Influence Of Bedding Planes Research Articles

Numerical simulation study of fracture height growth considering the influence of bedding planes

Hydraulic fracturing, a production enhancement technique, is widely used in the development of unconventional oil and gas reservoirs. The formation of a complex network of hydraulic fractures that connect with natural fractures is crucial for hydraulic fracturing in unconventional reservoirs. However, the current understanding of vertical fracture propagation behavior under the influence of variations in the mechanical properties of interbedded rock is insufficient to meet the requirements for simulations of unconventional reservoirs under complex geological conditions. In this study, a three-dimensional discrete grid method was employed to establish a model of three-dimensional fracture propagation. Numerical simulations were conducted to investigate the vertical growth of fractures while considering the influence of bedding planes. The effects of formation factors (stress, Young's modulus) and bedding planes (cohesion, density) on the height growth of hydraulic fractures were explored. The results indicated that the interbedded stress contrast, Young's modulus contrast, and bedding planes collectively controlled the height growth of hydraulic fractures. The height of hydraulic fractures decreased with increasing minimum horizontal principal stress of adjacent layers. When the minimum horizontal principal stress of adjacent layers exceeded the vertical stress, hydraulic fractures gradually deflected into the horizontal plane. Adjacent layers with large values of Young's modulus promoted the height growth of hydraulic fractures, while adjacent layers with small values of Young's modulus inhibited the height growth of hydraulic fractures. The presence of bedding planes further suppressed height growth, and the degree of suppression was related to the cohesion and density of the bedding planes. Weaker cohesion and higher density resulted in greater suppression. The results of this study provide a reference for the design and optimization of hydraulic fracturing treatments in unconventional oil and gas reservoirs.

Influence of bedding plane on the tensile properties and crack propagation of soft and hard laminated rock-like under Brazilian test

To investigate the tensile properties and crack propagation of anisotropic rock-like specimens, soft and hard laminated rock-like specimens were made for the Brazilian splitting test. Five bedding plane inclination angles (θ = 0°, 30°, 45°, 60° and 90°) and three thickness ratios of soft and hard layers (1:1, 1:3 and 3:1) were taken into consideration for the rock-like concrete specimens. The effects of different θ and rock layer thickness ratios on the tensile strength, brittleness and energy characteristics were analyzed. The variation in tensile strain and crack opening displacement of tensile fracture were obtained by the digital image correlation (DIC) technique for the specimens. The crack initiation position and propagation path were determined. It was found that the fracture process zone appeared before the initiation of macroscopic cracks owing to the accumulation of tensile strain. The crack initiation position occurs near the two loading points regardless of the different angle θ. When θ = 0° and 90°, the crack propagates toward the center of the specimen along the loading direction. However, when θ = 30°, 45° and 60°, the cracks tend to propagate along the bedding plane. The influence of the crack propagation path on the final fracture pattern was analyzed. The results indicated that the angle θ has a strong influence on tensile crack initiation and propagation under the Brazilian test. The fracture process zone appears at the location of crack initiation and the expanding cracking path before the formation of macroscopic cracks. The absorbed energy increases with increasing tensile strength, and it decreases as θ increases. The brittleness index of the specimen increases gradually with increasing θ.

Mechanical behavior and failure characteristics of double-layer composite rock-like specimens with two coplanar joints under uniaxial loading

To study the combined action of joints and bedding plane on crack evolution of flawed layered rock masses, uniaxial compression tests and digital image correlation (DIC) tests were carried out on double-layer composite rock-like specimens with multi-inclination two coplanar joints under different bedding angles. Based on the experimental results, the existence of bedding plane and joints performs a significant effect on characteristics of stress-strain curves and strength parameters. The failure modes are divided into four different types. The existence of bedding plane inhibits the coalescence of cracks in rock bridge area, and the variation of bedding angle affects the crack propagation path and failure mode transition greatly. For specimens with bedding angle of 30° and 45°, the larger the joint angle, the less the influence of bedding plane on failure characteristics. Besides, a new tensile crack type which initiates from the bedding plane and is easier to propagate in the upper layer was observed.

