Anisotropic Coal Research Articles

Effect of adsorption-induced matrix swelling on coal permeability evolution of micro-fracture with the real geometry

Gas transport in coal induces effective stress variation, matrix swelling/shrinkage, and significantly affects matrix and fracture deformation, resulting in porosity and permeability evolution. However, the heterogeneity and anisotropy of coal are neglected in dual porosity models, which can lead to the deviation from the real physical mechanisms. To uncover the permeability evolution, especially the influence of dynamic matrix-fracture interaction for real fracture distribution, advanced virtual simulation is proposed. In this study, real fracture geometry is taken into account in the physical model based on the CT-scan image, while the mathematical models for coal deformation and gas flow are established. Our calculations are verified against a long-term experimental data under the same boundary condition. Accordingly, the real matrix-fracture interaction caused by adsorption-induced matrix deformation has been visually exhibited, and some new insight into the behavior of fracture permeability in real materials is offered. The results indicate the non-uniform distribution of fracture geometry is responsible for the nonmonotonic change of permeability. It also found that injection pressure, Langmuir strain constant and initial matrix permeability have important influences on the fracture permeability evolution. This research provides valuable insight into the understanding of the permeability change for the real fracture spatial distribution in coal.

Fracturing behaviours and AE signatures of anisotropic coal in dynamic Brazilian tests

Anisotropy caused by bedding structures has a great effect on tensile properties and fracture behaviours of coal. Acoustic emission (AE) monitoring is a powerful tool in understanding the anisotropy of inherent failure mechanisms. Brazilian disc tests in a split Hopkinson pressure bar (SHPB) system were conducted on coal specimens with bedding orientations β (0°, 30°, 45°, 60°, and 90°) to investigate the damage evolution and AE characteristics. Lead Zirconate Titanate (PZT) sensors were attached on the specimens to monitor AE activities during dynamic fracturing. To obtain complete AE signals, an attenuator formed by a linear resistor was innovatively used in the circuit, which can reduce the power of a signal without distorting its waveform. Meanwhile, two high-speed cameras and the digital image correlation (DIC) technique are synchronously used with AE monitoring to record real-time coal fracturing processes and verify AE results. Results show that the jump of AE energy can be observed when the dynamic load approaches its peak, which indicates the onset of crack growth. The time to fracture tf determined by AE energy against β shows an inverted “V” shape, with the lowest and highest values at β = 0° and 60°, respectively. AE parameters like amplitude, counts and duration are heavily affected by anisotropy, among which AE count-duration relationship can be used to classify different failure modes. Moreover, the influence of anisotropic bedding planes on the evolutions of AE energy with crack growth is quantitatively presented.

Modeling of anisotropic coal permeability under the effects of matrix-fracture interaction

Research relating to changes in coal's permeability has a positive significant effect for coal mining safety. The construction of an accurate permeability model through appropriate methods would help to understand the flow characteristics of coalbed methane (CBM). However, it is worth noting that it is necessary to include the influence of anisotropy and heterogeneity when analyzing changes in coal's permeability, otherwise the results would distort the actual conditions. In this work, we considered, fully, the interaction of coal matrix-fractures, and established a coal permeability model. In addition, the effects of stress compaction, sorption-induced deformation, including the slippage effect have been incorporated into the permeability model. With respect to this proposition, the anisotropic strain law for coal has been analyzed based on the generalized Hooke constitutive model. And the anisotropic sorption-induced deformation mechanism in coal has been analyzed through an improved sorption-induced deformation equation. Following experimental results, an anisotropic coal permeability model was constructed. Through the constructed permeability model, we described further the change characteristics relating to anisotropic permeability in coal. In addition, the permeability model was verified through an anisotropic seepage laboratory test. The influence of the elastic modulus reduction coefficient, and the ratio of the elastic modulus to adsorption expansion modulus on coal's permeability have also been analyzed. Further, the elastic modulus reduction coefficient that determines the fracture width and porosity, thereby affecting coal's permeability, has also been considered. Moreover, the ratio of the elastic modulus during adsorption expansion that determines sorption-induced deformation, and indirectly affects permeability, has also been calculated. This work aims to further understand changes in gas' permeability under the combination of multiple fields during coal mining through the theoretical results from the proposed anisotropic permeability model.

