Anisotropic Coal Research Articles

Influence radius of gas extraction borehole in an anisotropic coal seam: Underground in‐situ measurement and modeling

AbstractCoal seam degasification by in‐seam drilling borehole is one of the popular techniques for coal mine methane (CMM) control. How to address the conflict between borehole numbers and drainage durations always arouses researchers’ interests. Furthermore, since coal has anisotropy structures, how the anisotropy feature of coal associated with bedding and cleat structure affects the influence radius of the drainage borehole has rarely been considered. In order to address the above‐mentioned issues, this work not only conducted underground in‐situ measurement of the borehole influence radius within 200 days in an anisotropy coal seam at Jiulishan coal mine in China, but also modeled the influence radius evolvement around borehole in an anisotropy coal seam using finite‐element based COMSOL package. The underground in‐situ measurement revealed an anisotropy feature of borehole influence radius associated with coal bedding and cleat structures. The influence radius of borehole parallel to the bedding plane in butt cleat direction is much larger than that perpendicular to the bedding plane. Ten permeability measurements of coal show that the permeability of coal parallel to the bedding plane in butt cleat direction is higher than that perpendicular to the bedding plane. Simulation results of the gas transport model in coal are consistent with underground in‐situ measurement in determining the influence radius of drainage borehole in the anisotropy coal seam. Based on modeling results, the ellipse influence area of borehole in an anisotropy coal seam is proposed, and the relationship between the influence radius of borehole and drainage time is obtained. On the basis of these findings, the proper in‐seam borehole arrangements for a specific mining panel at different drainage durations are determined. The finding of this work will be fundamental for addressing the conflict between minimizing the cost of borehole arrangement and increasing coal mining productions by adjusting CMM drainage durations.

Aging characteristic of effective distance on injecting N2 to displace CH4 in anisotropic coal seam

ABSTRACTGas injection is an effective way to enhance methane drainage in a low permeability coal seam, and the reasonable distance between injection holes and drainage holes depends on effective influence distance of injection gas in a coal seam. For anisotropic permeability of coal seam, the variation of the gas-flow rate from a test hole in the anisotropic coal seam is measured as nitrogen is injected into the injection hole near the test hole. The effective distance regularity is revealed by nitrogen injection in an anisotropic coal seam, and the model of nitrogen injection is established to numerically analyze the aging characteristic of effective influence distance in the anisotropic coal seam. The results show that the permeability in coal seam reduced exponentially with the increase in gas pressure, and it is larger in the direction of parallel to bedding than that in the direction of perpendicular to bedding. The effective distance with nitrogen injection increased exponentially in the direction of parallel/perpendicular to bedding, and the effective distance in the direction of parallel to bedding is bigger than that in the direction of perpendicular to bedding. The optimum distances between injection hole and test hole, with 0.5 MPa of injection gas pressure and 8.36 h of injection gas time, are 2.05 m in parallel to bedding direction, and 1.53 m in perpendicular to bedding direction. This study will have important theoretical and practical significance for a reasonable layout of injection-drainage holes during injecting gas to enhance methane drainage.

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.

Anisotropy of coal at various scales and its variation with sorption

For most flow modeling work, coal is considered transversely isotropic, factors responsible for the anisotropy being fabric, pore structure at different scales and stress, among others. This study characterizes anisotropy of coal at two scales, matrix, and bulk. Matrix anisotropy was estimated under varying conditions of hydrostatic pressure, measuring strains in the three principal directions. Using the results, Anisotropy ratio (An), defined as the ratio of derivative of strains in horizontal and vertical directions with respect to pressure, and its variation were estimated for helium, methane and carbon dioxide. Next, Anisotropy ratio was estimated for bulk scale and its evolution was discussed for hydrostatic reservoir condition with methane depletion.The Anisotropy ratio was found to be constant for helium, but less than one, suggesting a definite anisotropy, to begin with. For the two sorbing gases, methane and CO2, the ratio was the same at very low pressure but varied differently with changes in pressure, its value is higher at high pressure. Hence, sorption phenomenon decreases the anisotropy of coal with an increase in pressure. Since the variation of coal matrix, as well as bulk anisotropy, varied for methane and carbon dioxide pressure differently, the variation under in situ conditions would depend on both gas composition and pressure. The implication of a variable Anisotropy ratio is that it results in an underestimation of dynamic stresses in the reservoir or, overestimation of in situ strength.