The Influence of Bedding Planes on Tensile Fracture Propagation in Shale and Tight Sandstone

The Influence of Bedding Planes on Tensile Fracture Propagation in Shale and Tight Sandstone

Influence of bedding planes on fracture characteristics of coal under mode II loading

Due to bedding planes and natural fractures, the mechanical behavior of coal under different loading modes is usually anisotropic. The anisotropy usually influences the strength and stability of coal and rock mass in engineering. In this research, we used the asymmetric semi-circular bend (ASCB) method to carry out fracture tests of coal specimens with various bedding angles (0°, 22.5°, 45°, 67.5°and 90°) between loading direction and bedding planes under mode II loading. The influence of the bedding angle on the fracture toughness and fracture pattern of coal specimens under mode II loading was investigated. Moreover, based on the acoustic emission (AE) technology and digital image correlation (DIC) technique, the influence of the bedding plane on the failure process of coal specimens was studied. The research results indicate that the effect of the bedding plane is significant. The apparent mode II fracture toughness decreases and then gradually increases with the increase of bedding angle, picturing a check mark shape. The minimal value was obtained at the bedding angle of 22.5°. The obtained fracture patterns can be classified into fracture growth along the bedding and across the bedding. The influence of the bedding plane on the crack propagation is the most obvious at the bedding angle of 22.5° and 45°, where the cracks mainly grow along the bedding planes. The failure process of ASCB coal specimens can be divided into the compaction stage, the stable crack propagation stage, the rapid crack propagation stage, and the unstable crack propagation stage. The bedding angle has more influence on the crack initiation load while has less influence on the crack damage load between 80 %Pmax to 90 %Pmax.

Experimental investigation of the influence of bedding planes on the mechanical characteristics of saturated sand

This paper presents an experimental study concerning the relevance of mechanical characteristics and bedding planes of sand. The study was carried out using a new method of specimen fabrication based on the capillary effect. A series of triaxial tests were performed to study the stress-strain behavior of sand under different bedding plane tilting angles. The results show that the peak deviatoric stress and the volumetric strain are significantly different under different bedding plane tilting angles due to the inherent anisotropy of sand, and the difference between their peak strengths can reach 30%. The specimen reached the failure stage more easily and the peak strength was lower, reaching the residual state earlier when the bedding plane tilting angle was close to 45°-ϕ/2. The equation proposed in this study can thus reasonably reflect the relationship between the bedding plane tilting angle and the peak strength. The inherent anisotropy due to the bedding plane of sand may have a significant influence on the deformation of building foundations; therefore, neglecting this factor may lead to unsafe designs.

The Analytical Model for Horizontal Wellbore Stability in Anisotropic Shale Reservoir

Wellbore collapse is a frequent problem during drilling in shale gas reservoir, restricting efficient exploration of shale gas. Due to the presence of bedding plane, conventional homogeneity model is inappropriate to conduct wellbore stability analysis in shale reservoir. Therefore, in this study, under the influence of bedding plane, stress distributions including in situ stress components, pore pressure, seepage stress have been calculated. Meanwhile, shale strength is expressed using the Jaeger's criterion, which assumes shale is isotropic body with one group of weak planes. In combination with stress distribution and shale strength, an analytical model has been developed to investigate wellbore failure region. By using this model, influence factors of wellbore stability have been fully analyzed. Results demonstrate that the difference between bedding plane and rock matrix will modify in situ stress components, transport equations and rock mechanical properties around wellbore. As a result, there are heterogeneous distributions of shale strength and stress state. When the shear failure along bedding plane happens, shale strength has clearly reduction and breakout region has increment, changed from two-lobed failure to four-lobed failure shape. The different intersections between bedding plane and wellbore make variable anisotropy in wellbore plane, modifying the pore pressure propagation and seepage stress distribution. When the bedding plane angle in wellbore plane is low or high value, shear failure along bedding plane doesn’t happen and wellbore stability is depended on rock matrix strength. For middle bedding plane angle, failure region clearly increases and shear failure along bedding plane occurs, aggravating the instability. Furthermore, drilling fluid reduces shale strength, thus increasing region area and changing failure mode. This drilling fluid impact is strong in initial stage and becomes stable after 96 h. By having comprehensive investigation on bedding plane influence, findings in this study are advantageous for drilling operations in shale reservoir.