Mechanical anisotropy of coal under coupled biaxial static and dynamic loads

Understanding the mechanical properties and fracturing behaviours of coal is significant for the safety of underground mining engineering. Coal failure characteristics are highly influenced by the anisotropy and loading conditions. In this study, the coupled biaxial static and dynamic tests are conducted on coal specimens with five bedding orientations θ (i.e., 0°, 30°, 45°, 60°, and 90°) with respect to the normal direction to loading. A Triaxial Hopkinson bar (Tri-HB) system is adopted to apply the biaxial quasi-static stress first and then dynamic loading at four impact velocities (i.e., 10, 13, 17, and 21 m/s). Real-time processes of coal fracturing are recorded by two high-speed cameras, and accordingly, full-field deformation and ejection velocities are identified by the three-dimensional digital image correlation (3D-DIC) technique. Moreover, the internal fracture morphology of coal specimens is characterised using synchrotron-based X-ray computed tomography (CT). Experimental results show that at similar strain rates, the peak stress against θ shows a “U” shape with the lowest value at θ = 60°. The peak stress increases with increasing impact velocity, while its growth rate exhibits a downward trend revealing a decreasing sensitivity to strain rate. Coal ejection velocities are positively rate-dependent, and the highest ejection velocity is found shifting from θ = 45° to θ = 90° with increasing impact velocity. The average fragment size of coal specimens is negatively related to impact velocities and energy absorption, and the finest fragmentations are observed at around θ = 45°. Dynamic behaviours of coal under biaxial pre-stresses are dependent on bedding structures and strain rates, while the bedding effect becomes weak as strain rate increases.

Experimental Investigation of CS2 Extraction to Enhance the Permeability of Coal

To examine the permeability enhancement effect of the carbon disulfide (CS2) extraction on block coal samples, in this study, different cylindrical samples of anthracite coal were selected to conduct CS2 extraction experiments. Comprehensive measurement techniques, including Fourier transform infrared spectroscopy, X-ray diffraction, low-temperature nitrogen adsorption, industrial computed tomography, and permeability measurements, were employed to analyze the variations of in the chemical structure, microcrystalline structure, pore–fracture structure, and coal permeability before and after the CS2 extraction treatment. The results showed that CS2 extraction can improve the connectivity of multi-scale migration channels, with which the coal permeability can be enhanced. Also, it was found that the permeability enhancement performance by the CS2 extraction is related to the coal anisotropy, as the coal permeability in the vertical bedding direction is smaller than that in the parallel bedding direction before and after CS2 extraction. Also, the CS2 extraction had a time effect on enhancing the coal permeability, and the permeability enhancement was more significant after a longer period of CS2 treatment. It can be predicted that the permeability can be enhanced up to more than 1 mD when the extraction time exceeds 6 h. Also, we concluded that CS2 fracturing is worth exploring by the on-going studies to enhance the permeability and CBM recovery in low-permeability reservoirs.

A permeability model for anisotropic coal masses under different stress conditions

The heterogeneous and anisotropic nature of coal can have an important impact on its permeability. Coal permeability is usually directly related to the stress and hence, it is of great significance to understand the evolution of anisotropic coal permeability under different stress conditions. In this work, based on the stress-strain relationship of coal and the interaction between coal fractures and matrix, an anisotropic permeability model was constructed considering different stress conditions. The proposed model is further improved by considering the combined influence of the effective stress, pore pressure and slippage effect. The proposed permeability model is verified using available experimental data from previously published studies, which has shown close agreement with the experimental results in predicting coal permeability under different stress conditions. Cleat compressibility exhibited anisotropy and was dependent on the orientation of gas flow. The variation rule between coal permeability and effective stress was determined to be in accordance with a negative exponential function. The model clearly illustrates strong anisotropy in both magnitude and variation of directional permeability.

