Bedding Angle Research Articles

Investigation of anisotropic strength criteria for layered rock mass

Layered rock mass is a type of engineering rock mass with sound mechanical anisotropy, which is generally unfavorable to the stability of underground works. To investigate the strength anisotropy of layered rock, the Mohr-Coulomb and Hoek-Brown criteria are introduced to establish the two transverse isotropic strength criteria based on Jaeger's single weak plane theory and maximum axial strain theory, and parameter determination methods. Furthermore, the sensitivity of strength parameters (K1, K2, and K3) that are used to characterize the anisotropy strength of non-sliding failure involved in the strength criteria and confining pressure are investigated. The results demonstrate that strength parameters K1 and K2 affect the strength of layered rock samples at all bedding angles except for the bedding angle of 90° and the angle range that can cause the shear sliding failure along the bedding plane. The strength of samples at any bedding angle decreases with increasing K1, whereas the opposite is for K2. Except for bedding angles of 0° and 90° and the bedding angle range that can cause the shear sliding along the bedding plane, K3 has an impact on the strength of rock samples with other bedding angles that the specimens' strength increases with increase of K3. In addition, the strength of the rock sample increases as confining pressure rises. Furthermore, the uniaxial and triaxial tests of chlorite schist samples were carried out to verify and evaluate the strength criteria proposed in the paper. It shows that the predicted strength is in good agreement with the experimental results. To test the applicability of the strength criterion, the strength data of several types of rock in the literature are compared. Finally, a comparison is made between the fitting effects of the two strength criteria and other available criteria for layered rocks.

Investigation of Damage and Creep for Bedding's Carbonaceous Slate with Chemical Erosion Effect.

Tunnel projects in the southwestern mountainous area of China are in full swing. According to the tunnel burial depth, structural characteristics, and chemical erosion environments of the Lixiang railway tunnel, carbonaceous slate specimens obtained in the field were taken to experimentally investigate the physical, mechanical, and creep characteristics of the bedding's slate specimens after chemical erosion. The results of scanning electron microscopy (SEM) indicate that chemical erosion leads to internal damage in the carbonaceous slate specimens, and the internal damages are increasing with the increase of erosion days. Moreover, the specimens' ultrasonic test (UT) results prove that specimens with smaller bedding angles suffer a more serious erosion and induce more internal cracks. Under conventional triaxial compression conditions with 40 MPa of confining pressures, the conventional triaxial compressive strength (σs) decreases with the decrease of the bedding angle and the increase of erosion days, and the failure modes of the specimens are mainly controlled by oblique shear fractures and accompanied by the occurrence of slip dislocation fractures between the bedding inclination. Under creep conditions with 40 MPa of confining pressures, the final deformations of specimens are increasing with the increase of erosion days, which means the longer the erosion days, the greater the deformations. The failure modes of the specimens under creep conditions are controlled by shear fractures, and for the specimen with a 60° bedding angle and long-term erosion, there are block separations and many cavities along the shear planes. Therefore, more attention should be paid to prevent serious failure of the surrounding rock if the surrounding rock has a bedding angle of 60° or suffers long-term erosion.

Mechanical properties and failure characteristics of anisotropic shale with circular hole under combined dynamic and static loading

In deep hard rock mines, the drilling and blasting method is mainly used for mining, which results in a complex environment of high static stress and frequent dynamic disturbance in some underground engineering surrounding rocks for a long time. In addition, a large number of underground engineering are in the geological tectonic zone with poor stability, which often triggers rock disasters when stress waves propagate to the bedding. Therefore, it is significant to study the failure process and mechanism of underground engineering in bedded rock masses under combined dynamic and static loading. In this study, a combined dynamic and static impact experiment was conducted on circular-hole-containing shale specimens with different bedding angles using a modified split Hopkinson pressure bar (SHPB) device, and the damage process was recorded using Digital Image Correlation (DIC) technique. The experimental results show that the stress-strain curves of shale specimens under combined dynamic and static conditions include three stages elasticity, yielding, and failure. With the increase of the bedding angle, the dynamic strength and Young's modulus of the specimens showed a trend of decreasing and then increasing, with obvious anisotropy. However, with the increase of pre-static load, the dynamic strength and Young's modulus of the specimens showed a tendency to increase first and then decrease. In addition, repeated refraction (transmission) and reflection of the stress wave between the beddings and the stress concentration phenomenon of the circular holes resulted in different failure characteristics of the specimens with different bedding angles.

