Effects Of Bedding Planes Research Articles

Effect of bedding planes and property contrast between layers on the propagation mechanism of hydraulic fracture height in shale reservoirs

Bedding planes and property contrast between layers plays an important role in preventing hydraulic fracture height extension in shale reservoirs. However, the intrinsic causes of fracture containment have not been elucidated, and few studies have considered the effects of mixed factors on fracture height extension. Based on the continuum-discontinuum element method, a 3D hydraulic-mechanical coupling model was established to explore the influence mechanism of bedding planes, stress and modulus difference between layers, and mixing factors on fracture height extension in a transversely isotropic shale reservoir. Shear failure usually occurs preferentially when the fracture approaches the bedding plane, and the essential reason for fracture containment is that the shear slip on the bedding plane blunts the fracture tip. The stronger stress barrier formed by the higher minimum principal stress in adjacent formation effectively enables the fracture to terminate at the interlayer interface. The lower modulus in adjacent formation is more prone to rock initiation, but the larger elastic deformation absorbs the fracture tip stress and impedes height propagation. The comprehensive effect of bedding planes and property contrast between layers on fracture propagation is more complex than that of a single factor. When different factors have positive effects on fracture containment, the fracture is easier to terminate at the bedding plane. Otherwise, the dominant factor determines whether the fracture can be captured. No matter what the conditions, the pressure relief in the bedding plane can slow down the propagation rate. The findings can offer theoretical support for predicting fracture height during field fracturing in shale reservoirs.

Experimental investigation of the coupled effects of bedding planes and flaws on fracture evolution of soft bedded rocks based on 3D printing and DIC technologies

Bedded rocks are a typical geological formation with significant implications for understanding tectonic evolution, rock engineering, and geological hazard mitigation. This study introduces a novel method for fabricating bedded rocks using 3D printing (3DP) technology. By comparing the mechanical anisotropic properties and failure modes of 3DP bedded rocks with natural bedded rocks, we validate the feasibility of this approach. Compression tests are conducted on 3DP bedded rocks containing a single flaw to investigate the coupled effects of bedding planes and flaws on the mechanical properties. Digital image correlation (DIC) is employed for full-field deformation measurement during loading, enabling the study of deformation and crack evolution patterns. The results indicate that bedding planes dominate the brittleness of 3DP bedded rocks containing a single flaw. Significant plastic characteristics and prolonged nonlinear deformation stages are observed when the bedding angle (α) is between 0° and 45°, while brittleness becomes apparent beyond 45°. Both strength and elastic modulus are decided by bedding planes and initial flaws. By comparing stress–strain curves and rate of effective variance change (REVC)-strain curves, crack initiation can be quantitatively identified, revealing a linear correlation between strength and stress values at crack initiation points. The bedding plane also dominates crack propagation and failure modes: when α ≤ 30°, cracks propagate through the bedding, indicating tensile failure; when 45° ≤ α＜90°, cracks propagate along the bedding, indicating tensile-shear or shear failure; and when α = 90°, cracks propagate along bedding planes, indicating tensile failure. The findings of this study provide insights for explaining and predicting failure modes in engineering projects involving bedded rock formations, such as slopes, tunnels, and coal mines.

Study on the Effect of Bedding Plane and Loading Style on Fracture Process Zone in Coal

Study on the Effect of Bedding Plane and Loading Style on Fracture Process Zone in Coal

The effect of bedding planes on the bending strength of rock-like material and evaluation of the crack propagation mechanism

Bedding planes affects various material properties, including the bending strength and the crack propagation mechanism of rocks. In this paper, laboratory studies on rock-like material specimens were conducted to investigate the effect of bedding planes on the failure and crack propagation mechanism and to reduce the effect of secondary factors on the results. To this end, semi-circular bend (SCB) specimens were prepared with two concrete mixing designs using aggregate and geopolymer concretes as materials. Next, they were subjected to compressive and three-point bending tests. The crack propagation mechanism, changes in the bending strength, maximum fracture load, relative midspan displacement, fracture energy, and fracture toughness were analyzed for the anisotropic and isotropic specimens. Also, different bedding angles were examined for the anisotropic specimens. The results revealed that bedding planes generally affected the crack propagation mechanism and changed the fracture mode from pure opening mode for the isotropic specimens to mixed modes for different bedding angles. Hence, the strength and fracture parameters were generally smaller in the anisotropic specimens than in the isotropic ones. With an increase in the bedding angle in the specimens, the maximum fracture load and the fracture surface energy increased, and the midspan displacement behaved differently. Moreover, the fracture toughness displayed an increasing trend with increasing the bedding angle in the anisotropic specimens. The rate of changes in the parameters were different for different bedding angles. This may be attributed to the changes in the stress distribution and to the fracture and crack propagation modes in the anisotropic specimens compared to the isotropic ones.

