Coalbed Methane Extraction Research Articles

Mechanism of Nozzle Position Affecting Coalbed Methane Mining in High-Pressure Air Blasting

The use of clean energy is an important part of promoting sustainable energy development. As a clean energy source, coalbed methane, during the mining process, the position of the nozzle can influence coalbed methane extraction efficiency by affecting the cracking effect of coal. To investigate the impact of nozzles on the effect of coal fracture, a test of simulated coal by high-pressure air blasting was executed using nozzles 100 mm, 200 mm, and 250 mm from the orifice. Based on the test results and theories of fracture damage mechanics, two damage fracture models were established for the nozzles located in the middle-upper and middle-lower of the blasthole, respectively. The fracturing process and increased permeability mechanism of the coal were revealed by these two models. The results show that: when the nozzle is 100 mm from the orifice, the high-pressure air impacts the blasthole wall first, similar to a uniform expansion. Multiple longitudinal cracks are formed penetrating the coal. The permeability of the coal seam is greatly improved. When the nozzle is 200 mm and 250 mm from the orifice, the high-pressure air first impacts the bottom of the blasthole. The bottom hole angle and apex hole angle first form horizontal cracks while longitudinal cracks only appear at the same depth as the blasthole. The nozzle is 250 mm from the orifice to form a compaction zone at the bottom of the blasthole. The crack density is small and the tangential depth is shallow, which is not conducive to coalbed methane mining. The results of the research offer a theoretical framework and point of reference for the use of high-pressure air blasting technology in the extraction of coalbed methane (CBM).

Study on the Movement of Pulverized Coal Particles in Fractal Fracture Network.

Coalbed methane (CBM) exploitation is a complex multiphase flow process. With the development of CBM extraction technology, it is recognized that the migration of reservoir fluid in the fracture network is affected by the matrix fracture structure, fracture width, and pulverized coal particles plugging, therefore the fluid-particle coupling problem has received increasing attention. In this paper, six groups of fracture models with different fractal dimensions are established using the Weierstrass-Mandelbrot function, and the fluid-particle coupling phenomena in fractures with different roughness and aperture are simulated using an immersed boundary-lattice Boltzmann method (IB-LBM), by comparing the numerical simulation results with the theoretical analysis, it is proved that the IB-LBM numerical method has high accuracy and effectiveness. The effects of fractal dimension, particle size, and particle concentration on the plugging effect were investigated. The results show that the plugging effect on the fracture is enhanced with the increase of the pulverized coal particles content. When the concentration and size of pulverized coal particles are the same, the migration path of pulverized coal particles in complex fractures is more tortuous and more likely to be plugged. It may lead to a shift in the main seepage network of the fracture and the formation of new seepage channels. The research in this paper provides a data basis for the formation and plugging pattern of pulverized coal particles in coal seam fractures, which can provide a basis for the control of pulverized coal particles concentration in CBM drainage.

The effect of organics transformation and migration on pore structure of bituminous coal and lignite during biomethane production.

Biomethane generation by coal degradation not only can increase coalbed methane (CBM) reserves, namely, microbially enhanced coalbed methane (MECBM), but also has a significant effect on the pore structure of coal which is the key factor in CBM extraction. The transformation and migration of organics in coal are essential to pore development under the action of microorganisms. Here, the biodegradation of bituminous coal and lignite to produce methane and the cultivation with inhibition of methanogenic activity by 2-bromoethanesulfonate (BES) were performed to analyze the effect of biodegradation on coal pore development by determining the changes of the pore structure and the organics in culture solution and coal. The results showed that the maximum methane productions from bituminous coal and lignite were 117.69μmol/g and 166.55μmol/g, respectively. Biodegradation mainly affected the development of micropore whose specific surface area (SSA) and pore volume (PV) decreased while the fractal dimension increased. After biodegradation, various organics were generated which were partly released into culture solution while a large number of them remained in residual coal. The content of newly generated heterocyclic organics and oxygen-containing aromatics in bituminous coal was 11.21% and 20.21%. And the content of heterocyclic organics in bituminous coal was negatively correlated with SSA and PV but positively correlated with the fractal dimension which suggested that the retention of organics contributed greatly to the decrease of pore development. But the retention effect on pore structure was relatively poor in lignite. Besides, microorganisms were observed around fissures in both coal samples after biodegradation which would not be conducive to the porosity of coal on the micron scale. These results revealed that the effect of biodegradation on pore development of coal was governed by the combined action of organics degradation to produce methane and organics retention in coal whose contributions were antagonistic and determined by coal rank and pore aperture. The better development of MECBM needs to enhance organics biodegradation and reduce organics retention in coal.