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.

Permeability changes in coal seams: The role of anisotropy

The permeability of coal seams is pore pressure-dependent. A number of analytical models have been proposed to investigate this problem, but many disregard a crucial factor: the anisotropy of coal. This paper is devoted to investigating the role of anisotropy in modeling the change of horizontal permeability with pressure. Analysis is conducted using a fully anisotropic model that incorporates both the anisotropies of mechanical properties and of the permeability dependence on stresses. Analytical expressions of the pressure-permeability relationship are derived in oedometric and isochoric geomechanical conditions, and validations are conducted against both laboratory and field data. Then, the roles of the anisotropy of stiffness and of permeability dependence on stresses in the permeability-change model are explored. We demonstrate that the mechanical anisotropy can be simplified to an isotropic model without introducing significant errors in prediction of pressure-permeability relationship, while neglecting the anisotropy of permeability dependence on stresses leads to considerable errors. When both anisotropy sources are disregarded, the pressure-permeability curve can be exactly reproduced by a totally isotropic material in both oedometric and isotropic conditions. However, the material properties (e.g., bulk modulus) are skewed; moreover, such an equivalent material might lead to significant errors in other geomechanical conditions. Finally, the permeability change is investigated at the reservoir scale, and the reservoir simulation results confirm the conclusions obtained from the analytical analysis.

Gas flow field evolution around hydraulic slotted borehole in anisotropic coal

Currently, hydraulic slotted borehole technology for strengthening gas drainage is one of the main measures to control gas disasters in deep mines. In this paper, to analyze the evolution of gas flow field around the hydraulic slotted borehole, the borehole gas drainage was simulated based on the anisotropy of coal structure and the established multi-field (namely, coal deformation field, diffusion field and seepage field) coupling model. Then, the influences of characteristic parameters, including, initial diffusion coefficient, diffusion attenuation coefficient, initial gas pressure and initial permeability, on the gas production rate and reservoir parameters of hydraulic slotted borehole gas drainage were analyzed. Finally, the results were verified by comparing the model simulation with field tests in Yangliu Mine. The results show that the anisotropy of coal affects the effect of borehole gas drainage. The initial diffusion coefficient D0 and the diffusion attenuation coefficient λ have opposite effects on key parameters of coal. With a larger D0, it is easier for gas in matrix pores to diffuse into the fracture system, and accordingly the gas production rate and relative permeability grow. A larger λ makes it harder for gas in matrix pores to diffuse into fractures, thus decreasing the gas production rate and relative permeability. The initial gas pressure pm0 and the initial permeability kfx0 and kfy0 exert the same effect on the key parameters of coal. The larger their values are, the more easily the gas in fractures can be extracted under the same negative pressure of drainage, which contributes to the diffusion of gas from the matrix into the fracture system under the action of pressure difference. Through combining actual monitoring results of 3# and 4# drilling fields in Yangliu Mine and the results of numerical simulation, it is acquired that the gas production rate measured in situ coincides with the rate simulated by the model, verifying the feasibility and engineering applicability of the model.

Experimental study of impact of anisotropy and heterogeneity on gas flow in coal. Part II: Permeability

Coal is highly anisotropic and heterogeneous, affecting coal permeability. As permeability is one of the most important reservoir properties for coalbed methane production, it is useful to understand the impact of coal anisotropy and heterogeneity on coal permeability. In this work, anisotropic permeability measurements were performed in the laboratory on three cubic samples from the same coal block from the Bowen Basin, Queensland, Australia. The permeability was measured at a series of gas and confining pressures. Cleat compressibility, a measure of permeability sensitivity to stress, was also calculated from the experimental results. Each sample was then scanned using microscopic X-ray computerised tomography after permeability measurements to study its cleat system. The results show that permeability is strongly anisotropic and heterogeneous among the three samples and is correlated with the cleat system. A permeability model, which incorporates stress, gas pressure and swelling effects, is used to describe the experimental results. At last, numerical simulations were conducted to demonstrate the impact of coal permeability heterogeneity on coalbed methane production.

