Gas Flow In Coal Seam Research Articles

A CLUSTERED FRACTAL DISCRETE FRACTURE NETWORK MODEL FOR FRACTURED COAL

The fracture network in fractured coal is the main channel of coal seam gas flow. Not only the geometric topology properties (such as fractal characteristics) of a single fracture but also the connection topology properties (interconnection characteristics between fractures) of the fracture network have an important impact on the fluid flow in fracture networks. In this study, the connection topology properties of the fracture network in the fractured coal are explored based on the complex network theory for the first time. The property parameters such as the fracture node degree, the clustering coefficient, and the average path length are analyzed. It shows that the average clustering coefficient of the fracture network in fractured coal is larger, and the average path length is smaller. The connection property of the fracture network in the fractured coal presents a typical “small-world” clustering model. Further, by considering the fractal characteristics of the single fracture and the clustering characteristics of the fracture network, an improved clustered fractal discrete fracture network (DFN) model is developed. Then, based on the lattice Boltzmann method, the permeability properties of the generated clustered fractal DFNs are analyzed. The results show that the permeability of DFNs is positively correlated with the average clustering coefficient of fracture network, and negatively correlated with the fractal dimension of fracture. Therefore, the topological clustering characteristics of fracture networks and the fractal characteristics of fractures cannot be ignored in describing the fluid flow in the fracture network, and our clustered fractal DFN model provides a new idea for guiding the optimization design in DFN engineering.

Prediction of gas drainage changes from nitrogen replacement: A study of a TCN deep learning model with integrated attention mechanism

Accurate prediction of pure gas volume changes in nitrogen injection replacement coal seams is of great significance to increase coalbed methane (CBM) production and prevent underground gas disasters in coal mines. However, the mechanism of coal seam gas flow during nitrogen injection replacement is complex, and it is extremely difficult to achieve quantitative prediction of the pure volume of gas in nitrogen injection replacement coal seams. In this study, we propose a new model for predicting the change of pure amount of gas in coal seam of nitrogen injection replacement in coal mines (Attention-TCN). Meanwhile, we believe that a stable and reliable predictive model should have the ability to capture the underlying patterns and relationships of regular and noisy datasets during the learning process, rather than memorizing training data. Therefore, in this paper, the Attention-TCN model is trained and tested using the results of nitrogen injection and replacement methane double diffusion test (regularity dataset) and the results of nitrogen injection and replacement experiment in coal mine underground engineering (noisy dataset), respectively. The difference between the Attention-TCN model and the baseline models of TCN, gated recurrent neural network (GRU) and long and short-term memory neural network (LSTM) was also compared. The validity of the model was demonstrated using ten-fold cross-validation results. The results show that the proposed Attention-TCN prediction model achieves good prediction performance on both regular and noisy datasets, with prediction R2 greater than 0.99, and is highly flexible and robust. The model is better able to extract the basic features and structures in different types of datasets, enabling it to perform well on unseen data. The ten-fold cross-validation results clearly show that the Attention-TCN model is more accurate and all outperforms the TCN, GRU and LSTM prediction models. In addition, the prediction results of all models for different mine underground nitrogen injection replacement coal seam pure gas volume datasets further validate that the Attention-TCN model has better ability to generalize to new datasets and is more effective and practical for predicting the variation of pure gas volume in underground nitrogen injection replacement coal seams in coal mines.

The influence mechanism of effective stress, adsorption effect and Klinkenberg effect on coal seam permeability

Permeability is an important parameter in the process of coalbed methane exploitation. To improve the production efficiency of coalbed methane and explore the control mechanism of the gas flow law in coal, the permeability of helium and nitrogen in the same coal sample was tested under different effective stress (the difference between external stress and pore pressure of coal mass) and pressure by using the seepage device. Based on the gas flow theory, the interaction mechanism of effective stress, adsorption effect and Klinkenberg effect in controlling the permeability has been analyzed. Increasing the gas pressure will enhance the adsorption and deformation ability of coal, causing the reduction of pore size, while it will also cause the reduction of effective stress and stress deformation. There is a certain competition between them under the same external stress condition, which will lead to the change of pore and then affect the permeability of coal seam. The Klinkenberg effect will lead to more complex change factors of permeability, especially in laboratory experiments. Both adsorption deformation and stress deformation will affect the pore structure of coal body, which will also lead to changes in the influence degree of Klinkenberg effect on apparent permeability. Under the influence of adsorption effect, the Klinkenberg effect may be a variable. The experimental results in this work elaborate the microscopic control mechanism of gas permeability change in coal. It can not only provide important guidance for gas injection technology, but also enrich the theory of coal seam gas flow.