Dynamic Evolution and Quantitative Characterization of Fractures in Coal at the Eastern Edge of Ordos Basin under Axial Loading

Understanding the evolution of pore-fracture networks in coal during loading is of paramount importance for coalbed methane exploration. To shed light on these dynamic changes, this study undertook uniaxial compression experiments on coal samples collected from the eastern edge of the Ordos Basin, complemented by μ-CT scanning to obtain a 3D visualization of the crack network model. The compression process was divided into three stages, namely, micro-crack compaction, linear elasticity, and peak failure. An increase in stress resulted in greater concentration and unevenness in fractal dimensions, illustrating the propagation of initial cleats and micro-cracks in the dominant crack direction and the ensuing process of crack merging. These results provide valuable insights into the internal structure and behavior of coal under stress, informing more efficient strategies for coalbed methane extraction.

Thermal Expansion and Adsorption Characteristics of Coal Based on Temperature Change and Its Influence on the Slippage Effect

The slippage effect is an important phenomenon of coalbed methane (CBM) migration. As the exploitation of CBM, the slippage effect becomes more and more obvious. For exploring the evolution of the slippage effect as CBM extraction by heat injection, the thermal expansion test and the temperature increasing desorption test under different stress states, as well as the seepage test under variable temperature conditions, were conducted by operating the high-temperature multifunctional triaxial test system. The effect rules for temperature and pore pressure on the slippage effect were analyzed, and then the evolution mechanism of the slippage factor at the united action of pore pressure and temperature was comprehensively analyzed based on thermal expansion deformation and adsorption characteristics. The results showed that: first, the influence of pore pressure on the slippage effect is mainly caused by the molecular mean free path and effective stress when pore pressure is greater than 0.8 MPa in the range of 30–150 °C. With the pore pressure increase, the slippage effect decreases. Second, the temperature influence on the slippage effect is mostly induced by the variation of cracks in coal due to thermal expansion deformation created by temperature. With the increase in temperature, the slippage factor average increases first from 30 to 60 °C is 19 times and then decreases from 60 to 90 °C is 36%. Finally, at the combined influence of temperature and pore pressure, the slippage effect is mainly influenced by thermal expansion deformation, desorption deformation, and effective stress. At 30–60 °C, the slippage factor is principally affected by thermal expansion deformation. At 60–90 °C, the slippage factor is mostly influenced by desorption deformation and effective stress. The research results provide a foundation for the technology of CBM exploitation by heat injection.

Simulation Study on Molecular Adsorption of Coal in Chicheng Coal Mine.

To study the importance of the adsorption mechanism of methane (CH4) and carbon dioxide (CO2) in coal for coalbed methane development, we aimed to reveal the influence mechanism of adsorption pressure, temperature, gas properties, water content, and other factors on gas molecular adsorption behavior from the molecular level. In this study, we selected the nonsticky coal in Chicheng Coal Mine as the research object. Based on the coal macromolecular model, we used the molecular dynamics (MD) and Monte Carlo (GCMC) methods to simulate and analyze the conditions of different pressure, temperature, and water content. The change rule and microscopic mechanism of the adsorption amount, equal adsorption heat, and interaction energy of CO2 and CH4 gas molecules in the coal macromolecular structure model establish a theoretical foundation for revealing the adsorption characteristics of coalbed methane in coal and provide technical support for further improving coalbed methane extraction.

μCT quantitative assessment of the pore–fracture structures and permeability behaviors of long-flame coal treated by infrared rapid heating

To explore the evolution features of the pore–fracture structures and permeability behaviors of coal under high temperatures and nitrogen atmospheres, an infrared rapid heating apparatus was employed to heat coal samples up to 100–500 °C at intervals of 100 °C. The coal samples were then scanned using high-precision micro-CT, and a three-dimensional (3D) pore–fracture structure and equivalent pore network model of the coal samples were established. The changing laws of Ep, Cn, Et, and Lc that varied with the heating temperature were first quantitatively analyzed. The distribution of the surface area and volume of the pore fracture was then statistically characterized. The results showed that the number of pore fractures and throats reached a maximum at 200 °C. Pore and fracture structures in coal tend to develop more with increasing temperature. In addition, the variations in the fractal dimension, porosity, connectivity, and permeability of coal with temperature are discussed. The fractal dimension and porosity increased significantly with increasing temperature. Dc and φc are more linearly dependent on temperature than Df and. φf The interconnection between the pores and fissures caused by the temperature increase is responsible for the increase in connectivity and permeability. This study provides theoretical guidance for coal fire prevention and coalbed methane extraction.