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.

Influence of Bedding Strength and Angle on Fracture Characteristics of Sandstone under Three-Point Bending Conditions

The fracture characteristics of bedded sandstone determine the stability and safety of in situ coal gasification technology. Four semicircular stratified sandstone specimens with different strengths (0.3, 0.6, 1.0, and 1.5 times that of rock matrix) and seven different bedding angles (θ = 0°, 15°, 30°, 45°, 60°, 75°, and 90°) were numerically simulated using RFPA2D-Basic V2.0 software. The SCB specimen had no prefabricated crack, and its radius was 25 mm. The loading rate was 0.000001 m/step. The results show that the fracture characteristics of the sandstone are affected by both the strength of the laminae and the angle; the fracture toughness and peak strength of the ultra-weak sandstone, as well as the weak sandstone, are reduced and more easily affected by the bedding angle; the strength rate of the strong sandstone is higher than that of the homogeneous sandstone, and the difference between the fracture characteristics of the two is not significant. This paper suggests that the key mechanism of this phenomenon is the anisotropy between the bedding and the sandstone, along with the competition/synergy between the main crack and the bedding plane bias crack during fracture propagation. These research results can provide a theoretical reference for the safety and stability of underground engineering in China.

Crack deflection in shale-liked layered rocks under three-point bend loading

The bedding angle has a significant influence on the fracturing of anisotropic geomaterials, such as shale rock, and crack deflection usually takes place during crack propagation. To better understand and estimate the tensile crack deflection in shale, 35 large notched deep beam (NDB) specimens with different bedding angles—0° (Short-Transverse), 15°, 30°, 45°, 60°, 75°, and 90° (Arrester)—were tested under three-point bend (3PB) loading. For mode I dominated loadings, significant expected anisotropies in both fracture toughness and crack deflecting behavior were observed. The nominal mode I fracture toughness showed a progressive decrease from the arrester to the short-transverse orientation, with values from 1.371 MPa·m1/2 to 0.731 MPa·m1/2. When the bedding angle was less than or equal to 60°, the crack deflection induced the difference between the nominal and actual fracture toughness. By introducing the weak plane model to describe the fracture toughness of shale, an anisotropic K-type maximum tangential stress criterion (KMTS criterion) was presented. Using the KMTS criterion, the deflection angle and fracture load for an arbitrary bedding angle could be obtained conveniently with fracture toughness values in both arrester and short-transverse orientations. The comparison between theoretical predictions and experimental data demonstrated that the KMTS criterion could provide a dependable prediction, particularly for the slight deflection when the crack propagates within the matrix.

Experimental study on the shear failure of layered rock bridges

The structural face of rock plays a decisive role in the stability of geotechnical engineering, and the mechanism of failure destabilization of layered rock masses is still unclear. By preparing rock-like materials, shear failure tests on layered rock masses were carried out under different bedding dip angles, different rock bridge width, different normal forces, and forward and reverse shear effects. With the increase in the rock bridge width, the influence of the joint inclination angle on the shear failure process of the rock sample decreases; the peak shear strength of the specimen is the largest when the joint inclination angle is 60°, followed by 90°, 30° and 0° from large to small; the failure mode in forward shear is mainly tensile failure, and the failure mode in reversed shear is mainly shear failure; the peak number of acoustic emission events is proportional to the joint inclination angle and the rock bridge width of the specimen, and the number of acoustic emission events is proportional to the joint inclination angle and the rock bridge width of the specimen. The number of acoustic emission peak events is proportional to the bedding angle of the sample and the width of the rock bridge. The peak number of acoustic emission events in the reverse shear of the layered rock mass is greater than that in the forward shear. The angle of the lamina and the width of the rock bridge are both important factors affecting the strength of the rock. The results of this study provide a basis for identifying the shear failure mechanism of rock bridge in layered rock masses.

