Methane Desorption Research Articles

Hydrogeological Challenge for Coal Bed Methane (CBM) Production in Structurally Complex Reservoirs: Case Study for the Synclines of Umbita and Checua-Lenguazaque, Colombia

Colombia is advancing toward pilot production of coal bed methane (CBM) at the Umbita syncline (US) and Checua-Lenguazaque syncline (CLS) in the Cundiboyacense plateau of the Eastern Cordillera. Although attractive, the development of CBM resources remains hydrogeologically challenging due to infinite acting aquifer (IAA) conditions, a sustainable production of ineffective water, which removal is critical to achieve methane desorption. Here, we assessed the implications of IAA on methane production prior to pilot development by comparing single-phase flow numerical simulations of the prospects. The results suggest IAA would extend the dewatering period in the Umbita syncline up to 20 years to potentially compromise the commercial recovery of methane. The Chequa-Lenguasaque, on the other hand, appears to have reached the phase of production of effective water; therefore, it is deemed as the most attractive prospect for pilot testing the production of CBM in Colombia.

Experimental investigation of gas diffusion kinetics and pore-structure characteristics during coalbed methane desorption within a coal seam

The desorption-diffusion properties of coalbed methane (CBM) are not only the theoretical basis for investigating gas outburst hazards in a coal mine, but also impose a non-negligible impact on CBM recovery performance. In this paper, the pore structure of coal with five different metamorphism degrees was qualitatively and quantitatively characterized with the scanning electron microscopy (SEM) and low-temperature nitrogen gas adsorption (LTNGA) measurements. The isothermal methane ad/desorption experiments were performed with four initial gas desorption pressure scales. Subsequently, the matching degree between eight empirical correlations (ECs) and the measured methane desorption data was determined, and the methane diffusion coefficient (Dfg) was numerically calculated combining with an FGDG (free gas density gradient) diffusion model. Importantly, we evaluated and discussed the sensitivity of pore structure parameters, initial gas desorption pressure, and volatile fraction to Dfg. The results show that pore density distribution differs among coal samples and the pore structure, especially the micropores, is developed better in high-rank coals than in low- and medium-rank ones. Initial gas release velocity increases with an increase in coal rank and decreases negatively with the increasing particle size in an exponential manner. Among the eight ECs, EC-VIII can be applied to more accurately describe gas desorption process during the whole time scale, and its correlation coefficient with the measured data is larger than 0.9900. Dfg increases with an increase in pore volume and specific surface area (SSA), and it is negatively correlated with the average pore diameter. Additionally, Dfg exhibits an asymmetric U-shaped pattern with an increase in the volatile fraction, which is mainly related to the switch between the dominant roles of mesopores and micropores during coal metamorphism. At a lower volatile fraction, the diffusion coefficient is more sensitive to micropores, while, at a higher volatile fraction, mesopores contribute more to the diffusion coefficient.

Effect and mechanism of ultrasonic mechanical vibration on methane adsorption

Ultrasonic stimulation technology with mechanical vibration effect and thermal effect offers unique advantages for promoting coalbed methane desorption and improving reservoir permeability. Recently extensive studies have been conducted on the effects of ultrasound on reservoir physical characteristics and methane adsorption behavior. However, most studies have neglected the influence of each effect as an independent variable on methane adsorption. Particularly, there has been inadequate attention given to the adsorption behavior of methane by coal under ultrasonic mechanical vibration. The coupling relationship among coal-water-methane molecules under ultrasonic mechanical vibration has yet to be revealed. Here, anthracite and bituminous coal were used as research objects, and the variable separation of the ultrasonic mechanical vibration was carried out using a constant temperature device. The effect and mechanism of ultrasonic mechanical vibration on methane adsorption were revealed by isothermal adsorption experiments. The results showed that ultrasonic mechanical vibration had a certain promotion effect on methane adsorption, which was opposite to the comprehensive effect. However, this promotion effect was influenced by the coal rank, moisture condition, and pressure. The effect of ultrasonic mechanical vibration on the molecular structure of the coal was an internal mechanism that enhanced methane adsorption. Among them, the methyl structure of the aliphatic side chain, the hydroxyl structure, the aromatic hydrogen structure, and the oxygen-containing heteroatom structure were sensitive to ultrasonic mechanical vibration. This influence improved the orientation degree of the coal molecular structure and increased the coal's surface free energy and adsorption potential, resulting in the ability of the coal to adsorb more methane molecules under the same pressure conditions. The content of sensitive functional groups was the main reason for the difference in promotion effect. There were more sensitive functional groups in bituminous coal, leading to a more significant promotion of methane adsorption by ultrasonic mechanical vibration. Furthermore, the moisture content would also affect the promotion effect. Moisture in coal strengthened the effect of ultrasonic mechanical vibration on sensitive functional groups by facilitating ultrasonic propagation through the coal. Therefore, in moisture-equilibrated samples, ultrasonic mechanical vibration had a greater effect on methane adsorption.

