Bedding Angle Research Articles

Evaluation on the anisotropic brittleness index of shale rock using geophysical logging

The brittleness index plays a significant role in the hydraulic fracturing design and wellbore stability analysis of shale reservoirs. Various brittleness indices have been proposed to characterize the brittleness of shale rocks, but almost all of them ignored the anisotropy of the brittleness index. Therefore, uniaxial compression testing integrated with geophysical logging was used to provide insights into the anisotropy of the brittleness index for Longmaxi shale, the presented method was utilized to assess brittleness index of Longmaxi shale formation for the interval of 3155–3175 m in CW-1 well. The results indicated that the brittleness index of Longmaxi shale showed a distinct anisotropy, and it achieved the minimum value at β = 45°-60°. As the bedding angle increased, the observed brittleness index (BI2_β) decreased firstly and increased then, it achieved the lowest value at β = 40°–60°, and it is consistent with the uniaxial compression testing results. Compared to the isotropic brittleness index (β = 0°), the deviation of the anisotropic brittleness index ranged from 10% to 66.7%, in other words, the anisotropy of brittleness index cannot be ignored for Longmaxi shale. Organic matter content is one of the main intrinsic causes of shale anisotropy, and the anisotropy degree of the brittleness index generally increases with the increase in organic matter content. The present work is valuable for the assessment of anisotropic brittleness for hydraulic fracturing design and wellbore stability analysis.

Characterization of anisotropic mode II fracture behaviors of a typical layered rock combining AE and DIC techniques

Shear-sliding mode fracture, i.e., mode II fracture, is a very common rupture mode of layered rocks. To achieve a better understanding of the anisotropy in the real mode II fracture, which propagates in a self-similar manner, a series of direct shear tests were conducted on single-notched specimens made of shale with three bedding angles, i.e., β = 0°, 45° and 90°. The ranking of the anisotropic mode II fracture toughness (KIIc) with different bedding angles is KIIc,45° >KIIc,90° >KIIc,0°, which is different from that of the anisotropic mode II fracture energy release rate (GIIc), i.e., GIIc,90° >GIIc,45° >GIIc,0°, suggesting that, unlike isotropic rocks, the crack propagation resistance of a layered rock cannot be completely characterized by its fracture toughness. From the acoustic emission monitoring results, it is found that the fracture process initiates and accelerates from the notch front once the applied mode II stress intensity factor (SIF) reaches its first and second thresholds, KII, FPI (approximately 50%∼70% of the fracture toughness) and KII, FPA (approximately 85%∼95% of the fracture toughness), respectively; generally, both KII, FPI and KII, FPA show noticeable anisotropies. The results of the b value indicate that more small-scale events occur in the specimen with β = 0°, and large-scale events account for a greater proportion in the specimen with β = 45°. A dominant high strain zone along the prefabricated crack direction and a secondary high strain zone along the bedding planes were discovered. Combining the acoustic emission and digital image correlation results, three typical fracture mechanisms were observed: (1) shear fracture along bedding planes when shearing along beddings; (2) shear fracture in matrix and along bedding planes for β = 45°; and (3) shear fracture in matrix and tensile fracture of bedding planes when shearing perpendicular to the beddings.

Experimental Research on the Effect of Bedding Angle on the Static and Dynamic Behaviors of Burst-Prone Sandstone

Abstract In order to understand the mechanism of some unconventional failures such as rockburst caused by deep rock excavation, the failure characteristics of burst-prone sandstone specimens under static and dynamic loads were studied by using the MTS816 rock mechanics testing system and the split Hopkinson pressure bar (SHPB) experimental system, as well as the effects of bedding angle and impact pressure on rock mechanical properties and failure patterns. The uniaxial compression test used a cylindrical specimen with a height of 50 mm and a diameter of 100 mm, and cylindrical specimens with height and diameter of 50 mm were adopted in the SHPB tests. The bedding angles in the tests are 0°, 45°, and 90°. In the dynamic impact test, three different impact pressures were applied to observe the magnitude of impact load on the mechanical behaviors of the burst-prone sandstone specimens. The results show that with the increase in the bedding angle, the uniaxial compressive strength firstly decreases and then increases. When the bedding angle is 45°, the uniaxial compressive strength is the lowest. The uniaxial compressive strength is highest when the bedding angle is 0°. The burst-prone sandstone specimens with different bedding angles had three different failure pattern types. Under the dynamic loading, the stress-strain curves show springback phenomenon; with the increase in impact pressure, the dynamic strength of the burst-prone sandstone specimens with each bedding angle increases; the fracture degree of the bedding sandstone specimens gradually increases. The dynamic strength of the 45° burst-prone sandstone specimen is the lowest, and it has the highest fracture degree.