True triaxial strength and failure characteristics of cubic coal and sandstone under different loading paths

In deep underground coal mining, engineering activities are performed within anisotropic in-situ stress fields due to engineering disturbances and tectonic stress. Many such activities involve the development of excavations in soft rock and anisotropic coal. Accordingly, studying the mechanical properties of soft rocks is important for the stability of deep underground excavations. In this study, the deformation, strength, and failure characteristics of soft sandstone and raw coal under two different true triaxial loading paths were investigated using a self-developed true triaxial test apparatus. The results indicated that the inelastic strain in the pre-peak stage of sandstone and coal gradually increased with increasing intermediate principal stress. Also, the strength-drop in the post-peak stage increased. The crack initiation stress, crack damage stress, and peak strength of sandstone and coal first increased and then decreased with increasing intermediate principal stress for a given σ3. Moreover, with increasing intermediate principal stress, the failure mode of sandstone and coal changed from shear to tensile shear, and from brittle to semi-brittle. The linear Mogi criteria as found to characterize the true triaxial strength of coal well, while the modified Lade criteria was more applicable to soft sandstone. Owning to the symmetry assumption, the linear Mogi criteria predicted low strength when the intermediate principal stress coefficient exceeded to 0.5. In addition, the peak strength curve of rock on the π plane and the influence of weak structures on the failure mode of anisotropic coal was discussed. Weak structures have an important influence on the failure mode of coal, which depends on the strength difference between the structural plane and the coal rock mass. The strength envelope on the π plane had a significant stress Lode angle effect, which gradually decreased as the mean stress increased.

Mixed Hardening Characteristics of the Anisotropic Coal under Cyclic Loading

Pulse fracturing has been used to increase permeability and weaken the strength of the coal seam, making the coal fracture under cyclic loading. During the cyclic loading, the rock-like materials tend to present mixed hardening (mixed mode of isotropic and kinematic hardening) from initial yielding to failure (critical yielding). At present, understanding of the mixed hardening characteristics in anisotropic coal involving massive cleats remains challenging and crucial. In this paper, the cylinder specimens (diameter: 50 mm; length: 100 mm) of the coal were tested under cyclic uniaxial loading (loading rate: 900 N/s), and acoustic emission (AE) was employed to characterize the hardening process. The samples were drilled at the angle (α) of 0°, 45° and 90° with the coal surface cleat respectively. The upper-stress limit (P max) increases by 2 kN at each loading cycle, and the lower-stress limit (P min) was kept at 1 kN. Several findings were obtained based on the experimental results. (1) Uniaxial compression strength and the cyclic number increase with α, presenting isotropy. (2) The remarkable accumulation of AE energy is the feature of identifying plastic hardening. Both the isotropic and kinematic hardening processes are significant for the specimen with α dip angle (α) of 90°, validated by the dramatically increased isotropic and kinematic hardening indexes. However, the coal presents a slight isotropic and kinematic hardening, with a ranging from 0° to 45°. (3) The remarkably mixed hardening (α=90°) corresponding to a complex fragmentation of the coal, which is supposed to be caused by the continuous weakening of coal matrix (including the butt cleat). In contrast, at α of 0° and 45°, the mixed hardening characteristics are slight. Accordingly, the final fracture surfaces of coal tend to be relatively single, roughly along with the surface cleats. Thus, we can infer that the slightly mixed hardening is due to the weakening of surface cleats. Based on the mixed hardening characteristics of anisotropic coals, conducting pulse fracturing to apply cyclic loading perpendicular to the cleat surface is supposed to be beneficial for generating more complex fractures, to improve coal permeability.

Analysis of hydraulic fracture behavior and well pattern optimization in anisotropic coal reservoirs

Macrolithotypes control the pore-fracture distribution heterogeneity in coal, which impacts stimulation via hydrofracturing and coalbed methane (CBM) production in the reservoir. Here, the hydraulic fracture was evaluated using the microseismic signal behavior for each macrolithotype with microfracture imaging technology, and the impact of the macrolithotype on hydraulic fracture initiation and propagation was investigated systematically. The result showed that the propagation types of hydraulic fractures are controlled by the macrolithotype. Due to the well-developed natural fracture network, the fracture in the bright coal is more likely to form the “complex fracture network”, and the “simple” case often happens in the dull coal. The hydraulic fracture differences are likely to impact the permeability pathways and the well productivity appears to vary when developing different coal macrolithtypes. Thus, considering the difference of hydraulic fracture and permeability, the CBM productivity characteristics controlled by coal petrology were simulated by numerical simulation software, and the rationality of well pattern optimization factors for each coal macrolithotype was demonstrated. The results showed the square well pattern is more suitable for dull coal and semi-dull coal with undeveloped natural fractures, while diamond and rectangular well pattern is more suitable for semi-bright coal and bright coal with more developed natural fractures and more complex fracturing fracture network; the optimum wells spacing of bright coal and semi-bright coal is 300 m and 250 m, while that of semi-dull coal and dull coal is just 200 m.