Influence of deposited methane and microstructure of bituminous coal on the characteristics of its oxidation and gas production under different oxygen concentration

The coupling of coal spontaneous combustion and high methane is ubiquitously observed in coal mining and poses a significant jeopardy to production safety. The extant studies are largely focused on altering external methane conditions and fail to investigate the spontaneous combustion properties of methane-laden coal samples. This study conducted an evaluation of the microstructure of typical coal samples, followed by a fractal analysis. It discovered that the surface structure of the specimen is relatively homogeneous, and the mesopores (2–4 nm) exhibit a significant influence on the specific surface distribution. The overall pore structure is characterized by a multiplicity of peaks, which renders the coal sample markedly segmented during methane desorption, oxygen consumption, and gas products generation. The low-temperature oxidation tests under various working conditions reveal that methane desorption is a crucial factor in determining the spontaneous combustion pattern of the coal sample. The degree of methane desorption bears a close association with the oxygen concentration, and at an oxygen concentration of approximately 2 %, there is a turning point for gas generation and oxygen consumption. The present investigation provides a qualitative and quantitative exposition of the relevant laws, offering guidance for fire prevention and suppression in pertinent mines.

Quantitative Evaluation of the Influence of Injected Water on Methane Desorption Behaviors of Coal in the Ordos Basin

Quantitative Evaluation of the Influence of Injected Water on Methane Desorption Behaviors of Coal in the Ordos Basin

Structure I methane hydrate confined in C8-grafted SBA-15: A highly efficient storage system enabling ultrafast methane loading and unloading

Confinement of water and methane in mesopores of hydrophobized SBA-15 is demonstrated to promote methane hydrate formation. In comparison to as-synthesized SBA-15, hydrophobization by C8 grafting accelerates the kinetics of methane storage in and delivery from the hydrate. C8 grafting density was determined at 0.5 groups nm−2 based on TGA and quantitative NMR spectroscopy. Multinuclear 1H-1H DQSQ and 1H-1H RFDR NMR provided spectroscopic evidence for the occurrence of C8 chains inside the mesopores of SBA-15, by showcasing close spatial proximity between the grafted C8 chains and pore-intruded water species. X-ray diffraction demonstrates formation of Structure I hydrate on SBA-15 C8. At 7.0 MPa and 248 K, the water-to-hydrate conversion on hydrophobized SBA-15 C8 reaches 96% as compared to only 71% on a pristine SBA-15 sample with comparable pore size, pore volume and surface area. The clathrate loading amounted to 14.8 g/g. 2D correlation NMR spectroscopy (1H-13C CP-HETCOR, 1H-1H RFDR) reveals hydrate formation occurs within pores of SBA-15 C8 as well as in interparticle volumes. Following the initial crystallization of SBA-15 C8-supported methane hydrate taking several hours, a pressure swing process at 248 K allows to desorb and re-adsorb methane from the structure within minutes and without thawing the frozen water structure. Fast loading and unloading of methane was achieved in 19 subsequent cycles without losses in kinetics. The ability to harvest the gas and regenerate the structure without the need to re-freeze the water represents a 50% energy gain with respect to melting and subsequently recrystallizing the hydrate at 298 K and 248 K, respectively. After methane desorption, a small amount of residual methane hydrate in combination with an amorphous yet locally ordered ice phase is observed using 13C and 2H NMR spectroscopy. This effect offers an explanation for the enhanced hydrate formation kinetics in adsorption-desorption cycles. These findings open new perspectives for clathrate hydrate-based methane storage.