Acoustic and mechanical tests of sandstone-shale composites in Songliao Basin and prediction of uniaxial compressive strength

Rock uniaxial compressive strength is a vital rock mechanics parameter, which accurate assessment is critical for high-efficiency unconventional reservoir development. However, its realization for standard cores is quite problematic due to difficulties and high costs related to the collection and preparation of samples. Unconventional fracturing treatment have evolved from single to composite lithology. This study collected shale and sandstone samples at different bedding angles (0°, 45°, 90°)to prepare the sandstone-shale composite samples at different interlayer ratios. It measured the prepared composites’ longitudinal wave and transverse wave velocities with ultrasonic velocity testing. The uniaxial compressive strength of the sandstone-shale composites with various bedding angles were tested via a servo rock triaxial testing machine. The anisotropic characteristics of the prepared sandstone-shale composites with various bedding angles were analyzed, and relation between the uniaxial compressive strength of the anisotropic sandstone-shale composite and P-wave velocity was derived and experimentally validated. The results show that: the wave velocity test results of the sandstone-shale combination show that with the increasing proportion of shale, the wave velocity of the longitudinal wave and the transverse wave both increase; The wave velocity results of different bedding planes in shale are as follows: Vp(0°)> Vp(45°)> Vp(90°). The average P-wave and S-wave velocities at 45° are 0.92, while the average P-wave and S-wave velocities at 90° are 0.85 compared to 0°; the P-wave and S-wave anisotropy of shale are both 0.14, and the shale shows significant anisotropy. In the sandstone-shale composite, the uniaxial compressive strength gradually decreases and the strain increases with the increase of shale proportion. The value of uniaxial compressive strength is UCS(0°) > UCS(90°) > UCS(45°). Based on the P-wave modulus, a fitting relationship formula for the uniaxial compressive strength of sandstone-shale composites with different bedding angles and interlayer ratios was established and verified. The anisotropic strength predictive formula established based on experimental data is simple, precise, and easy to obtain, with strong practicability. The present research results can provide proper technical support for sandstone-shale composite fracturing optimization and reservoir development.

Particle flow simulation of Brazilian splitting failure characteristics of layered shale

Shale reservoir is the research hotspot in the development of unconventional oil and gas resources. The properties of bedding plane have an important influence on the mechanical behavior of shale. According to the fracture characteristics of layered shale under Brazilian splitting load, the effects of bedding angle, bond strength ratio and natural fractures on the tensile strength and fracture pattern of layered shale in Brazilian splitting test were studied by particle flow code. The evolution mechanism of meso–macro fracture of shale is analyzed by combining the dynamic change process of particle mesoscopic force chain and crack. The results show that three types of fracture patterns are observed: splitting tensile failure of shale matrix, splitting tensile failure along bedding plane and tensile–shear composite failure along shale matrix and bedding plane. Mesoscopic cracks initiate at the top and bottom loading points of the disk. After the peak load, mesoscopic cracks propagate through and appear dense force chains until macroscopic failure occurs. The bond strength ratio of bedding plane affects the failure pattern and peak strength of the specimen. With the increase in bedding angle, the tensile strength decreases gradually. Considering the influence of natural fractures, the tensile strength of shale is lower, and the crack propagation is more complex. The research model and results provide theoretical basis for the mechanical properties evaluation and fracture pattern analysis of shale.

Experimental Study on the Mechanical Properties and Acoustic Emission Characteristics of Different Bedding High-Rank Coals.

