Anisotropic Coal Permeability Research Articles

Experimental and numerical model of anisotropic permeability and CO2 injectivity of coal during CO2 enhanced coalbed methane recovery process

Guaranteeing CO2 injectivity has been the precondition for implementing the CO2-enhanced coalbed methane recovery (CO2-ECBM), however, it dramatically decreases during the CO2 injection process because it is influenced by the dynamic change of the anisotropic permeability of coal. To reveal and evaluate the CO2 injectivity of coal, the anisotropic permeability test and the CO2 injectivity simulation test were first conducted, then the corresponding numerical models were established and verified by the experimental data. The results show that the permeability of coal in parallel face cleat direction is the largest, followed by the permeability of coal in parallel butt cleat direction and that in vertical bedding direction is the minimum. The peak value of the instantaneous injectivity rate is enhanced and the injection time is prolonged with the increase of the CO2 injection pressure. The total CO2 injectivity rate is nonlinearly increased from 13.61 to 311.87 cm2/MPa min when the CO2 injection pressure raises from 2 to 10 MPa. The anisotropic permeability model is appropriate to describe the dynamic evolution of permeability under different boundary conditions, the CO2 injectivity prediction model can be used to evaluate the CO2 injectivity during the CO2 injection process. Increasing the CO2 injection pressure may temporarily promote CO2 injectivity, while the CO2 injection increment is limited. The CO2 fracturing by phase transition may be an available reservoir stimulation method for enhancing the CO2 injection and should be focused on in the future.

Characterization of anisotropic coal permeability with the effect of sorption-induced deformation and stress

Changes in permeability are related to coalbed methane (CBM) production activities, and to coal mining safety. However, inherent properties, i.g. anisotropy and heterogeneity make the gas flow mechanism more complex. In this work, an anisotropic coal permeability model with synergistic stress compression and gas adsorption was constructed to simulate gas flow behavior in all directions in coal. Changes in coal anisotropic permeability are restricted by the fracture deformation in coal in all directions. Wherein, the fracture deformation in coal is controlled by the effective volume strain coordinated by gas adsorption and stress. The influence of stress can be expressed by employing the generalized Hooke’s law. Then, based on the coal matrix bridge structure, an internal swelling coefficient has been proposed to assess the control of the matrix swelling on fracture width during the gas adsorption process to further obtain the fracture deformation caused by gas adsorption. The anisotropic coal permeability model was verified by laboratory test data under different effective stresses, gas pressures, with respect to their combined influences. Subsequently, based on the experimental conditions of uniaxial strain, the permeability model of anisotropic coal during gas depletion was further deduced, and changes in anisotropic coal permeability under uniaxial strain were also calculated. In addition, the sensitivity of the reduction rate of the elastic modulus, and the internal swelling coefficient was also studied, to reveal the matrix-fracture interaction mechanism.

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.

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.

A Stochastic Method to Estimate the Anisotropic Stress-Dependent Coal Permeability by Pore-Volume Distribution and Stress-Strain Measurements

Summary A novel, simple, economical, and time-effective method to estimate the anisotropic permeability of coal is presented in this paper. This method estimates the coal’s anisotropic permeability by avoiding the tedious experimentation using triaxial permeameter or history-matching exercises. This method calculates the absolute magnitude of the permeability of the sample. In this regard, it is unlike other analytical permeability models, such as given by Palmer and Mansoori (1998) and Shi and Durucan (2014), that only calculate the permeability ratio (k/k0). The motivation is to find a method by which the permeability of the coal may be determined with reasonable accuracy by using only two easy measurements: mercury intrusion porosimetry (MIP) and anisotropic stress-strain (σ-ɛ) measurement. The main blocks of the method are based on cleat size that is obtained from MIP and randomly allocated to form flow channels/cleats through the coal; these cleats form parallel paths in the orthogonal face and butt cleat directions that provide the permeability; and the cleat width (b) is stress dependent. This method is further validated by comparing with the experimentally measured stress-dependent permeability of Surat Basin (Australia) coal and German coal in face cleat and butt cleat directions.

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.

