Coal Gas Flow Research Articles

Feasibility of carbon dioxide geological storage in abandoned coal mine: A fully coupled model with validated multi-physical interactions

It has gained wide attention that Carbon dioxide (CO2) is to be injected into abandoned coal mines for geological storage of CO2-enhanced coalbed methane recovery. Although abundantly evidences in literature indicate that the injection of CO2 will cause lots of interactions among the mechanical characteristics of coal and the properties of CO2 flow, further studies on these multi-physical interactions are still necessary. In this work, a series of laboratory experiments to elucidate the multi-physical interactions of CO2 adsorption, softening effect of coal and the non-Darcy gas flow were conducted. Based on the experimental results, theoretical and empirical models to describe these coal-CO2 interactions were meticulously proposed and validated, the results turned out to be satisfactory. Consequently, the compressive strength and elasticity modulus of coal decrease exponentially with the increased injection CO2 pressure. The gas flow in coal obeys the Izbash non-Darcy model, and coal permeability can be well modified by the volumetric stress. By taking these coal-CO2 interactions into account, this study established of a fully coupled model for coal deformation and CO2 conservation. The model was then implemented into the numerical simulations of CO2 storage in abandoned coal mine by using the finite element method. A series of scenario-based numerical simulations was conducted to investigate the feasibility and limitation of CO2 storage in abandoned coal mine. The conducted experiments, models and numerical simulation will offer implications on the multi-physical interactions between coal and gas especially in CO2 storage in abandoned coal mines.

Experimental Study on Flame Chemical Composition of Coal and Ammonia Gas-Solid Jet in Flat Flame Burner.

Ammonia as a fuel to partially or completely replace fossil fuels is one of the effective ways to reduce carbon dioxide, and the research on ammonia coal cocombustion is of great significance. The combustion characteristics of ammonia are very different from those of pulverized coal, resulting in the ignition and emission characteristics of ammonia and pulverized coal gas flow that is different from traditional pulverized coal flame. In this paper, the effect of pulverized coal concentration in coal and ammonia mixed combustion jet on the ignition distance and gas-phase components at different positions of the jet flame were studied experimentally on the flat flame burner, and the conditions of ignition and ignition stability of coal and ammonia gas-solid fuel were expounded. It was found that the ammonia mixed with pulverized coal changed the temperature field of the flat flame burner and therefore the ignition characteristics of the jet were changed. The ignition delay time at the same jet speed was positively correlated with the pulverized coal concentration, but when the pulverized coal concentration continued to decrease, the influence on the ignition delay time gradually became smaller. The composition of coal ammonia gas-solid fuel changed the heat transfer path and share during combustion, and finally, the flame temperature was negatively correlated with the concentration of pulverized coal. Therefore, the reduction of the pulverized coal concentration was conducive to the stable combustion of coal ammonia mixed fuel. When HAB = 100 mm, the conversion rate of fuel N to NOx per unit mass of coal ammonia mixture increased with the increase of pulverized coal concentration. The NOx production amount first increased and then decreased with the increase of pulverized coal concentration, and the amount of N2O and NO2 decreased rapidly with the increase of HAB. The proportion of NOx in NO exceeded 94%, which was conducive to achieving low nitrogen combustion of coal and ammonia gas-solid fuel. In general, the O2 concentration in the ammonia coal jet flame decreased, the flue gas temperature, and NOx and CO generation increased after mixing ammonia, and the optimal pulverized coal concentration in this experiment was 0.41 kgc/kga (mass ratio of pulverized coal to the sum of N2 and NH3).

Investigation of Formation Process and Intensity of Coal and Gas Outburst Shockwave

The shock wave of a coal and gas outburst is a high-pressure and high-speed impact airflow formed rapidly after the outburst. The propagation destroys the ventilation facilities and causes the destruction of the ventilation system. The theoretical research on the outburst shock wave is of great significance. In order to deeply understand the formation mechanism of the outburst shock wave, this paper draws on the shock wave theory to theoretically analyze the microscopic formation process of the outburst shock wave. The main difference between the formation process of a coal and gas outburst shock wave and the formation process of a general shock wave is that the outburst shock wave has a solid–gas flow zone in the high-pressure zone. The calculation formulas of pressure, density, temperature and other parameters before and after the outburst shock wave are derived. After the outburst shock wave passes through, the pressure, temperature and density of the roadway air will change suddenly. The relationship expression between outburst gas pressure and outburst shock wave intensity is derived, which can reflect the role of pulverized coal in the formation process of a shock wave. In order to facilitate the understanding and calculation, the concept of equivalent sound velocity of coal-gas flow is proposed, and the direct calculation of the impact strength of a coal and gas outburst is attempted. This paper is helpful to improve the understanding of the essence of a coal and gas outburst shock wave. It is also of great significance to outburst disaster relief.