Bedding has an important influence on the macroscopic and microscopic mechanical properties of coal, and the mechanical properties of coal and rock mass and acoustic emission characteristics are very important for rock burst monitoring and warning. In order to explore the influence of different beddings on the mechanical properties and acoustic emission characteristics of high-rank coal, using the RMT-150B electrohydraulic servo rock mechanics test system and the DS5 acoustic emission analyzer, the uniaxial compression and acoustic emission characteristics of high-rank coals with different beddings (parallel bedding 0°, oblique bedding 30, 45, 60°, and vertical bedding 90°) were investigated. The results show that the uniaxial compressive strength and deformation modulus of vertical stratified coal samples are the largest, which are 28.924 MPa and 2.95 GPa, while the average values of uniaxial compressive strength and deformation modulus of oblique stratified coal samples are the smallest, which are 10.91 MPa and 1.776 GPa, respectively. With the increase of the bedding angle, the uniaxial compressive strength of high-rank coal decreases first and then increases. The stress-strain process of coal varies greatly with different high stratification grades (parallel bedding 0°, oblique bedding 30°, 45°, 60° and vertical bedding 90°). The loading times of parallel, oblique, and vertical beddings are 700, 450, 370, 550, and 600 s, and the acoustic emission mutation point values are 495, 449, 350, 300, and 410 s. The mutation point value can be used as precursor information to judge the failure of high-rank coal in different beddings. Research results to find the high-rank coal destruction instability prediction method and the index provide a basis; further damage to the acoustic emission testing high order coal provides reference significance; and the use of acoustic emission in the monitoring and early warning of percussive ground pressure, the bedding surface of coal, and the actual stress on site should be considered.

Fracture damage characteristics of hard roof with different bedding angles induced by modified soundless cracking agents

Soundless cracking agents (SCAs), as an environment-friendly cracking material, are widely used to fracture the hard roof of coal seams. Under the long-term action of geological structure, the bedding characteristics of hard roof are obvious, and the roof bedding will affect the cracking effect of SCA. In order to study the fracture damage characteristics of hard roof with different bedding angles under SCA fracturing, four kinds of similar specimens with different bedding angles (0°, 30°, 45° and 60°) were prepared. The modified SCA fracturing experiments were carried out under the maximum principal stress, and ultrasonic measurement, acoustic emission (AE) monitoring and full-field strain measurement were performed to analyze the dynamic fracture process and damage characteristics of the samples. The results showed that the specimen with 45° (CL-3) had the largest damage and complex fracture morphology, and the 45° bedding angle was conducive to the fracture of the sample. The fracture of the specimens was dominated by tensile damage. With the increase of bedding angle, the cumulative AE energy released by fracture, fractal dimension and fracture density first increased and then decreased, and the distribution of the surface strain concentration area was more closely related to the bedding angle. The specimen fracture was divided into four stages based on the variation of AE parameters, and the percentage of tensile cracks in stage II decreased and that of shear cracks increased. With the increase of bedding angle, the damage degree of the specimen first decreased, then increased and then decreased. The damage value of sample CL-3 is 0.543, which is 7.73%, 29.1% and 12.89% more than that of samples CL-1 (0°), CL-2 (30°) and CL-4 (60°), respectively. As the weak structural surface of the sample, the bedding plane contributes to the generation of cracks under the action of modified SCA.

Impacts of bedding angle and cementation type of bedding planes on mechanical behavior of thin-layer structured bedded rocks under uniaxial compression