Effects of hydrostatic pressure on hydraulic fracturing properties of shale using X-ray computed tomography and acoustic emission

The study of the hydraulic fracturing characteristics in shale reservoirs is crucial for improving the permeability of shale gas reservoirs. In this study, hydraulic fracturing tests are conducted on full-diameter shale cores, and the hydraulic fracturing characteristics of shale with different bedding angles under high hydrostatic pressure are studied. The results show that under a hydrostatic pressure of 59 MPa, the shale sample will rupture twice in the breakdown stage during hydraulic fracturing. When the bedding inclination increases from 0° to 90°, the shale fracture breakdown pressure first decreases and then increases, showing an overall downward trend. Under high hydrostatic pressure, shale specimen surface fracture morphology and X-ray computed tomography reveal that a bedding angle close to 45° is more conducive to forming a complex hydraulic fracture network, which makes the fractal dimension of the fracture network larger. The bedding plane of shale is the direct cause of the diverse propagation patterns of hydraulic fractures. The b-value is a parameter that characterizes the magnitude–frequency relationship of an earthquake, and it is closely related to the rock rupture process. The b-value varies with different trends with the expansion of hydraulic fractures, but they all reach the lowest point before the failure of the shale sample. The higher the hydraulic fracture network complexity, the lower the peak b-value is. This work provides a qualitative understanding and scientific explanation of the hydraulic fracturing characteristics of anisotropic shale under hydrostatic pressure.

Macro- and meso- failure mechanism analysis for shale-like brittle materials under uniaxial compression

Understanding the failure mechanisms of the widespread shale-like brittle materials is of great significance to engineering safety. In this research, the discrete element model which can reflect the real rock mechanical properties and failure mode of shale specimens with different bedding angles has been reconstructed. First, microcrack has been introduced to characterize the meso-damage. The relationship between the microscopic crack numbers in the material and the macroscopic stress has been revealed. In addition, the relative movement between different deformed blocks has been analyzed by introducing the residual velocity field. The control factor of material failure varies with the bedding angle (β) in the macro scale. It manifests from the bedding plane and the matrix synergistically control the failure mode of rocks to mainly the bedding plane and then back to the bedding plane and the matrix synergistically control the rock failure mode. The competitiveness of the shear and tensile damages in the failure mechanism varies with the bedding angle in the meso scale. When 30°<β<75°, Shear damage dominates the failure mechanism. The established numerical model is capable of predicting the mechanical behavior of brittle rock, which may have guiding merits to the application of shale-like brittle materials.

Experimental Study on Mechanical Properties of 3D-Printed Specimens of Iron Oxide, Quartz, and Bedded Composites Under Uniaxial Compression and Indirect Tensile Strength.

The heterogeneity of natural rocks produces increased variations in the results of geomechanical and metallurgical tests making the repeatability of experimental work questionable. Fabricated test specimens have, therefore, become more attractive for fundamental studies. In this study, quasi-identical 3D-printed (3DP) specimens with 10 and 16 mm diameters were fabricated and tested to study material strength and understand the breakage characteristics at a scale more suitable for comminution. Cylinder specimens composed of quartz (named Si) and iron oxide (named Fe), with sorted grains of ∼100-150 μm in the form of homogeneous specimens (3DP-Si and 3DP-Fe) and heterogeneous specimens (bedded) (3DP-SiFeSi, 3DP-SiFe, and 3DP-FeSiFe) were tested. This article presents the results for experimental Unconfined Compressive Strength (UCS) and Brazilian Tensile Strength (BTS) tests. The elastic property was obtained from the UCS tests, while tensile strength was obtained from BTS tests. The strength of 3DP specimens of similar diameter decreases following the types: 3DP-Si (most competent), 3DP-Si-Fe, 3DP-SiFeSi, 3DP-FeSiFe, and 3DP-Fe (less competent). The results show that heterogeneous 3DP specimens were influenced by bedding angle, thickness, and mineral group composition. It also seems that the sequence of mineral composition and the number of beds play a role, rather than the overall grain percentage area for each cylinder, in influencing the strength and variability of fragments. Finally, the brittleness indices for 3DP specimens were calculated as a function of UCS and BTS.