Investigation of strain fields and anisotropy in triaxial tests on Callovo-Oxfordian claystone by X-ray micro-tomography and digital volume correlation

The objective of the present study is to investigate deformation behavior of anisotropic claystone in relation with material heterogeneity. For this purpose, a set of five triaxial compression tests are performed on samples of Callovo-Oxfordian (COx) claystone under in-situ X-ray micro-tomography monitoring. Complete 3D images of the samples full loading history were recorded and analyzed by digital volume correlation method. Full non-uniform strain fields were determined and averaged global strains were calculated. As novelty, we have particularly investigated the effects of bedding planes, initial cracks and confining stress on strain fields. The results indicate that the structural anisotropy of COx claystone has a strong influence on the local strain distribution and cracking process. The confining pressure has also an important impact on the strain fields and failure modes of samples in the triaxial tests. The cracking process of samples is strongly correlated with radial strain distribution.

Effect of bedding plane on mechanical properties, failure mode, and crack evolution characteristic of bedded rock-like specimen

To study the influence of bedding planes on mechanical properties and failure characteristics of specimens, an experimental test was conducted first on bedded rock-like specimens by uniaxial compression test with acoustic emission and digital image correlation. Experimental results show that both uniaxial compression strength (UCS) and failure strain (εf) present a tendency to first decrease and then increase with the increase in bedding angle (α). Moreover, the existence of bedding planes changes failure mode and fracture process of bedded specimen. Based on experimental results, three-dimensional numerical models containing bedding planes were used to determine the effect of orientation angle (α), spacing (s), and strength (λ) of bedding planes. Numerical results are as follows: (1) For α = 0° and 30°, the increase in s leads to the decrease in translaminar tensile crack and increase in translaminar shear crack, and the trend is reversed for α = 45° and 60°. Besides, (2) the variation of s leads to the variation of the percentage of the crack in bedding and rock matrix. However, (3) with the increase in s, the UCS and εf of bedded specimen at the same α remain almost unchanged. Unlike the influence of s, (4) the UCS and εf of bedded rock for α = 30°–60° are positively correlated with λ. (5) Moreover, with the increase in λ, the BP-crack percentage decreases dramatically, leading to the variation of the failure mode of bedded rock.

Effects of bedding planes on progressive failure of shales under uniaxial compression: Insights from acoustic emission characteristics

It is believed that the bedding plane is one of controlling factors in progressive failure of shales and its characterization is of great significance in the gas shale engineering. However, it is often difficult to directly capture and visualize the progressive failure process of shales in practice. Although the microseismic method is feasible and economical to overcome this problem, to date, the effect of bedding plane on progressive failure of shales is still not fully clarified by this method. Here, we use the bonded-particle model (BPM) embedded with moment tensor algorithm to explore the progressive failure process and acoustic emission (AE) characteristics of shales under uniaxial compression. The macro mechanical behaviors such as stress-strain response and failure modes of shales were well reproduced by BPM. Meanwhile, AE characteristics were systematically monitored and analyzed during the progressive failure process. Then, the failure type dependent on layer orientation was determined by the moment tensor decomposition. And bedding-orientation-controlled crack stress thresholds as well as the degree of progressive failure could be captured by the AE cumulative number and hits. It was found that b-value in post-peak stage was susceptive to layer orientation and could be used as the indicator of the fracture complexity. Also, proportion of shear AE sources have a good consistence with fracture complexity, which suggests that shear failures favor the complex fracture formation. Our study provided a new insight into effects of bedding planes on progressive failure of shales from the viewpoint of AE characteristics.