A multiphysics field-scale investigation of gas pre-drainage in sorptive sediments

Gas pre-drainage through underground-to-inseam (UIS) horizontal boreholes is one of the common practices for the management of environmental and safety concerns in underground coal mining. In the design of UIS boreholes, local experience is invaluable; however, an experienced-based design is not necessarily optimal from both economic and technical perspective. In this study, we present an informed, data-driven design and optimization methodology that employs a multiphysics coal seam gas flow model for simulating gas pre-drainage on different scenarios of UIS borehole pattern. The implementation of the methodology is presented through a case study from an Australian underground coal mine. The methodology uses an in-house three-dimensional finite element (FEM) model (referred to as NETCoal) that has been developed based on the non-equilibrium thermodynamics and continuum mechanics. NETCoal considers the interacting processes of matrix gas diffusion, fracture gas flow and rock stress evolution in a coupled manner. This allows the permeability tensor of each grid element to be updated in response to evolving stresses on fractures induced by matrix shrinkage due to gas desorption (sorptive-mechanical effect) and gas pressure depletion (hydro-mechanical effect). The proposed methodology for the UIS borehole design requires a three-dimensional geological/structural grid model to be constructed for the area under consideration using available field data. Material and flow properties of coal (e.g., Isotherm, flow, mechanical properties etc.) are then calculated particularly using downhole geophysical logs (DGL) and physical equations/empirical correlations at borehole locations and distributed onto other grid cells using the kriging method to build a property model. NETCoal imports these geological and property models in a corner point gridding format and generates its own finite element mesh that suits the UIS borehole pattern under consideration. A Warren-Root type shape factor is used for the calibration of the numerical results of gas content with actual borehole flow data. Once calibrated, the model simulates gas drainage for different scenarios to determine an optimal borehole pattern which is the one satisfying the threshold limit value (TLV) and lead time constraints while requiring least drilling length. In addition to introducing an innovative practical methodology for the design of UIS pre-drainage boreholes, this study provides insights into the effect of coal seam and geometry parameters on gas pre-drainage through a sensitivity analysis.

Research on Influencing Factors of Effective Gas Extraction Radius in Coal Mine Based on Multiple Linear Regression

In order to analyze the influence degree of different factors on gas extraction radius, on the basis of coal seam gas flow theory and elastic mechanics, considering the influence of coal seepage rate change on gas flow, the COMSOL Multiphysics numerical simulation software is secondarily developed by using the stress-seepage coupling mathematical model of coal and rock containing gas. Taking coal seam No.2 of Juji Coal Mine as an example, the effective extraction radius under different extraction negative pressure, extraction borehole diameter, extraction time, and initial permeability of coal seam are simulated, respectively. Then, the orthogonal test method is used to design different calculation schemes, and the effective extraction radius is used as the judgment index. The simulation software is used to calculate the effective extraction radius corresponding to different influencing factors. SPSS statistical software is used to analyze the parameters. The multivariate linear regression equation of the influencing factors of the effective extraction radius is obtained, and the multivariate linear regression model is established. The results show that the negative pressure of extraction has little effect on the effective extraction radius; the effective extraction radius gradually expands with the increase of extraction time and extraction aperture, but it has certain timeliness. The initial permeability of the coal seam has a great influence on the effective extraction radius. The influence degree of different influencing factors from large to small is the initial permeability of coal seam, extraction time, borehole diameter, and extraction negative pressure. Therefore, the key to improving the effectiveness of gas extraction is to increase the initial permeability of the coal seam before preextraction, ensure sufficient extraction time, appropriately increase the borehole diameter, and improve the negative pressure of extraction. Finally, the gas flow rate is used for a field test, and the measured results are basically consistent with the simulation results, which proves the correctness of the mathematical model of gas-containing coal and the feasibility of using numerical simulation methods to study the effective extraction radius of coal seams.