Influence of microwave irradiation on the permeability of coal with different bedding dip angles

Microwave irradiation (MI) is a prospective method for increasing CBM production. Due to the permeability anisotropy caused by bedding structural plane, the spatial relationship between underground microwave irradiation drillings (MID) and the coal bedding plane has become an important factor affecting the effect of MI on enhancing production. The self-developed MI device numberer WY3S02 was used to carry out the MI experiment on samples with different bedding angles (BDA). The permeability of coal before and after MI was tested by the steady-state seepage method. The surface temperature and ultrasonic data of samples during the test were collected, and the influence of MI on the permeability of coal with different BDA was studied. The experiments indicate that the permeability of samples with different BDA increases by 30.2–100.1 times after MI under external axial load, and the increase of permeability is positively correlated with the BDA. The permeability after MI shows the variation law of K (90°) > K (60°) ≥ K (45°) > K (30°) > K (0°), showing an obvious bedding effect. The standard deviation of the lateral surface temperature of the specimen increases with the increase of the BDA, which makes the thermal fracture degree of the sample with large BDA higher, resulting in more dominant seepage channels, which is manifested as greater permeability growth. These results provide a scientific basis for the design of MID in the method of enhanced coalbed methane by microwave irradiation (MI-ECBM).

Low Nuclear Magnetic Resonance Experimental Study on Gas Adsorption of High-Rank Coals with Different Beddings.

In order to deeply study the influence of the coal bedding structure on coal gas adsorption, low nuclear magnetic resonance (LNMR) and a confining pressure loading system were used to carry out the LNMR experiment of gas adsorption of high-rank coals with different beddings under different confining pressures. The results showed that the amount of gas adsorption of high-rank coals with different beddings increases with time and decreases with the increase of confining pressure. In the process from low confining pressure to high confining pressure, the coal sample with oblique bedding (bedding angles 30°, 45°, and 60°) has the largest average increment of gas adsorption, followed by the coal sample with vertical bedding (bedding angle 90°), and the coal sample with parallel bedding has the smallest increment of gas adsorption (bedding angle 0°). The linear function relation between the different-bedding high-rank coal gas adsorption state and the confining pressure is y = a – bx. The relation between the free peak area and the confining pressure conforms to the exponential function y = a + bexp(cx). Different-bedding high-rank coal adsorption peaks and the peak area decrease with the increase of confining pressure, and the free peak continues to move to the left; that is, the large pores gradually shrink. With the increase of angle and bedding, the area of the adsorption peak increases first and then decreases, presenting an “inverted V” shape on the whole. The area of the free peak decreases first and then increases, presenting a “V” shape on the whole.

Wellbore Stability Analysis of Horizontal Wells for Shale Gas With Consideration of Hydration

Abstract With the wide application of extended reach wells and horizontal wells, the geological conditions faced during the drilling process have become increasingly complicated, and the resulting wellbole collapse problem has become serious. Based on an L-1 horizontal well, this study provides a method for calculating collapse pressure considering the influence of formation hydration. This method is used to obtain the stress distribution around the well and collapse situation at 4100 m underground. The wellbore stability can be evaluated by the method. The factors affecting the stress and collapse of the well are analyzed by changing the construction parameters and geological parameters. Studies have shown that the rock near the well has collapsed to a certain extent and shale hydration increases the formation Poisson's ratio, resulting in more serious collapse. The study also found that hydration, drilling fluid density, and borehole size have the greatest impact on the stability of the borehole wall, followed by the well deviation angle, and the bedding angle the least. Carefully exploring the situation of the shale gas reservoir in the drilling design stages, considering the impact of hydration, and selecting appropriate construction parameters are necessary to avoid the collapse of the shaft wall and affect the shale gas production process. This study can provide a reference for ensuring the stability of the wellbore.

