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Nowadays, coalbed methane recovery faces challenges such as high gas content, micro-porosity and low permeability. An exploratory study on improving coal porosity by acetone treatment was carried out. Pore size distribution and fracture development before and after acetone treatment were evaluated using a suite of integrated diagnostic techniques including rock acoustical test, weight analysis, Scanning Electron Microscope (SEM), Nuclear Magnetic Resonance (NMR), Fourier Transform Infrared Spectroscopy (FT-IR) and mercury intrusion porosimetry (MIP). After acetone treatment, micropores are partially converted into mesopores and macropores. The porosity of coal increases by 2.66% after acetone treatment. Functional groups including hydroxy, methylene, oxygen functional groups and aromatic hydrocarbons reduce significantly and the removal of these hydrophilic groups significantly weakens the hydrophilicity of coal. New fractures appeared within 32.89% of the eroded zone, even large through-going fractures may appear after acetone treatment. The results indicate that acetone treatment is effective in improving gas productivity thus has the potential to become a new coalbed methane reservoir stimulation technology.

Microwave heating has gained widespread popularity in coal processing. Numerical simulation is a promising tool to visualize and quantize microwave-coal interactions. However, coal is always assumed as a solid continuum in the current models, which cannot accurately predict the thermodynamic behavior. In this study, a fully coupled electromagnetic, heat transfer and multiphase porous media model was developed to investigate microwave heating of coal. Results show that microwave absorption by coal induces significant redistribution of the electromagnetic field in the cavity, forming high- and low-energy regions. The nonuniform electromagnetic distribution and microwave selective heating generate hot and cold spots, whereas heat convection tends to homogenize the thermal field. The temperature rise during microwave heating is characterized by “fast-slow-fast”. In addition, water evaporation and surface heat convection can exert profound impacts on the thermal evolution of coal. When water evaporation is included in the model, the temperature increases nonlinearly because the dielectric property of coal is constantly changing during microwave heating. Outcomes of this study can be used to identify the heating behaviors of coal during microwave processing and thereby help to optimize the microwave applicators.

Carbon dioxide injection, gas flow, and adsorption of CO2 in a coal matrix are modelled by a large-scale physical experiment. The process primarily comprises an increase in the gas pressure from an unsaturated to a saturated status during gas injection followed by a reduction in the gas pressure due to the adsorption of CO2 after the cessation of gas injection. We developed a straightforward and effective calibration-based gas pressure model for the spatio-temporal prediction of CO2 in a coal matrix. The Nash–Sutcliffe efficiency of the models for predicting gas pressure is between 0.931 and 0.998. The results indicate that the spatio-temporal distribution of gas pressure in the coal matrix mainly relates to the gas pressure injected, geological boundary conditions, initial geo-stress, and gas type. The gas concentration tends to increase in high Earth stress and boundary areas during gas injection, whereas the gas adsorption rate is high in areas of low Earth stress. The gas adsorption rate of the coal matrix decreases as the gas injection content and gas injection time increase; improvement of the gas injection pressure can boost the gas adsorption to some extent. This model can explain or predict the behaviour of gas in a coal matrix depending on the matrix density, and it has the potential to simulate gas behaviour in a large-scale undisturbed coal sample considering the structure (joints and cleats) of the coal.

An innovative microwave irradiation device is developed to investigate effect of microwave irradiation on pore morphology of coal powder. This device with several distinguished highlights, such as real-time infrared temperature measurement, gas replacement, process parameters controls, and large size samples testing. Specifically, results of effect of temperature (25–300 °C) and microwave power (1–6 kW) on pore morphology was evaluated by the mercury intrusion porosimetry method. The pore connectivity decreases at first then increased with incremental processing temperature or microwave power. The percolation threshold increases with increased heating temperature or microwave power. The total pores volume increases with increased processing temperature or microwave power. The total specific surface area increases firstly then decreases with increased temperature or microwave power. These suggest that effect of microwave irradiation on pore morphology is obvious. Several possibilities for microwave energy application in the field were discussed, including drilling borehole from the ground to the payzone, drilling up-hole or down-hole from the roadway to the payzone, drilling borehole along the coalbed. The process economics were also discussed. It implies that as a new technology microwave energy may have the potential for the degassing 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.