Role of pore irregularity in methane desorption capacity of coking coal

Gas desorption property of coal is closely related to pore distribution, and the accurate characterization of pore structures is of great significance for gas flow in coal seams. In this work, nine typical coking coal samples were collected in Xishan coalfield, North China. Both qualitative and quantitative analysis were made on these samples regarding the pore structure characteristics. Fractal theory was also used to evaluate the pore irregularity and its influence on methane desorption capacity of coal. Significant hysteresis loops are observed for all the LPNA isotherms due to the delay of gas desorption. Pore volume and pore specific surface area of these coking coals are changed in the wide ranges of 1.35–8.15 × 10−3 cm3/g and 0.87–2.59 m2/g, respectively. Micropores can provide tremendous surface area but have less pore volume, and the largest contribution of pore volume is made by macropores. Fractal dimensions are determined based on LPNA data. D1 fluctuates in the small range of 2.6036–2.7937, while D2 is changed in the wide range of 2.0898–2.9268. Pore structure has obvious impact on gas desorption of coking coal. Positive correlations have been recognized between gas desorption capacity and pore volume, pore specific surface area and the average pore diameter. Gas desorption behaviors within coking coal is sensitive to the change of D2, but exhibits little relationship with D1, indicating that the increased structure irregularity of macropores can hinder the gas flow. Besides, the increase of moisture content and f of coking coal can also inhibit gas rapid release from coal matrix. This work will provide theoretical support for gas extraction underground coalmine.

Numerical modeling of gas extraction from coal seam combined with a dual-porosity model: Finite difference solution and multi-factor analysis

Gas extraction from coal seams is an extremely significant technical measure for preventing gas hazards and predicting coalbed methane (CBM) reservoir. Gas diffusion and seepage properties during the extraction process jointly dominate the CBM migration and production. How to integrate diffusion and seepage theories to construct numerical models of coalbed methane flow and evaluate extraction performance has become one of the hot spots of competitive research. In this work, we developed an integrated mathematical model of seepage and diffusion during borehole gas extraction combined with a dual-porosity model. Emphatically, the gas diffusion driver in coal matrix is no longer the concentration gradient (CG) associated with Fick's Law, but the innovative free gas density gradient (FGDG). The driving force for gas seepage in fractures remains the pressure gradient (PG) with respect to Darcy's law. Here, seepage and diffusion were coupled by a gas source term equation. Subsequently, a numerical simulator based on finite difference method (FDM) was programmed to compute the mathematical equations of gas extraction. Finally, the simulation results were validated by field gas extraction data and the contribution of some key parameters to the gas flow rate was discussed. The findings indicate that: (i) the simulation results are generally consistent with field data, and the developed model as well as simulator both have the capability to predict methane gas extraction. (ii) The gas diffusion process in coal matrix can be considered to be dominated by the FGDG. (iii) The gas permeability coefficient in fractures mainly controls the gas flow rate in the early extraction stages, while the coal matrix radius and microchannel diffusion coefficient contribute more in the later periods of gas extraction. (iv) Fracture porosity is positively correlated with gas flow rate and negatively correlated with the magnitude of flow rate decay. (v) The higher the original gas pressure or the lower the extraction pressure, the greater the pressure gradient between the borehole and the coal seam, contributing to an increase in gas flow. This work is expected to provide some support in modelling the gas flow in coal seam and assessing the dynamic CBM production.