A lattice Boltzmann model for simulating gas transport in coal nanopores considering surface adsorption and diffusion effects

The gas flow in coal is extremely complicated as a combined results of multiple flow mechanisms and complex pore structures. Comprehensive understanding of gas flow behaviors in coal is of great importance for the prediction of coalbed methane (CBM) production and the elimination of gas-induced disasters in coal mines. Based on the lattice Boltzmann method (LBM), this study constructs a LB model considering surface adsorption and diffusion effects, which involves multiple mechanisms of gas flow, including viscous flow, enhanced gas slippage (gas slippage is enhanced by surface diffusion) and surface diffusion. The results show that in micropores (<2 nm) and mesopores (2–50 nm), under the effect of adsorbed gas, bulk gas flow velocity increases at a low pressure but decreases at a high pressure. Adsorbed gas surface diffusion permeability, enhanced gas slippage permeability and viscous flow permeability decrease with the rise of pore pressure. In nanopores in coal reservoirs, surface diffusion more significantly enhances gas slippage at a low pressure and in smaller pores. Besides, in the flow calculation, gas slippage without the considering effect of adsorbed gas and Knudsen diffusion can replace each other. According to the results from the LB model, the apparent diffusion coefficients of gas in coal particles decreases with the increase of pore pressure in smaller pores(<10 nm) but increases in larger pores(>10 nm). The study further investigates flow mechanisms controlling the CBM extraction yield in different extraction stages. In the early stage of extraction, the CBM extraction yield is controlled by Darcy seepage in fractures; in the middle stage, it is controlled by viscous flow in larger matrix pores; in the late stage, it is controlled by gas diffusion induced by the superposition of surface diffusion and enhanced gas slippage in matrix micropores and mesopores.

Experimental study of the radial multi-scale dynamic diffusion model for gas-bearing coal

Purpose. The purpose of this paper is to solve the scientific problem that the classical diffusion model in columnar coal cores cannot accurately describe the whole process of gas diffusion. Methods. The diffusion-percolation experiments were carried out using the laboratory’s homemade experimental equipment with standard ϕ 50mm×100 mm columnar raw coal cores under different air pressures. Findings. The classical diffusion model was used to fit the experimental data. The experiment has found that the classical diffusion model of the columnar coal core can only partially describe the gas diffusion process. The longer the experimental time, the larger the error between the model and the experiment, and the analysis has found that the apparent diffusion coefficient shows decay changes with time. The dynamic diffusion coefficient concept is then proposed in order to con-struct a radial multi-scale dynamic prominent diffusion-percolation model for columnar coal cores. The theoretical curve of the new model nearly coincides with the experimental curve, and the new model can describe the gas diffusion-percolation process of columnar coal cores more accurately. The multi-scale dynamic diffusion-percolation model covers the classical diffusion model. It explains the mechanism of gas diffusion-percolation in multi-scale pores, i.e., at the beginning of the flow, gas flows out from the large external pores first, from the surface inwards. Over time, the pore size through which gas flows gradually becomes smaller, the diffusion resistance gradually increases, and the apparent diffusion coefficient slowly decreases. Originality. This paper proposes a new multi-scale dynamic diffusion-percolation model to compare the old and new model analysis, as well as carefully studying the mechanism of gas flow in coal. Practical implications. This research has important engineering significance for the accuracy of measuring the gas content of coal seams, as well as predicting coal and gas content.

