Diffusion Coefficient Of Methane Research Articles

Study on the Mechanism of Gas Diffusion in Coal Particles under the Coupled Effects of Gas Pressure and Temperature.

To investigate the gas diffusion mechanism in coal particles under the coupled effects of gas pressure and temperature, a low-temperature liquid nitrogen adsorption experiment was first conducted to obtain the pore structure characteristics and pore size distribution patterns of the coal samples. Subsequently, a methane diffusion experiment was performed on coal particles with simultaneous increases in gas pressure and temperature using a unipore model to solve the diffusion coefficient. Finally, models for the diffusion coefficient of free-state methane and adsorbed-state methane were established, with their applicability discussed. The results indicate that the pore structure of the coal samples exhibits an approximately unimodal distribution, predominantly consisting of mesopores. As the gas pressure and temperature increase, the tortuosity of the coal sample pores also increases. Both the desorption volume and desorption rate of gas rise with the simultaneous increase in gas pressure and temperature, and overall, the diffusion coefficient of coal particles tends to increase under these conditions. Methane diffusion in coal particles is primarily governed by the surface diffusion of adsorbed methane, with gas pressure exerting a greater influence on surface diffusion than temperature. The findings of this study contribute to the understanding of the gas diffusion mechanism in coal and are of significant importance for improving gas extraction rates and preventing gas-related disasters.

Enhancing shale gas recovery and CO2 sequestration: Microscopic mechanisms of adsorption and diffusion in shale pores

The global energy imbalance has created significant challenges and environmental consequences. Integrating carbon sequestration with shale gas development provides a promising solution to address both issues simultaneously. This study explores the adsorption, desorption, and diffusion behaviors of methane and CO2 in shale pores using a high-precision, high-pressure adsorption apparatus alongside molecular simulation techniques. Results demonstrate that CO2 shows a markedly higher adsorption capacity in shale pores than methane, particularly at high pressures. Simulations reveal that CO2 molecules nearly saturate pore spaces at pressures above 10 MPa. Methane's diffusion coefficient, initially high at low pressures, drops sharply with increasing pressure, decreasing by approximately 90% from 5.6 × 10−10 m2/s at 154 psi to 1.38 × 10−11 m2/s at 1032 psi. Conversely, CO2 diffusion remains stable under pressure changes, suggesting that methane diffusion is constrained by intermolecular interactions at high pressures, while CO2 retains stable movement. Under a reservoir condition of 30 MPa with adsorbed CH4, CO2 injection at varying pressures continues to enhance CO2 adsorption, underscoring CO2's dual role in boosting shale gas recovery while achieving carbon sequestration. This study highlights CO2-methane displacement mechanisms, providing theoretical insight that support both effective shale gas recovery and CO2 storage.

Investigation on the methane emissions from permeation of urban gas polyethylene pipes under the background of hydrogen-mixed natural gas transportation

Methane, a more potent greenhouse gas than carbon dioxide, aggravates the greenhouse effect of ecological environment through its emissions. When transporting hydrogen-mixed natural gas through urban polyethylene pipes, methane will be emitted due to the permeation characteristic of polyethylene pipe material. Therefore, it is crucial to pay attention to the methane emissions from permeation of polyethylene pipes for protecting the environment and climate. In this paper, the solubility and diffusion coefficients of methane within hydrogen-mixed natural gas in polyethylene pipe materials are calculated through the Monte Carlo method and molecular dynamics simulation to determine the permeation coefficient of methane. Simultaneously, the methane emissions and equivalent carbon dioxide emissions of the hydrogen-mixed natural gas in polyethylene pipes are investigated. Results indicate the methane solubility coefficient decreases with increasing temperature and hydrogen mixing ratio. The methane diffusion and permeation coefficients increase with the rising temperature and pressure, and decreasing hydrogen mixing ratio. The pressure exerts minor influence while the temperature and hydrogen mixing ratio have more significant impact. That is, the lower the temperature and the higher the hydrogen mixing ratio, the less methane emissions caused by permeation. Therefore, the effective reduction of methane emissions can be achieved by adding the insulation layer to mitigate the impact of high temperature and increasing the hydrogen mixing ratio. For example, for the polyethylene pipe with the operating pressure of 4 bar and the size specification of SDR11, when the temperature drops from 300 K to 260 K, the methane emissions of hydrogen-mixed natural gas with hydrogen mixing ratios of 0 vol%, 20 vol%, 50 vol% and 80 vol% will be drastically reduced by 73.3%, 74.4%, 77.2% and 78.5% respectively, and when 20 vol%∼80 vol% hydrogen are mixed in the natural gas at 260 K, 300 K and 310 K, the methane emissions will be effectively reduced by 26.7%–85.6%, 23.3%–82.1% and 23.9%–82.4% respectively. The findings of this study can serve as guidance for reducing the methane emissions from permeation of polyethylene pipes transporting hydrogen-mixed natural gas.