Influence of coal deformation on the Knudsen number of gas flow in coal seams

The effective stress and pore pressure can change the fracture aperture, which in turn can change the methane Knudsen number and transitions between gas flow regimes. A novel Knudsen number (Kn) model for gas flow under coal deformation was established; a revised permeability model was formulated and used to obtain an equation for the Klinkenberg coefficient b. The predictions of the revised permeability model were in good agreement with experimental data, and the parameters used to determine the gas Kn were obtained. An increase in the pore pressure under coal deformation causes the fracture aperture to increase and the gas molecular mean free path to decrease, thereby decreasing the Kn. Thus, the critical pore pressure for the transition between gas flow regimes is reduced. The gas Kn for a deformed fracture is higher than that for a constant fracture aperture. The critical fracture aperture for the transition between gas flow regimes increases under coal deformation. The Klinkenberg coefficient b for methane is a variation value that increases as Kn decreases. The Kn variation is very important for determining the coal permeability and which can be used to resolve problems in CBM recovery.

Study on the pressure relief effect in gob‐side coal body during mining an inclined longwall panel

AbstractThis study combined multi‐field coupled numerical calculations and field measurements to analyze the gas pressure relief and the outburst elimination range of the coal body in the lower side of a gob induced by excavating an inclined seam. A coal seam gas flow solid‐gas coupled model allowing for elastoplastic damage was established based on an inclined longwall panel of the Hongyan Coal Mine in Chongqing, China. To inspect the gas depressurization effect, this study simulated the collapse and damage of strata surrounding the longwall panel after mining by using a constitutive model of the interlayer weak plane, the changes in the damage, gas pressure, and stress concentration in the gob‐side coal seam were analyzed numerically. Besides, variation in gas pressure within 10‐23 m away from the panel margin was measured on‐site during the advancement of the longwall face to validate and modify the results of modeling. The simulation results showed that the advancing of the panel is highly effective in releasing gas from the gob‐side coal body. The rupture penetration, stress concentration, and original elastic zone will be formed in the gob‐side coal body with increasing distance away from the gob after mining the 3603‐2 panel. The permeability increased dramatically and the gas was almost exhausted in the rupture penetration zone. The field measurement results indicated that gas pressure decreased in three stages with advancement. After completion of excavation activities, the drop rates of gas pressure and gas content within 10‐23 m from the gob were 26.55%‐73.58% and 34%‐71%, respectively. Additionally, it was proved that the numerical simulation results were instructive for predicting the range of gas outburst‐proneness elimination belt by comparing simulation and field results, and the outburst elimination range of the inclined longwall panel of the Hongyan Coal Mine was determined to be 10 m.

Effects of permeability anisotropy on coal mine methane drainage performance

Coal mine methane (CMM) drainage using in-seam boreholes is an effective method to reduce the methane content in coal seams, ensure coal mine safety production and exploit fossil fuel resources. The dominant direction of gas seepage changes due to the anisotropy of coal seam permeability, which causes differences in the amount of methane extracted from boreholes drilled in different directions. The main purpose of this study is to investigate the control effect of coal permeability anisotropy on CMM drainage performance. First, a mathematical model coupling the orthotropic permeability and gas flow in coal seams was established. The methane drainage parameters of the coal mass surrounding in-seam boreholes were simulated. The simulation results showed that the amount of methane extracted from a borehole perpendicular to the permeability advantage direction was much greater than that from a borehole parallel to the permeability advantage direction, and the amount of methane increased with the increase of the angle between the boreholes and the permeability advantage direction. Then, permeability differences of coal samples in three directions, with Gaohe Coal Mine as the test site, were obtained through laboratory tests, and it was determined that the direction of methane flow along the inclined direction of the coal seam was the permeability advantage direction. Finally, five groups of boreholes with different angles from the coal seam inclination, were drilled to investigate their drainage performance. The field test results showed that the group of the largest-angle boreholes had the most positive effect of methane drainage, and the amount of extracted methane was positively proportional to the angle between the borehole and the coal seam inclination. This behavior is consistent with the simulation results. These study outcomes can provide scientific guidance to maximize the methane drainage performance by designing a reasonable layout of boreholes considering the coal permeability anisotropy.