Quantitative evaluation of the effect of gas adsorption and desorption hysteresis on gas flow in coal

• A calculation method for inversing adsorption parameters is proposed by using the modified hysteresis index DHI. • The new gas flow coupling model considering the adsorption–desorption hysteresis effect is constructed in this paper. • The reliability and superiority of the new model is proven by laboratory experiments. • The effect of different degrees of hysteresis on gas extraction is quantitatively evaluated using the new model. Coal mining methane (CMM) is a major hazard source and a typical greenhouse gas in the coal industry. Gas extraction is an effective solution to eliminate coal seam gas disasters and reduce greenhouse gas emissions. Multi-field coupling model of gas flow in coal (MCMGF) is an important theoretical basis for guiding gas extraction, and the adsorption model is an important component of MCMGF. Numerous studies have shown that there is a hysteresis effect in the desorption of gas in coal compared to the adsorption process. Since gas extraction is a depressurized desorption process, HE should be taken into account when building the adsorption model. However, most of the current MCMGFs ignore the hysteresis effect, resulting in an overestimation of the amount of gas involved in the desorption process. Therefore, in this paper, the calculation method of the degree of hysteresis index (DHI) is revised and the adsorption model is modified based on systematically sorting out the adsorption–desorption hysteresis phenomenon. On this basis, a coupled gas flow model considering the hysteresis effect of adsorption and desorption was constructed, and the reliability of the model was verified by isothermal adsorption and desorption experiments on coal pillars. Eventually, the influence of different hysteresis degrees (DHI = 0, 0.25 and 0.59) on gas extraction was quantitatively evaluated by using the new model. The results show that hysteresis effect has a significant influence on gas extraction, and the larger DHI value is, the smaller the daily extraction is. Ignoring the influence of adsorption–desorption hysteresis will make the gas extraction capacity of the borehole overestimated. After 160 d of extraction, the difference between the residual gas content when adsorption–desorption hysteresis is considered and when it is ignored is up to 2.05 m 3 /t. This is because the adsorption–desorption hysteresis limits the ultimate amount of gas that can participate in the flow. In our previous studies, it has been found that the additional resistance to gas desorption is intrinsic to the hysteresis. With the increase of the mining depth, the in-situ stress increases rapidly, therefore the influence of coal adsorption–desorption hysteresis on gas extraction under high stress is one of the problems that need to be considered emphatically.

An image-based coal network model for simulating hydro-mechanical gas flow in coal: An application to carbon dioxide geo-sequestration

CO2 geo-sequestration is a practical approach to achieve net-zero carbon target. However, one of the main challenges for successful CO2 geo-sequestration is the reduced coal permeability and injectivity that are caused by coal swelling. Coal has complex and heterogenous internal pore and fracture structure. The processes of gases adsorbing, desorbing, and transporting within multiscale heterogeneous coal structures are more complicated compared with conventional rocks. This paper aims to gain insights about the gas transport behaviours in coal by developing a coupled model to simulate gas flow multiphysics as well as dynamic coal deformation. This work develops an image-based 3D fracture network model, called Fracture Box Model (FBNM). In this model, each fracture is described by arrays of box elements such that the regional change of fracture opening widths can be preserved. Compared with other fracture models (e.g. discrete fracture network), FBNM can simulate complicated multiphysical gas transport more efficiently, but also be able to simulate corresponding coal matrix deformation. By comparing permeability results between direct simulation method with FBNM, it is found that FBNM can effectively estimate the permeability of original fracture networks, but requiring significantly less computational cost. To study the implications of gas types, effective stress, gas adsorption, and thermal expansion on coal permeability, gas injection pressures, gas types, coal seam temperatures are varied and investigated in the simulations. In addition to the advantage of computational efficiency, FBNM is more preferable for complicated flow transport simulations where direct simulation methods are still challenging. This work provides a promising framework which could be further developed for multiphase and multicomponent flow simulations for CO2 geo-sequestration projects.

Effects of chemical solvents on coal pore structural and fractal characteristics: An experimental investigation

