Methane Adsorption Isotherms Research Articles

Diffusion characteristics of methane in various rank coals and the control mechanism

The methane diffusion behavior is one of the key factors that influence the production potential of coalbed methane (CBM) reservoirs. In this study, low temperature nitrogen, carbon dioxide adsorption and methane isotherm adsorption experiments for various rank coal samples were carried out to investigate the characteristics of pore size distribution and gas adsorption-diffusion behavior. A mathematical model similar to the Langmuir equation was established to describe the methane diffusion in coals. The variation law of the Langmuir effective diffusion coefficient (DL) with coal rank and its influencing factors, control mechanism were analyzed. With the increase of coal rank, the development degree of micropore rises, and both the specific pore volume and specific surface area first decrease and then increase. The diffusion coefficient of methane in coals generally rises with the rise of pressure. The Langmuir volume rises with the rise of Ro,m (maximum vitrinite reflectance with oil). The influence of moisture and coal molecular structure on gas diffusion varies with coal rank. The moisture content has a more significant influence on the gas diffusion in the medium-rank coals than in the low- and high-rank coals. The outcomes may deepen our understanding of gas transport in coals and benefit the exploration of CBM.

Using Low-Pressure Methane Adsorption Isotherms for Higher-Throughput Screening of Methane Storage Materials.

A useful correlation between the low-pressure (up to 1.2 bar), low-temperature (195 K) and high-pressure (up to 65 bar), room temperature (298 K) methane storage properties of a range of porous materials is reported. Methane isotherms under these two sets of conditions show a remarkable agreement in both equilibrium adsorption and deliverable capacities for materials with pore volumes that are less than approximately 0.80 cm3/g. This trend holds well for the suite of metal-organic frameworks and porous coordination cages we studied, in addition to a zeolite and porous organic cage. Although it is well known that gravimetric gas storage capacity trends with gravimetric surface area, the 1.2 bar, 195 K excess adsorption capacity of a given framework is a better indicator of its room temperature, 65 bar capacity. Given the significantly smaller sample quantities needed for low-pressure measurements, greater accessibility to researchers around the world, accuracy of the measurement, and higher throughput, we envision this method as a rapid screening tool for the identification of methane storage materials. As excess/total adsorption and gravimetric/volumetric adsorption can be interconverted by simple utilization of the scalar quantities of pore volume or density, respectively, this method can be easily adapted to obtain both gravimetric and volumetric total adsorption capacities for a given adsorbent. In terms of volumetric methane adsorption, we further investigate the relationship between crystallographic and bulk density for the adsorbents studied here. With this analysis, it becomes apparent that in the absence of novel synthetic approaches, reported volumetric storage capacities should be viewed as an optimistic upper limit for a given material and not necessarily a true reflection of its actual adsorption properties as most MOFs have bulk densities that are less than half of their crystallographic values.

Machine-Learning-Based Prediction of Methane Adsorption Isotherms at Varied Temperatures for Experimental Adsorbents

Metal–organic frameworks (MOFs) are crystalline materials and one of the optimal materials for large-scale grand canonical Monte Carlo (GCMC) simulations. Recently, there have been trials for apply...

Adsorption of H2S from Hydrocarbon Gas Using Doped Bentonite: A Molecular Simulation Study.

Hydrogen sulfide is a commonly occurring impurity in hydrocarbon gases such as natural gas or landfill gas. Apart from its toxicity, H2S can cause problems in downstream processing because of corrosion of piping in the presence of moisture. Removing this contaminant using a cost-effective and energy-efficient technique such as adsorption using commonly occurring adsorbents would be beneficial both for processing and refinement of hydrocarbon gases and for their use as an energy source. In this work, grand canonical Monte Carlo simulations were performed using an ab initio forcefield to predict adsorption isotherms for methane, hydrogen sulfide, and nitrogen in bentonite doped with K+, Li+, and Na+ cations with a view to aiding the development of low-cost pressure-swing adsorption systems for the targeted removal of H2S from landfill gas or natural gas. Pure species simulations were done, in addition to considering mixtures at conditions approximating real-world natural gas fields. Highly selective targeted adsorption of hydrogen sulfide was achieved for all three doped bentonites, with the adsorbed phase consisting of almost pure H2S, although the volume of gas adsorbed differed between adsorbents. The results suggest the following ranking for the three doped bentonite adsorbents in terms of their overall performance: K+ > Li+ > Na+. By considering both the composition of the adsorbed phase and the total quantity of adsorbed gas, there may be an interplay between the gas–gas and gas–solid interactions that becomes somewhat noticeable at low pressures.