Ultrasonic-Assisted Coalbed Methane Recovery: A Coupled Acoustic–Thermal–Mechanical–Hydrological Model

Ultrasonic propagation in coal seams is accompanied by heat transfer, which has the potential to increase coal seam permeability, enhance coalbed methane (CBM) recovery, and prevent coal and gas outbursts. Therefore, it is important to analyze the effects of heat transfer by ultrasonic vibration on CBM recovery and evaluate the prospects of engineering applications of ultrasonic heating technology. In this study, the factors affecting heat transfer by ultrasonic vibration were analyzed theoretically; then, CBM recovery under the condition of ultrasonic heating was simulated by establishing a coupled acoustic–thermal–mechanical–hydrological model. The correctness of acoustic–thermal model was validated by matching simulated data with experimental data. And the accuracy of the gas flow model was verified by comparing the production data from CBM extraction boreholes with the simulated data. The research results were as follows: heat transfer by ultrasonic vibration was affected by frequency and sound pressure. When the ultrasonic frequency varied from 30 to 40 kHz and the sound pressure varied from 0.1 to 0.12 MPa, the lower the frequency, the higher the sound pressure, and the better the ultrasonic vibration heat transfer effect. In addition, the thermal expansion volumetric strain of the coal matrix caused by the ultrasonic heating of coal seams was weaker than the shrinkage volumetric strain of the coal matrix caused by gas desorption, improving the porosity and permeability of the coal seams. Furthermore, the gas drainage standard area increased by 20.8 m2 after 720 days of CBM recovery when replacing conventional CBM recovery with ultrasonic-assisted CBM recovery. With a production time of 720 days, the maximum production of CBM after ultrasonic excitation at a frequency of 40 kHz and a sound pressure of 0.10 MPa increases from 3744 to 9740 m3/day compared to conventional excitation. Our fully coupled acoustic–thermal–mechanical–hydrological model can improve current understandings of heat and mass transfer in thermal simulation of ultrasonic-enhanced CBM recovery.

Experimental study of shock pressure and erosion characteristics of high‐pressure gas–liquid two‐phase jet: Exploration for improving coalbed methane extraction efficiency

AbstractCoalbed methane extraction promotes energy exploitation and alleviates the greenhouse effect. This study aimed to analyze the technical potential of a high‐pressure gas–liquid two‐phase jet in increasing coalbed permeability and promoting coalbed methane extraction. A well‐designed experimental setup was used to investigate the erosion characteristics of a high‐pressure gas–liquid two‐phase jet. Through the shock pressure monitoring and erosion tests of the jet, the effects of pump pressure, mixer structure, and nozzle structure on the shock pressure and rock‐breaking performance were studied, and the rock‐breaking mechanism of the high‐pressure gas–liquid two‐phase jet was examined. The results showed that the nonuniformity of mixing and the expansibility of high‐pressure gas continually alters high‐pressure gas–liquid two‐phase jets between single and multiple streams, which makes the jet show obvious pulse effects and considerably improves the rock‐breaking performance. Furthermore, reasonable air pump pressure and water pump pressure settings significantly increase the shock area, whereas the maximum shock pressure only decreases slightly. Although the shock pressure of a high‐pressure gas–liquid two‐phase jet decreases with the increase in air pump pressure, its shock efficiency first increases and then decreases with the increase in air pump pressure. To improve the erosion efficiency of a high‐pressure gas–liquid two‐phase jet, the settings of the air pump pressure and water pump pressure is crucial, but the optimal air pump pressure differs for different water pump pressures. Additionally, the greater the angle between the gas and liquid inlets, the greater the energy loss of the fluid during the mixing. Finally, the rock‐breaking performance showed that the optimal nozzle structure is the single‐cone nozzle.