Chemical solvent treatment is one of the most effective means to improve pore structure of low-permeability coal and increase production of coalbed methane. Low-temperature liquid nitrogen adsorption method, X-ray diffraction (XRD) mineral analysis and fractal theory were adopted to study variations in coal pore structural parameters and fractal characteristics affected by hydrochloric acid (HCl), tetrahydrofuran (THF) and carbon disulfide (CS2). Liquid nitrogen adsorption results of coal samples were analyzed using density functional theory (DFT), Brunauer-Emmett-Teller (BET) equation and Barrett-Johner-Halenda (BJH) model. Meanwhile, changes in fractal dimensions of pore surface (D1) and pore structure (D2) were calculated based on Frenkel-Halsey-Hill (FHH) model. Relationship between pore volume, average pore diameter, specific surface area and two fractal dimensions were established. Results show that: (1) Hysteresis lines exist in all isothermal adsorption-desorption processes of coal. Pores of raw coal are mainly semi-open pores with poor connectivity. The types of pores do not change significantly after THF and CS2 treatment, while the number of pores rise. In contrast, affected by HCl treatment, ink-bottle pores and open pores increase and pore fracture network connectivity becomes better. (2) Pore sizes of treated coal mainly distribute in range of 0–10 nm. After HCl treatment, many closed pores are opened, with growth in proportion of micropores. Meanwhile, specific surface area (SSA) of pores increases by 55.21% and 31.36%, respectively. THF extraction results in an increase in pore volume and SSA. After CS2 extraction, dissolution of organic material increases pore size and further improve volume of transition pores and mesopores. (3) HCl treatment causes decrease in D1 value of pores. However, in organic-solvent-treatment cases, solvent residues retain on coal pores, resulting in growth in D1 value and pore surface becoming rougher. Proportion of total pore volume of transition pores and mesopores of acidized coal decreases and D2 value rises. Pore distribution is more uneven. Relevant results could provide theoretical references for revealing mechanism of chemical solvents effects on gas flow in coal.

On swelling stress–strain of coal and their interaction with external stress

Gas sorption and its induced volumetric strain can significantly influence gas flow in coal by controlling the change in the conductivity of coal cleats. A significant number of studies have been conducted to date to describe the coal swelling stress with sorbing gases and several theoretical models have been proposed to estimate swelling stresses. Despite these efforts, the accurate prediction of coal swelling stress remains a challenge hence a comprehensive theoretical and experimental investigation of swelling stress is required. This is particularly important when swelling stress of fractured coals develops under external loading. We, therefore, perform two sets of time-dependent diffusion and volumetric strain experiments under various stress conditions and gas pressures on coal specimens (a highly cleated and a dense sample) exposed to helium and carbon dioxide. These experiments are conducted to investigate i) the performance of commonly used theoretical models for swelling stress estimation, ii) the validity of thermodynamics swelling coupling coefficient defining the swelling stress and iii) the effect of external stress on coal volumetric strain response (hydromechanical versus swelling/shrinkage). Our analysis shows that the developed models based on non-equilibrium thermodynamics can predict both the evolution and the final swelling stress closely. We further show that the new definition of swelling coupling coefficient leads to greater performance of non-equilibrium thermodynamics-based models and further suggest a new experimental technique for characterizing this coupling coefficient. Finally, the results reveal that the effect of external stress on the hydromechanical volumetric strain of coal (during gas adsorption by unloading and desorption by loading) has a considerable hysteresis, especially at high pore pressures.

Experiment Study of Outburst Pulverized Coal-Gas Two-Phase Flow and Characteristic Analysis of Outburst Wave

Coal and gas outburst is still a major safety problem in the process of coal production in China. Correctly understanding of the migration law of outburst high gas and pulverized coal is an important basis for accurately predicting the occurrence time and possible scope of outburst. To reveal the airflow disturbance characteristics and coal-gas flow rule in coal and gas outburst process, outburst coal-gas migration simulations under different gas pressures were conducted using a self-developed visual outburst dynamic effect test device. The results showed that coal-gas flow state at the outburst port is divided into subcritical flow, critical flow, and supercritical flow state. The pulverized coal-gas flow migration in the roadway space can be divided into coal gas two-phase flow area, air compression area, and undisturbed area. Under the experimental conditions, the maximum propagation velocities of wave are 342.22~359.21 m/s, and the coal gas two-phase flow is far less than the propagation velocities of outburst wave, just 3.68~33.33 m/s. When the outburst energy is large, multiple compression waves can superimpose to form shock waves. The peak value of the wave does not necessarily appear in the first boosting range. The presence of pulverized coal leads to a faster attenuation of shock wave, but it makes a greater dynamic destructive force at the same speed.