To investigate the mechanical behavior and crack evolution properties of the anisotropic bedded rocks, a series of uniaxial compression tests were conducted using bedded limestone, phyllite, and shale specimens, and the bedding angle α of specimens was designed as 0°, 15°, 30°, 45°, 60°, and 90°. The acoustic emission (AE) and digital image correlation (DIC) techniques were applied to monitor the crack evolution mechanism. The cementation types of bedding planes were also different, which greatly affected the mechanical properties of bedded rocks. In this study, the cementation types of bedding planes for limestone, phyllite, and shale were defined as embedded cementation, oriented cementation, and cracked cementation, respectively. The experimental results showed that with the increase of the α, the peak strength σucs, elastic modulus E, AE hit rate, cumulative AE hit and cumulative AE energy all exhibited a decreasing firstly and then increasing trend, and all showed obvious anisotropy. A novel trigonometric strength criterion for bedded rocks was developed. The fluctuation of the b-value indicated that the propagation of the internal micro-cracks was unstable, and showed an overall downward trend. Meanwhile, a tensile and shear crack classification criterion combining the AE parameters, clustering idea and genetic algorithm was proposed, and then the rational dividing line of tensile and shear cracks was determined in AF-RA maps. The α and cementation types both affected the failure mode of bedded rocks. For limestone with α = 0°– 90°, the longitudinal splitting, forming approximate vertical macro tensile cracks, was the primary failure mode, and local shear slipping along the bedding plane occurred in the specimens with α = 30°– 60°. For phyllite and shale, when α≤ 30°, the inclined macro-cracks were not along the bedding planes but showed a shear-dominated failure mode. When 45° ≤α≤90°, the main macro-cracks were along the bedding planes, and some macro-sub cracks penetrated the bedding planes in shale, showing shear-dominated failure for specimens with 45°≤α<90° and tensile-dominated failure for specimens with α=90°. The method of rating scores corresponding to the uniaxial compressive strength in RMR has been improved and an optimization method of the revised rating scores of bedded rocks in different situations considering α and σucs has also been proposed.

A Review of Mechanical Properties and Rockburst Investigation of Transversely Isotropic Rocks by Experimental Technique.

Under complex geostress caused by long-term geological evolution, approximately parallel bedding structures are normally created in rocks due to sedimentation or metamorphism. This type of rock is known as transversely isotropic rock (TIR). Due to the existence of bedding planes, the mechanical properties of TIR are quite different from those of relatively homogeneous rocks. The purpose of this review is to discuss the research progress into the mechanical properties and failure characteristics of TIR and to explore the influence of the bedding structure on the rockburst characteristics of the surrounding rocks. First, the P-wave velocity characteristics of the TIR is summarized, followed by the mechanical properties (e.g., the uniaxial compressive strength, the triaxial compressive strength, and tensile strength) and the related failure characteristics of the TIR. The strength criteria of the TIR under triaxial compression are also summarized in this section. Second, the research progress of the rockburst tests on the TIR is reviewed. Finally, six prospects for the study of the transversely isotropic rock are presented: (1) measuring the Brazilian tensile strength of the TIR; (2) establishing the strength criteria for the TIR; (3) revealing the influence mechanism of the mineral particles between the bedding planes on rock failure from the microscopic point of view; (4) investigating the mechanical properties of the TIR in complex environments; (5) experimentally investigating the rockburst of the TIR under the stress path of "the three-dimensional high stress + internal unloading + dynamic disturbance"; and (6) studying the influence of the bedding angle, thickness, and number on the rockburst proneness of the TIR. Finally, some conclusions are summarized.

Effect of particle shape and bedding angle on suffusion in gap‐graded granular soils by coupled CFD‐DEM method

AbstractThe natural deposition of soils results in significant fabric anisotropy, and its influence on suffusion remains unclear. In this study, a series of gap‐graded soil samples with different particle shapes and bedding angles are generated and implemented into the coupled computational fluid dynamics‐discrete element method (CFD‐DEM) simulations to investigate the migration of fine particles within the coarse grain matrix. Macroscopic results, including mass loss, void ratio variation, and soil deformation, as well as microscopic particle trajectory, force chain network, and contact orientation are analyzed. In addition, drained triaxial tests on samples before and after erosion are carried out. All results turn out that the greater the bedding angle, the more severe the loss of fines and the more significant the soil deformation. Three different migration characteristics can be identified according to soil anisotropy degree, that is, when the bedding angle is 0°, the migration of fine particles remains relatively stable; when the bedding angle is 30° and 60°, the particle migration shows a gradual change throughout the erosion; when the bedding angle is 90°, the fine particles migrate significantly in the very early stages of suffusion. Besides, the aspect ratio (AR) of the particle is found to be beneficial for both erosion resistance and soil strength.