Effects of bedding planes on mechanical characteristics and crack evolution of rocks containing a single pre-existing flaw

The existence of bedding planes in rock materials has an obvious impact on the material mechanical properties and the failure process of engineering projects. In this study, a series of pre-cracked models with different bedding plane properties (i.e., strength reduction factor, bedding plane angle and spacing) were established and simulated to fail under the uniaxial compression. The results indicate that the increases of the strength reduction factor and the bedding plane spacing lead to the increasing peak strength, the change of peak strength regarding the bedding plane angle presents a concave upward trend. In terms of crack evolution, the proportion of cracks formed in rock matrix increases concerning the increasing strength reduction factor and bedding plane spacing, the abovementioned crack proportion in models with increasing bedding plane angles firstly decreases and then increases. The rock matrix has higher mechanical characteristics than bedding planes, the change of crack proportion can account for the changing peak strength from the microscopic viewpoint. By summarizing the failure patterns, the failure of models with the 45° ~ 75° bedding plane angle is induced by cracks formed in bedding planes, which can be depicted and discussed by the plane of weakness model. The above results imply that the rock slope stability can be evaluated by analyzing bedding plane properties in the field.

Effects of bedding planes on the fracture characteristics of coal under dynamic loading

To investigate the influence of bedding planes on its fracture characteristics under dynamic loading, testing and numerical simulations are carried out on semi-circular bend specimens of coal. It is shown that the fracture load and dynamic initiation fracture toughness decrease as the increase of bedding-plane angle. The crack propagation direction is jointly controlled by the maximum principal stress and bedding planes. Based on the digital speckle correlation method, it is found out that, due to stress concentration, strain at the initial loading stage concentrates around crack tip. After crack initiation, there is a specific strain gradient with a candle flame-like shape on the surface of a specimen. The opening displacements at crack tip can be divided into stable and linearly increasing phases. Further, a continuum-based discrete element method is applied to virtually reproduce these fracture characteristics, which are instructive to study dynamic anisotropy in fracture of coal.

A New Wellbore Stability Model for Shale Gas Horizontal Wells with Effects of Bedding Planes and Water Content

Shale gas exploration and development have become a hot spot in the natural gas industry. Providing wellbore stability is one of the major engineering problems occurring in the process of drilling of horizontal shale gas wells. In this paper, based on circumferential stress analysis of horizontal wells and analysis of the geometrical relationship of bedding planes and the wellbore, a new horizontal wellbore stability model is introduced. The proposed model takes into account the effect of bedding planes and water content on the strength parameters of shale. Based on actual field data of a shale gas well, the proposed model is compared with the wellbore stability model for an intrinsic rock formation. The results show that the existence of bedding planes affects the stability of a wellbore, making it less stable. Meanwhile, the maximum and minimum values of collapse pressure density for different azimuths of wellbore increase linearly with increase in water content. Moreover, the results also show that factors of the wellbore azimuths and the bedding planes dip angle and dip direction have a significant impact on the wellbore stability of horizontal shale gas wells.

Effect of Bedding Plane on the Permeability Evolution of Typical Sedimentary Rocks Under Triaxial Compression

Effect of Bedding Plane on the Permeability Evolution of Typical Sedimentary Rocks Under Triaxial Compression

Digital Volume Correlation Applied to X-ray Micro-Tomography Images in Uniaxial Creep Tests on Anisotropic Clayey Rock

Creep tests are commonly performed to characterize time-dependent deformation of geological materials. Classical measuring methods are not suitable for long term tests and not able to provide full three-dimensional strain fields. In this study, Digital Volume Correlation (DVC) is applied to X-ray micro-tomography (XRMT) images from creep tests on a hard clayey rock. In situ uniaxial compression creep tests are performed under different levels of stress and with different loading orientations with respect to the structural anisotropy of rock. Based on the XRMT images taken during the creep tests, DVC is applied to compute the full three dimensional strain fields and global averages strains of tested samples. The effects of bedding planes and hard inclusions on the non-uniform distribution of strains are analyzed.