Roadways excavated through a coal seam can exert an adverse effect on roadway stability. To investigate the effects of in-situ stress on roadway stability, numerical models were built and high horizontal stresses at varying orientations were applied. The results indicate that stress concentrations, roadway deformation and failure increase in magnitude and extent as the excavation angle with respect to the maximum horizontal stress increases. In addition, the stress adjacent to the coal-rock interface sharply varies in space and evolves with time; coal is much more vulnerable to deformation and failure than rock. The results provide insights into the layout of roadways excavated through a coal seam. Roadways should be designed parallel or at a narrow angle to the maximum horizontal stress. The concentrated stress at the top corner of the face-end should be reduced in advance, and the coal seam should be reinforced immediately after excavation. Keywords: Roadway stability, Numerical simulation, In-situ stress, Stress concentration, Failure, Deformation

Microwave drying of coal has the advantage of volumetric and selective heating, which saves energy and time. However, comprehensive assessment of the moisture migration during microwave drying is lacking. In this study, the low-field nuclear magnetic resonance (LF-NMR) was used for moisture characterization during microwave drying. Moisture migration occurs first in larger pores, whereas the smaller pores appear to remain water-saturated. As the microwave power increases, the drying capacity and drying rate both increase because high-power microwave can open the blind pores to promote moisture seepage. Furthermore, moisture migration during microwave drying is usually directional (i.e., along the bedding plane). The LF-NMR method provides a framework for the determination of moisture migration of coals.

Realized by the conversion of the electromagnetic energy into thermal energy, microwave heating can be a sound method to enhance coalbed methane (CBM) recovery. Although the effects of microwave heating on the petrophysical characteristics of coal have been comprehensively investigated, few studies considered the complex coal-gas interactions under non-isothermal conditions. In this study, a fully coupled electromagnetic-thermal-mechanical model was developed to investigate the influence of coal compaction, thermal expansion, thermal-induced gas desorption, and sorption-induced coal deformation on CBM extraction. The results indicated that the sorption-induced coal deformation makes the predominant contribution to the porosity evolution. In addition, microwave heating can enhance the cumulative gas production by 43.9% by promoting gas desorption and seepage. The sensitivity analysis showed that the microwave operating at 2450 MHz under lower powers is preferred to maintain efficient stimulation and to avoid overheating. Furthermore, the stimulation efficiency of microwave heating increases with the initial permeability and gas pressure. Outcomes of this study can provide valuable insights into microwave-assisted CBM recovery.

Coalbed methane (CBM) reservoirs generally have very low permeability and require stimulation to make gas extraction economical. Hydraulic fracturing has been widely applied to enhance CBM productivity, but this technology has a number of key limitations, including the drawback that fractures only propagate along existing joints, the large amount of water usage, and the potential environmental impact. Microwave irradiation technology can likely overcome the above limitations. In this study, the effect of microwave irradiation on the petrophysical properties of an unconstrained bituminous coal was comprehensively investigated through a suite of integrated diagnostic techniques including nuclear magnetic resonance and X-ray computed tomography. A series of experiments were conducted both on centrifuged samples and on samples with different water contents ranging from 1 to 15%. The mineral removal and moisture evaporation due to the microwave selective heating lead to the enlargement, opening, and interconne...

Temperature is an important index to evaluate the microwave heating characteristics and affection of Formation microwave heating treatment. Experiments on microwave heating of coal powder were conducted to investigate the temperature rising characteristics of coal powder. The results indicated when the microwave heating power ranged from 1 kW to 5 kW, the temperature rising rate of the coal sample with grain size 0.18 mm–0.25 mm was larger than that with grain size 1.00 mm–2.00 mm. However, this relation was the opposite when the microwave power was 6 kW. The temperature rising rate increased with the increased moisture and pyrite content of the coal sample, while it decreased with the increased calcium carbonate, silicon dioxide, and potassium chloride of coal samples. Moreover, the correlation between oxygen concentration and the temperature rising was not obvious in this study. Finally, empirical equations between various factors and the heating temperature are given.