Effects of geo-stress on the dynamic response of multi-physical field parameters during coal and gas outbursts under true triaxial stress

With the increase of the mining depth in coal mining operations, more coal and gas outburst (CGO) disasters are likely occur due to the increase in geo-stress. The limitations of conventional experimental methods prevent the observation of multi-physics parameters and dynamic effects during CGO. In this work, a customized coal mine dynamic disaster test system was used to analyze the evolution of multi-physics parameters such as gas pressure, temperature, and geo-stress during CGO. The influences of geo-stress on the energy sources of CGO were discussed and the response characteristics of the coal-gas flow in the roadway during the CGO were analyzed. Results show that during the CGO process, an increase in the geo-stress will decrease the gas pressure and rate of the pressure drop. The rate of geo-stress change gradually increases with an increase in the initial stress, and the peak impact force of the coal-gas flow increases with increasing geo-stress. The elastic strain energy released during CGO also increases with increasing geo-stress, however, it is less valuable than gas expansion energy. The contribution of geo-stress to the CGO depends on its influence on the gas expansion energy. Coal seam temperature variations are mainly manifested in the stress relief and abutment stress zones, and the flow velocity of CGO coal demonstrates alternating deceleration and acceleration cycles. Additionally, the obtained distribution characteristics of CGO coal exhibit log-normal patterns. These findings can help further understand the CGO mechanism and provide a reference for passive disaster reduction measures after outbursts.

Experimental investigations of CO2 seepage behaviorin naturally fractured coal under hydro-thermal-mechanical conditions

Gas-flow in coal or rock is hypersensitive to the changes of temperature, confin?ing pressure and gas pressure. This paper implemented a series of experiments to observe the seepage behavior, especially the permeability evolution of CO2 in naturally fractured coal sample under coupled hydro-thermal-mechanical conditions. The experimental results show that coal permeability increases exponentially with the increasing gas pressure, and tends to be linear when the confining pressure is high. Coal permeability decreases exponentially with the increasing confining pressure. Coal permeability decreases with the increasing temperature generally, but it may bounce up when the temperature rises to high. The results provide reference for the projects of coal gas extraction and carbon dioxide geological sequestration.

Coupling of transient matrix diffusion and pore network models for gas flow in coal

Processes of sorption and diffusion play a critical role in the analysis and simulation of coal seam gas (CSG) reservoirs since most of the gas reserves are held by the coal matrix. Thus, it is essential to understand the displacement of gas in coal matrix and fractures. This can be formulated in terms of sorption and diffusion processes inside the matrix and advective flow in the cleats. The multi-scale porous structure and multi-physics nature of gas flow in coal complicate numerical modelling of these phenomena. A novel approach is developed to solve this problem by coupling conventional pore network modelling (PNM) with transient diffusion flow. The coupling is undertaken for each pore network throat separately by utilising the Finite Volume Method and Fick's second law of diffusion. The Langmuir isotherm is employed to describe sorption processes within the coal matrix and to link the concentration and pressure values when coupling the diffusion model with PNM. The proposed approach provides the possibility to account for the transient nature of gas transport across the different scales, while limiting the number of unknowns in the simulation to an effective diffusivity parameter. Our results demonstrate that steady-state pore network models can be effectively coupled with a transient phenomenon such as Fick's diffusion allowing to estimate the diffusive constitute of the matrix to the total gas flow in coal over time. The obtained gas sorption capacity and its release rate can then be used to accurately estimate the time-dependent production profile of CSG reservoirs.

The Energy Principle of Coal and Gas Outbursts: Experimentally Evaluating the Role of Gas Desorption

Outburst energy is a major factor influencing coal and gas outbursts, albeit its estimation is difficult owing to the lack of amenable means for quantification of gas desorption. In the past decades, determining the mechanism of outbursts is one of the most challenging issues in rock mechanics. In this study, a triaxial coal and gas outburst simulation system was employed to perform simulated experiments using He (to rule out the influence from gas ad-desorption), N2, and CO2. This facilitated understanding of the energy principle underlying the said outbursts and evaluation of the effects of gas desorption on outburst development. Results of this study indicate that outburst energy and energy consumption are influenced by several factors, including outburst pressure, outburst intensity, ejection distance, and particle size of ejected coal. Among these, gas desorption demonstrates the greatest influence when performing controlled tests (using He). Considering the effects of gas desorption, the total outburst energy can be increased by 1.35–2.95 times, thereby causing an enormous increase in the destructive potential of outbursts. Additionally, values of the coal crushing and transport energies can be enhanced by the order of 118.9–206.6% and 157.8–406.6%, respectively, thereby resulting in a stronger conveying capacity of outburst coal–gas flow along with severe coal fragmentation. A further analysis of the energy distribution indicated that in the development stage, gas desorbed from coal acts as the force driving coal transport, whereas free gas energy is mainly consumed during coal crushing. Findings of this study highlight the importance of quantifying contributions of coal gas towards effective interpretation of outburst-causing mechanisms.