Effects of Fracturing Fluids Imbibition on CBM Recovery: In Terms of Methane Desorption and Diffusion

Summary Hydraulic fracturing technology has been widely used to improve the productivity of the coalbed methane (CBM) reservoir, during which tons of fracturing fluids infiltrate the coal seam. However, the effects of fracturing fluids imbibition on CBM recovery are still unclear. In this study, spontaneous and forced water imbibition experiments in methane-bearing low-volatile bituminous (LVB) coal were conducted at various gas adsorption equilibrium pressures, following which methane desorption and diffusion experiments were performed. These experiments simulated the complete process of fracturing fluid imbibition during well shut-in and subsequent methane production upon reopening, which is helpful in understanding the impact of fracturing fluid imbibition on CBM production. The results show that water imbibition displaces adsorbed methane in the coal matrix, and with reservoir pressure increasing, the displaced effect decreases. Furthermore, the forced imbibition (FI) displaces less methane than the spontaneous imbibition (SI) due to water rapidly filling fractures and blocking methane migration out of the matrix in the FI. In the initial stages of gas production following spontaneous or forced water imbibition, the displaced methane diffuses out of the coal at a rapid rate and then slows down. Furthermore, in the case of FI, a significant amount of residual gas remains after desorption and diffusion due to the water blocking effect. However, the water blocking effect has a minimal impact on coal undergoing SI. In terms of desorption and diffusion, this study provides a comprehensive understanding of the effects of fracturing fluids imbibition on recovery of CBM, which is useful for practical shut-in operations following hydraulic fracturing in LVB coal seams.

Short-term variation in Mars atmospheric methane concentrations driven by barometric pumping

Measurements of atmospheric methane by the Curiosity rover's SAM-TLS instrument are providing evidence of seasonality and diurnal variation in concentration. Given methane's short atmospheric lifetime relative to geological timescales, its presence implies a replenishing source, and the observed variation demands the proposition of a modulation mechanism. This paper focuses on the modulation mechanism on a diurnal scale, extending earlier modeling of seasonal variation. Our modeling shows that barometric pumping driven by both diurnal and seasonal variation of atmospheric pressure, along with possible adsorption and desorption of methane in the shallow subsurface driven by temperature and pressure change, can explain the variation in methane concentration. In the model, an active, continuous, steady-state deep source of methane is assumed, and carbon dioxide serves as the carrier gas for drawing methane and possible trace gaseous constituents into the atmosphere.

Permeability evolution and production characteristics of inclined coalbed methane reservoirs on the southern margin of the Junggar Basin, Xinjiang, China

The thick and steeply inclined coal seams of the Junggar Basin of Xinjiang, China, are unique with dip angles generally >50° but over the range 0°–85°. Initial and evolving permeability and pressures change drastically around wells down-dip within the steeply inclined reservoir as a result of the depth differential. Hence, the evolution of permeability and fluid pressures during drainage exhibits significant differences from those of flat-lying or even slightly inclined reservoirs. We apply a hydro-mechanical model to evaluate the interaction of two-phase flows of gas and water in the inclined system. The influence of different reservoir inclinations (15°, 30°, 45°, 60°, and 75°) on the evolution of permeability, reservoir pressure, and gas production are explored through finite element modeling of this system. The results show that: 1) Reservoir inclination induces differences in permeability, reservoir pressure, gas content and methane production between the shallower updip reservoir and deeper downdip reservoir. The difference in permeability between the updip and downdip reservoirs is amplified as the dip angle increases and as drainage proceeds in the presence of the varying stress gradient. 2) An apparent asymmetric distribution of reservoir pressures results for wells along dip. The difference in reservoir pressure between the updip and downdip reservoirs intensifies as the inclination increases but lessens with the progress of drainage. The larger the dip angle, the smaller the final reservoir pressure. 3) The pressure reduction in the updip reservoir is larger than that in the downdip reservoir, resulting in the unsynchronized desorption of methane in the updip and downdip reservoirs. Methane within the updip reservoir desorbs preferentially over that in the downdip reservoir. For reservoir dip angles <45° a single peak in methane production rate is apparent but this is supplanted by dual peaks for inclinations >45°. The time gap in gas desorption between the updip and downdip reservoirs results in the “dual-peak” on gas production profile. 4) A larger well spacing along the dip of a more highly inclined reservoir results in more efficient water drainage and gas production. An inverted trapezoidal well pattern is recommended to facilitate the drainage and gas production of reservoirs with significant dip angles.