Heat-dependent properties of methane diffusion in coal: an experimental study and mechanistic analysis.

Coalbed methane thermodynamic extraction, as an emerging ECBM recovery method, can effectively improve gas recovery rates. And clarifying methane diffusion and migration law in coal under thermal stimulation is crucial for the selection of its process parameters. Based on laboratory methane adsorption-release experiments, the evolution law of methane diffusion characteristics with temperature and pressure was studied, and the control mechanism of heat-dependent methane diffusion behavior was explored. The results show that both thermal stimulation and high adsorption pressure accelerated the methane diffusion rate in coal. Adsorption pressure had little effect on methane diffusion percentage, but thermal stimulation promoted a significant increase in diffusion percentage and improved the net methane yield. The influence mechanisms of adsorption pressure and thermal stimulation on methane diffusion characteristics are elucidated in relation to the amount and proportion of methane-activated molecules in the diffusion process. The constant diffusion coefficient of methane is heat-dependent, based on which a diffusion model is derived to accurately predict the methane release process in coal. Additionally, temperature has a more important effect on transient diffusion coefficient than pressure. Thermal stimulation leads to a net increase rather than a decrease in diffusion coefficient in the early diffusion stages and can also accelerate the attenuation of diffusion coefficient, with this intensifying effect becoming more pronounced at higher temperatures. The research results can provide some reference for the determination of coal seam gas content and the selection of heat injection process parameters.

Determination of the Diffusion Coefficients of Binary CH4 and C2H6 in a Supercritical CO2 Environment (500–2000 K and 100–1000 atm) by Molecular Dynamics Simulations

The self-diffusion coefficients of carbonaceous fuels in a supercritical CO2 environment provide transport information that can help us understand the Allam Cycle mechanism at a high pressure of 300 atm. The diffusion coefficients of pure CO2 and binary CO2/CH4 and CO2/C2H6 at high temperatures (500 K~2000 K) and high pressures (100 atm~1000 atm) are determined by molecular dynamics simulations in this study. Increasing the temperature leads to an increase in the diffusion coefficient, and increasing the pressure leads to a decrease in the diffusion coefficients for both methane and ethane. The diffusion coefficient of methane at 300 atm is approximately 0.012 cm2/s at 1000 K and 0.032 cm2/s at 1500 K. The diffusion coefficient of ethane at 300 atm is approximately 0.016 cm2/s at 1000 K and 0.045 cm2/s at 1500 K. The understanding of diffusion coefficients potentially leads to the reduction in fuel consumption and minimization of greenhouse gas emissions in the Allam Cycle.