A hybrid fracture-micropore network model for multiphysics gas flow in coal

Coal is not only a combustible black sedimentary rock, but also a source and reservoir rock for natural gas, known as coal seam gas (CSG). A thorough understanding of gas transport through coal matrix micropores as well as coal cleats is of significance in both mining and CSG industries. Multiple physical mechanisms are identified during gas flow in coal. However, there is not a comprehensive modelling framework, where multiscale flow behaviours with multiphysics are coupled. In this paper, we develop a hybrid Fracture-MicroPore Network Model (FM-PNM), coupling viscous gas flow in fractures and gas diffusion in coal matrix. This model includes multiphysics gas flow mechanisms in coal, including gas sorption, diffusion, slip flow, and compressibility. With FM-PNM, methane flowing through fractured coal is simulated to compute apparent permeability and gas flow rates, which are critical for CSG developments. Besides, the process of gas emission from coal over time is simulated, where desorption curves are obtained and analysed at different scenarios. This multiphysics FM-PNM gives a promising framework for studying gas flow in fractured coal, which can be integrated with other works to study different coal internal structures, flow models and stress conditions. This model has a wide range of applications, including prediction of greenhouse gas (GHG) emission of coal mines, ventilation design during mining, CSG development, and CO2 storage management.

Thermo-hydro-mechanical couplings controlling gas migration in heterogeneous and elastically-deformed coal

Abstract Revealing gas field evolution and distribution in coal seams is of great significance to coalbed methane (CBM) recovery. It is believed that gas migration in coal involves multifield coupling processes. In this study, an improved thermo-hydro-mechanical (THM) coupling model is developed to investigate the physical fields evolution during gas flow in coal. In this model, the effect of internal swelling deformation is considered to modify the conventional coal deformation equation and coal permeability model. A dynamic diffusion model is introduced to accurately characterize the dynamic evolution of gas diffusion in matrix. Besides, a new matrix porosity model is developed based on the assumption that the pore volume change equals to the matrix volume change. To reveal the gas field evolution and distribution in coal core during coal permeability test in the lab, we build two heterogeneous coal models based on numerically reconstructed method and CT scanning. With the THM model, the gas pressure distribution in the coal core is quantitatively characterized. It is found that in the heterogeneous coal core, gas streamline fluctuates in space. The gas pressure nonlinearly distributes in the coal core, rather than the linear distribution assumed when testing permeability in the lab. Aiming at the confliction between the theoretical result and the assumption used in permeability testing, we propose to improve the accuracy of lab testing result by reducing the gas pressure difference between the gas inlet and outlet.

Fluid–Solid Coupling Model and Simulation of Gas-Bearing Coal for Energy Security and Sustainability

The optimum design of gas drainage boreholes is crucial for energy security and sustainability in coal mining. Therefore, the construction of fluid–solid coupling models and numerical simulation analyses are key problems for gas drainage boreholes. This work is based on the basic theory of fluid–solid coupling, the correlation definition between coal porosity and permeability, and previous studies on the influence of adsorption expansion, change in pore free gas pressure, and the Klinkenberg effect on gas flow in coal. A mathematical model of the dynamic evolution of coal permeability and porosity is derived. A fluid–solid coupling model of gas-bearing coal and the related partial differential equation for gas migration in coal are established. Combined with an example of the measurement of the drilling radius of the bedding layer in a coal mine, a coupled numerical solution under negative pressure extraction conditions is derived by using COMSOL Multiphysics simulation software. Numerical simulation results show that the solution can effectively guide gas extraction and discharge during mining. This study provides theoretical and methodological guidance for energy security and coal mining sustainability.