Experimental and theoretical study on the dynamic effective stress of loaded gassy coal during gas release

In the process of mining coalbed methane (CBM), an unsteady state often arises due to the rapid extraction, release and pressure relief of CBM. In this case, the effective stress of coal changes dynamically, affecting the stability of the gassy coal seam. In this paper, gas release tests of gassy coal under conventional triaxial compression were performed, and the dynamic effective stress (DES) during gas release was obtained indirectly based on a constitutive equation and deformation of coal. The results show that the maximum increases in DES caused by the release of free gas and adsorbed gas under the stress of 1.1 MPa were 0.811 and 5.418 MPa, respectively, which seriously affected the stress state of the coal. During the gas release, the free gas pressure and the adsorbed gas volume were the parameters that directly affected the DES and showed a positive linear relationship with the DES with an intercept of zero. The DES of the coal sample increased exponentially with time, which was determined by the contents of free and adsorbed gas. Based on the experimental results and theoretical analysis, an effective stress model was obtained for loaded gassy coal during gas release. The results of verification indicated accuracy greater than 99%.

Analysis of the Change Feature and Action Mechanism of Coal Pore Structure under the Action of Supercritical CO2

In this study, supercritical CO2- (ScCO2) induced permeability enhancement tests were performed on two different-rank coal samples (i.e., coking coal and anthracite) by a self-developed ScCO2-induced permeability enhancement test device for the purpose of exploring the change feature and action mechanism of ScCO2 on coal pore structure during the improvement of coalbed methane extraction. The following results were obtained: after the injection of ScCO2, the original pore structure of coal changes. Specifically, the connectivity between macropores and fractures increases, that between mesopores and small pores decreases and that between small pores and micropores increases. Besides, the total pore volumes of the two types of coal samples both grow primarily due to the increase in macropores and mesopores, and the sample of a higher rank corresponds to a higher growth rate. The growth in the number of effective pores conduces to dredging and enlarging pores in some ways, promoting the surface roughness of macropores and mesopores and reducing that of small pores and micropores. With respect to the action mechanism, ScCO2-induced changes in the coal pore structure are jointly induced by pore adsorption swelling, dissolution-migration, and dissolution-precipitation. Among them, the internal cause of pore adsorption swelling is the surface-free energy of fine pores. The sample of a higher rank contains more fine pores and greater surface-free energy; resultantly, it experiences stronger adsorption swelling and thus greater changes in its pore structure. The carbonic acid generated by ScCO2 and the strong acid minerals existing in the coal matrix and pore space dissolute and migrate, which stands to dredge and expand pore space and meanwhile promote pore connectivity and volume. Moreover, dissolution-precipitation leads to the blockage of pore space and pore channels, hence reducing pore connectivity and pore volume.

Numerical Study on the Evolution of Reservoir Pressure and CBM Concentration Considering Hydraulic Fractures

Based on the theories of mass conservation and coalbed methane (CBM) adsorption/desorption, this paper first establishes a novel reservoir pressure model for CBM production, following which, the CBM concentration and production models are also proposed. Then, these models are programmed and solved by means of the finite element method. Taking the Hunchun CBM field in Jilin province, China, as an example, the reservoir pressure, gas concentration, and production characteristics under different hydraulic fracture forms are simulated and investigated. In conclusion, the reservoir pressure decreases very rapidly in a small region near the fracture tip, which we called the “reservoir pressure singularity”. The existence of a hydraulic fracture greatly reduces the reservoir pressure in the process of CBM exploitation. The permeability sensitivity coefficient of reservoir pressure, Rpk, is defined to quantitatively describe the influence of coal seam permeability on the evolution of reservoir pressure. Rpk decreases logarithmically as the distance from the CBM extraction well increases. The reservoir pressure and CBM recovery rate characteristics in the presence of multiple hydraulic fractures are also investigated. We believe these results could contribute to the design of hydraulic fracturing wells and the evaluation of gas production in a CBM reservoir.

Experimental investigation on pores and permeability variation characteristics of coal treated by chemical solvents