Dissolution of gas in a stationary liquid layer: A diffusion-kinetic model and experimental studies

An analytic model of gas dissolution in a stationary liquid layer is presented. This model considers for the first time the intrinsic kinetics of the process of gas dissolution in a liquid characterized by the rate of gas absorption/desorption reaction at the gas–liquid interface. Within the framework of the presented model, it is shown that the process of gas dissolution in a stationary liquid layer occurs either in the diffusion-kinetic regime or in the diffusion regime. In the diffusion-kinetic regime, the rate of gas dissolution in a stationary liquid layer is affected by both the intensity of gas diffusion through the liquid layer and the intrinsic kinetics of the gas dissolution process in the liquid; in the diffusion regime, the rate of gas dissolution in a stationary liquid layer is limited only by the intensity of gas diffusion through the liquid layer. Experiments were carried out to study the kinetics of methane dissolution in a stationary layer of water at different temperatures. From the comparison of calculated and experimental data, the values of the diffusion coefficient of methane in water were determined, and it was also established that the process of methane dissolution in a stationary layer of water occurs in the diffusion regime. A new empirical correlation for calculating the diffusion coefficient of methane in water in the temperature range from 273 to 323.15 K was obtained based on our and available data points: D[m2/s]=exp(−12.64−2250.5/T[K]).

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.

Investigation of methane and ethane diffusivity in the glass reinforced epoxy composite: Experimental and simulation

New polymeric pipes have been used to overcome difficulties created by metal pipes in gas transmission lines such as corrosion and leakage. In this work, diffusion and solubility cells are designed to determine the diffusivity and possible leakage of methane and ethane gases in the glass-reinforced epoxy (GRE) composite. A composite sample with a diameter of 5.95 mm and thickness of 1.40 mm to determine the diffusivity and a composite powder to estimate the gas solubility are used. The experiments are done under temperatures varied from 25 to 40 ℃ and a pressure of 20 bar. The results show that the diffusivity of both gases increases with increasing temperature; while gas solubility decreases. The highest diffusion coefficient of methane and ethane are observed at 8.23 × 10−11 and 7.94 × 10−11 m2/s at 40 ℃, respectively. Moreover, novel correlations for estimating gas diffusion coefficient and mass flow rate in terms of temperature are developed based on the experimental data with the correlation coefficients (R2) of 0.98 and 1 and the average relative error percent (AREP) less than 4.5% and 0.5%, respectively. Interestingly, CFD (Computational fluid dynamics) results show a good consistency with experimental data for gas concentration profiles through the composite with AREP ranges of 1.95–15.78%.

Artificial neural networks in predicting of the gas molecular diffusion coefficient

The diffusion coefficient is one of the most important parameters for designing two-phase operations between liquid and gas phases in refineries and petrochemical industries, as well as for the gas injection process in oil fields to enhance oil production. Accurate knowledge of this parameter is essential for the prediction of the dissolution rate of the gas phase into the liquid phase. Ideally, this parameter should be obtained experimentally. Given that setting up laboratory equipment and conducting experiments can be costly and time-consuming, mathematical modeling is used as an alternative. In some cases, this data is not either available or reliable, which poses a challenge for designs. Hence, empirically derived correlations are used to predict molecular diffusion coefficients. However, the success of empirical models depends mainly on the range of data at which they were originally developed. Empirical models are not comprehensive for applying to the other data. Recent studies demonstrated that the alternative approach to modeling complex processes and identifying the effective parameters is the artificial neural network (ANN), a suitable prediction method. This study presents a new model developed to predict the molecular diffusion coefficient of methane in crude oil. The model is developed using 172 data points collected from recent literature. Out of the total laboratory data, 90% (155 data points) were used for training the desired neural network, while 10% (17 data points) were reserved for testing and evaluating the performance of the network. The multi-layer perceptron (MLP) neural network architecture with back-propagation (BP) training algorithm was used successfully for the prediction of diffusion coefficients of methane in crude oil. The developed model is compared with the empirical data, which shows the developed model predicts the methane molecular diffusion coefficient in crude oil with an average absolute error of 4.18%.