Methane Adsorption on Fе–BDC Metal–Organic Porous Structures at High Pressures

The isotherms of absolute methane adsorption in an Fe–BDC metal–organic porous structure after long-term storage were measured at a pressure as high as 40 MPa and the temperatures of 303, 313, 323, and 333 K. The dependences of differential molar isosteric heats of adsorption and entropies of adsorption on methane uptake were calculated. It was shown that taking into account the nonideality of a gaseous phase and steepness of the isotherm of adsorption leads to a temperature dependence of thermodynamic functions of adsorption. Both a sharp drop in the heat of adsorption and a local maximum of the entropy at the maximum values of methane uptake are indicative of rearranging the adsorbate structure and the probable formation of molecular methane associates in micropores.

Impacting Factors, Dynamic Process, and Correction of Adsorption Reduction in Shale Reservoir: A Case Study on Shale Samples from the Western Guizhou.

Adsorption reduction occurring during isothermal experiment leads to the failure of telling the true adsorption capacity of shale reservoir. A correct understanding of this will be helpful in improving the accuracy of resource estimation and economic evaluation of shale gas reserves. Six shale samples were collected from the Permian Longtan Formation in the western Guizhou Province, China. Volumetric methane isotherm adsorption experiment and data processing were conducted in this research. The study investigates the effect of free space volume reduction (FSVR), excess adsorption amount conversion (EAAC), and blank test correction (BTC) on adsorption reduction, the understanding of the dynamic process of adsorption reduction, and the evaluation of the way of weakening and correcting this phenomenon. The conclusions are as follows. (1) Adsorption reduction does exist in the shale sample. The adsorption process of methane in the shale sample can be divided into the strong adsorption stage, approximate saturation stage, and adsorption reduction stage. (2) Shale adsorbing methane has a positive effect on the experimental adsorption amount. Comparatively, free space volume, excess adsorption amount, and blank test have negative effects. Adsorption reduction is the result of combined influence of positive and negative effects above. (3) At the first two stages of methane adsorption, the positive effect is greater than the negative effect, resulting in the hidden of adsorption reduction, and the experimental adsorption amount increases with the growth of experimental pressure. While at the adsorption reduction stage, the former effect is smaller than the latter, and their difference increases as the experimental pressure increases. It leads to the occurrence of adsorption reduction, and the phenomenon becomes increasingly obvious. (4) FSVR has the strongest impact on the weakening of adsorption reduction, followed by EAAC and BTC. The adsorption reduction in shale reservoir can be corrected effectively by BTC and EAAC.

Methane resaturation in Barnett Formation core plugs and new approach for determination of post-coring gas loss

Understanding the physiochemical mechanisms that control the loss of gas during coring processes is critical to accurately determining gas-in-place (GIP) resource assessments of unconventional shale-gas plays. Our study uses an experimental approach, utilizing methane (CH4) adsorption isotherms and degassing curves of methane-resaturated Barnett Formation core plugs, to determine the amount of lost-gas based on mass-balance analysis at different CH4 re-saturation pressures and varied exposure times. Several readily available empirical methods for estimating lost-gas were evaluated, quantified, and compared with the mass balance–derived lost-gas values in our experiments.A CH4 isotherm measurement on 3/8-inch Barnett Formation core plugs was performed at 35.4 °C; the amount of gas adsorbed in excess was then quantified and fitted to the modified Langmuir equation to determine the Langmuir maximum, Langmuir constant, and adsorbed gas-phase density. Two sets of CH4 gas-resaturation and degassing measurements, one varying saturation pressures and the other varying exposure times, were performed on 3/8-inch Barnett Formation core-plugs at an isothermal temperature of 35.4 °C. Degassing curves, the plot of the released gas yield versus the square root of degassing time, display three stages that correspond to different gas-releasing mechanisms. The rapid increase of released gas yield at the beginning of degassing represents that nonlinear gas expansion is dominant and that degassing evolves into a linear desorption-dominated phase over time. Experimentally derived values for lost gas were determined by subtracting the sum of the desorbed and retained gas at the peak of the degassing curve from the amount of gas initially charged into the samples at equilibrated resaturation pressure. Lost gas varies linearly with increasing gas-resaturation pressure and nonlinearly by a greater magnitude with increasing exposure time, indicating that lost gas is more sensitive to exposure time.The uncertainty evaluation of lost gas determined by three empirical methods was conducted by direct comparison with mass-balance-derived lost-gas values from our experiment. Nonlinear least-squares extrapolation overestimates, and both linear extrapolation and polynomial equation fitting underestimate, mass-balance lost-gas control points. Among the three empirical methods, the polynomial-fitted lost-gas values most closely agree with mass-balance lost gas, revealing that polynomial fitting to degassing curves is a viable way to accurately estimate lost gas and, more importantly, to estimate GIP values with up to 85% accuracy. As free gas is the dominant storage mechanism in the Barnett Formation, the trends of experimentally measured cumulative desorbed gas content versus the square root of elapsed time match neither the calculated curves derived from USBM Direct Method nor Amoco Method.