Effects of bedding planes on fracture behavior of sandstone under semi-circular bending test

The mechanical behavior of layered rock is seriously influenced by the bedding planes that have lower tensile strength and shear strength than rock matrix. We conducted experimental and numerical three-point-bending tests on semi-circular sandstone specimens with an opening mode fracture to study the influences of bedding parameters on their fracture behavior. Experimental study mainly focuses on the influences of the inclination angle between loading direction and bedding plane on fracture load, fracture patterns and failure process. The numerical simulations based on cohesive element method aim to explain how the shear strength, tensile strength and spacing of bedding planes influence the fracture load, crack propagation and fracture patterns. Results show that stronger bedding strength, larger bedding spacing or larger inclination angle will induce a larger fracture load. The anisotropy of sandstone becomes stronger for the weaker bedding strength and larger bedding spacing (≤5.5 mm). The fracture patterns of sandstone are greatly governed by the inclination angle and partly controlled by bedding strength and bedding spacing, which is exceptional for the specimen whose inclination angle is 0°. In addition, the length percentage of fracture along the bedding plane and the proportion of tensile or shear cracks are analyzed to further address the effects of bedding parameters on the fracture properties.

Shale bedding planes control rock removal behaviors of PDC cutter: Single cutter experiment

Shale bedding planes arise with the complexity of rock removal by the PDC cutters. A series of single cutter experiments were conducted to understand the effects of bedding planes of shale on chip formation behaviors. Six bedding inclinations were taken into account by recording the variation of cutting force, cutting size, topography of debris, and high-speed images. Results demonstrate that the average cutting force highly correlates with bedding inclination where a parabolic relation is observed. The maximum value of cutting force is presented while perpendicular cutting to the bedding plane. Crushing and chipping are approved to be two mechanisms of rock removal of the laminated shale. Particularly, the bedding plane controls the chip formation behaviors. While the cutting force is perpendicular to shale bedding, the crushing dominates the rock removal process in which more powder debris are created. While the cutting force is parallel to shale bedding, the chipping dominates the rock removal and more chunk-like cuttings are generated. Shear cracking along the weak bedding planes promotes chipping that is controlled by the bedding inclination. In summary, the bedding plane attributes to the variation of cutting force response, cutting size distribution, and even rock removal mechanisms.

Transfer of anchoring load in layered roadway roof under different lithological sequences

Roadway roofs of coal measures strata mainly occur in layered structures. Due to high frequency of shearing or bending failures of rockbolt induced by layer separation or interlayer sliding of roadway roofs during the tunneling process, this study presents an intensive analysis of load distributions of rockbolts in layered roofs under different lithological sequences based on numerical simulations. First, numerical simulation models of cross-layer anchoring roofs of four different lithologic sequences (including hard-hard, soft-hard, hard-soft, and soft-soft) with effects of bedding planes taken into consideration were established. Then, the distributions of axial forces, shear forces, and shear stresses on rockbolts in different locations in each model were investigated. Finally, loading mechanisms and failure modes of rockbolts in layered roofs of different lithological sequences were elaborated based on the results of finite element analysis. Results indicate that cross-layer rockbolt is easy to produce combined deformation of tension and bending near the bedding plane. In this area, three internal forces of axial force, shear force, and bending moment exit on the cross-sections of rockbolt, meaning that the rockbolt is in a complicated stress state. In this case, the lithological sequence, thickness, and stress level of soft strata, as well as location of the bedding plane, have a significant effect on tension-bending failure of rockbolt.

Numerical Study of Fracture Network Evolution during Nitrogen Fracturing Processes in Shale Reservoirs

This paper develops a numerical model to study fracture network evolution during the nitrogen fracturing process in shale reservoirs. This model considers the differences of incompressible and compressible fluids, shear and tensile failure modes, shale heterogeneity, and the strength and permeability of both shale matrix and bedding planes through the coupling of mechanical-seepage-damage during fracturing fluid injection. The results show that nitrogen fracturing has a lower breakdown pressure and larger seepage zone than hydraulic fracturing under the same injection pressure. Tensile failure was identified as the major reason for the initiation and propagation of fractures. Ignoring the effect of bedding planes, the fracture initiation pressure, breakdown pressure, and fracturing effectiveness reached their maxima when the stress ratio is 1. Under the same strength ratio, the propagation path of the fractures was controlled by the stronger effect that was casused by the bedding angle and stress ratio. With increasing the strength ratio, the fracture number and shearing of the bedding plane increased significantly and the failure pattern changed from tensile-only mode to tensile-shear mode. These analyses indicated that the fracture network of bedding shale was typically induced by the combined impacts of stress ratio, bedding angle and strength ratio.