Shale reservoir storage of hydrogen: Adsorption and diffusion on shale

In this study, a series of hydrogen adsorption and diffusion tests were conducted on shale samples at high pressure and elevated temperatures to assess the potential hydrogen geo-storage in shale reservoir. Experimental results show that hydrogen can be adsorbed on shale surfaces which accounts for approximate 65% to 80 % amount of methane adsorption for the tested samples. The hydrogen adsorption capacity correlated positively with pressure and negatively with temperature. The Freundlich model can be used to predict the hydrogen adsorption behavior of the tested shale samples. Hydrogen diffusion coefficients are much larger than the equivalent methane diffusion coefficients, and both of them show an increasing trend versus temperature and a decreasing trend versus pressure. The results of this study illustrate that hydrogen can be stored as an adsorbed phase on kerogen and clay minerals, which could provide fundamental data for hydrogen storage on shale reservoirs.

Importance of the average radius of coal particles on determining the methane diffusion coefficient.

The average radius of coal particles is an estimate of the diffusion path in the particle method for determining the diffusion coefficient. It is currently calculated using the arithmetic mean of coal particle sieved intervals. This calculation, however, ignores the coal particle size distribution, resulting in significant deviations when calculating the gas diffusion coefficient. An appropriate average radius calculation method should consider the particle size distribution and the physical essence of diffusion. To accomplish this, a series of methods for calculating the mean particle diameters and their physical significance were reviewed. Next, coal samples were sieved into three intervals, and gas diffusion tests and laser particle size distribution were conducted. Results show that coal particles are within the sieving interval, ranging from 42.01 to 76.18%. By solving the diffusion coefficients using four mean particle diameters based on particle size distribution and diffusive mass transfer, the difference between the arithmetic mean value and these diameters is up to 89.06%. [Formula: see text] and [Formula: see text] are preferred for the calculation of the average radius since they are compatible with coal particle shape and the physical meaning of diffusive mass transfer.

Regularity of change in the volmer diffusion coefficient of methane adsorbed in the microsorption structure of the elastic zone of the coal seam bearing pressure

The Volmer diffusion coefficient of methane adsorbed in the micropores of coal in the elastic zone of the coal seam bearing pressure, which normally is under conditions of significant compressive stress, was calculated with taking into account the energy of the methane sorption connection with coal, the energy of Volmer diffusion activation in the porous space of coal, and the stressed state of the elastic zone with its influence on the change of Volmer porosity. During the calculations, such parameters as the diameter of Volmer micropores and the length of the descending branch of the bearing pressure diagram were varied. As a result of the approximation of these calculations, both pairwise dependences of the Volmer diffusion coefficient on the listed parameters and its multifactorial relationship with them were established. Therefore, it is concluded that the process of methane diffusion in the elastic zone of bearing pressure is not blocked by the rock pressure, as previously thought, but is actively developing. The diffusion of free methane will be determined by the established regularity of changes in the Volmer diffusion coefficient in the elastic zone of the coal seam bearing pressure. The calculations show that as the distance from the maximum of the bearing pressure increases, the Volmer diffusion coefficient of methane in the coal seam increases, which is due to a decrease in the pressure of rocks in the descending branch of the bearing pressure diagram. However, this growth is not great due to the weak compressibility of pores. Therefore, for pores of the same diameter, the Volmer diffusion coefficient in the elastic zone of the coal seam bearing pressure for the given mining geological conditions can be considered a constant. For depths of, for example, 1000 m and pore diameters of 10 Å, the value of the Volmer diffusion coefficient will be approximately 3.77·10-8 m2/s. This confirms that methane gas release is caused not only by filtration of free gas, but also by Volmer diffusion of adsorbed methane. In turn, the reserves of the latter are known to be the main reserves of methane in coal. Therefore, the established regularity makes it possible to more accurately calculate the volumes of methane, which will be released from the coal massif during mining operations, in order to assess safety of conditions for coal deposits mining and to develop technologies for coal mine methane production.