The experimental work described in this paper discusses the effect of microwave irradiation on the petrophysical characteristics of coal cores. Nuclear magnetic resonance (NMR) is a non-destructive approach employed in this research. This paper verifies that the membrane-bound water model is appropriate for the explanation of the state of bound-water in pores of coal samples. The paper also verifies fractal dimension as an intrinsic property of a surface that does not depend on porosity. The pore size, pore number, connectivity and porosity increased after microwave irradiation. Furthermore, a theoretical equation was proposed to calculate fractal dimensions based on the transverse relaxation time (T2). The fractal dimensions of coal cores decreased after microwave irradiation, increasing the permeability of the sample. These results suggest that microwave irradiation is a potential method for degassing coal seams in the future.

The experimental work described in this paper aims to study the effect of microwave irradiation on petrophysical characterization of coals. Twenty coal samples were irradiated at 2.45 GHz with variable power (2, 4, 6 kW). The temperature, mass and specific heat capacity of coal samples were measured and calculated. The effect of microwave irradiation on the porosity of coal samples was evaluated by the gravimetric method and nuclear magnetic resonance (NMR) measurements. The porosity obviously increases after microwave heating. Interestingly, growth rate of the porosity decreases at first then increases with microwave energy. The turning point is approximately 100 kJ. The influence of microwave irradiation on pore size, throat size and pore numbers of coal samples were also evaluated by NMR measurements. It suggest that the pore size, throat size and pore numbers are obviously increase with microwave energy. In a word, it appears likely that microwave energy may have the potential for the degassing coal seams.

The technology for combining CO2 injection with alkaline solution injection into coal bodies is proposed and investigated to address the disadvantages of traditional CO2 injection technologies, such as the production of dangerous amounts of excess residual gas and prevention of water injection into the deeper coal seams. After the CO2 injection, the wettability of coal sample is greatly improved, especially the alkaline solution. CO2 injection can effectively displace methane in coal, while alkaline solution can dissolve CO2 molecules adsorbed in coal and improve the wetting effect of coal. The increase of moisture will lead to the decrease of coal permeability, which can reduce gas emission. The permeability loss ratios for coal samples injected with CO2 were more than 90% much higher than those for coal samples not injected with CO2. After alkaline injection, the moisture content of coal into which CO2 had been injected increased 6.13 times compared to raw coal. Therefore, this technology may consume excess CO2 gas from the coal body and improve the effect of water injection into deeper coal seams.

Formation microwave heating treatment (FMHT) is a potential method for degassing coalbed methane. Camera pictures and ultrasonic method were used to evaluate the crack propagation on the surface and in the interior of the coal cores, respectively. The results suggest that the fracture induced by microwave energy would remarkably increase with increased heating time. New fractures and pore sizes produced by microwave energy decreased P- and S-wave velocity in coal cores. The amplitude of the frequency spectrum of coal cores decreased after microwave energy treatment. The frequency distribution domain in the frequency spectrum of coal cores was also affected by microwave energy treatment. The density, bulk modulus and shear modulus of coal decreased with microwave energy treatment. Specifically, the slopes of declines in the bulk modulus and shear modulus were larger than that in the coal density. The increasing fracture network could improve the permeability of coal for degassing coal seams. In a word, the microwave energy has potential for assisting coalbed gas drainageing.

To maximize coalbed methane recovery, the reservoir is often stimulated because of its low permeability. An exploratory study on improving coal porosity and permeability by microwave treatment was proposed. Pore size distribution of four unconstrained coals (lignite, subbituminous, bituminous, and anthracite), before and after microwave treatment, were evaluated using the nuclear magnetic resonance. With continuous exposure to microwaves, the pore size distribution of coals (especially the lignite coal) extends, and the pore volume and connectivity increase. In addition, coal porosity and permeability evaluated based on the Schlumberger Doll Research model increase by 33–72% and 73–181%, respectively. The mechanism was revealed by combining P-wave tests, thermal imaging and X-ray computed tomography scanner. The moisture and minerals bounded in pores are selectively heated and then detached, and, as a result, the pore structure is opened. Continued exposure to microwaves rapidly converts the mobilized moisture into super-heated steam. Under the steam pressure, pores and fractures generally expand. Borrowing from microwave-assist oil recovery, we presented a conceptual design of a CBM extraction borehole with microwave irradiation.