Dynamic tensile failure of layered sorptive rocks: Shale and coal

Shale and coal samples were selected to clarify the tensile failure of layered sorptive rocks under the co-influence of bedding planes and loading conditions. Results indicate that, for these two rocks, tensile strength exhibited anisotropy. As loading rates rises, inverted Trumpet-shaped trend lines is proposed to reflect bedding effects with loading effects. The tensile strength of shale increases linearly with the bedding angle rises, while the value of coal shows great dispersion. The former is greater than the latter. An ultimate loading rate exists for shale and coal. Empirical relations were established to describe tensile strength with varying loading rate and bedding angle. Under the co-influences of bedding plane and loading conditions, more complex failure modes happened for coal and shale samples. Hydraulic fracturing along the direction of 60° (for shale reservoirs) or 45° (for coal reservoirs) to the bedding plane is more likely to produce a complex fracture network.

Micro and macro evaluation of tensile characteristics of anisotropic rock mass after high temperatures treatment-a case study of Lingshi Gneiss

The study of macroscopic and microscopic fracture behaviour and tensile strength evolution of anisotropic rock mass at high temperatures are of great significance to evaluation of stability of enhanced geothermal system (EGS). In this paper, Brazilian split test was adopted to study the action mechanism of different temperatures and bedding angles on the tensile strength of gneiss; the digital image correlation (DIC) was used for tracking the principal strain evolution throughout the whole split. Furthermore, the micro X-ray computed tomography and thin-section petrographic analysis were combined in studying the microscopic crack propagation trend and mineral component physicochemical changes of gneiss at different temperature stages. Results show that: (1) under the same bedding angle, the evolution of tensile strength of gneiss within 25–700 ℃ can be divided into three stages: at low temperature stage (25–200 ℃), the tensile strength of gneiss slowly increased, at medium-temperature stage (200–400 ℃), the tensile strength of gneiss slowly decreased, and at high temperature stage (400–700 ℃), the tensile strength of gneiss drops sharply. Under the same temperature, with increased bedding angle, the tensile strength of gneiss first decreased and then increased, with a minimum value when the bedding angle is 30° and a maximum value when it is 90°; (2) under the same temperature, with the increase of bedding angle, gneiss shows a single bedding plane tensile failure (0°), shear-type composite failure (15–30°), tensile-shear failure (45–75°), and base particle-bedding tensile failure (90°); (3) when the temperature is under 400℃, the initiation location is distributed at both ends of disc, whereas when the temperature is equal or above 400 ℃, it is distributed at the centre of disc. The results obtained are of great significance to safety evaluation of EGS and other high-temperature rock engineering.

Mechanical characteristics of thick-bedded limestone with different bedding angles subjected to acid corrosion

Mechanical characteristics of thick-bedded limestone with different bedding angles subjected to acid corrosion

Water Content and Bedding Angle Effects on the Mechanical Properties and Micro-/Macro-Failure Mechanism of Phyllite

Water Content and Bedding Angle Effects on the Mechanical Properties and Micro-/Macro-Failure Mechanism of Phyllite

Evaluating the mechanical properties of anisotropic shale containing bedding and natural fractures with discrete element modeling

Natural fracture data from one of the Carboniferous shale masses in the eastern Qaidam Basin were used to establish a stochastic model of a discrete fracture network and to perform discrete element simulation research on the size effect and mechanical parameters of shale. Analytical solutions of fictitious joints in transversely isotropic media were derived, which made it possible for the proposed numerical model to simulate the bedding and natural fractures in shale masses. The results indicate that there are two main factors influencing the representative elementary volume (REV) size of a shale mass. The first and most decisive factor is the presence of natural fractures in the block itself. The second is the anisotropy ratio: the greater the anisotropy is, the larger the REV. The bedding angle has little influence on the REV size, whereas it has a certain influence on the mechanical parameters of the rock mass. When the bedding angle approaches the average orientation of the natural fractures, the mechanical parameters of the shale blocks decrease greatly. The REV representing the mechanical properties of the Carboniferous shale masses in the eastern Qaidam Basin were comprehensively identified by considering the influence of bedding and natural fractures. When the numerical model size is larger than the REV, the fractured rock mass discontinuities can be transformed into equivalent continuities, which provides a method for simulating shale with natural fractures and bedding to analyze the stability of a borehole wall in shale.