ДИНАМИКА ЗМІН КОЕФІЦІЄНТА ФОЛЬМЕРІВСЬКОЇ ДИФУЗІЇ МЕТАНУ, АДСОРБОВАНОГО У МІКРОСОРБЦІЙНІЙ СТРУКТУРІ ПРУЖНОЇ ЗОНИ ОПОРНОГО ТИСКУ ВУГІЛЬНОГО ПЛАСТА

Purpose. Set the possible change in the coefficient of Volmer diffusion of methane in the microsorption structure of the elastic zone of the supporting pressure of the coal seam. Methodology. The energy of the sorption bond of methane with the coals, the activation energy of the Volmer diffusion in the porous expanse of coal, as well as the stresses of the elastic zone and its effect on the change of the Volmer porosity, was taken into account. In the calculations, such parameters as the diameter of the Volmer micropores and the length of the descending branch of the support pressure diagram were varied. Results. Approximation of calculations made it possible to establish both the pairwise dependences of the Volmer diffusion coefficient on the listed parameters and its multifactorial relationship with them. It was concluded that the diffuse process of methane in the elastic support pressure zone is not blocked by rock pressure, as previously thought, but is actively developing. In this case, the diffusion of free methane will be determined by the established pattern of change in the Volmer diffusion coefficient in the elastic zone of the coal seam bearing pressure. Calculations showed that as the distance from the maximum reference pressure increases, the Volmer diffusion coefficient of methane in the coal seam increases, which is due to a decrease in rock pressure in the descending branch of the reference pressure graph. However, this growth is not strong due to the weak compressibility of pores. Therefore, for pores of the same diameter, the Volmer diffusion coefficient in the elastic zone of the supporting pressure of the coal seam for the given mining geological conditions can be considered a constant. For depths of, for example, 1000 m and pore diameters of 10 Å, the value of the Volmer diffusion coefficient will be approximately 3.77×10-8 m2/s. Scientific novelty. Calculations of the Volmer diffusion coefficient of methane adsorbed in the micropores of coal in the elastic zone of the supporting pressure of the coal seam, which, of course, is under significant compressive stresses, have been performed. The established phenomenon confirms that methane gas release is determined not only by free gas filtration, but also by Volmer diffusion of adsorbed methane. Practical significance. Since adsorbed methane reserves are known to be the main methane reserves in coal, the regularity established in the article makes it possible to more accurately calculate the volumes of methane that will be released from the coal massif during mining operations for the assessment of safe working conditions for coal deposits and for the development of technologies for the production of shale methane.

Investigating the concentration distribution characteristics of methane in large storage facility using desorption-concentration gradient method and CFD simulation

The continuous methane emission from the coal piles is associated with many environmental problems and safety risks, in large storage facility. The methane emission parameters of coal piles and the concentration distribution characteristics of methane, which are the basis of ventilation design, are rarely reported in large storage facilities. In this study, based on the mass-transfer diffusion theory, desorption-concentration gradient method for determining methane emission parameters of coal piles in large facilities is proposed. In this method, firstly, the coal pile methane diffusion coefficient was deduced by conducting the coal particles absorption-desorption experiments and the particle-size distribution. Then, combing the diffusion coefficient and methane concentration-gradient on the coal pile surface, the coal pile methane diffusion flux was calculated. Further, when methane diffusion flux was utilized as the source item for CFD calculations, the results were consistent with the onsite measured values, which demonstrated the accuracy of this method. The CFD simulation results show that methane has diffusion characteristics that adhere upward along the slope of the coal pile and converge on the coal ridge. The methane flow reaches the dome upwards, where a higher concentration is formed. Both the coal pile volume and the methane diffusion flux will affect the concentration distribution of methane in large storage facility. For a larger diffusion flux, the explosion limit will be exceeded, and measures need to be taken to eliminate the safety risk. These findings provide principal data and analysis guide for optimized design of ventilation performance in closed industrial building.