Experimental Study on Coal Damage Subjected to Microwave Heating

Coalbed methane (CBM) reservoirs usually require stimulation to enhance the permeability, thereby promoting CBM production. Realized by the conversion of the electromagnetic energy into heat, microwave heating may be an effective CBM stimulation method. In this study, the effects of microwave heating on the damage of coal were investigated through scanning electron microscope and ultrasonic wave tests. The results indicate that the mineral grain in coal generally disappears after microwave heating. For low-permeable coals with highly mineralized fractures, the dissolution of mineral could potentially enhance permeability and improve stress resilience. Microwave selective heating can lead to the opening and extension of original fractures and the generation of secondary fractures, reducing P-wave and S-wave velocity of coal. The stronger the anisotropy of coal is, the greater the microwave fracturing effect becomes. In a word, microwave heating has potential for assisting coalbed methane exploitation.

Dynamic mechanical properties of structural anisotropic coal under low and medium strain rates

The static mechanical properties of coal rock show anisotropism, which makes the permeability have anisotropic characteristics partly. The dynamic impact mechanical characteristics of structural anisotropic coal under low and medium strain rates were studied by using self-made vertical Split Hopkinson Bar (SHPB) equipment. The peak stress, the strain rate, dynamic elastic modulus and failure characteristics of raw coal with three coring directions were analyzed under the influence of five impact loads and structural anisotropy. The peak stress increases linearly with impact load, and the maximum strain rate and the dynamic elastic modulus increase exponentially with impact load. The coal samples display anisotropic mechanical characteristics. The values of maximum strain rate, peak stress and dynamic elastic modulus are ranked with directions by the perpendicular to bedding direction (Z direction), the parallel to bedding direction (X direction), and the oblique 45° to bedding direction (Y direction). Dynamic mechanical properties of structural anisotropic coal provide a theoretical basis for gas seepage in far-blasting field.

Fractal and pore structure analysis of structural anisotropic coal under different impact loads

Pore structure is the key factor that affects the adsorption/desorption of coal. In order to study the change characteristics of pore structure in structural anisotropic coal under different impact loads, the low- temperature liquid nitrogen adsorption tests were carried out after the split Hopkinson bar (SHPB) impact, then the pore structure variation and fractal dimension were calculated and analyzed. The results show that before and after impact, the adsorption isotherms change from type II (raw coal) to type III (impacted coal). With impact loads increasing, adsorption and desorption isotherms change from hysteresis loops to overlap which are determined by open pore structure and dead end pore structure. Meanwhile, mesoporous and micropores in coal samples are much more obviously damaged by larger impact load, and the adsorption capacity becomes smaller, especially for coal samples in parallel to the bedding. Pore volume mainly distribute in mesoporous and macropores, and SSA mainly distribute in mesoporous and micropores. On the whole, the pore volume and SSA decrease with the increase of impact load. Not only pore volume but also SSA in perpendicular to the bedding are larger than that in parallel to the bedding under same impact load. Fractal dimension D2 decreases with impact load increasing, which causes the adsorption capacity linearly reduces.

A new anisotropic coal permeability model under the influence of stress, gas sorption and temperature: Development and verification

Modeling approach to permeability especially anisotropic permeability of coal in front of working face is of significance for gas extraction in coal mining. Considering three-dimensional stress condition and multi-field influence, in this study, an anisotropic permeability model was suggested for coal in front of working face. The anisotropic permeability was assumed to be controlled by the directional fracture strain under the condition of multi-field influence caused by effective stress, adsorption-desorption and thermal stress. The directional fracture strain induced by effective stress was governed by the generalized Hooke constitutive model. Based on the matrix-fracture interaction, the constitutive model of directional fracture strain induced by adsorption-desorption was established from the improved Langmuir equation. Assuming that the coal matrix was connected by the matrix bridge, the constitutive model of directional fracture strain induced by thermal stress was constructed by proposing the thermal internal swelling coefficient. Consequently, a new anisotropic coal permeability model was developed based on the directional fracture strain under the influence factor of stress, gas sorption and temperature. Moreover, the new permeability model was verified by laboratory tests under different effective stresses, gas pressures, temperatures and their combined influence. The theoretical results of proposed anisotropic permeability model may provide a better understanding of permeability evolution in multi-field coupling geological environment during coal mining.

