a long active period, the number of AE count surges is large, and the AE peak is small, and the failure mode is relatively complicated. The research results provide important theoretical guidance for shale gas fracturing mining.

Study on anisotropy of Longmaxi shale using hydraulic fracturing experiment

Hydraulic fracturing reservoir reconstruction technology is crucial in the development of shale gas exploitation techniques. Large quantities of high-pressure fluids injected into shale reservoirs significantly alter compressional (P) and shear (S) wave velocities, rock mechanical parameters, and anisotropic characteristics. In this study, differentiated hydraulic fracturing petrophysical experiments were carried out on Longmaxi Formation shale under pseudo-triaxial stress loading conditions. The effects of stress loading methods, and water-rock physical and chemical reactions on P- and S-wave velocities and rock mechanical parameters were compared. The experimental results showed that isotropic stress loading may increase the P- and S-wave velocities and Young’s modulus of dry shale kldnsample. Furthermore, it may lead to a weakening of the corresponding anisotropy. In contrast, differential stress loading was able to improve the anisotropy of Young’s modulus and accelerate the decrease in the compressive strength of shale in the vertical bedding direction. The water-rock physical and chemical reactions prompted by hydraulic fracturing was found to “soften” shale samples and reduce Young’s modulus. The influence of this “soften” effect on the compressional and shear wave velocities of shale was negligible, whilst there was a significant decrease in the anisotropy characteristics of Thomsen parameters, Young’s modulus, and Poisson’s ratio. The negative linear relationship between the Poisson’s ratios of the shale samples was also observed to lose sensitivity to stress loading, as a result of the “soften” effect of fracturing fluid on shale. The results of this study provide a reliable reference point and data support for future research on the mechanical properties of Longmaxi shale rocks.

The influences of super-critical CO2 saturation on tensile characteristics and failure modes of shales

To investigate the mechanism of super-critical CO2 saturation on the tensile properties of shale, shale specimens were saturated with Sc-CO2, and inert gas (Ar) was employed as control group. Brazilian splitting experiment with different layering inclinations (0°, 30°, 60°, 90°) showed that after Sc-CO2 saturation, the Brazilian splitting strength (BSS) and Brazilian splitting modulus (E) of shale were decreased by 11.9% and 5.7%, respectively. Regardless of any direction of stress loading, the bedding planes of shale had been damaged. The failure mode of shale transformed from single tensile to tensile-shear mixed failure after Sc-CO2 saturation and less energy was required for shale failure. The weakening of carbonate cementation made the bedding planes more prone to damage, and the number of shear cracks after Sc-CO2 saturation was twice as much. The X-ray diffractometer, scanning electronic microscope and nuclear magnetic resonance results showed that the increase of porosity caused by calcite dissolution and organic matter extraction was the fundamental reason for the deterioration of shale tensile properties. Furthermore, gas pressure made shale more compact, and also induced new cracks formation in the meanwhile. Therefore, after Ar saturation, E and BSS of shale were increased and decreased, respectively.

Study on the Damage Evolution Process and Fractal of Quartz-Filled Shale under Thermal-Mechanical Coupling

Filling of brittle minerals such as quartz is one of the main factors affecting the initiation and propagation of reservoir fractures in shale fracturing, in order to explore the failure mode and thermal damage characteristics of quartz-filled shale under thermal-mechanical coupling. Combining the theory of damage mechanics and thermoelasticity, RFPA2D-Thermal is used to establish a numerical model that can reflect the damage evolution of shale under thermal-solid coupling, and the compression test under thermal-mechanical coupling is performed. The test results show that during the temperature loading process, there is a temperature critical value between 60°C and 75°C. When the temperature is less than the critical temperature, the test piece unit does not appear obvious damage. When the temperature is greater than the critical temperature, the specimen unit will experience obvious thermal damage, and the higher the temperature, the more serious the cracking. Under the thermal-mechanical coupling of shale, the tensile strength and elastic modulus of shale show a decreasing trend with the increase of temperature. The failure modes of shale under thermal-solid coupling can be roughly divided into three categories: “V”-shaped failure (30°C, 45°C, and 75°C), “M”-shaped failure (60°C), and inverted “λ”-shaped failure (90°C). The larger the fractal dimension, the more complex the failure mode of the specimen. The maximum fractal dimension is 1.262 when the temperature is 60°C, and the corresponding failure mode is the most complex “M” shape. The fractal dimension is between 1.071 and 1.189, and the corresponding failure mode is “V” shape. The fractal dimension is 1.231, and the corresponding failure mode is inverted “λ” shape.