Desorption and diffusion characteristics of methane in the non-ventilation working face

Based on the rapid development of intelligent unmanned mining technology in coal mines, a new concept of non-ventilation working face is put forward. In the absence of ventilation, it is more possible to extract high purity methane and eliminate coal mine accidents. There are few studies on the desorption and diffusion characteristics of methane under constant positive pressure (higher than atmospheric pressure) in the non-ventilation working face. In this study, a set of self-designed gas positive pressure desorption equipment was used to carry out desorption experiments under various positive environmental pressures. The diffusion coefficient of methane in coal was calculated by using the unipore diffusion model. The results indicate that the extended Langmuir EXT1 model can well describe the variation of methane desorption capacity with time under different positive pressures. Under the same temperature and equilibrium pressure, the limit desorption capacity and desorption rate of methane in coal decrease with the increase of external positive pressure. With the increase of temperature and equilibrium pressure, the negative effect of positive pressure is weakened. At the same temperature and equilibrium pressure, the diffusion coefficient of methane in coal decreases with the increase of positive pressure. The higher equilibrium pressure increases the methane concentration in coal matrix and weakens the influence of positive pressure on the concentration difference, which slows down the decreasing rate of diffusion coefficient. On the contrary, the increase in temperature reduces the methane concentration in coal matrix and accelerates the decreasing rate of diffusion coefficient. These results provide theoretical guidance for the realization of the non-ventilation working face.

Shale’s Pore Structure and Sorption-Diffusion Characteristics: Effect of Analyzing Methods and Particle Size

The delineation of pore size and surface area distribution and methane sorption-diffusion capacity in gas shale reservoirs is crucial for the estimation of storage capacity, anticipating flow characteristics, and field development. A systematic approach and guidelines are needed for the analysis of the pore size and surface area distribution of shale formations. The effect of shale sample size on the gas sorption and diffusion properties is not well understood either. The low-pressure nitrogen adsorption technique is a prevalent method for pore characterization of nanoporous shale formations. Although researchers adopted some corrections to the classical method for the analysis of pore size and surface area distribution, there is a significant mismatch between different approaches in depicting fine mesopore size and surface area (2–10 nm). In this study, the classical methods and density functional theory are employed to comparatively analyze the pore characteristics of some shale and clay samples for their applicability, efficacy, and consistency issues. Furthermore, the effect of shale particle size on the methane sorption capacity and diffusion is being investigated. It seems that confinement stress has less of a considerable effect on methane sorption (6% decrease). However, crushing shale rocks into smaller particles can significantly overestimate the methane adsorption capacity. The methane diffusion coefficient also increases with increasing the shale particle size by more than an order of magnitude.

Mutual diffusion coefficients from NMR imaging

• A method of studying diffusion of dissolved gases in liquids is presented. • The concentration field of the diffusing dissolved gas is monitored by NMR imaging. • Selective excitation enables quantitative measurements at low concentrations. • The mutual diffusion coefficient is determined from modeling the diffusion process. • The results agree well with reference data. In the present work, we use nuclear magnetic resonance imaging (MRI) to measure the mutual diffusion coefficient of methane in toluene at 298 K. The concentration field obtained upon dissolving gaseous methane in liquid toluene was monitored with two-dimensional MRI. To cope with the low concentration of methane, a chemical shift-selective pulse sequence was employed. The diffusion coefficient was determined from the resulting temporally and spatially resolved concentration data based on Fick’s second law. The resulting diffusion coefficient is in good agreement with reference data. We conclude that MRI experiments are well-suited for quantitative studies of mutual diffusion in liquid mixtures, also in challenging applications as the one studied here.

Effect of water on methane diffusion in coal under temperature and pressure: A LF-NMR experimental study on successive depressurization desorption