Experimental Study on the Influence of Confining Pressure and Bedding Angles on Mechanical Properties in Coal

Extensive bedding planes have a great influence on the mechanical properties of coal. In order to study the mechanism of the effects of bedding angles on the mechanical properties and failure characteristics of coal in the Shanxi Baode coal mine, uniaxial and triaxial compression tests and numerical simulations were conducted. The strength deterioration and microstructural changes in the samples were then analyzed with discrete element method (DEM) numerical simulation. The experimental results reveal that the power function strength criterion has good applicability to the strength characteristics of this coal. It was also found that the bedding angles have a great influence on the mechanical properties of coal. The properties of peak strength at different bedding angles roughly showed a U-shaped changing trend. The maximum strength occurred at a bedding angle of 0°, whereas the minimum strength occurred at a bedding angle of 60°. The numerical simulation and test results prove that the forms of failure of different bedding coal samples are complicated and are mainly represented by tensile and shear failures.

Investigation on the Deformation and Failure Characteristics of Shale Samples Containing Circular Hole considering the Bedding Angle Effect under Uniaxial Compression

The drilling and blasting method (DBM) is widely used in the excavation of tunnel engineering, and it is essential to first drill circular holes. This paper experimentally explored the influence of bedding angles on crack propagation of shale containing a circular hole. Specifically, the uniaxial compression tests were carried out on shale samples with a single prefabricated hole combined with digital image correlation technology (DIC) to obtain the real-time deformation process near the macro damage. The results show that the peak strength, global shear strain field, failure mode around the circular hole and macro crack failure mode of shale with prefabricated circular hole are closely related to the bedding dip angle. The strength generally increases with the decrease of the bedding angle. Two transitions of failure mode were observed at the point of 45° and 75°: tensile shear mixed failure around the circular hole (less than 45°) first transits into no damage around the circular hole (60° and 70°), and then, tensile failure occurs around the circular hole (90°). In addition, for the cases of 60° and 75°, tensile failure away from the circular hole occurs along the weak surface of bedding, while for the other cases, the tensile failure through the circular hole mainly occurs. These findings have implied that the bedding angle is one essential parameter that needs to be considered for the stability of tunnel surrounding rock.

Effect of liquid nitrogen cooling on mechanical characteristics and fracture morphology of layer coal under Brazilian splitting test

Liquid nitrogen (LN 2 ) fracturing, as an environmentally friendly waterless fracturing technology, is given more and more attention in coalbed methane (CBM) exploitation. In this paper, a series of microscopic tests and a thermal damage simulation were combined to evaluate the changes in the internal structure of the coal caused by the LN 2 cooling, firstly. Then, Brazilian splitting tests were carried out on the bedding coal, and influences of cryogenic cooling fracturing and bedding orientation on mechanical properties and fracture morphology of the bedding coal were analyzed. The results show that the micro-structure of the coal is obviously damaged under the LN 2 cooling due to the thermal tensile stress. The P-wave velocity, tensile strength , Brazilian splitting modulus, and brittleness index of the coal sample have significant anisotropic characteristics and their values are reduced after the LN 2 cooling. The weakening degree in mechanical characteristics of the coal induced by the LN 2 cooling is also closely related to the bedding direction of the coal. After the LN 2 cooling treatment, the maximum and minimum reduction of the tensile strength is at the bedding angle 0° and 45°, respectively, with the reduction of 55.81% and 6.98%, respectively. The maximum and minimum changes of the Brazilian splitting modulus and brittleness index before and after the LN 2 cooling locate at 0° and 60°, respectively. The LN 2 cooling treatment can increase the ductility of the coal. The length and surface complexity of the induced fracture increase after the LN 2 cooling. The increase in the fracture length is particularly evident at the high bedding angle, while the significant increase of the crack surface complexity is more likely to occur at the low bedding angle. The results contribute to the design of LN 2 fracturing for coal beds.