Effect of Desorption Damage on the Kinetic Characteristics of Coal Particle Gas Desorption.

There is a positive feedback mechanism of desorption, pulverization, and redesorption in the process of coal and gas outburst. The coal pore structure changes to some extent in the process of coal pulverization and has a related impact on the dynamic parameters of coal particle gas desorption. To understand the impact of desorption damage on the dynamic characteristics of coal particle gas desorption, in this paper, a self-developed coal particle gas desorption test device is used to measure the amount of methane desorption of different coal samples repeatedly desorbed under the same adsorption equilibrium pressure. The results show that the methane desorption kinetic curves of the four coal samples based on desorption damage basically have the same trend. Nie's diffusion model can better describe the methane desorption characteristics of coal particles. After desorption, the methane desorption amount, initial desorption velocity, diffusion ability, and ultimate amount of methane desorption of the coal samples are greater than those before desorption. The desorption damage affects the relevant dynamic parameters of the coal particle gas diffusion model, and its impact on the outburst coal is greater than that on raw coal. In addition, the pore size distribution and change characteristics of the coal samples before and after desorption are analyzed quantitatively via a low-temperature liquid nitrogen adsorption test and fractal dimension-related theory. It was found that the pore volume peak area of the coal samples affected by desorption damage within each pore size range was significantly larger than that before desorption. Among them, micropores have the most significant impact on the desorption damage of coal samples, and the peak area of the pore volume of protruding coal is greater than that of raw coal, indicating that the internal pores of coal after desorption damage are more developed than those of raw coal.

Influences of Evolution of Pore Structures in Tectonic Coal under Acidization on Methane Desorption.

Experiments on corrosion reactions of pulverized coal with monomeric and polymeric (mixed) acid solutions reveal that monomeric acids are listed in a descending order as HF, HCl, and CH3COOH according to their corrosion effects on tectonic coal collected in Faer Coal Mine (Liupanshui City, Guizhou Province, China). In addition, the optimal mixing ratio of mixed acids is 6% HCl + 6% HF + 3% CH3COOH + 2% KCl. The mineral grains filled in pores in coal samples treated with mixed acid solutions are dissolved, so the porosity increases. The volumes of transition pores and mesopores are obviously affected by acidization, and some transition pores are transformed into mesopores and macropores to form dissolved pores. At the same time, inkbottle-shaped pores reduce, while slit pores or open pores increase. The coal samples after acidization show a higher aromatization degree and an increased relative content of oxygen-containing functional groups, with a generally lower hydroxyl content, so the methane (CH4) adsorption capacity of coal declines, which promotes CH4 desorption. The control effect of pore structures after acidization reactions on CH4 desorption was revealed from perspectives of the diffusion coefficient (Kn), adsorption volume (ω), average pore-throat ratio (PT), and average sinuosity (τav). That is, CH4 molecules in tectonic coal after acidization turn from Knudsen diffusion to transitional diffusion, the adsorption volume of CH4 molecules shrinks, the average pore-throat ratio decreases, and the average sinuosity reduces, which promotes CH4 desorption from tectonic coal.

Low-Temperature Response Characteristics of Coalbed Methane Desorption under High and Low Destruction Degrees.