Liquid CO 2 high‐pressure fracturing of coal seams and gas extraction engineering tests using crossing holes: A case study of Panji Coal Mine No. 3, Huainan, China

Owing to the low coal permeability coefficients and poor gas extraction efficiencies associated with the extraction of coal in the Huainan coal mining area, the aim of this study was to design, for the first time, a high-pressure low-temperature liquid CO2 (LCO2) pump for engineering tests with respect to the fracturing of coal seams and the improvement of CH4 recovery in China. To reasonably calculate the initiation pressure for the coal seam fracturing as well as the gas flow parameters, theoretical analysis with field testing were integrated in the design. In addition, considering the interim gas extraction standard, the gas extraction equivalent radius based on this standard was calculated. The test results revealed a maximum LCO2 pressure of 20.6 MPa, which surpasses the theoretical maximum pressure (19.5 MPa) required for the fracturing of coal seams. During the entire gas extraction process, the CO2 concentrations in boreholes K1 and K3 decrease exponentially with time, showing attenuation coefficients of 0.0205 and 0.0231, respectively. The attenuation coefficients of the three attenuation stages of the original coal seam gas flow rate were 0.0314, 0.2142, and 0.1198, which were higher than those corresponding to the test area. The CH4 concentration in boreholes K1 and K3 in the test area were both higher than that in the original coal seam, and the maximum CH4 flow rates in these boreholes were 1.31- and 1.72-fold higher than that in the original coal seam, respectively. The comparative analysis of CH4 concentrations and flow rates indirectly showed an improvement in the permeability of the coal seam. Further, a quantitative equivalent gas extraction evaluation method based on LCO2 fracturing of coal seam and gas displacement was proposed. Furthermore, based on a comprehensive evaluation, the effective displacement radius caused by the LCO2 fracturing test was 9 m, with a displacement efficiency of 23.95%, and a displacement volume ratio of 0.21. This method offered the possibility of realizing the quantitative evaluation of the LCO2 injection amount and the gas extraction effect, and provides a basis for optimizing the LCO2 fracturing of coal seams as well as the gas extraction process.

Numerical assessment of the critical factors in determining coal seam permeability based on the field data

Accurate acquisition of the coal seam permeability is of great significance to gas drainage. Field measurements are more reliable to determine the coal permeability than laboratory. The Radial Flow Permeability (RFP) measurement is the most used field measuring method to measure the coal seam permeability in Chinese coal mines. The theoretical basis of the RFP method is the single-porosity medium flow theory and the gas content in coal seams is described by a parabolic equation. However, coal is a typical dual-porosity medium. The simplifications may lead to nonnegligible influence on the accuracy of the RFP method, which are not well documented in the literature. In this paper, three different mathematical models of gas flow in coal seams are established. The field gas flow data was matched by these three models to prove the reliability of these models. Then the influences of single-porosity simplification and the parabolic gas content simplification on the accuracy of the RFP method were investigated by comparing the differences of gas flow rates and gas pressures. Results show that there are apparent deviations of gas flow rate and gas pressure between these three models. The precision of the RFP method rapidly reduces with the increase of cleat spacing, which is induced by the single-porosity simplification; the precision decreases with the increase of Langmuir volume, first increases and then declines with the increase of Langmuir pressure, which are led by the gas content simplification. Moreover, because of the imperfections of the numerical calculus in the derivation of the RFP method, the calculated value is always smaller than the true value of coal seam permeability.

Analysis of the coal seam spalling-failure mechanism based on the seepage instability theory.

Coal and gas outburst is a common coal-rock dynamic disaster. Such accidents frequently occur, and the mechanism underlying the occurrence of these outbursts is complex. As a typical failure mode of a gas-filled and pressure-relieved coal body, the spallation mechanism should be investigated to reveal the mechanism of coal and gas outburst and guide outburst-prevention strategies. In this paper, a fluid-solid coupling model for coal seam gas flow is established. This model considers the adsorption characteristics of coal. Numerical calculations are used to simulate the stress field distribution and evolution of gas-filled coal bodies under different boundary conditions. The mechanical mechanism of the spallation occurrence after the pressure relief of coal is explained from the perspective of seepage breaking coal. The control of the flow and stress state of the gas to the spallation failure is analyzed. The mechanical-quantitative conditions for the initial failure of the coal body under seepage and the mechanical-qualitative conditions for the continuous advancement and termination of spallation are studied based on numerical solution results. The numerical calculation results show that the formation of a flow field after pressure relief will apply a drag force (tensile stress) on the porous media of coal. The presence of this force plays a crucial role in promoting the spallation and cracking of coal and, thus, the promotion of spallation. The tensile strength, initial adsorption pressure, and pressure relief rate of the coal body jointly control whether the initial failure can occur and the thickness of the fracture layer cracks. Spallation propulsion is mainly determined by the pressure relief conditions of the undestroyed coal body and pressure changes in the spallation space; the former can be quantitatively obtained by numerical calculations, whereas the latter is related to the thickness of the spalled layer and the degree of the layer-crack structure.