Chemical-Mechanical Coupling Effects on the Permeability of Shale Subjected to Supercritical CO <sub>2</sub>-Water Exposure

The permeability of shale reservoir rock and caprock is the key parameters influencing the shale gas production and the storage security of CO2. In this study, the ScCO2-water exposure induced chemical-mechanical coupling effects on the permeability of shale was analyzed by a systematic research, including mineralogical analysis, pore structure analysis, uniaxial compression and permeability test at different stress and injection pressure of shale before and after ScCO2-water exposure. After ScCO2-water exposure, the pore volume and average pore size of shale increased, and the initial permeability of shale increased. At the stressed condition, the porosity and permeability of CO2-water treated shale sample is lower than the untreated shale sample. The uniaxial compressive strength and elastic modulus of shale were decreased after ScCO2-water exposure. The shale permeability was increased with the increase of injection pressure, and decreased with the increase of confining stress. The stress sensitivity of permeability in shale is stress-dependent, a higher effective stress corresponding to a lower stress sensitivity of permeability. The damage in the permeability (Δkd) of shale caused by the ScCO2-water exposure is also depending on the confining stress and injection pressure, at a higher effective stress condition, the damage in the permeability of shale is more significant.

Effects of Bedding Geometry and Cementation Strength on Shale Tensile Strength Based on Discrete Element Method

To study the effects of anisotropy and heterogeneity on the shale failure mode and tensile strength, Brazilian splitting tests were performed from both directions of the bedding and layer thickness. Layers containing different bedding and loading angles and layer thicknesses were obtained separately. The results show that, at 0° and 90° angles, the shale cracks grow “linearly”; at 15°, the shale cracks have “arc type” growth; and at 30°–75°, the shale‐splitting displays “broken line” crack propagation. The tensile strength from 0° to 90° exhibits an increasing trend. Water has a significant softening effect on the tensile strength of shale—the higher the water content, the lower the tensile strength. In addition, a 3DEC numerical simulation was used to simulate the tests, establishing shale specimen particles with random blocks. In the shale disc, uneven parallel bedding and uniform parallel bedding were set up with different loading angles and layer thicknesses to generate simulated stress‐displacement curves, and the effect of layering on shale cleavage was analyzed from a mesoscopic perspective. The tensile strength of shale with uniform parallel bedding was found to be higher under the same conditions, which is consistent with the experimental results. By comparing the experimental and simulation results, from both the macro‐ and mesoperspectives, the Brazilian splitting crack growth of shale is affected by bedding, displaying a process from disorder to order. This study is of great significance for further exploration of the mechanical properties of shale under loading failure.

Residual shear strength of shales from the Gemas Formation based on ring shear tests

Fresh, dark grey to black, shales with minor sandstones of the Triassic Gemas Formation outcrop at the slope cut between Km 81.30 and 81.05 (southbound) of the North-South Expressway near Ayer Hitam in Johore State. The beds strike 165o with eastward dips of 35o to 43o and have thickness of between 0.2 and 1.5 m. Over-lying the fresh bedrock along an irregular boundary is a bleached zone, some 14 to 18 m thick, comprising light grey to white, in situ oxidized shales and sandstones. Three samples were collected from a thick shale bed; fresh samples A and B at 18 m and 15 m depth, respectively, and a bleached shale sample C at 11 m depth. All samples were air dried, finely ground, and then passed through a wire mesh sieve with 180 μm aperture sieve. The remoulded samples were tested with the Bromhead ring shear apparatus, employing the pre-shearing test procedure with multi-stage loading. Plots of shear stress versus cumulative horizontal shear displacement under low normal stresses (<150 kPa) yield failure envelopes with residual friction angles of 27.4o, 27.4o and 27.9o for samples A, B, and C, respectively. Under moderate to large normal stresses (150-350 kPa), plots of shear stress versus linear displacement sometimes slope upward due to increased friction as a result of sample extrusion and settlement of the top platen. It is concluded that the pre-shearing test procedure with multi-stage loading is best suited for low to moderate, effective normal stresses (<250 kPa). Based on the test results, the residual friction angle for shales from the Gemas Formation is between 27o and 28o.

Study on the Microscale Tensile Properties of Lower Cambrian Niutitang Formation Shale Based on Digital Images

Tensile strength is an important parameter that affects the initiation and propagation of shale reservoir fractures during hydraulic fracturing. Shale is often filled with minerals such as calcite. To explore the effect of calcite minerals on the tensile strength and failure mode of shale, in this paper, lower Cambrian shale cores were observed by microslice observations and core X-ray whole-rock mineral diffraction analysis, and 7 groups of numerical direct tensile tests were performed on simulated shale samples with different azimuth angles. The test results show that as the azimuth angle α increases, the tensile strength of the samples gradually decreases, and the fracture rate also shows a decreasing trend. The failure modes can be summarized as root-shaped (0° and 15°), step-shaped (30 and 45°), fishbone-shaped (60°), and river-shaped (75° and 90°) fracturing. The smaller the azimuth angle α, the easier it is for hydraulic fractures to propagate along the direction of the calcite veins and inhibit the formation of fracture networks in the shale matrix. Considering the correlation between the acoustic emission characteristics and failure mode, the fractal dimension is used to reflect the microscopic failure mode of shale. The larger the fractal dimension, the higher the fracture rate is, the more microcracks exist at the edge of the main crack, the more severe the internal damage is, and the more complex the failure mode of the sample is. As the azimuth angle α increases, the fractal dimension shows a decreasing trend, and the crack becomes smoother. This research has important reference value for the study of hydraulic fracture initiation mechanisms and natural fracture propagation.