Experimental investigation of in situ stress relaxation on deformation behavior and permeability variation of coalbed methane reservoirs during primary depletion

Dynamic evolution of coal deformation and permeability, induced by changes in in situ stresses with continued depletion in coalbed methane (CBM) reservoirs, has been extensively investigated both experimentally and theoretically. However, the impact of stress-/strain-controlled mechanics on permeability variation is somewhat unclear. To examine this behavior, gas flow experiments were conducted under best replicated in situ conditions. Considering that coal anisotropy may lead to changes in deviator stress and coal failure behavior during primary depletion, core flooding experiments were carried out to investigate the anisotropic behavior of coal. Using the experimental results, Mohr's circle of strain was first employed to facilitate the process of analyzing the dynamic deformation behavior of coal. Next, a strain-based failure criterion was developed to analyze the failure tendency of CBM reservoirs in order to explain the sudden increase in permeability observed during experimental work. The results show that the abrupt increase in permeability can be attributed to shear failure of coal due to increased deviator stress resulting from in situ stress redistribution after significant depletion. This study sheds light on the influence of stress relaxation on coal deformation behavior, failure tendency and permeability evolution, with practical implication for gas flow modeling of CBM reservoirs.

Experimental study on the mechanical property of coal and its application

Brazilian splitting tests, uniaxial compression tests and triaxial compression tests are carried out on the coal samples cored from Shanxi group II1 coal seam of Jiaozuo coal mine, Henan province, China, to obtain their property parameters. Considering the bedding has notable effect on the property parameter of coal, the samples with different bedding angles are prepared. The effects of bedding on the anisotropic characteristics of the coal seam are investigated. A geological geomechanical model is built based on the geology characteristics of the Jiaozuo coal mine target reservoir to study the effects of bedding on the fracture propagations during hydraulic fracturing. The effects of injection pressure, well completion method, in-situ stress difference coefficient, and fracturing fluid displacement on the fracture propagations are investigated. Results show bedding has notable effects on the property parameters of coal, which is the key factor affecting the anisotropy of coal. The hydraulic cracks trends to bifurcate and swerve at the bedding due to its low strength. Induced fractures are produced easily at the locations around the bedding. The bedding is beneficial to form a complicated fracture network. Experimental and numerical simulations can help to understand the effects of bedding on hydraulic fracturing in coalbed methane reservoirs.

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.

Anisotropic Model for Permeability Change in Coalbed-Methane Wells

Summary With depletion of coalbed-methane (CBM) wells in the San Juan basin, permeability rises by 10–100 times. Similar permeability increases are found in other CBM basins around the world. Models previously advanced to explain this behavior combine a compaction mechanism (which closes cleats) with matrix shrinkage (which opens cleats). In this paper, the original model—Palmer and Mansoori (1996, 1998)—is extended to include the transversely isotropic elastic response found in vertically cleated coals. This new version of the model has been calibrated by laboratory measurements on San Juan basin coal loaded under uniaxial-strain conditions to simulate reservoir depletion. The main goal/result is a predictive model characterized by two measured Young's moduli and three measured Poisson's ratios. Although there exist many models that seek to explain the huge permeability increases in CBM wells, the significance of this logical extension to the Palmer-Mansoori model (Palmer and Mansoori 1996, 1998) (i.e., the addition of coal anisotropy) is that it has the ability to predict permeability increases in wells in the field by use of measurable rock moduli and other quantities, thus reducing the need for empirically derived field matching parameters. Note that this new model also confirms an empirical constant that was invoked in a previous version of the Palmer-Mansoori model (Palmer and Mansoori 1996, 1998) to account for cleat anisotropy. To illustrate the model results, permeability increases with depletion from a few wells in one area of the San Juan basin have been matched by the new model. The new model should allow better forecasting of gas rates by reservoir simulators, as well as improved predictions of gas production in fields yet to be developed.