Freeze-coring technology can effectively reduce the amount of gas loss during the sampling process and improve the accuracy of gas content measurements in underground coal seams. In this study, high- and low-damage coals were selected as test objects to investigate whether the freeze-coring technique is universally applicable to inhibit gas desorption in high- and low-damage coals. In this paper, the pore structure of the test coal samples was first tested using an ASAP2020 specific surface area analyzer, and then a nonfreezing and freezing simulation test was carried out on high- and low-damage coals using a self-developed freezing coring response test platform. The results showed that the gas desorption curves of both high- and low-damage coal samples followed the pattern of rapid increase in the early stage, slow increase in the middle stage, and stability in the late stage under both conditions; freezing conditions significantly reduced the gas desorption during the sampling process, and the difference in gas desorption between high- and low-damage coals was reduced; the gas desorption inhibition rate of high-damage coals was higher at an external heating temperature of 60 °C under freezing conditions; at an external heating at an external heat temperature of 90 °C, the gas desorption inhibition rate of low-damaged coal was higher in the early stage, and the gas desorption inhibition rate of high-damaged coal was higher in the later stage; freeze coring had a significant inhibition effect on the gas desorption of both high- and low-damaged coal types, which verified that the inhibition effect of freeze coring on the gas desorption of high- and low-damaged coal samples was universal. It provides a basis for the future application of freeze-coring technology in coal mines.

Mechanisms and field application of in-situ heat injection-enhanced gas drainage

The low permeability of coal seams and strong adsorption of methane considerably inhibit gas drainage; thus, developing a method for efficient gas drainage is crucial. The heat injection-enhanced gas drainage tests were carried out in the laboratory and coal mine, and the gas drainage effect and mechanism of heat injection method were studied. Then, through numerical simulation, the gas production law of heat injection method was analyzed from the perspective of water and gas migration. Indoor experiments demonstrated that heat injection strengthened methane desorption and relieved the inhibitory effect of water on gas. The gas drainage effect of field tests was remarkable, and the gas concentration and daily gas production increased by over 10 and 100 times, respectively. During heat injection, water occupied the migration channel of gas, and the inhibition of water on gas was greater than the promotion of temperature, resulting in the reduction of gas production; After heat injection, high temperature promoted gas desorption and relieved the inhibition of water on gas, resulting in a significant increase in gas production. The gas production law obtained from the numerical simulation showed a high degree of consistency with field tests. The results can provide a reference for gas control.

Study on Gas Adsorption-Desorption and Diffusion Behaviour in Coal Pores Modified by Nano Fracturing Fluid.

Nanoparticles are added to clean fracturing fluids to formulate nanoparticle-modified clean fracturing fluids, compared with ordinary clean fracturing fluid, it has the advantages of good temperature resistance, low loss of filtration, and so forth, and has good application prospects in coal-bed methane. However, the current research on nanoparticle-modified clean fracturing fluids is mostly focused on the study of their rheological properties. The mechanism of nano-fracking fluid influence on methane adsorption-desorption characteristics is not clear. Therefore, this study chooses Jiulishan anthracite coal (high-rank coal), Pingdingshan coal (medium-rank coal), and Geng village mine long bituminous coal (low-rank coal) of the three rank coal samples. Using indoor experiments and molecular simulation methods, a study on the influence of methane adsorption and desorption capacity and diffusion ability of coal samples provides a modified fracturing fluid formulation of 0.8% CATB + 0.2% NaSal + 1% KCl + SiO2. The experimental results show that nanofracturing fluid-treated coal samples compared to clean fracturing fluid treated coal samples, both methane adsorption and desorption capacities, were increased to some extent. Construction of methane adsorption systems with different apertures and calculation of isosteric heat of adsorption, indicating that the interaction force between methane and coal molecules is smaller after nanofracturing fluid treatment, which facilitated methane desorption. A simulation study of methane diffusion in coal samples treated with two systems of fracturing fluids at different aperture was carried out using molecular dynamics methods, indicating that nanoparticle-modified clean fracturing fluids can reduce the damage of clean fracturing fluids to the desorption-diffusion ability of coal reservoirs. Comparison of 6 MPa as the most suitable pressure for nanofracturing fluids to function provides a basis for the future development of nanofracturing fluids and their popularization.