Energy Evolution and Acoustic Emission Characteristics of Uniaxial Compression Failure of Anchored Layered Sandstone

To study the energy evolution and acoustic emission characteristics of layered sandstone under anchorage in the process of deformation and failure, the sandstone samples from Chuxiong Yi Autonomous Prefecture, Yunnan Province were selected for uniaxial compression testing. The energy evolution in the process of sandstone failure and the spatial fractal characteristics of acoustic emission events in the process of deformation and failure were investigated. Research results show that anchoring can make layered sandstone store more energy, the stored energy first increases, then decreases with the increase of bedding angle; the B value of sandstone under anchorage is generally higher than that of unanchored sandstone in the whole deformation and failure process, and the continuous decline in B value can be used to indicate a precursor to instability and failure; under the action of anchoring, the D value of sandstone (its fractal dimension) also increases, then decreases with the increase of bedding angle. The D value changes within [2, 3]. At a given bedding angle, the D value of anchored sandstone is greater than that of unanchored sandstone, the D value of 30° anchored sandstone increased the most (by 12.33%); the maximum D value occurred in 45° anchored sandstone (reaching 2.72) and the spatial distribution of acoustic emission events and damage of sandstone under anchorage is also more uniform; under increasing stress, the number of acoustic emission events is less widely distributed in the early stage and more densely distributed in the later stage. The growth rate of the D value varies across different peak stress ranges, which is more significant under the action of anchorage. The acoustic emission event counts grow evenly and slowly in the space, and the toughness of sandstone is improved to a certain extent under the action of anchorage.

Regional Characteristics of Porosity and Permeability of Dahebian Syncline Coal and Its Application

In order to study the basic properties of coal seam thus to provide reference for coalbed methane (CBM) production, the porosity and permeability of coal from DH and WJZ regions of Dahebian syncline in Liupanshui Coalfield were systematically studied. The results shows that the porosity and permeability of coal samples in DH region are between 7.4–10.1% and 0.01 mD∼0.04 mD respectively. The permeability is obviously anisotropic, and it is distributed in a U-shape along bedding angle. The porosity and permeability in the WJZ region are between 12.4–24.85% and 0.3 mD–4.8 mD, but the correlation of permeability and bedding angle is not obvious. There are many primary cracks in coal matrix of WJZ region, while the coal of DH region is relatively complete. As a result, the porosity and permeability of WJZ coal samples are much higher than that of DH coal samples. With the change of confining pressure, obvious change has taken place in the permeability of coal in the two regions. When the confining pressure decreases from 2 to 8 MPa, the permeability loss rate of coal samples is close to 80%. The internal mechanism of the permeability loss is related to crack closure and plastic deformation caused by confining pressure. In contrast, the porosity and permeability of coal in WJZ region are much higher than those in DH region, which indicates that the permeability of the same coal seam in the same structural will have great differences and inhomogeneities. Therefore, the WJZ region is the preferred area, which can be included in the preferential exploitation plan of CBM. In addition, for the typical syncline gas reservoir in Liupanshui Coalfield, it is necessary to consider the block division according to the porosity and permeability indexes, and select the optimal block and give priority to development of CBM. Furthermore, we should pay attention to the difference and adaptability of CBM development in different regions.

Research on Evolution Characteristics of Shale Crack Based on Simultaneous Monitoring of Multi-Parameters

Meso-crack evolution mechanism of shale is a key factor affecting the mechanical properties of shale. In order to explore evolution laws of cracks in shale during loading, a meso-crack monitoring system, loading test equipment and an automatic ultrasonic data acquisition system were set up. On this basis, a set of experimental apparatus simultaneous monitoring multi-parameters of shale micro-crack was designed, and destruction experiments of shale samples with different bedding angles were carried out to find out evolution characteristics of cracks. The results show the following: (1) The designed apparatus can monitor ultrasonic, mechanical and video information simultaneously of crack evolution in the entire process of shale destruction under load to provide information for analyzing acoustic and mechanical characteristic responses of crack propagation at key time nodes. (2) With an increase in load, shale will undergo four stages of destruction: crack initiation, propagation, penetration and overall failure. In the course of these stages, acoustic characteristics and mechanical characteristics are in good agreement, which proves the validity of predicting rock mechanical parameters with acoustic data. (3) During the loading process of shale, the main amplitude of acoustic wave is more sensitive than mechanical parameters to the change of rock cracks. Research results have important theoretical reference value for evaluating wall stability of shale gas horizontal well with ultrasonic data.