Coal Gas Flow Research Articles

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.

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

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

Controlling factors of symbiotic disaster between coal gas and spontaneous combustion in longwall mining gobs

Underground methane-rich and combustion-prone longwall mining gobs are usually subjected to the symbiotic thread between coal gas and combustion disasters. Quantitative understanding of this symbiosis is essential to mitigate these potential disasters. However, previous mathematical models mainly focused on the simulation disaster of coal gas flow in gobs or spontaneous combustion, less attention is paid to the symbiosis between them. In this study, we extended a fully coupled coal-oxygen-heating model for the spontaneous combustion into a symbiotic model between coal gas flow and spontaneous combustion. This was achieved through the introduction of an accumulative gas release function in the set of coupled governing equations. The function defines the mass exchange of coal gas between gob and surrounding areas. We applied the symbiotic model to investigate the sensitivity of the symbiotic disaster to the controlling factors, including (1) ventilation flux; (2) face advance rate; (3) longwall panel width; and (4) coal-oxidation rate. Our results show that enhancing ventilation flux can dilute the CH4 concentration of return airway or working face, but increase the risk of gob spontaneous combustion. On the contrary, accelerating face advance rate can suppress gob spontaneous combustion, but easily cause CH4 concentration overrun of return airway or working face. That is, if parameters such as ventilation flux and face advance rate were not designed properly, a disaster (i.e. gas disaster) might be suppressed, but another disaster (i.e. spontaneous combustion) could be triggered or aggravated. In addition, widening longwall panel could aggravate gob leakage flux and improve the CH4 concentration of return airway, while it might exert less influence on coal self-heating. These results may provide some guidelines on how to mitigate the symbiotic disaster between coal gas flow and spontaneous combustion in longwall gobs.

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.

Evaluation of gas sorption-induced internal swelling in coal

Gas sorption plays an important role in permeability evolution and gas flow in coal. Many efforts have been made to evaluate the effect of gas sorption on permeability. For example, the total gas sorption matrix swelling is divided in to two pars, one acts on cleat (internal swelling) and the other contributes to coal block deformation. Nevertheless, this effect has not been fully identified yet. This paper proposed a method that regressed the internal swelling by using a proposed permeability model. The internal swelling values under different boundary conditions were calculated by using the well-measured permeability from two published papers. The results indicated that the internal swelling was positively proportional to pore pressure and negatively proportional to confining stress for gaseous and supercritical methane and gaseous carbon dioxide. For supercritical carbon dioxide, the internal swelling increased with increasing pore pressure but was almost independent of confining stress. The internal swelling may be affected by sorbate type, coal structure and coal lithotypes in addition to pore pressure and confining stress. The internal swelling ratio that is defined as the ratio of the internal swelling to the gas sorption-induced swelling of coal matrix or coal block was also calculated. The internal swelling ratio may roughly keep constant during coalbed methane recovery when the in situ stress was greater than 5MPa. This assumption was practically useful for engineering application as it linked the effect of gas sorption on in situ permeability to the easy-measured sorption strain. Although the constant value cannot capture the full nature of the in situ internal swelling ratio, it was still capable of improving the accuracy of permeability prediction to some extent.

A fully coupled coal deformation and compositional flow model for the control of the pre-mining coal seam gas extraction

Fully coupled compositional (coal seam gas and air) model of coal deformation, coal gas flow and transport, and air flow in coal seams is developed to better understand the gas drainage processes and the coal gas–air mixing mechanisms during pre-mining coal seem gas extraction. The model was first verified by showing that the modelled gas concentration profiles match reasonably with the in-situ measured ones. The verified model was then applied to evaluate how the drained gas concentration could be controlled under different conditions of the sealing length, the leakage rate and the leakage fracture width. These modelled results provides the basis of a new in-situ control technology of gas–air mixing, which uses fine expansive particles to seal the leakage fractures around the borehole. This technology has been commercially applied to enhance the concentration of the pre-mining gas drainage. The field test shows that the characteristics of the leakage fractures are greatly changed after the particles are injected into the in-seam drainage borehole when the gas concentration declines. Once the leakage fractures are blocked with the particles, the outside air is prevented from entering the coal seam. Thus, an ideal gas concentration can be maintained, and the duration of higher gas concentration is extended. For boreholes with an originally low gas concentration (0–30%), the new technology can increase the gas concentration by 10–65% and extend the production time by approximately two to three months. The total amount of gas drainage increases to 2000–3000m3 per borehole.

Improved apparent permeability models of gas flow in coal with Klinkenberg effect

Klinkenberg effect is an important phenomenon for gas flow in low permeability reservoirs, and its influence increases with the reduction of gas pressure. Unlike conventional gas reservoirs, coal seam is unique with its high compressibility and sorption-induced-swelling features. During the coalbed methane (CBM) recovery process, coal cleat width varies due to the net effect of effective stress and coal matrix shrinkage, thus the Klinkenberg coefficient cannot be treated as a constant like rock reservoirs. A brief review of previous studies shows that the influence of coal seam characteristics on Klinkenberg effect was ignored by other researchers. By using the bundled matchstick conceptual model of coal, two improved models are proposed in this paper, one is under constant effective stress and the other is under reservoir condition. The former shows that the proportion of permeability change due to Klinkenberg effect is greater than the result from original model, and the Klinkenberg coefficient varies substantially albeit the influence of effective stress is eliminated. The latter links apparent permeability and coal porosity together, and the results show good agreement with field data, especially when the gas pressure is relative low. It can be concluded that apparent permeability model is crucial for explicit prediction of gas permeability changes during CBM recovery process, which may be underestimated without due consideration of Klinkenberg effect. The development of the improved apparent permeability models has significant value in the accurate prediction of CBM production as a result of permeability changes.