Laboratory experiments on the sublimation of methane through ice dust layers and applications to cometary activity

Context. Comets are small celestial bodies made of ice, dust, and rock that orbit the Sun. Understanding their behavior as they warm up at perihelion unveils many pieces of information about the interior and general morphology of the ices hidden under the dust. Aims. The goal of this research is to study the sublimation of CH4 through amorphous solid water (ASW), with a focus on the structural changes in water and the influence of a layer of indene (as a proxy of the crust) during a period of thermal processing, which we use in a controlled laboratory setting to simulate cometary environments. Methods. Ices at a CH4/H2O abundance ratio of about 0.01 are deposited and layered, or co-deposited, at 30 K and are heated until 200 K (or 140 K) with a ramp of either 1 or 5 K min−1. We use mass spectrometry and infrared spectroscopy to analyze the results. Results. Depending on the heating ramp and type of deposition, the sublimation of methane through ASW varies, being lower in intensity and higher in temperature when the co-deposited structure is considered. When two temperature cycles are applied, the second one sees less intense CH4 desorptions. When indene is placed above the ice mixtures, we find that the thicker its layer, the later the methane desorption. However, this later desorption sees a greater quantity of methane released due to water reorganization and higher desorbed material pressure. Conclusions. The structural changes of water ice drive volatile and hyper-volatile desorption because of the transition from high to low intrinsic density and transformation from amorphous to crystalline. This desorption indicates that such material has been deposited at low temperatures in agreement with previous theories on cometary ices formed in the pre-stellar cloud. During the two temperature cycles of our experiments, most of the released material is seen to be pristine and the processed part, if any, is of a negligible quantity, in agreement with dust-rock cometary studies.

Experimental investigation on coal desorption characteristics and spontaneous combustion properties evolution under the coupled effect of temperature and pressure

Gas desorption patterns and microscopic pore structure morphology in coal are influenced by the coupled effects of temperature and pressure, and their effects on the spontaneous combustion (SC) characteristics of coal cannot be ignored. In this paper, two types of coal samples, lean coal and fat coal from high gas-prone SC mines, are used as test objects. A self-designed experimental platform for coupling gas adsorption and desorption with coal spontaneous combustion (CSC) and heating and oxidation was used. Firstly, orthogonal tests on the desorption of methane from coal under the coupling effect of different temperatures and pressures were carried out to obtain a gradual decrease of methane desorption from coal with increasing temperature under the same pressure conditions. The results show that the pore volume and specific surface area of coal show a monotonic decreasing pattern with increasing temperature and pressure, while the percentage of micropores and mesopores, which promote the oxygen reaction of coal, show an increasing trend. In addition, a comparative analysis of the characteristic temperature points of the first stage showed that the characteristic temperature points of the desorbed coal were higher than those of the original coal, reflecting the higher SC activity of the desorbed coal; and the crossover temperature points of the lean coal and the fat coal were 79 °C and 88 °C, respectively, after desorption during the heating and oxidation process, which also confirmed the high SC activity of the desorbed coal. Based on the above findings, the methane concentration calculation model of the mining area was applied to redraw the oxidation zone using the mining area of the 41,201 working face of Hongyang No. 2 Mine as an example, and the area was reduced by 39.5%. The research results of the paper can provide guidance for the prevention and control of fire hazards in mines with high gas-prone SC.

Electrocatalytic Scission of Unactivated C(sp3)-C(sp3) Bonds through Real-Time Manipulation of Surface-Bound Intermediates.