Mechanical characterization of Laffan and Nahr Umr anisotropic shales

Geomechanics-related wellbore instability has become a major source of non-productive time in highly deviated wells drilled in oil and gas fields in offshore Abu Dhabi. These wells are drilled through two highly anisotropic shale formations, namely the Laffan shale formation and the Nahr Umr shale formation. Most of the models used in the oil and gas industry do not account for the strength and elastic anisotropy of shale. Therefore, a laboratory study was conducted to examine the strength and elastic anisotropy of shales using uniaxial compression tests and triaxial compression tests.Jaeger's Plane of Weakness (JPW) model was used to understand the anisotropic failure behavior of highly laminated shales of the Laffan and Nahr Umr formations. This model assumes that for an anisotropic rock, there exists a plane of weakness that has strength properties (cohesion and angle of internal friction) different than the strength properties of the intact rock. By minimizing the Root Mean Square Error (RMSE), the experimental strength values of the samples, as measured at different orientations, were fitted to the JPW model.Elastic moduli were also measured on these shales, as a function of orientation angle. The results showed that the moduli vary with angle according to the expected tensor transformation law. Therefore, the transverse isotropy assumption is a reasonable model to be used when dealing with these laminated sedimentary rocks.

Mechanism of wellbore instability in continental shale gas horizontal sections and its water-based drilling fluid countermeasures

The wellbore instability in the horizontal sections of continental shale gas wells in the Ordos Basin is a major engineering and technical problem that restricts the exploration and development of the shale gas resources in the Yanchang Formation of Upper Triassic of Mesozoic in the Yanchang Block. To solve this problem, this paper analyzed the characteristics of mineral components in shale by means of X-ray diffraction. In addition, its physical and chemical characteristics, specific surface area and microstructure were analyzed. On this basis, a shale water-based drilling fluid system (PSW-2) of low free water activity based on nano plugging was developed. And it was applied on site at five horizontal wells in continental shale gas reservoirs to ensure the wellbore stability of their long horizontal sections. And the following research results were obtained. First, the Yanchang Formation shale in this area has a high content of clay mineral, and it is a fractured formation of weak expansion, easy dispersion and multiple bedding, so the wellbore instability here is the result of the comprehensive action of mechanical factors, physical and chemical factors, drilling mechanical disturbance, etc. Second, the shale has average pore diameter is 4.494–8.502 nm and is characterized by obvious capillary action, strong water absorption capacity and uneven hydration, which result in the decrease of local shale strength, so sudden collapse tends to happen easily. Third, the API loss of PSW-2 system is less than 2.8 mL, the rolling recovery rate is 95.15% (close to the recovery rate of oil-based drilling fluid 98.25%), the linear expansion rate is as low as 1.38%, and the wetting angle increases from 26° of dry rock samples to 56.5°. Fourth, the compressive strength of the system after immersion increases to 95.806 MPa, which is close to the original rock strength (110.70 MPa). It is concluded that this water-based drilling fluid system can effectively ensure wellbore stability by blocking the pores of shale with micro nano components, reducing the activity of liquid phase to increase the inhibition, and weakening the multi-element collaboration of the capillary imbibition effect of shale.

Coupled Damage-Plasticity Modelling of Saturated Shale under Undrained Condition

Shale is classified as a poroelastic material as the shale matrix has comparable stiffness with its constituent solid grains. Also, shales at large depth are often fully saturated. Due to their low permeability, which can be in the nano-Darcy range, shales remain undrained during typical loading conditions. Additionally, shales are quasi-brittle materials which can exhibit very pronounced peak shear stress followed by pronounced strain softening during shearing. All these unique aspects of shale behavior must be considered in developing a model that predicts the change of the pore pressure, the effective stress response and the undrained shear strength of shales during loading. This study presents a coupled plastic-damage constitutive model which considers poroelastic effect with the objective to predict the mechanical behavior of saturated shales under undrained condition. The proposed model couples the plastic mechanism, which is formulated on the effective stress space, and the damage mechanism which is responsible for the elasticity degradation and the post-peak softening of shale. Validation of the proposed model against triaxial est results demonstrates that the proposed model can predict the stress-strain behavior, the effective stress path and the pore pressure evolution of shale specimen during shearing.