Modeling and simulation on heat-injection enhanced coal seam gas recovery with experimentally validated non-Darcy gas flow

Abstract Heat-injection into coal seam has gained great attention as a supplementary method for the improvement of unconventional gas recovery from deep reservoirs in recent years. However, it involves a lot of ambiguous interactions among the deformation of formation, the transport of gas and the diffusion of heat in the porous coal media, especially the property of gas. In this paper, the seepage behavior of coal seam gas (CSG) that obeys non-Darcy flow law was directly observed in a conducted laboratory experiments. Then, a general Forchheimer equation was confirmed and strictly validated by the experimental data to characterize the non-Darcy seepage phenomenon. Subsequently, a fully coupled thermo-hydro-mechanical model was mathematically established. Based on the finite element approach, a numerical simulation for heat-injection enhanced coal seam gas recovery was then implemented. The susceptibilities of coal permeability and gas production to the gas sorption, heat-mass transfer, and the porosity-seepage characteristics of coal were quantified by a series of numerical scenarios. Final modeling and simulation results reveal that: (1) coal seam gas flow obeys non-Darcy law when the gradient of gas pressure is higher, but trends to be laminar flow that obeys Darcy law when the gradient is lower, and (2) heat-injection to stimulate coal seam could improve the production efficiency, increasing the Langmuir volume sorption parameter, the temperature correction coefficient and the initial values of permeability can improve coal permeability and enhance gas production.

Orthogonal Experimental Study on Multifactor Conditions for Gas Desorption in Coal

The isothermal adsorption experiment of coal is an important method to study the mechanism of coal gas desorption. The orthogonal experiment is used to compare the gas desorption mechanism of coal under multifactor conditions, such as the particle size, temperature, pressure, moisture content, and molding pressure. The sensitivity of five factors was used to conduct regression analysis. The sensitivity and influence degree of five factors on the coal gas desorption capacity were analyzed. The results showed the following: (1) the effect of the coal sample particle size, temperature, pressure, moisture content, and molding pressure of coal on the sensitivity of desorption capacity is shown as C (pressure) > B (temperature) > A (particle size) > D (moisture content) > E (molding pressure); (2) the regression analysis of various factors for gas adsorption indicates that the degree of correlation of the multivariate quadratic regression equation is higher compared to that of the multivariate one‐time regression equation; and (3) the coal sample particle size, temperature, pressure, moisture content, and type of gas desorption can well represent the gas desorption capacity of a coal sample under various conditions. The smaller the particle size of the coal sample, the higher the ambient temperature, the higher the gas pressure, the lower the moisture content of the coal sample, and the greater the gas desorption per unit mass in 30 min after coal sample gas adsorption equilibrium. The orthogonal test results have important theoretical significance for guiding gas adsorption and desorption tests of coal. For coal and gas outburst prediction, the coal seam gas flow mechanism, coal gas content prediction, and calculation of the mining coal gas emission have important practical significance for gas explosion accident prevention.

Gas drainage radius of single-seam and cross-seam boreholes

In high gas mine, gas needs to be pre-pumped through single-seam boreholes to prevent gas outburst in the working face. The effective drainage radius is an important parameter of borehole drilling and has a direct impact on the borehole design and pumping effect. In accordance with the coal seam gas flow theory, the relationship between the effective drainage radii of the cross- and single-seam boreholes is analyzed. A field test is conducted by using the pressure drop method. Experimental results are in line with the conclusions drawn from the theory. When accurate measurement is not required, the effective drainage radius of the cross-seam borehole is slightly equal to that of the single-seam borehole. Therefore, when the effective drainage radius of the single-seam borehole is difficult to determine, it can be replaced by the effective radius of the cross-seam borehole.