Electrocatalysis plays a critical role in future technologies for energy storage and sustainable synthesis, but the scope of reactions achievable using electricity remains limited. Here, we demonstrate an electrocatalytic approach to cleave the C(sp3)-C(sp3) bond in ethane at room temperature over a nanoporous Pt catalyst. This reaction is enabled by time-dependent electrode potential sequences, combined with monolayer-sensitive in situ analysis, which allows us to gain independent control over ethane adsorption, oxidative C-C bond fragmentation, and reductive methane desorption. Importantly, our approach allows us to vary the electrode potential to promote the fragmentation of ethane after it is bound to the catalyst surface, resulting in unprecedented control over the selectivity of this alkane transformation reaction. Steering the transformation of intermediates after adsorption constitutes an underexplored lever of control in catalysis. As such, our findings widen the parameter space for catalytic reaction engineering and open the door to future sustainable synthesis and electrocatalytic energy storage technologies.

Molecular simulation on the desorption and extraction of methane in the slits with varying surface activity

Understanding the production process of shale gas in the complex geological environment is important for the recovery prediction of shale gas development. The adsorption/desorption and extraction of methane in the slits with varying surface activity simplified by tuning the force field parameters are studied with molecular dynamics (MD) simulations. The density of free gas in the slits does not change with the surface activity, while the density of adsorbed gas increases with the surface activity. A minimum pressure difference to initiate the desorption is characterized by the non-equilibrium molecular dynamic (NEMD) simulation. The high surface activity results in not only the slow flow of free gas, but also the slow desorption of the adsorbed gas. Thus, the extraction presents a slow rate and long-tail with extraction times. The adsorption/desorption and the extraction computations in the MD simulations provide a useful reference for the production prediction of shale gas.

Methane Adsorption in Anthracite Coal under Different Pressures and Temperatures—A Study Combining Isothermal Adsorption and Molecular Simulation

In situ gas content is an important parameter associating coalbed methane, while the influence of pressure and temperature on methane adsorption and desorption still needs to be revealed. In this study, the molecular structure and methane adsorption capacity of anthracite coal collected from Diandong Coalfield (China) were studied based on 13C nuclear magnetic resonance (13C NMR), Fourier transform infrared spectroscopy (FT-IR), and methane isothermal adsorption experiment. The results show that the carbon skeleton of coal sample is mainly composed by aromatic carbon (72%), followed by aliphatic carbon structure (14.2%). Carbons connected to the oxygen atoms contribute 13.7% of the total carbons in coal molecule, and the oxygen atoms are mainly in the form of carbonyl. The 2-dimension structure and 3-dimension molecular structure of coal sample was also reconstructed. The average chemical formula of the coal molecule is C200H133O21N3. The experimental methane adsorption isothermal data of the coal sample under different temperatures shows that with increasing the temperature, the methane adsorption amount at each pressure decreases obviously. At 7 MPa and 20°C, the methane adsorption amount of the coal sample is 28.5 cm3/g. Comparably, at 100°C and 7 MPa, the methane adsorption amount is only 15.9 cm3/g, decreasing by 44%. In mesopores, temperature has stronger influence on methane adsorption under higher pressure than that of lower pressure. On the contrary, in micropores, temperature has weaker effects on methane adsorption at higher pressure than that at lower pressure. The results can be beneficial for understanding methane adsorption characteristics of deep coal.

Evolution of the pore structure and fractal characteristics of coal under microwave-assisted acidification

Coal reservoir usually requires stimulation to enhance coalbed methane (CBM) production. As a novel stimulation method, microwave heating contributes to methane desorption and coalbed fracturing. However, the microwave-induced fractures are easily closed under in-situ stresses. Acidic stimulation is a process of demineralisation in the coal reservoir using hydrochloric, acetic, or formic acids, resulting in opening and interconnecting pores and fractures to flow gas. However, it is difficult for acids to enter microfissures under normal pressures. Microwave-asisted acidification may offer an alternative. In this work, the evolution of the pore structure and fractal characteristics of coal under microwave-assisted acidification was evaluated using low-temperature nitrogen adsorption and Scanning electron microscope. Compared to microwave heating or acidification, microwave-assisted acidification greatly increases the pore volume of coal and extends the length and width and connectivity of fractures. In addition, the isolated pores were interconnected and the methane migration was facilitated. This work demonstrates the potential of microwave-assisted acidification for CBM recovery.