A numerical model for simulating single-phase gas flow in anisotropic coal

Numerical simulation has become an important approach for investigating coalbed methane (CBM) recovery. This demand has enabled the development of numerical simulators and each simulator has its own features. This paper first modifies the mathematical model of single-phase gas flow in coal by incorporating the anisotropic permeability model presented in our previous work. A numerical model is then developed based on this modified mathematical model by using the finite difference method. The numerical model is solved by using the Gauss-Seidel iteration method. A simulator is developed with the C++ programing language to solve the iteration equation. The numerical model is validated with a set of published experimental data and a set of published production data modeled by commercial CBM simulators. The results show that the numerical model developed in this paper agrees well with both data sets. A series of numerical simulation is conducted to evaluated the effects of permeability anisotropy on dry CBM production by using the numerical model developed in this paper. The results show that if the equivalent permeability is equal to the isotropic permeability, the permeability anisotropy has few effects on gas rate but impacts the distributions of pore pressure and gas content significantly. These results may be meaningful for dry CBM production with multiple vertical wells.

Experimental study on dynamic elastic parameters of coal samples

Dynamic elastic parameters of coal are closely linked to crack characteristics and are lithology indicators in seismic exploration. This experimental study measured ultrasonic wave velocity of coal samples considering both parallel (90°) and perpendicular (0°) to bedding planes, and then calculated the dynamic elastic parameters (Ed and μd) and their anisotropy values (AEd and Aμd). The variations of Ed and μd, as well as AEd and Aμd were analyzed under various confining stresses. The results show that: Firstly, a critical confining pressure exists, and significant variation in the parameters can be seen below this point and weak variation appears above it. Secondly, a positive correlation exists between Ed and the square of P-wave velocity (VP2), and between AEd and the P-wave velocity anisotropy (AEP) as well; however, there is no clear correlation between μd and P-wave velocity (VP). Thirdly, according to the major controlling factors of anisotropy, the coal samples with different Ed and μd as well as AEd and Aμd can be divided into two types: one is mainly controlled by bedding and cracks and the other one is mainly controlled by differences of mineral compositions in directions. Consequently, this study can provide theoretical basis for future research on the dynamic elastic parameters and anisotropy of coal.

Numerical investigation of the scale effect and anisotropy in the strength and deformability of coal

The decrease in strength and stiffness of coal with increase in specimen size has long been recognized as scale effect. In this study, a synthetic rock mass (SRM) approach has been used to investigate this scale effect and the anisotropy of coal. A discrete fracture network (DFN) for the coal was created and then embedded into bonded-particle models to form a range of varying size of SRM coal specimens. Unconfined compression tests on the SRM specimens showed that the SRM approach is capable of quantitatively reproducing the dependency of the strength and stiffness on the size of the coal specimens. The strength and stiffness anisotropy of coal can also be reproduced. The numerical results are shown to be in good agreement with published laboratory and field test results.

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.

Evaluation of the reservoir permeability of anthracite coals by geophysical logging data

Permeability is one of the most significant reservoir parameters. It is commonly obtained by experiment, history simulation, injection/falloff well test and geophysical logging. Among these, geophysical logging remains as the most economic and efficient technique in evaluating coal permeability in the vicinity of an open-hole. In this paper, geophysical logging data are used to evaluate the coal reservoir permeability for the No. 3 coal seam in the southern Qinshui Basin (Fanzhuang and Zhengzhuang coal zones). Ideally coal reservoirs consist of coal matrix and fracture networks that can be represented by a model called a collection of sheets. Based on the model, coal reservoir permeability can be quantitatively calculated using the theoretical formula of kf = 8.50 × 10− 4 w2φf, in which fracture width (w) and fracture porosity (φf) were obtained by dual laterolog and density logging data, respectively. Calculative results show that coal reservoir permeability ranged from 0.017 mD to 0.617 mD for the Fanzhuang coal zone and from 0.047 mD to 1.337 mD for the Zhengzhuang coal zone. The permeability decreases with coal burial depth, reflecting variations in penetration capability of coal reservoirs at varying depths. Comparing results with those from injection/falloff well tests, however, shows that the model-calculated permeability is slightly higher. This is expected because the model did not include the influence from coal anisotropy.