Anisotropic coal permeability estimation by determining cleat compressibility using mercury intrusion porosimetry and stress–strain measurements

This paper presents a novel method to calculate the anisotropic and stress-dependent coal permeability by determining cleat compressibility using the Mercury Intrusion Porosimetry (MIP) and stress–strain measurements. Cleat compressibility is often assumed isotropic and constant in the literature and is usually obtained by numerical fitting to a matchstick permeability model e.g. the model by Seidle et al. (1992) despite the gross simplification of this representation of the coal pore network. This paper provides a method to calculate anisotropic cleat compressibility using only MIP and stress–strain measurements which are easy to conduct, and without permeability information, which is harder to come by, requiring laboratory experiments on the core or through fitting field data. We report the measured stress–strain behaviour of a coal sample, with hydrodynamic loading/unloading over the range 0.5–4.0 MPa, and the permeability in face cleat (kF) and butt cleat (kB) directions using the Triaxial Stress Permeameter (TSR). The stress–strain measurement is used to calculate the anisotropic modulus of elasticity (EF, EB, and EV) in face cleat, butt cleat and bedding plane directions, and cleat compressibility in the face cleat (CfF) and butt cleat (CfB) directions using the fractal dimension analysis with MIP measurement. Finally, the cleat compressibilities are used to calculate the anisotropic coal permeability by Seidle et al. (1992) permeability model and compared with the measured permeability of the coal sample.

Anisotropic coal permeability and its stress sensitivity

This paper focuses on anisotropy of coal permeability and its stress sensitivity. Coal blocks were taken from Xinjing Coal Mine in Yangquan Coal District. Coal cores were then drilled along the strike, dip, and vertical directions. Coal permeabilities were measured with respect to stress by using a self-developed coal permeability measurement system. The used samples exhibited significant permeability anisotropy. The permeability along the strike direction was greatest among the three directions, the permeability along the vertical direction was the smallest, and the permeability along the dip direction was between the other two directions. The sensitivity of coal permeability to stress was transversely isotropic. The stress sensitivity coefficient was greater along the horizontal directions than along the vertical directions. Coal permeability exhibited anisotropic stress sensitivity due to anisotropy in Young’s modulus and porosity. The results obtained in this study are useful for optimizing the arrangement of pre-drainage boreholes.

Anisotropic Permeability of Coal Subjected to Cyclic Loading and Unloading

An experimental investigation on the anisotropic permeability of coal under cyclic loading and unloading conditions is presented in this article. The results show that coal permeability decreases with increasing hydrostatic stress. The relationship between coal permeability and applied stress was quantified using an improved power model. The cyclic loading and unloading were found to induce an irreversible permeability reduction, which tended to be diminished with increasing loading/unloading cycles. The anisotropic permeability of the studied coal is notable, and it was found to nonlinearly decrease with an increase of the angle between the direction of flow and the direction of bedding planes (θ). The relationship between permeability and θ can be quantified using an exponential model with an anisotropic parameter (λ). Several factors (e.g., loading cycles, stress) influencing the anisotropic permeability are discussed in this article.

An anisotropic coal permeability model that considers mining-induced stress evolution, microfracture propagation and gas sorption-desorption effects

Anisotropic coal permeability plays an important role in the simultaneous extraction of coal and gas from deep underground. In this study, first, the representative elementary volume of a coal mass is determined based on the natural microfracture distribution in coal. Then, an anisotropic coal permeability model that considers the complex stress evolution process, microfracture propagation and gas sorption-desorption effects was established to comprehensively study the coupled process of the actual mining-induced anisotropic mechanical behaviors and the gas seepage phenomena in the coal. The main characteristic of this model is coupled the stress-fracture-seepage analyses on the mechanical behavior of coal in terms of complex stress evolution. The presented model is verified using two different experimental observations, and a precise mathematical expression of the stress evolution processes induced by different mining layouts is presented to perform the coupled analyses using the presented model. The simulation results exhibit good agreement with the experimental data and reflect the overall trends of the test results fairly well, and the simulation results can appropriately represent the actual mining-influenced coal mechanical behavior. Thus, the model is highly adaptable for simulating various stress boundary conditions and engineering disturbance processes and is applicable for conducting numerical studies on coal fracture propagation, gas sorption-desorption effects and complex mining-induced mechanical behavior. Therefore, this model provides a theoretical basis for further analyses.