Coalbed methane production is a process of desorption, diffusion, and seepage with the successive decrease of reservoir pressure. However, desorption at atmospheric pressure has been mainly used to investigate the gas diffusion of water-bearing coal. The influence of confining pressure and gas pressure on methane diffusion is not considered in this case. So, the question is, how does the variation of the methane diffusion coefficient of water-bearing coal during desorption? How is it different from dry coal? What is the difference between the methane diffusion behavior of anthracite and bituminous coal? To address these problems, we select low-volatile anthracite in the south of Qinshui Basin and high-volatile bituminous coal in the south of Junggar Basin. The methane diffusion of water-bearing and dry coal during the successive depressurization desorption is compared by low field nuclear magnetic resonance. The influence mechanism of water on methane diffusion is explored from the pore structure, wettability, and external stress. The results show that the methane diffusion coefficient of water-bearing anthracite and bituminous coal increases with the decrease of gas pressure. The methane diffusion of water-bearing coal is affected by the combination of the increase of methane desorption volume (positive effect) and the increase of effective stress (negative effect). When the gas pressure decreases from 6 MPa to 2 MPa, the negative effect is dominant, and the effective diffusion coefficient of methane in coal is poor; When the gas pressure is less than 2 MPa, the positive effect is dominant and the effective diffusion coefficient increases significantly. The homogeneity of methane effective diffusion path of water-bearing anthracite is enhanced, resulting in higher methane desorption rate and effective diffusion coefficient than dry coal. The methane effective diffusion pore size of water-bearing bituminous coal decreases, resulting in the effective diffusion coefficient of methane less than that of dry coal. These results are of great significance to reveal the methane diffusion behavior of coalbed methane in the actual production.

Dynamic model for the simultaneous adsorption of water vapor and methane on shales

Water is almost ubiquitous in shale gas reservoirs. To analyze simultaneous water vapor and methane adsorption in shales, a dynamic model with a time-dependent boundary and diffusivity and a noninstantaneous adsorption mechanism was proposed. The analytical solution of the model was obtained via integral transformation method. The model was applied to match the dynamic data of simultaneous CH4–H2O adsorption. The results suggest that the strength for gas adsorption to shale is characterized by adsorption rate coefficient and desorption rate coefficient, where the adsorption strength of shale to water vapor is stronger than that to methane, and the conversion rate from water vapor to adsorbed water on adsorption sites is larger. The methane and water vapor diffusion coefficients first increase to a maximum and then decrease with time. The maximum methane diffusion coefficient ranges from 5.83 × 10−13–1.08 × 10−11 m2/s; the maximum water vapor diffusion coefficient ranges from 2.06 × 10−19–4.53 × 10−13 m2/s. The methane diffusivity and its variation rate are larger than those of water vapor. The methane and water vapor diffusion coefficients first increase with the density of free gases in pores and then decrease in the late period of the adsorption process. Water vapor reduces the adsorption strength of shale to methane and free methane-to-adsorbed methane conversion rate on adsorption sites. The methane diffusion coefficient in dry shale increases with time, and its growth rate reaches zero at adsorption equilibrium, which ranges from 6.60 × 10−14–4.82 × 10−11 m2/s; the methane diffusivity increases exponentially with the methane adsorption amount and free methane density in pores.

Experimental and molecular dynamics studies of zwitterionic inhibitors of methane hydrate dissociation

Hydrate dissociation inhibitors can be widely applied in the field of natural gas exploitation. A combined experimental and molecular dynamics study into a new type of hydrate dissociation inhibitor is presented herein. The polymeric kinetic inhibitor is zwitterionic and was characterized by Fourier transform infrared spectroscopy and thermogravimetric analysis. When evaluated by methane hydrate formation and dissociation tests, the zwitterionic compound showed an inhibitory effect on hydrate dissociation of up to 37.3%. A kinetic model for the hydrate dissociation process in the presence of hydrate dissociation inhibitors was validated by the experimental data. The difference in dissociation efficiency can be represented by the methane diffusion coefficient in the liquid phase. Molecular dynamics simulations were performed to investigate the effect of the inhibitor on methane hydrate and water molecules. The results showed that the charged functional groups improved the interaction energy between the inhibitor and methane hydrate through Coulomb interactions, and the sulfonate group had a strong hydration ability, which could affect the distribution of water molecules in the water phase. The molecular-level understanding of the dissociation inhibition mechanism is instructional for the design of new and effective hydrate dissociation inhibitors.