Experiment and Modeling of Permeability under Different Impact Loads in a Structural Anisotropic Coal Body.

A mount of bedding and cleat in a coal body causes that the mechanical property and gas permeability are anisotropic in a coal seam, partly. To reveal the permeability change law of the impacted coal, a self-developed vertical split Hopkinson pressure bar (SHPB) device is used to carry out the dynamic impact mechanical property tests of coal samples in three different coring directions under five impact loads and then the permeability of the impacted coal samples is measured by a permeability measuring instrument under different gas pressures. Finally, a calculation model for the anisotropic coal permeability is established to analyze the permeability distribution law in any direction with different angles to the bedding plane. The results show that with an increase in impact height the dynamic peak stress of coal samples increases gradually, which shows a linear growth relationship. The permeability of the impacted coal samples is much larger than that of raw coal samples, and the bigger the impact load, the larger the permeability. Moreover, under the same impact load and gas pressure, the permeability is the largest in parallel to the bedding direction, followed by that in oblique 45° to the bedding direction, and the smallest in perpendicular to the bedding direction. The permeability calculated by the anisotropic model in oblique 45° to the bedding direction is in good agreement with the measured results, and the errors are no more than 10%, which will provide a theoretical basis for the permeability distribution law of the coal seam after deep-hole blasting.

The seepage properties and permeability enhancement mechanism in coal under temperature shocks during unloading confining pressures

To understand the gas penetration properties during unloading confining pressures and permeability enhancement mechanism in coal, temperature shocks, seepage tests, NMR, CT and SEM scanning tests were carried out with trimmed coal samples. The results showed that the coal permeability increased with the decrease of confining pressures after temperature shocks performed, and that the coal permeability fully followed an exponential change law. To achieve a higher gas permeability, heating-cooling shocks should be executed in low stress regions and stress-releasing areas in coal. According to the NMR results, the increase in pore volume improved the gas diffusion coefficient of coal. The CT and SEM scanning results declared that the growth and extension of internal and surface cracks eventually enhanced the coal permeability. After temperature shocks performed, it can be concluded that coal anisotropy and excessive thermal stresses are the essential causes of the coal damage and gas permeability enhancement.

Mechanical Properties and Failure Behavior of Dry and Water-Saturated Anisotropic Coal Under True-Triaxial Loading Conditions

In underground coal mining, coal failure generally occurs due to the relatively weak strength of the coal and the high applied mining-induced stresses. The outer complex geological conditions (i.e., tectonic structure and water intrusion) and internal structural anisotropy of the coal introduce uncertainty in predicting its mechanical properties and failure behavior. In this study, laboratory investigations of the mechanical properties and failure behavior of dry and water-saturated anisotropic coal samples subjected to different true-triaxial loading stresses were conducted. The effects of water weakening, intermediate stress, and structural anisotropy on the mechanical properties and failure behavior of the coal were systematically studied. The results indicate that the presence of water significantly reduced the strength, elastic modulus, and strength anisotropy of the coal. The maximum stress at failure first increased and then decreased with increasing intermediate stress. The residual strength-to-peak strength ratios and failure plane angles of the coal showed a linear increase with increasing intermediate stress. When the coal samples were loaded in the bedding plane direction, the brittleness of the coal was higher than when they were loaded in the other two cleat plane directions. In addition, when the coal samples were loaded in the butt cleat plane direction, the brittleness of the coal decreased with increasing intermediate stress. Two typical failure modes of the dry and water-saturated coal samples were observed: shear and mixed splitting and shear failures. The dominant failure mode of the coal also varied with the loading direction relative to the weakness planes, which could be well recognized and predicted by the acoustic emission (AE) characteristic curves. To further reveal the fracture mechanism, the microcrack patterns of the coal were further identified based on the AE parameters.