Microscopic Study of Shale Anisotropy with SEM In Situ Compression and Three-Point Bending Experiments

The microscopic anisotropy of shale has an important impact on its mechanical properties and crack behavior, so it is essential to understand the microscopic origin of anisotropic growth with a more effective laboratory work scheme. Uniaxial compression test and three-point bending test are considered to be efficient means to study the elastic properties and crack behavior of rocks. In this paper, uniaxial compression experiments and three-point bending experiments were conducted on shale outcrops in the Changqing area using field emission scanning electron microscopy (SEM) and in situ tensile testing, and the microscopic deformation and crack processes were quantitatively characterized by the digital image correlation (DIC) method. For the compression experiments, the observation of the first principal strain fields indicated that the microscopic anisotropy of shale was related to the bedding planes, and the microscopic deformations were mainly concentrated in the clay mineral accumulation area and at the microcracks. Elastic moduli and compressive strengths of specimens with different bedding angles were affected by the strong shear stress effects. The specimens with a bedding angle of 30° showed lower peak loads and compressive strengths, and the specimens with a bedding angle of 60° had lower elastic moduli. Three-point bending experiments were conducted for studying the effects of crack-bedding orientation relationships on cracking processes, and four critical fracturing mechanical properties were calculated. The short transverse-type cases were prone to break and had a lower peak load, tensile strength, fracture toughness and elastic-bending modulus. The divider-type cases were more difficult to break, formed a more tortuous crack and had a higher tensile strength, fracture toughness and elastic-bending modulus. The arrester-type cases had a middle range of mechanical parameters but developed the longest cracks. This study provides a feasible experimental and analysis method for understanding the microscopic anisotropy of shale samples. The small specimen size also makes the requirements of core samples easier to be satisfied considering the field application. Furthermore, the anisotropy of cracking processes can be understood better by building the connections between microstructural characteristics and mechanical performances.

Creep Characteristics of Layered Rock Masses after Water Absorption Due to Structural Effects.

Affected by the "three highs and one disturbance" (high ground pressure, high ground temperature, high permeability pressure, and strong mining disturbance), deep layered rock mass roadways often display large deformations, resulting in accidents and disasters from time to time. This paper aims to study creep characteristics of layered rock masses after water absorption due to structural effects, combined with acoustic emission energy and dominant frequency value analysis. Experimental results show that as the water content decreases, the long-term strength of the rock sample increases, and the damage becomes more severe. Under the same water content state conditions, the rock samples with bedding angles of 0°, 30°, and 90° have high long-term strength and undergo severe failure, whereas rock samples with bedding angles of 45° and 60° have low long-term strength and undergo mild failure. Under the same water content, the initial energy release increases with the bedding angle. Under the same water content, the energy release during failure decreases first and then increases with the increasing bedding angle. The initial energy, the cumulative energy, the initial main frequency, and the main frequency at the time of failure tend to decrease with the increase in water content.

Experimental and Numerical Investigations of Fracture Behavior for Transversely Isotropic Slate Using Semi-Circular Bend Method

Slate with inherently transverse isotropy is abundant in metamorphic rock, in buildings, and in geotechnical engineering worldwide; the tensile and shear fracture behavior of layered slate is vital to know for engineering applications. In this paper, the Brazilian and semi-circular bend (SCB) tests of layered slate were performed. The fracture characteristics of the slate were investigated by numerical simulations developed by the hybrid finite and cohesive element method (FCEM). Results showed that the measured experimental tensile strength, and mode I fracture toughness of layered slate all showed a typical V-type trend as the bedding angle increased from 0° to 90°, and with divider type. The developed empirical relationship between tensile fracture toughness and tensile strength KIC = 0.094σt + 0.036 fitted experimentally and strongly correlated. The mechanical response and fracture patterns predicted by FCEM agreed well with those of the laboratory experiments. Moreover, the shear fracture behavior and mode II fracture toughness of the layered slate were explored by systematic numerical simulations. Research results provide potential insights for further prediction and improvement of the complex fracture behavior of anisotropic rock masses for rock engineering.