Simulation of the interaction of methane, carbon dioxide and coal

Gas adsorption has an important influence on gas flow in a coal body. Research on the characteristics of coal and gas adsorption is the theoretical basis for studying gas flow in coal. In this paper, the interaction between methane, carbon dioxide and surface molecules of anthracite was simulated using the quantum chemistry method. Adsorption energy and adsorption configurations of different quantities of gas molecules absorbed on the coal surface were calculated. The results show that adsorption between coal and the two kinds of gas molecules is a physical adsorption process and there is an optimal configuration. Gas molecules are more easily adsorbed in the hydroxyl-containing side chain, while it is difficult for them to be adsorbed at the position of the benzene ring. Besides, carbon dioxide molecules are more readily adsorbed on the coal surface than methane molecules. The findings have an important significance in revealing the nature of gas adsorption in coal.

An Experimental Study on Permeability Properties of Outburst Coal under the Coupling Effect of Multiple Factors

Through the experiments investigation on complete stress-strain of coal and the permeability of coal under different stress and gas pressures, the rules of permeability of coal samples under different conditions and the corresponding permeability evolution equation were established. The results show that the permeability of coal samples under certain gas and axial pressure will exponentially decrease when the confining pressure increase. The gas flow in coal has significant Klinkenberg effect. As the volume stress increase, the scope of Klinkenberg effect can also become larger. The relationship of confining pressure & permeability and gas pressure & permeability were obtained by fitting experimental data. Base on permeability test of complete stress-strain, we recognized that the curves between gas permeability and strain under different confining pressure have the similar change trends. Changes of permeability are closely related to the damage evolution of coal. According to the characteristics of permeability curves, considering the integrated effect of confining pressure, axial compression, and gas pressure, the permeability evolution equations of complete stress-strain was established with effective stress.

A Mathematical Model of Coal-gas Flow Conveying In the Process of Coal and Gas Outburst and Its Application

Taking the coal-gas flow in the process of coal and gas outburst as the study object, the coal particles movement process is divided into acceleration process and balance process according to the solid particle suspension mechanism in the gas flow and the energy conservation law. A mathematical model for outburst coal conveying in the roadway in one-dimensional case is established, and its reasonableness is proved through resolving an on-site example. The mathematical model which is a new method for engineering calculation of outburst two-phase flow can both estimate the coal particles conveying distance according to the initial state of coal seam and inverse the initial energy of coal-gas flow according to the coal particles conveying distance record on spot.

Study on the formation mechanism of shock wave in process of coal and gas outburst

According to the research results of motion parameters of coal-gas flow, analyzed the formation mechanism of shock waves at different states of coal-gas flow in the process of coal and gas outburst, and briefly described the two possible cases of outburst shock wave formation and their formation conditions in the process of coal and gas outburst, and then pointed out that a high degree of under-expanded coal-gas flow was the main reason for the formation of a highly destructive shock wave. The research results improved the shock wave theory in coal and gas outburst.

Numerical analysis of coal-gas flow and pressure relief on the mining method of extreme short-range protective strata

The coal-gas existing condition was ameliorated in the coal seams prone to coal-gas outburst adopting the mining method of protective strata. The gas volume and the gas pressure were reduced synchronously in the protected coal seam, and the coal seam of high permeability prone to the coal-gas outburst was changed into that of low permeability with no proneness to the coal-gas outburst. The D15 coal seam was treated as the protective strata, and the D16–17 coal seam was treated as the protected strata in the Fifth coal mine in the Pingdingshan Coal Mining Group. The distance between the two coal seams was 5 m averagely, clarified into the extreme short-range protective strata. The numerical analysis was based on the theory of the porous media flow with the finite element method. The gas flow process and the change mechanism of the coal-gas pressure were analyzed in the process of mining the protective strata.

97/01849 Research on methods for metering pipeline coal-gas flow rate

97/01849 Research on methods for metering pipeline coal-gas flow rate

97/01011 Research on methods for metering pipeline coal-gas flow rate

97/01011 Research on methods for metering pipeline coal-gas flow rate

石炭の非定常ガス透過性に対する間隙ガス圧の影響 き裂網モデルによる石炭のガス透過性の研究（第２報）

The purpose of this study is to prove the effect of internal pore pressure on unsteady-state gas flow in coal. Practically, the gas permeation model of coal which consists of the gas permeable and non-permeable crack network was proposed. Then the numerical simulations of gas permeation by this model were carried out by changing the parameters such as inlet pressure in the crack network, confining pressure against coal, closing probability of cracks, etc.The results of these simulations on fluctuations of unsteady-state gas flow were in agreements with the experiments of gas permeation of coal. From these considerations it was found that the above gas permeation model was accurate enough for practically. Moreover, the results of the simulations on unsteady-state gas flow in coal gave an explanation to the deviation from the Darcy's law in the relationsbetween gas flow and difference pressure.