Study on the influence of bedding plane on the fracturing behavior of sandstone

Study on the influence of bedding plane on the fracturing behavior of sandstone

The Influence of Bedding Planes and Permeability Coefficient on Fracture Propagation of Horizontal Wells in Stratification Shale Reservoirs

The complexity of hydraulic fractures (HF) significantly affects the success of reservoir reconstruction. The existence of a bedding plane (BP) in shale impacts the extension of a fracture. For shale reservoirs, in order to investigate the interaction mechanisms of HF and BPs under the action of coupled stress-flow, we simulate the processes of hydraulic fracturing under different conditions, such as the stress difference, permeability coefficients, BP angles, BP spacing, and BP mechanical properties using the rock failure process analysis code (RFPA2D-Flow). Simulation results showed that HF spread outward around the borehole, while the permeability coefficient is uniformly distributed at the model without a BP or stress difference. The HF of the formation without a BP presented a pinnate distribution pattern, and the main direction of the extension is affected by both the ground stress and the permeability coefficient. When there is no stress difference in the model, the fracture extends along the direction of the larger permeability coefficient. In this study, the in situ stress has a greater influence on the extension direction of the main fracture when using the model with stress differences of 6 MPa. As the BP angle increases, the propagation of fractures gradually deviates from the BP direction. The initiation pressure and total breakdown pressure of the models at low permeability coefficients are higher than those under high permeability coefficients. In addition, the initiation pressure and total breakdown pressure of the models are also different. The larger the BP spacing, the higher the compressive strength of the BP, and a larger reduction ratio (the ratio of the strength parameters of the BP to the strength parameters of the matrix) leads to a smaller impact of the BP on fracture initiation and propagation. The elastic modulus has no effect on the failure mode of the model. When HF make contact with the BP, they tend to extend along the BP. Under the same in situ stress condition, the presence of a BP makes the morphology of HF more complex during the process of propagation, which makes it easier to achieve the purpose of stimulated reservoir volume (SRV) fracturing and increased production.

Anisotropic energy and ultrasonic characteristics of black shale under triaxial deformation revealed utilizing real-time ultrasonic detection and post-test CT imaging

SUMMARY For evaluating the fracturing-related activities in a deep shale formation, it is important to investigate the effect of anisotropy on its geomechanical properties. Many effects have been performed to reveal the strength and deformation anisotropy of shale, however, the influence of bedding planes on the anisotropic energy evolution and velocity-energy dependency are still not well understood, especially under high confinement condition. In this study, triaxial compression tests with a high confining pressure of 60 MPa in combination with real-time ultrasonic detection and post-test CT scanning were performed to the shale samples cored along an angle of 0°, 30°, 60° and 90° with respect to bedding planes. The effect of the bedding orientation on the shale geomechanical, ultrasonic, energy dissipation and energy release characteristics are explored. The experimental results show that shale structural features highly affect the total energy, elastic energy and dissipated energy. The increasing trend of elastic energy shows a slow, fast and slow mode, and the dissipate energy increases rapidly near sample failure. Good correlations have been found among the P- and S-wave velocities and the elastic and dissipated strain energy. The mesostructural changes during deformation are considered to be the primary factor controlling the energy sensitivity to the velocities. CT images further reveal the anisotropic fracture pattern which is in good agreement with energy release and dissipation analysis. The analysis of the strain energy and velocities suggests that the strain energy evolution and fracture anisotropy are bedding orientation dependent.