Microstructure and mechanical properties alterations in shale treated via CO2/CO2-water exposure

CO2 injections into shale reservoirs can enhance the recovery of oil and gas, while simultaneously sequestrating CO2. However, with the injection of CO2, the microstructure and mechanical properties alteration of shales caused by CO2/CO2-water exposure is should be noticed, which is of great importance to predicting the efficiency of CO2-enhanced shale oil/gas recovery and safety of CO2 sequestration in shale reservoirs. Therefore, this study clarifies the influence of sub/super-critical CO2-water exposure on the microstructure and mechanical properties of shale. Results show that after Sub/ScCO2 and Sub/ScCO2 water exposure, the contents of carbonate and clay in shale were decreased. Additionally, the proportion of micropores and mesopores were decreased, while the proportion of macropores was increased, leading to the increase of porosity and the improvement of pore connectivity, and the decreases of the pore surface fractal dimensions D1 and pore volume fractal dimensions D2 in shale. The uniaxial compressive strength (UCS) and elastic modulus (E) of shale were also decreased. Acoustic emission (AE) analysis results indicated that the cracks closure stage and stable crack propagation stage was prolonged and shorten, respectively, and AE signals show obvious multifractal characteristics. ScCO2 induced greater alteration in pore structure and mechanical properties than SubCO2, and the alterations were enhanced for CO2-water exposure. UCS and E of shale samples show a good negative correlation with porosity, and a positive correlation with D1 and D2. UCS in shale is positively related to the proportion of micropores and mesopores, and negatively related to the proportion of macropores, which indicated that microstructure alterations inevitably affected the macroscopic mechanical properties of shale.

A modified Hoek-Brown failure criterion for unsaturated intact shale considering the effects of anisotropy and hydration

The mechanical strength of shale is highly influenced by anisotropy and hydration, which cannot be ignored for the stability evaluation of the geotechnical structures related to shale after water invasion. In the present work, a modified Hoek-Brown failure criterion for unsaturated intact shale considering the effects of anisotropy and hydration has been developed in order to obtain an accurate strength evaluation for unsaturated intact shale. In this proposed failure criterion (abbreviated as the proposed criterion), the effect of anisotropy on shale strength is determined by a piecewise functions for uniaxial compression and the parameter kβ added in Hoek-Brown (HB) failure criterion for triaxial compression condition, and the effect of incomplete hydration on shale strength is regarded as an effective stress decrease by incorporating a discount function related to moisture content. The proposed criterion is defined by 8 parameters, including a material parameter of HB failure criterion (m), 4 material parameters (ξ, η, R and r) describing the hydration effect and 3 model parameters (g, h and kβ) describing the anisotropy effect. The physical meanings and determination procedures of these parameters are described. The predicted values of the proposed criterion agree well with the experimental data, showing that R2 = 0.966 and AAREP (average absolute relative error percentage) = 4.393% for all the predicted results. The proposed criterion is then compared with other two failure criteria which consider the effects of water invasion and anisotropy on rock strength, showing that the proposed criterion has higher accuracy and better stability. The error of the predicted values for the proposed criterion compared with the tested results increases for the shale specimen for which β (bedding plane orientation relative to the axial loading in degrees, °) is closer to 30°, is bigger for the shale specimen with a higher moisture content, and is bigger for the shale specimen under a higher confining pressure. The sensitivity analysis of the 4 hydration parameters on the predicted results shows that the predicted values are more sensitive to R and r than ξ and η. This proposed criterion can improve the reliability for the stability evaluation of the geotechnical structures related to shale after water invasion.

Influence of water content on mechanical behavior of basalt in Baihetan Hydropower Station, China

Water content is a critical factor affecting rock strength in numerous geotechnical engineering applications. This study experimentally investigates the influence of water content on the mechanical behavior of basalt in the Baihetan Hydropower Station. Uniaxial compression tests were performed on basalt specimens at five moisture content levels. The strength evolution of shale and sandstone specimens with different water content levels were also analyzed through an extensive literature review. The results show that an increase in water content can exacerbate the initial damage to the basalt specimens, increasing the nonlinearity in the initial deformation stage of the stress-strain curve. As the water content increases, both uniaxial compressive strength and Young’s modulus gradually decrease, but Poisson’s ratio remains almost constant. Splitting failure is the dominant mode for all tested specimens. Because of the differences in pore structures and mineral compositions, the mechanism of water-weakening on the compressive strength of shale, sandstone, and basalt is different. The negative exponential function provides a good representation of the compressive strength versus water content of shale and sandstone, but unsuitable for describing that of basalt.