Enhancing coal permeability by adopting chemical solvents treatment is one of the most effective methods to improve coalbed methane production efficiency in deep coal reservoirs with low permeability. However, coal pores and permeability variation characteristics affected by chemical solvents is not quantitatively determined. In this study, after being treated with carbon disulfide (CS2), acetone (C3H6O) and N-methyl pyrrolidone (NMP), changes in coal ultrasonic characteristics were investigated. Relationship between ultrasonic parameter variations and coal fracture development was revealed. Meanwhile, through adopting nuclear magnetic resonance (NMR) and permeability testing equipment, coal pore structure, porosity and permeability changes were studied. Results show that after treated by three chemical solvents, ultrasonic velocity in coal reduces by 7.7%–51.2%, while ultrasonic attenuation coefficient increases by 4.22dB/m-123.8 dB/m, indicating that coal fractures experience good development. Moreover, NMR signal quantity of treated coal under saturated state rises significantly, with big growth in mesopore and macropore peaks. Compared with NMP and acetone, CS2-treated coal porosity experiences the maximum increase of 7.71%, while the corresponding permeability growth is also the largest of 69.565 × 10−15m2. Permeability changes obtained from NMP calculation model is basically consistent with that acquired by lab measurement. Furthermore, changes in extraction rate, ultrasonic parameters, pore fracture characteristics and permeability results of solvent-treated coal are mutually consistent. This verifies that chemical solvents could dissolve molecular substances in coal, effectively increase primary fractures and change small pores to mesopores and macropores, as well as improving pore connectivity. As a result, coalbed methane extraction performance is expected to be enhanced due to increased coal permeability.

Experimental investigations on effects of gas pressure on mechanical behaviors and failure characteristic of coals

The mechanical behavior of coal is the key factor affecting underground coal mining and coalbed methane extraction. In this study, triaxial compression and seepage tests were carried out on coal at different gas pressures. The mechanical properties and failure process of coal were studied, as well as the acoustic emission (AE) and strain energy. The influence of gas pressure on the mechanical parameters of this coal was analyzed. Based on the conventional energy calculation formula, the pore pressure was introduced through the effective stress formula, and each energy component of coal containing gas was refined innovatively. The contribution of gas pressure to the total energy input and dissipation during loading was quantitatively described. Finally, the influence of gas pressure on coal strength was theoretically analyzed from the perspectives of Mohr–Coulomb criterion and fracture mechanics. The results show that the total absorbed energy comprises the absorbed energy in the axial pressure direction (positive) and in the confining pressure direction (negative), as well as that induced by the pore pressure (initially negative and then positive). The absorbed energy in the axial pressure direction accounts for the main proportion of the total energy absorbed by coal. The quiet period of AE in the initial stage shortens, and AE activity increases during the pre-peak stage under high gas pressure. The fractal characteristics of AE in three stages are studied using the correlation dimension. The AE process has different forms of self-similarity in various deformation stages.

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.

A fractal power-law approach for investigating the interactions between reservoir microstructure and gas migration

As one of the most significant clean energy sources, the migration characteristics and extraction processes of coalbed methane (CBM) have been extensively studied. The structural distribution and evolutionary characteristics of reservoir fractures, as the main conduit for gas migration, significantly affect the permeability and gas production. However, few models have been able to quantitatively and accurately explore reservoir micro–macro interactions under coupled thermal-fluid–solid effects. This work develops a new highly coupled model based on the widely adopted power-law function to quantify reservoir thermal conduction effect, gas pressure evolution, reservoir deformation, in situ stress, the adsorption–desorption effect, and reservoir microstructure evolution. Three parameters are adopted to quantitatively characterize the reservoir structure: (1) fracture power index αf (to characterize the fracture density), (2) fracture length ratio rf (to characterize the fracture size), and (3) the maximum fracture length l. The results demonstrate that the fractal network is a special kind of network in the power-law length distribution. The proposed power-law seepage model is able to accurately characterize the evolution of reservoir microstructure and the impact of microevolution on extraction under multi-field coupling effects, compared to the traditional power-law model. The proposed model can provide a good theoretical and practical support for the study of CBM migration and extraction.

Characteristics of Gas Seepage and Pore Structure Response in CO2-ECBM Process of Low Permeability Coal Seam