Discrete element modelling of stress-induced instability of directional drilling boreholes in anisotropic rock

Directional drilling is a viable and cost-effective trenchless technology for the installation of pipelines and conduits in shallow formations and the extraction of unconventional energy resources in deep reservoirs. The performance of drilled boreholes is associated not only with drilling practice (e.g., borehole size and drilling direction) but also with geological environment (e.g., in-situ stress and geomaterial property). We conduct discrete element modelling on hollow anisotropic rock samples to elucidate the stress-induced instability along a directional drilling borehole with vertical, inclined, and horizontal sections. We systematically evaluate the influences of confining pressure, borehole diameter, and bedding orientation on the mechanical response of the hollow samples. When the confining pressure increases, the peak stress varies from a linearly increasing fashion for relatively small borehole diameters to an unstable variation trend for relatively large borehole diameters. Both peak stress and elastic modulus decrease with larger borehole diameter. The effects of bedding planes are negligible when the borehole exists perpendicular to bedding planes, while borehole instability likely occur when the borehole is oriented along bedding planes. The presence of bedding planes leads to the concentration of tangential stress along the normal direction of bedding planes, which ultimately dominates the failure mechanisms at the particle scale. With the increase of anisotropy angle, the dominant failure modes transform from the tensile failure of rock matrix to the shear failure along bedding planes.

Analysis of wellbore stability considering the effects of bedding planes and anisotropic seepage during drilling horizontal wells in the laminated formation

With the exploration and development of gas shale in recent years, layer structures (bedding planes) of laminated rock and its effects on mechanical properties of rock and wellbore stability have arisen great attention. In this paper, we mainly concerns the wellbore stability during drilling horizontal wells in the laminated formation with horizontal bedding planes, meanwhile, the effects of anisotropy of permeability paralleled to bedding planes and that perpendicular to bedding planes are taken into consideration as well. Models of pore pressure distribution and seepage stresses under anisotropic seepage are established. Combining the effects of anisotropic seepage and bedding planes, we have investigated the equivalent collapse density (ECD) around horizontal wellbores. The results based on the input data in this paper reveal that the ECD considering the weakness of bedding planes is far higher than that without considering bedding planes, and the shape of zones where surrounding rock is relatively easier to collapse is similar as two couples of cat ears. The point where ECD reaches its maximum value tends to move from wellbore wall to internal rock if drilling close to the direction of minimum in-situ stress, and the higher anisotropic permeability ratio is, the more obvious the phenomenon is. The effect of anisotropic seepage can be ignored if the weakness of bedding planes is omitted, it otherwise can not be left out and the maximum ECD around wellbore instead of the maximum value at wellbore wall should be used to design for drilling operation. This study simultaneously investigates the effects of anisotropic seepage and weak bedding planes on wellbore stability during drilling horizontal wells, it is promising to provide a theoretical basis and practical guidance for the further researches on wellbore stability in future.

Fully three-dimensional propagation model of horizontal borehole hydraulic fractures in strata under the effect of bedding planes

The bedding plane effect will occur when hydraulic fractures propagate to the bedding plane in sedimentary strata, resulting in the “≠,” “工,” or “/” shaped fracture morphology. Based on previous physical experiments results, this article analyzed the mecroscopic propagation mechanism of tensile failure and the mechanical conditions for main hydraulic fracture and the bedding plane fracture propagating, proposing the criteria for hydraulic fracture to penetrate through the bedding plane. A fully three-dimensional model of hydraulic fracture morphology in horizontal borehole hydraulic fracturing is established with the vertical water flow, water leak-off, and bedding plane effect taken into consideration. Basic equations of continuity, pressure decline, hydraulic fracture morphology, and others are solved. After that, true triaxial hydraulic fracturing experiments with samples containing bedding planes are conducted to verify the aperture, length, width, and height of hydraulic fractures in this model. The model is proved to be accurate and reliable.

Numerical investigation of the direct tensile behaviour of laminated and transversely isotropic rocks containing incipient bedding planes with different strengths

The anisotropic direct tensile behaviour of laminated and transversely isotropic Midgley Grit sandstone (MGS) was investigated in this paper using a particle-based discrete element method (DEM) with consideration of the incipiency of bedding planes. Laboratory experiments were conducted to quantify the direct tensile strength of the incipient bedding planes, and the results were used to calibrate the numerical model. The effects of bedding planes with different relative positions and spacing were discussed. A comparison between the simulated results of this study and the predicted results from failure criteria was completed, and a good agreement was observed.