Numerical investigation on the coupled gas-solid behavior of coal using an improved anisotropic permeability model

Coalbed methane (CBM) extraction uses coal seam depressurization to extract methane adsorbed into the coal matrix. The process involves complex interactions between solid and gas, and between mechanical and chemical effects. Permeability of coal is one of the most important parameters to evaluate the production of CBM reservoirs. Several models have been proposed to establish the relationship between permeability and poromechanical response, gas desorption, and coal shrinkage. However, there have been only a few attempts in describing the anisotropic permeability of coal, which is an important characteristic of gas flow in coal. This paper focuses on the anisotropic properties of coals and its effects on the coupled gas-solid behavior. We develop an improved anisotropic permeability based on linear elastic poromechanical theory and then implement this model into a finite element package to study the effects of the anisotropic permeability on the gas transport in coal. Cases with three boundary conditions including uniaxial strain, constant volume, and constant confining stress are analyzed in details. Numerical simulation results show that the proposed model can well represent the anisotropic permeability of coal. Permeability is closely related to the mechanical properties of coal. We find that modulus reduction ratio is a key parameter to control the anisotropic characteristics. Different modulus reduction ratios alter directional permeability significantly. Anisotropic permeability changes with time and pressure during gas desorption and gas flow. Further, the magnitude of directional permeability variations depends on both the modulus and the boundary conditions. Permeability evolution is controlled by the change in effective stress and matrix shrinkage due to desorption. The effects of anisotropic permeability on the coupled gas-solid behavior of coal during gas desorption and flow processes are demonstrated in this work.

Modeling anisotropic permeability of coal and its effects on CO2 sequestration and enhanced coalbed methane recovery

ABSTRACT This paper presents an experimental and numerical investigation on anisotropic permeability of coal in order to evaluate the effects of the anisotropic permeability variation on CO 2 sequestration in coal and CO 2 -sequestration enhanced coalbed methane (CO 2 -ECBM) recovery from coal seams. An alternative permeability model has been developed to improve reservoir simulation for CO 2 sequestration in coal and coalbed methane (CBM) recovery and provide better understanding of the CO 2 -ECBM process. Combined with experimental observations, the permeability model adopts a discontinuum medium approach, by which coal is treated as a discontinuum medium containing anisotropic matrixes and cleats. The permeability variations and anisotropic permeability ratios under isotropic net stresses were tested with relatively large coal samples to provide experimental information for model parameterization and validation. The model was further incorporated into a 3D coal reservoir model for reservoir simulations to investigate the impact of the anisotropic permeability of coal on coalbed methane (CBM) recovery. The simulations show good agreements with the experimental data, revealing that the developed model is superior for describing stress- and sorption-induced permeability variations in coals compared with commonly used permeability models that treat coal as isotropic media and use constant values for stress-dependent parameters. The results suggest that anisotropic permeability has significant effects on gas production and CO 2 breakthrough time, implying that it is a critical parameter in determining well pattern and orientation of horizontal wells.

Characteristic of anisotropic coal permeability and its impact on optimal design of multi-lateral well for coalbed methane production

Coal permeability is usually anisotropic and the permeability anisotropy ratio along the face cleats to the butt cleats can be up to 17:1 for some coals. The characteristic of the anisotropic coal permeability and its impact on the optimal well design for coalbed methane (CBM) production are important, but have not been well studied. This paper investigates this issue through numerical modeling and reservoir simulations. Various case studies are performed on the two commonly used multi-lateral well patterns including the quad-lateral well and the pinnate lateral well to investigate the impact of permeability anisotropy ratio on the layout of the multi-lateral well. The results demonstrate that the optimal well direction of the quad-lateral well is parallel to the butt cleat direction as expected. However, the optimal main well angle of the pinnate lateral well is significantly affected by the permeability anisotropy ratio. The orientations of the branches of the pinnate lateral well are less sensitive than the branch numbers and this indicates that more gas recovery efficiency can be effectively achieved by drilling more branches but not by varying the branch orientations. These conclusions are drawn by using a widely used stress-based permeability model with the fixed permeability anisotropy ratio. In order to investigate the permeability anisotropy change during the CBM production, a strain-based and a stress-based coal permeability models for isotropic condition are improved to incorporate the permeability anisotropy and to quantitatively study the impact of permeability anisotropy ratio change on lateral well pattern. In order to be consistent with the previous permeability model, we implement the improved stress-based model into the reservoir simulation model. The results show that the permeability anisotropy is not only caused by the initial differences in structure and tortuosity of the coal cleats in the two directions, but also induced by the anisotropic mechanical and swelling properties during the CBM production. The permeability anisotropy ratio change during production may also have a significant impact on the optimal design of the multi-lateral well.