This paper aims to predict the heating behaviors of coal sample by COMSOL Multiphysics software based on the finite element method. The effects of microwave frequency, microwave power and sample position on temperature profiles and electric field distributions are examined. The effect of microwave frequency on temperature is obvious. The maximum and minimum temperatures of the coal sample change with frequency in different laws respectively. There is just one optimal heating frequency to achieve the maximum or minimum value of temperature. The effect of microwave power on temperature is found evidently, too. The lower power microwave-treatment requires more energy than microwave-treatment with a higher power when the heating temperature is greater than 200 °C. And there is an optimal temperature between 100 °C and 200 °C, which energy input is equal to each other under different power inputs. The effect of sample position on the temperature is also remarkable. The maximum and minimum temperatures of the coal sample change with position in different laws respectively. The results can serve as references for the study on heating coal sample by microwave heating system.

The low gas permeability of coal formations with limited coal pores and fractures leads to difficulty in coalbed methane exploration. High-voltage electrical pulse has a potential application in enhanced coalbed methane recovery. In this study, we discuss the microscopic characteristics of anthracite coals treated by high-voltage electrical pulse. We find that C, O, and other coal elements constituting oxygenic groups, which mainly account for gas adsorption, decreased slightly after high-voltage electrical pulse treatment, indicating that elemental variation may have little influence on gas adsorption. The scanning electron microscopy and low-pressure nitrogen gas adsorption (LP-N2GA) results show that the cumulative micropore volumes of high-voltage electrical pulse-treated coals were much larger than those of original coals. The mercury intrusion porosimetry results show that the cumulative macropore volumes, which act as gas migration channels in coal increased. Additionally, high-voltage electrical pulse-treated coals were found to have smaller entrapment areas, indicating that gas migration was enhanced.

The experimental work in this paper investigated the effect of microwave energy on pore structure and gas permeability of coal cores. Fifteen coal samples were irradiated under the condition of 2.45 GHz with 6 kW. The effect of microwave irradiation on the pore structure of coal samples was evaluated by nuclear magnetic resonance (NMR). The water saturation degree has little influence on transverse relaxation time (T2) distribution before the microwave treatment. By contrast, water saturation degree obviously affects the T2 distribution after the microwave energy treatment. Coal permeability increased after microwave energy treatment. But fractal dimension decreased after microwave energy treatment. The results show that microwave energy has a potential for degassing coal seams.

Microwave-assisted pyrolysis (MWAP) provides distinctive heating for the conversion of coal into liquid fuels. This work investigated the effects of MWAP on the microstructure of bituminous coals by using Fourier transform infrared spectroscopy (FTIR), mercury intrusion porosimetry (MIP) and Scanning Electron Microscope (SEM) techniques. FTIR results showed that MWAP can lead to the decomposition of oxygen functional groups and aliphatic hydrocarbons. Removal of these hydrophilic groups significantly weakens the hydrophilicity of coal. With continuous exposure to microwaves, unstable components such as alcohol, phenol and carboxylic acid are converted into ether groups. Cyclization and aromatization induced by microwave heating improve the maturation of coal. MIP and SEM results indicated that the jet force exerted by the high-pressure steam and gaseous products under MWAP gives a great boost to the opening, enlargement and connection of pores. Furthermore, the thermal stresses induced by microwave selective heating enlarge the fractures and create new cracks, providing seepage space for fluids. MWAP of bituminous coals can not only release moisture and volatiles but also create seepage space by modifying the pore and fracture structure.

The modes of occurrence of mercury in coals from Guizhou, P.R. China were investigated using sequential extraction procedures and density separation experiments. The sequential extraction tests were carried out using 32 coals sampled from the Longtan Group of the Permian system in Guizhou Province. Eight coal samples collected from the same area were divided into 10 density fractions between 2.8 g/cm3 by separation in a heavy-liquid mixture. Combining the results of mineral transformation and of the amount of mercury leached during the leaching experiments with the results of the correlation between mercury and mineral phases and organic matter from density separation experiments, the authors concluded that mercury in the coals examined mainly exists in pyrite. The studies also showed that most of mercury in these coals can be brought into the environment easily by acid mine drainage. It is inferred that a rather large part of Hg in coals from Guizhou may be removed by using conventional coal cleaning techniques.