Investigation of Anisotropic Deformation and Stress-Dependent Directional Permeability of Coalbed Methane Reservoirs

Two types of experiments were conducted under different boundary conditions to characterize the anisotropic deformation and directional permeability of the coal sample. Gas sorption-induced strains in the three principal directions were studied to examine the anisotropic deformation. The results show that the coal sample can be treated as isotropic in those planes parallel to the bedding direction. However, the strong strain anisotropy of coal indicates that the gas sorption-induced matrix strain in the direction parallel to the bedding is smaller than that which is perpendicular to the bedding, under different hydrostatic pressures for unconstrained conditions. Permeability measurements were conducted under different effective stresses to determine the magnitude and orientation of directional permeability. It was found that cleat distribution plays a dominant role in the magnitude of the measured permeability. After the two principal permeabilities parallel to the bedding planes are calculated, Mohr’s circle of permeability can be then determined and used to estimate permeability in other directions of the bedding. The experimental results show that the principal permeability directions vary under different effective stresses, illustrating that coal cleat system would be reoriented during gas production. It was also found that cleat reorientation is attributed to three factors: stress contrast, the difference of Biot’s coefficient, and the difference in the sorption-induced stresses in the direction perpendicular to and parallel to the bedding planes. However, cleat reorientation had little influence on changes in the magnitude of coal permeability. Permeability anisotropy degree between the three principal directions varies dynamically under different effective stresses, and then possibly changes direction of gas flow. This study demonstrates that stress-dependent coal anisotropy should be considered for simulating gas flow behavior and predicting coalbed methane production.

Analysis of the strength and break state of anisotropic coal in indirect tensile test

The article is to perform the results of the indirect tensile test with two combination samples of dim coal (peat) which has the anisotropic angle of α = 00 and α = 900. The results showed that the tensile strength in both cases above was anisotropy. However, the case of α = 900 has the anisotropic angle larger than the case of α = 00, or in another word, the featured anisotropic tensile strength depends on the angle which was created by bedding and loading direction.

Mechanical anisotropy of coal with considerations of realistic microstructures and external loading directions

This paper investigates the anisotropy of coal under uniaxial compression conditions using both experimental and numerical methods. Primary wave (P-wave) velocity measurements, X-ray micro-computed tomography (CT) scanning, and uniaxial compression tests were conducted to assess the anisotropic characteristics of selected coal samples. Based on the experimental results, comprehensive analyses were carried out to study the influences of different contributing factors on the anisotropic properties of coal. A three-dimensional (3D) finite difference model was then built to further investigate the effects of failure initiation, stress redistribution and the presence of microstructures on the anisotropy of coal. The experimental and numerical results of uniaxial compression tests indicate that the coal strength anisotropy is affected by the directional distribution of the microstructures. Bedding planes directly control the uniaxial compressive strength of coal, and face cleats have a greater influence on the mechanical properties than butter cleats. The presence of mineral inclusions increases the heterogeneity of coal because of the significant differences in strength and deformation in the mineral inclusion zones. The mechanical and seismic anisotropies are both directly related to the directional distribution of the microstructures, which is demonstrated by the variations in the anisotropic strength and P-wave velocity of the coal. The correlations between the uniaxial compressive strength (UCS) and P-wave velocity are different when the coals are loaded in different directions, and an exponential correlation was suitable for defining this relationship.

Deformation and permeability evolution of coals considering the effect of beddings

Coal belongs to sedimentary rock with obvious bedding planes. The existence of bedding weakens the continuity and integrity of coal. Deformation and permeability anisotropy of coals, to a certain extent, is attributed to the bedding planes. We performed experiments of deformation and permeability evolution of coals under true triaxial stress conditions. The experimental results indicate that the volumetric strain and bedding jointly affect permeability. An analytical model for stress-strain relationship of coals containing beddings is developed to quantify the bedding effects under true triaxial stress conditions. In the model, the deformation and permeability of the bedding and non-bedding are separated through bedding ratio and bedding elastic modulus. The bedding ratio is defined as the ratio of the total length of bedding to the total length of coal normal to bedding planes, which is a parameter closely related to the porosity. The elastic modulus is divided into four parts: a bedding elastic modulus and a non-bedding elastic modulus in the direction normal to bedding planes, and two elastic moduli in the direction parallel to bedding planes. The four elastic moduli are calculated by the analytical model. The bedding elastic modulus is relatively small, which shows that the bedding effect cannot be ignored. The permeability is highly sensitive to the bedding of coal with low stress in the direction normal to the bedding. The bedding ratio always affects the volumetric strain. The permeability of the coal is divided into bedding and non-bedding permeability. The ratio of the bedding permeability to the total permeability is different due to different stresses in the direction normal to bedding planes, which is linking with the compression of bedding. A permeability model which can reflect the different effects of beddings and non-beddings to the total permeability is proposed and verified.