NMR experiments on the migration characteristics of water in bedding-bearing low rank coal

Coal seam water injection is an effective technique for dust reduction in coal mines. Imbibition is an important means of water transport in coal seams and is influenced by coal bedding. The spatial distribution of water in the seepage process was obtained, and the spontaneous seepage characteristics and water transport mechanisms of coals containing different layered angles were revealed by NMR. Results show that the bedding mainly serves as the water flow channels in the imbibition. When the angle increases, the imbibition rate of adsorption pores and seepage pores in the early stage decreases. The imbibition is firstly transported to the top quickly along the bedding flow channel, 1.5h for 0° bedding coal sample, 24h for 90°, when the water reached its apex, the curve no longer changed and imbibition proceeded slowly. With the increase of bedding angle, the imbibition equilibrium time increased from 3h to 30h, up to 9 times more, the maximum imbibition rate decreases significantly, reduced from 5.64 g/h to 1.13 g/h, the flow resistance increases, and the remaining capillary force after equilibrium increases from 0.007 Pa to 5.24 Pa. When the capillary force cannot overcome the flow resistance, the imbibition reaches equilibrium.

The invasion law of drilling fluid along bedding fractures of shale

In the process of drilling, the drilling fluid will invade into the bedding plane of shale under the action of pressure difference that will cause hydration collapse and wellbore instability. In order to ensure the wellbore stability during shale oil and gas drilling, it is necessary to clarify the invasion law of drilling fluid along bedding fractures during the drilling process. The immersion experiment method is often used to study the invasion law of drilling fluid, which is quite different from the actual invasion process of drilling fluid underground. In this paper, the depth of drilling fluid invasion into shale under different confining pressures and displacement times is intuitively and accurately determined by the displacement experiment and NMR scanning first. Also, then the mathematical relationships between drilling fluid invasion depth and invasion time, invasion pressure difference, confining pressure, bedding angle, and drilling fluid viscosity were established. The errors between the calculated values of the drilling fluid invasion depth and the experimental values were less than 15%, and the calculation accuracy was high. In addition to the influence of invasion time, formation pressure difference and confining pressure on invasion depth were researched through the method of numerical simulation. The results showed that the liquid invasion depth increased logarithmically with the increase of invasion time and formation pressure difference, but it grew slowly in the later period and tended to be stable; the invasion depth decreased exponentially with the increase of confining pressure, bedding plane angle, and drilling fluid viscosity. The results in the paper provide a basis for the subsequent determination of the collapse pressure and collapse period of bedding shale.

Mechanical aspects of semi-circular sandstone fractured specimens with U-notch in the presence of various bedding angles

In tunnel and underground structures, the deformation and damage of layered rock masses often exhibit substantial anisotropy, where the fracture pattern of layered rock masses containing notched cracks directly influences the construction stability of the project. By this virtue, this paper presents a theoretical formulation of the fracture toughness of anisotropic rock materials containing U-notches on the basis of the constitutive model of transversely isotropic materials. The anisotropic influence coefficients are also appropriately incorporated into the formulas of the point stress (PS) and mean stress (MS) criteria to obtain their corresponding modified relations. The fracture toughness (KIC) of sandstones with bedding angles is evaluated by utilizing semi-circular bend (SCB) and U-notched semi-circular bend (USCB) specimens. The theoretically obtained results and experimentally observed data display that the fracture toughness of the specimens gradually decreases with increasing bedding angle, irrespective of the notch tip radius. Further, the results of the mean stress criterion are closer to the experimental data in comparison with those of the point stress criterion. Additionally, the fracture damage patterns pertinent to the SCB and USCB sandstone specimens with various bedding angles are displayed and discussed. The achieved results could provide a solid basis for better realizing the engineering deformations and failure analyses of layered sandstones.