CO2-ECBM is a method of enhanced coalbed methane extraction followed by cutting greenhouse gas emissions and new energy development. In order to reveal the characteristics of gas flow in porous media and the pore structure response characteristics of coal rocks, the experiments were carried out to simulate the process of CO2 displacement of N2 at a buried depth of 900 m, including monitoring the changes in gas permeability and strain of coal samples along with a comparison of the pore structure of low-temperature liquid nitrogen adsorption on coal samples both before and after displacement were both done. The findings of the experiment are listed below. The N2 permeability of the LiuZhuang sample ranges from 0.0008mD to 0.0014mD, whereas the permeability of QiDong is around 0.0003mD. With an increase in gas injection duration and an expansion of the coal matrix for N2 adsorption, the permeability steadily decreases. The efficient stress compression of the coal pore fracture structure during sample preparation and testing avoids the visible fracture region, which results in poor permeability. The displacement stages of CO2 can be divided into three phases. Free nitrogen flows from the end of the position and the permeability diminishes during the phase of free nitrogen. When CO2 is introduced into the penetration stage, the permeability tends to rise, however when there is no penetration, the permeability test values are frequently low. During the CO2 steady displacement phase, gas permeability gradually declines. Axial and radial strains are progressively raised during the initial stage of the CO2 injection whereas they are gradually reduced during the initial stage of the N2 injection. While CO2 is continuously supplied through the coal body stage, there are modest axial and radial strain changes. The axial and radial stresses are stabilized by the CO2 displacement. The overall pore volume of the coal significantly rises following the displacement. The increase part of the pore volume is primarily focused on the pore of absorption and filling (aperture &lt; 10nm), whereas the decreased part is mainly concentrated in the diffusion pore of the Fick type and the permeability part (aperture &gt; 50nm). The increased in pore volume ratio surface area is centered mostly in the fill pore region (aperture 10 nm) and is four times greater than it was before the displacement. The CO2 injection exerts an expansion impact on the adsorptionfilled and diffusion pores during the CO2-ECBM process, whereas the compression effect on the percolation pores results in a reduction in permeability.

NUMERICAL ANALYSIS OF TWO IMMISCIBLE FLUIDS IN POROUS MEDIA BY THE MULTISCALE AND HOMOGENIZATION METHODS

This paper proposes a numerical model for simulating an immiscible compressible two-phase flow in a periodic heterogeneous porous media with application to a coal-bed methane extraction system. Methane flow in a coal sample submerged in water is simulated using the multiscale homogenization method and the averaged macroscopic approach. A weak formulation is used to discretize the governing equations coupled with the boundary conditions using the finite element method and solved using the open-source code Freefem&amp;#43;&amp;#43;. It was observed that the general behavior of both methods was in good agreement with actual gas saturation evolution. However, the multiscale approach provided more information regarding the global pressure evolution behavior of the mitigating gas.

An investigation into the modification of microwave-assisted oxidation in the macromolecular structure of coal via XRD and Raman spectroscopy

The macromolecular structure of coal governs the generation and expansion of micro/mesopores, contributing significantly to the storage and transport of coalbed methane (CBM). To reveal the modification mechanism of microwave-assisted oxidation at molecular scale, this paper presents a comprehensive analysis of the evolutionary characteristics of macromolecular structure of coal using X-ray diffraction and Raman spectroscopy tests. Results show that microwave irradiation triggers the conversion of ordered graphitic crystalline carbon to amorphous carbon, as well as the condensation and heterogeneous spatial arrangement of aromatic clusters. The oxidation of NaClO or Na2S2O8 destroys the initial aromatic structure and reduces the aromaticity of coal, whereas Fenton's reagent oxidation causes the cyclocondensation of macromolecular structure and contributes to the graphitization and crystallization of coal. Microwave-assisted oxidation not only facilitates the directional growth of aromatic rings and the cross-linking of aliphatic chains, but also induces the rotation, displacement, stacking and gradual parallel alignment of aromatic lamellae, thus improving the integrity of three-dimensional lattice of graphite structure and the orderliness of macromolecular structure. As the average size of microcrystalline structural units increases and align directionally to form molecularly oriented domains, the pore size and volume subsequently increases, thereby enhancing the gas transport capacity of coal and the extraction of CBM. This study provides theoretical guidance for the optimisation of microwave-assisted oxidant modification and the paramount consideration of CBM development strategy.

Numerical investigation of seepage characteristics of propped fracture in coalbed methane reservoirs

The investigation of the pore structure characteristics and the effect on fluid seepage in propped fractures are important considerations for improving coalbed methane (CBM) production and optimizing CBM extraction systems. In this paper, a numerical model was established to study the pore structure characteristics and distribution of fluid pressure, velocity and pathline fields of propped fractures. The results showed that, with the increase of the closure stress, the porosity decreased linearly while the effective pore connectivity ratio first increased, then decreased. The best flow conductivity of the fracture was achieved at the closure stress of 12.67 MPa. In addition, there were disparities in the pore distribution of the different layers due to the uneven transfer of the closure stress from top to bottom. Compared to the top and bottom layers, the effective pore connectivity ratio and fluid pressure were significantly smaller in the middle layer, while the flow velocity and pathline length were larger. The top and bottom layers were the main fluid seepage channels. The results of this study can provide a theoretical basis for CBM production prediction and real-time quantitative optimization of CBM extraction systems.