The influence of CO2 saturation time on the coal gas flow: Fractured bituminous coal

The permeability reduction due to the coal matrix swelling induced by the adsorption of CO2 is the major concern during enhanced coalbed methane recovery (ECBM) as well as CO2 sequestration in coal reservoir. Many problems arise in the context of this process regarding the coal permeability variations, and very few studies have been made with consideration of long-term CO2 saturations on gas flow behaviour alterations. The main objective of this study is therefore to examine the influence of both subcritical and supercritical CO2 flooding durations on N2 and CO2 permeability variation of naturally fractured low volatile bituminous coal under various tri-axial conditions. Five CO2 flooding durations, up to 65 h, both subcritical (6 MPa) and supercritical (14, 20 MPa), were introduced to the sample under three confinements, respectively. Results show that for 11 MPa and 17 MPa confining environment, most permeability reduction occurs during the first CO2 flooding process, and supercritical CO2 flooding causes significant higher N2 permeability reduction (from 14.38% to 50.18%) than subcritical CO2 (from 4.11% to 11.25%). The permeability reduction rate decreases with time but this reduction continues even after a total of 153 h flooding. Longer permeability reduction process is observed upon supercritical CO2 adsorption due to the much higher viscosity of supercritical CO2. Under deep underground condition (>1 km), CO2 exhibits a N2-like flow behaviour because of the dominant poroelastic effect. The influence of CO2 flooding on permeability alterations has been significantly compromised with much smaller permeability reduction after 20 MPa CO2 flooding compared with 14 MPa CO2 flooding (7.33% and 38.75% for 7 MPa CO2 injection pressure, respectively), along with no apparent permeability reduction for the further CO2 flooding scenarios.

Experimental study of impact of anisotropy and heterogeneity on gas flow in coal. Part I: Diffusion and adsorption

Gas is adsorbed in the pores of coal matrix and during gas production gas is desorbed from the pore surface and diffuses through the matrix pore structure and flows in the fracture/cleat system to the production well or boreholes. However, coal is highly heterogeneous and anisotropic. How heterogeneity and anisotropy affect the gas storage and especially the diffusion behaviour is not well studied. In this work, a series of measurements were performed on three dry cubic coal samples cut from the same coal block from the Bowen Basin, Australia, using an adsorbing gas, methane. For each sample, gas adsorption experiments with gas flowing from three principal directions were performed. The diffusion data was fitted with a bidisperse diffusion model to obtain diffusion coefficient. The three samples, although from the same coal block, showed difference in adsorption amount and significant difference in effective diffusivity. It was found that the effective macropore diffusivity increased with gas pressure and effective micropore coefficient decreased with gas pressure. The effective diffusivity showed difference among samples and directions, demonstrating coal heterogeneity and anisotropy both have a significant impact on gas diffusion behaviour. However, no generalisation can be obtained with any single pore structure parameter, such as pore size or surface area, as it may be related to all pore and fracture structures at various scales.

Experimental investigation on the formation and transport mechanism of outburst coal-gas flow: Implications for the role of gas desorption in the development stage of outburst

As the one of the most catastrophic hazards in underground mining, coal and gas outburst seriously threatens the safe mining of collieries. To understand the formation and transport mechanism of outburst coal-gas flow in roadway as well as evaluate the effects of gas desorption on its development, a new apparatus was developed to conduct simulated experiments with different gases of CO2 and N2. Results indicated that the outburst coal-gas flow was a high-speed (up to 41.02 m/s during tests) gas-solid two phases flow with extreme complexity, its transport/destructiveness characteristics were significantly influenced by a number of factors including the outburst pressure, coal sample composition, ejection distance and so on. Among these factors, the gas desorption showed the greatest impact when compared to the controlled tests which only considered the effect of free gas expansion. With the effect of gas desorption, especially the rapid gas desorption from powdered coal, the total outburst energy could be promoted by 1.30–2.43 times; the peak values of outburst shockwave could be enhanced by at least 13.67%–63.22%; the transport type of coal-gas flow could be changed from dynamic pressure pneumatic conveying to the static pressure conveying which providing higher capability for outburst coal/rock conveying; the motion of ejected coal flow could have higher speed, longer transport duration and could suffer secondary acceleration. As the result, the destructiveness of outburst coal-gas flow would be remarkably intensified.A further analysis for the energy consumption during the outburst coal-gas flow transport indicated that the free gas expansion energy was insufficient for the conveying of ejected coal (only accounting for less than half of the total energy), the difference of which was made up by the gas desorption, especially the rapid gas desorption from powdered coal (the average contribution ratio reached 56.0%, while the maximum reached 64.1%). Thus, it can be concluded that the rapid gas desorption from powdered coal played a decisive role on the promotion of outburst.