Heterogeneous development of micropores in medium-high rank coal and its relationship with adsorption capacity

In order to better understand the micropores (pore size less than 2 nm) in some medium-high rank coals, the development, distribution, and structure of micropores in five medium-high ranks coal samples (0.68% < Ro, max < 6.22%) were determined by low-pressure CO2/N2 adsorption (LP-CO2/N2 GA) experiments, and high-resolution transmission electron microscopy (HRTEM). The mathematical models and methods for accurately determining micropores were further discussed. Then, the high-pressure methane isotherm adsorption (HP-CH4 GA) experiment was carried out to analyze the gas adsorption capacity of the micropores. The results showed that LP-CO2/N2 GA in conjunction with non-localized and quenched-solid density functional theory models could more accurately determine the pore volume (PV), specific surface area (SSA), and pore size distribution (PSD) of the micropores. The PV and SSA of the coals were distributed between 0.046 and 0.089 cm3/g and 139.6–305.2 m2/g, respectively. Although the micropore development differed greatly between the samples, their PSD was very similar, with more than half of the micropore PV and SSA distributed in the size of the pore between 0.35 and 0.70 nm. The micropores could be mainly derived from pores in organic macromolecular structure and were evenly distributed in the coal matrix in complex form. The development and size of the micropores were controlled to some extent by the size and arrangement of the organic structure. Micropores accounted for 86.9%–98.5% of the total pore SSA in the coals, so most methane could be adsorbed in micropores. However, the SSA of micropore has not restricted the linear relationship with the methane adsorption capacity.

Characterization of adsorption isotherm and density profile in cylindrical nanopores: Modeling and measurement

Gas adsorption behavior in shale kerogen may not be fully described using the Simplified Local-Density (SLD) model with a slit-shaped pore. In this study, an extension of the SLD model is proposed to characterize the adsorption isotherm and density profile in shale nanopores with a circular pore geometry. Such an extension enables the pore structure associated with the SLD model to be comparable with the cylindrical micro- and mesopores in shale kerogen. To examine the accuracy and reliability of the extended SLD model, adsorption isotherms and density profiles of methane and carbon dioxide in single-walled carbon nanotubes (SWCNTs) calculated by the extended SLD model were compared with the corresponding quantities determined via the grand canonical Monte Carlo (GCMC) simulations. In addition, the extended SLD model was used to evaluate the measured adsorption of methane on the Marcellus shale core sample to demonstrate its practicability. The results of this study indicate that the adsorption isotherm and density profile calculated by the extended SLD model are in reasonably good agreement with those determined via the GCMC simulation. Moreover, the extended SLD model can properly characterize the measured adsorption isotherm of methane on shale. These findings illustrate that the extended SLD model is a robust engineering model, which is capable of predicting the adsorption isotherm and density profile in cylindrical nanopores. More significantly, the extended SLD model serves as a link to convey the microscopic details from the GCMC simulations to the interpretation of experimental measurements through rapid computations.

Supercritical methane adsorption and storage in pores in shales and isolated kerogens

Shale gas is an important hydrocarbon resource in a global context. It has had a significant impact on energy resources in the US, but the worldwide development of this methane resource requires further research to increase the understanding of the relationship of shale structural characteristics to methane storage capacity. In this study a range of gas adsorption, microscopic, mercury injection capillary pressure porosimetry and pycnometry techniques were used to characterize the full range of porosity in a series of shales of different thermal maturity. Supercritical methane adsorption methods for shale under conditions which simulate geological conditions (up to 473 K and 15 MPa) were developed. These methods were used to measure the methane adsorption isotherms of Posidonia shales where the kerogen maturity ranged from immature, through oil window, to gas window. Subcritical methane and carbon dioxide adsorption studies were used for determining pore structure characteristics of the shales. Mercury injection capillary pressure porosimetry was used to characterize the meso and macro porosity of shales. The sum of the CO2 sorption pore volume at 195 K and mercury injection capillary pressure pore volumes (1093–5.6 nm) were equal to the corresponding total pore volume (< 1093 nm) thereby giving an equation accounting for virtually all the available shale porosity. These measurements allowed quantification of all the available porosity in shales and were used for estimating the contributions of methane stored as ‘free’ compressed gas and as adsorbed gas to overall methane storage capacity of shales. Both the mineral and kerogen components of shale were studied by comparing shale and the corresponding isolated kerogens so that the relative contributions of these components could be assessed. The results show that the methane adsorption characteristics were much higher for the kerogens and represented 35–60% of the total adsorption capacity for the shales used in this study, which had total organic contents in range 5.8–10.9 wt%. Microscopy studies revealed that the pore systems in clay-rich, organic-rich and microfossil-rich parts of shale are very different, and also the importance of the inter-granular organic-mineral interface.