Characterization of coal porosity and permeability evolution by demineralisation using image processing techniques: A micro-computed tomography study

The permeability of coal is the key parameter both in primary and enhanced coalbed methane recovery. The natural cleat system in coal serves as the primary pathway for gas flow in coal seams though mineralisation in cleats and is known to significantly reduce coal permeability. This paper reports on a numerical simulation of the pore network evolution of coal subject to cleat demineralisation. A high-resolution micro-computed tomography scanner was used to characterize the micro-structures of three anthracite coal samples. The mineral phases available in the coal samples were selectively removed to different extents (20%, 40%, 60%, 80% and 100%) and merged into the pore space using image processing techniques. In this way, the coal demineralisation process could be simulated and its impact on porosity and permeability studied. Comprehensive pore structure characterizations, including porosity, connectivity and tortuosity, were then conducted on the reconstructed pore network using Avizo software. Pore network models were also extracted to investigate changes in the pore and throat attributes. The lattice Boltzmann method was adopted to identify the absolute permeability changes with cleat demineralisation. The results reveal that demineralisation can increase coal porosity and permeability up to a percolation threshold. Although porosity was enhanced prior to the percolation threshold, the coal permeability was not enhanced due to poor pore connectivity. The permeability changed rapidly close to the percolation threshold, depending on the degree of demineralisation, and an exponential relation was observed between permeability and the amount of demineralisation. According to the observations, complete removal of the mineral phase can significantly increase the connected porosity while reducing the pore tortuosity, resulting in several orders of magnitude increase in coal permeability. This study shows that cleat demineralisation is an effective permeability enhancement technique for coalbed methane recovery, if very high demineralisation can be achieved.

A simple method for solving unidirectional methane gas flow in coal seam based on similarity solution

The equation used to model the unidirectional flow of methane gas in coal seams is usually formulated as a nonlinear partial differential equation, which needs to be solved numerically with a computer program. Nevertheless, for people without access to the computer program, the conventional numerical method may be inconvenient. Thus, the objective here is to seek some method simpler than the conventional one for solving the flow problem. A commonly used model of the unidirectional methane gas flow is considered, where the methane adsorption is described by the Langmuir isotherm and the free gas is treated as real gas. By introducing the similarity solution, a simple method for solving the flow model is proposed, which can be done on a hand calculator. It is shown by two examples that the gas pressure profile obtained by the proposed method agrees well with the direct numerical solution of the flow model.

Study on gas permeability coefficient measurement of coal seam by radial flow method

For the accurate measurement of the coal seam permeability coefficient, the application range of the coal seam permeability coefficient was studied under various gas flow conditions with the guidance of the coal seam gas flow theory. Adopting the radial flow method, the measurement and calculation of the permeability coefficient of the coal seam C13-1 in Xinji No.1 Coal Mine shows that the permeability coefficient of the original coal seam C13-1 is less than 0.1, and the coal seam is difficult to extract.

Prediction of Coalbed Methane (CBM) Production Considering Bidisperse Diffusion: Model Development, Experimental Test, and Numerical Simulation

Gas flow in coal seam consists of laminar flow through coal cleat and diffusion through pores of coal matrix. Previous studies on the prediction of CBM production mostly focused on the impact of permeability change while the gas exchange between matrix and cleat was assumed to obey unipore diffusion assumption with a single diffusion coefficient. However, numerous scholars have found that a single diffusion coefficient cannot reproduce the sorption kinetic data precisely for a lot of coals, while bidisperse diffusion with fast and slow diffusion coefficients can represent the diffusion process well. Until now, attempts on studying the impact of bidisperse diffusion on CBM production are very limited and mathematical model describing the gas flow with bidisperse diffusion is unavailable. In this study, we propose a fully coupled coal seam gas flow model with consideration of bidisperse diffusion and the interaction between bidisperse diffusion, adsorption strain and geomechanical response of coal. A series...