Fracture evolution in artificial bedded rocks containing a structural flaw under uniaxial compression

The Observation and prediction of crack propagation are important in understanding rock behavior in engineering practice. Previous studies have focused on homogeneous and isotropic rocks, but the influence of bedding planes on rock fracture is sparingly documented. In this study, we investigate the fracturing response in uniaxial compression of artificial bedded rocks containing variable-inclination bedding planes and single structural flaws. The recorded stress-strain data and captured cracking patterns are examined together. Nine separate crack types are identified in which bedding-plane sliding and splitting are potentially new. The presence of bedding planes plays a decisive role in crack propagation. With a steepening of the bedding plane, tensile cracks initiating from the structural flaw are better able to propagate along bedding planes, and the accumulative length of bedding fractures accordingly increases. Failure transforms from a purely tensile mode to a bedding sliding mode and subsequently to a bedding splitting mode as the bedding inclination increases. Compared to pro-dip flaw tests, the fracturing of bedding planes is triggered by both the tensile and shear stress with more far-field tensile cracks initiating from bedding planes in the specimens containing anti-dip flaws. Finally, the results are applied to predict the failure mode and fracture evolution in bedded rock slopes.

Influence of Bedding Planes on the Mechanical Characteristics and Fracture Pattern of Transversely Isotropic Rocks in Direct Shear Tests

Bedding planes are the primary control on the anisotropy of mechanical characteristics and fracture patterns in rock. To analyze the influence of the geometrical properties of bedding planes on the direct shear strength characteristics and fracture patterns of transversely isotropic rocks, numerical models were established using an improved modeling method using Particle Flow Code. The results of the numerical model were in good agreement with those of the physical experiments of an artificial rock mass containing a single bedding plane. The results show that the shear fractures with a range of bedding plane geometries can be divided into two patterns. When the inclination angles of the bedding planes were larger or smaller, a thoroughgoing fracture plane was formed near the preexisting shear fracture plane. On the other hand, the intact rock was broken into many parallel sheets.

Fracture pressure model for inclined wells in layered formations with anisotropic rock strengths

Fracture pressure is one of the most important foundations of drilling engineering, and the anisotropy of tensile strength is also a distinct feature of layered formations, but the anisotropy of tensile strength has seldom received attention in the analysis of fracture pressure. In the present paper, a novel fracture pressure model was proposed for inclined wells in layered formations with anisotropic rock strengths. Firstly, the characteristics of anisotropic tensile strength were investigated. Secondly, four typical anisotropic tensile criteria were collected and assessed using the maximum absolute relative error (MARE), the average absolute relative error (AARE), and the standard error (SE). Thirdly, the fracture pressure model was proposed based on the stress distribution model and anisotropic tensile criterion. Finally, the influences of anisotropy ratio, bedding planes' occurrence, well path, in-situ stress and pore pressure on fracture pressure were investigated. The results indicated that the layered rocks display the distinct anisotropy in tensile strength, and the strength increases with the loading angle. The MARE, AARE and SE of the Nova-Zaninetti criterion is the lowest for all six kinds of typical layered rocks, followed by the Lee-Pietruszczak, Hobbs-Barron, and Single Plane of Weakness criteria, and therefore the Nova-Zaninetti criterion is recommended. The higher the anisotropy ratio in tensile strength, the higher the influence on fracture pressure involved. The influence of bedding planes’ occurrence mainly depends on the initiation position of the fractures, the influence increases with increasing the horizontal stress ratio and pore pressure. Thus, the influence of anisotropy cannot be ignored in the prediction of fracture pressure.

New Method for Calculating Wellbore Collapse Pressure in Shale Formations

We have developed a simple and accurate method for calculating the equivalent density of wellbore collapse pressure in shale formations, which can be used for well path optimization. The calculation takes into account in-situ stresses, occurrence of bedding planes (direction, dip), the strength of bedding planes, rock strength, wellbore trajectory (deviation, azimuth), and also the external stress caused by mud filtrate invasion. We consider the influence of bedding planes on the equivalent density.