Test on Elastic Anisotropic Coefficients of Gas Coal

AbstractGas coal is abundant in fractures and characterized by anisotropy. Determining the type and magnitude of anisotropy of gas coal is of great significance for studying the anisotropy of a coal field. The porosity of gas coal samples was tested. On the basis of this, we measured the P‐wave velocity (Vp) and the horizontal and vertical components of S‐wave velocity (VSH, VSV) in three directions parallel to, perpendicular to, and an angle of 45° to the coal seam. For the transverse isotropic medium model with a vertical symmetry axis, the average values of anisotropic coefficients are |ε| ≤ 0.2, |γ| ≤ 0.2 and |γ| ≤ 0.2, showing that the gas coal is a weakly anisotropic medium. Based on that the average value of the anisotropic coefficients in the transverse anisotropic medium model with a horizontal symmetry axis is ε(V) ≈ δ(V), Thomsen thought that the anisotropy of gas coal is caused by elliptical cracks. The relationship between the elastic anisotropic coefficients and the porosity of the gas coal samples is discussed, indicating that the value of the anisotropic coefficient is related with porosity to a certain degree. So, the anisotropy was thought mainly to be caused by cracks when the thickness of the coal seam and the lithology of adjacent rock strata do not change or only change a little. In this case, the direction and coefficient of elastic anisotropy of gas coal can reflect the direction and the density of cracks.

Laboratory Experiments on Environmental Friendly Means to Improve Coalbed Methane Production by Carbon Dioxide/Flue Gas Injection

Scaled in situ laboratory core flooding experiments with CO2, N2 and flue gas were carried out on coal in an experimental high P,T device. These experiments will be able to give an insight into the design of the injection system, management, control of the operations and the efficiency of an ECBM project. Although the experience gained by the oil industry represents a valuable starting point, several problems are still to be studied and solved before CO2 improved deep coalbed methane production may be operationally feasible. These are all related to the heterogeneous nature of the pore structure of coal, and in particular to the presence of fractures. More specifically, a number of questions need to be addressed, e.g. what are the conditions under which the fluid in the micro pores of the coal is displaced by the CO2 in the presence of competitive adsorption; what is the role of compositional heterogeneity and fracture anisotropy of coal for the injection design and the efficiency of the sequestration in relation to the swelling and shrinkage characteristics of coal; how does the mobile and the immobile water in the coal affect the exchange process. These questions can be answered by means of downscaled laboratory experiments that are capable of accurately describing the coupled process of multiphase flow, competitive adsorption and geo-mechanics. The laboratory conditions have been simulated to match pressure and temperature at depths of 800 to 1,000 m. Under those conditions the injected CO2 remains supercritical. Upto now, the results show that dewatering will be an essential step for successful ECBM combined with a CO2 sequestration process.

Structure and electrical resistivity of nano-carbon materials

The electrical resistivity and microstructure of nano-carbon materials, such as anisotropic coal tar pitch (pitch C) and polyparaphenylene (PPP), were investigated. The resistivity of the samples measured under applied pressure decreased as the packing density increased. The change ratio, ρ/PD (resistivity/packing density), for pitch C was larger than that for well-known petroleum pitch (Ashland 240), and the resistivity value of the former was about 30% smaller than that of the latter at the highest packing density. The difference in the resistivity under pressure is thought to be caused not only by the contact resistance of the grains but also by the difference in the domain structure of the two kinds of samples. The packing density of PPP, which is a disordered carbon material, is scarcely changed by applied pressure. The resistivity of PPP was found to be affected strongly by the heat treatment temperature.