Numerical Investigation into the Influence of Bedding Plane on Hydraulic Fracture Network Propagation in Shale Formations

Shale formations are often characterized by low matrix permeability and contain numerous bedding planes (BPs) and natural fractures (NFs). Massive hydraulic fracturing is an important technology for the economic development of shale formations in which a large-scale hydraulic fracture network (HFN) is generated for hydrocarbon flow. In this study, HFN propagation is numerically investigated in a horizontally layered and naturally fractured shale formation by using a newly developed complex fracturing model based on the 3D discrete element method. In this model, a succession of continuous horizontal BP interfaces and vertical NFs is explicitly represented and a shale matrix block is considered impermeable, transversely isotropic, and linearly elastic. A series of simulations is performed to illustrate the influence of anisotropy, associated with the presence of BPs, on the HFN propagation geometry in shale formations. Modeling results reveal that the presence of BP interfaces increases the injection pressure during fracturing. HF deflection into a BP interface tends to occur under high strength and elastic anisotropy as well as in low vertical stress anisotropy conditions, which generate a T-shaped or horizontal fracture. Opened BP interfaces may limit the growth of the fracture upward and downward, resulting in a very low stimulated thickness. However, the opened BP interfaces favor fracture complexity because of the improved connection between HFs and NFs horizontally under moderate vertical stress anisotropy. This study may help predict the HF growth geometry and optimize the fracturing treatment designs in shale formations with complex depositional heterogeneity.

Analysis of Fracture Initiation Pressure of Horizontal Well at Anisotropic Formation Using Boundary Element Method

With the influence of bedding plane, the mechanical properties of bedded rocks, such as shale and coal rock at vertical and parallel bedding direction show comparatively significant difference. Therefore, compared with homogeneous rocks, bedded rocks exhibit obvious anisotropic mechanical properties. Currently, most fracture initiation pressure prediction models simplify rock to homogeneous dielectric elastomer, which is inapplicable to calculate wellbore fracture initiation pressure at bedded stratums. Given that the mechanical properties of bedded rocks at bedding plane direction are the same, this paper established a calculation model of horizontal well fracture initiation pressure in the anisotropic formation and set up a numerical solution method of circumferential stress around the borehole by means of using boundary element method. In addition, a calculation method for fracture initiation pressure based on tensile strength criterion and simplex algorithm was also proposed. The research results indicate that: In the case of high elasticity modulus anisotropy ratio, the wellbore fracture initiation pressure is low; when the elasticity modulus at vertical bedding direction is greater than that at parallel bedding direction, fracture initiation pressure decreases with the increase of bedding dip angle; the shape of wellbore has imposed an significant influence on fracture initiation pressure. For non-circular wellbore, fracture initiation pressure increases with the increase of length-width ratio in the case of smaller ovality. When wellbore shape transforms from equiaxial circle to long and narrow ellipse, fracture initiation pressure decreases with the increase of stress concentration level. Thus, this paper performed calculation and comparison with classical solutions for particular cases similar to isotropic body of stratum using the model established which verified the validity of the proposed theory. According to the research results, a new method for precise calculation of fracture initiation pressure of horizontal well at bedded stratum was provided.

The influence of bedding-planes on the development of karst caves (a study of Velika Dolina at Škocjanske Jame caves, Slovenia)

There have been fewer researchers looking for initial flow paths along karst bedding-planes than those who deduced the origin of cave channels from tectonic structures. The aim of my research was to focus scientific attention on the lithology where answers might be expected. The study has conformed that the basic idea of bedding-planes being important in the initiation of cave channels but also has shown that the interrelation is different to what had been supposed. Single lithological, petrological or stratigraphical parameters of the inception are only partly known, or merely guessed. My researches threw light on the problem of initial channels met in the Velika Dolina collapse doline at Skocjanske Jame caves. Cave passages or their fragments and other traces of the underground karstification do not appear scattered at random on the walls of the doline but they are obviously gathered along small number of so calledformative bedding-planes.

Influence of bedding planes on stability of boreholes : Desai, CS Army Waterw. Expt. Stat. Vicksburg, Miss, USA Johnson, LD Army Waterw. Expt. Stat. Vicksburg, Miss, USA Conference. 6F, 1T, 8R. Proc. Third Congress, Int. Soc. Rock Mech. Denver 1974, V2, Part B, 1974, P997–1002

Influence of bedding planes on stability of boreholes : Desai, CS Army Waterw. Expt. Stat. Vicksburg, Miss, USA Johnson, LD Army Waterw. Expt. Stat. Vicksburg, Miss, USA Conference. 6F, 1T, 8R. Proc. Third Congress, Int. Soc. Rock Mech. Denver 1974, V2, Part B, 1974, P997–1002