Methane Adsorption Interpreting with Adsorption Potential and Its Controlling Factors in Various Rank Coals

Water content, metamorphism (coal rank) particle size, and especially pore structure, strongly influence the adsorption capacity of coal to methane. To understand the mechanism of methane adsorption in different rank coals, and its controlling factors, isothermal adsorption experiments with different coal ranks, moisture contents and particle sizes at the temperature of 303.15 K were conducted. In addition, the pore structures of coals were investigated through N2 adsorption/desorption experiments at the low-temperature of 77 K for selected coals from the Junggar Basin of NW China, Qinshui Basin and Ordos Basin of north China. Moreover, the adsorption potential of methane on the surface of the coal matrix was calculated, the controlling factors of which were discussed. The obtained methane isothermal adsorption result shows that the Langmuir volume (VL) of coal is independent of the particle size, and decreases with the increase of moisture content, which decreases first and then increases when the coal rank increases. Combined with the pore structure by the N2 adsorption at 77 K, VL increases with the increase of pore surface area and pore volume of coal pores. Besides, the adsorption potential of all selected coals decreased with the increase of the methane adsorption volume, showing a negative relationship. The interesting phenomena was found that the surface adsorption potential of the coal matrix decreases with the increase of moisture content, and increases with the decrease of particle size at the same pressure. With the same adsorption amount, the adsorption potential on the surface of coal matrix decreases first, and then increases with the increase of coal rank, reaching a minimum at Ro,m of 1.38%, and enlarging with the increase of pore surface area and the pore volume of coal pores. These findings may have significant implications for discovering CBM accumulation areas and enhancing CBM recovery.

Effects of helium adsorption in carbon nanopores on apparent void volumes and excess methane adsorption isotherms

A void volume, which is measured by helium expansion tests and used in the calculation of methane adsorption amounts, is always overestimated due to helium adsorption. In this study, by comparing void volumes of carbon nanopores determined under different temperatures and pressures using GCMC (Grand Canonical Monte Carlo) simulation, suitable experimental conditions for helium expansion tests are obtained. Five volumes, including one apparent volume Vapp, three referred volumes Vref and one physical volume Vphy, are recognized. The apparent volume Vapp corresponds to the volume directly determined under traditional experimental conditions, while three referred volumes are determined at 500 K with different pressure ranges (low, moderate, high). The physical volume is calculated by multiplying a pore width and a surface area. Besides, a volume determined by using a helium probe is named an accessible volume Vacc and used as a criterion for a determined volume through mass balance. It is found that use of a void volume determined under traditional experimental conditions or a physical volume leads to negative adsorption amounts at high pressures. Considering an economic effect and measurement accuracy, determining a void volume by helium expansion tests within a moderate pressure range at 500 K is suggested. Excess isotherms of methane calculated by the suggested volume are more appropriate and of great physical meanings for further investigation of adsorption mechanisms.

A novel method to calculate the gas adsorption of mixed minerals

Five groups of methane isotherm adsorption tests were carried out at 61 °C under a pressure range of 0–25 MPa, including four groups of pure clay minerals (illite, montmorillonite, chlorite, kaolinite, for brief they are expressed as Ill, Mont, Chl, and Kaol respectively) and a group of mixed minerals by four pure clay minerals in equal mass ratio. The experimental results showed the order of methane adsorption of pure clay minerals: Kaol > Ill > Mont > Chl. And the composition of clay minerals was obtained by XRD experiments. The result showed that the four samples were of high purity, both of which were greater than 90%. In addition, the results of low-pressure nitrogen adsorption experiments showed the order of specific surface area: Kaol > Ill > Mont > Chl, which is consistent with the order of methane adsorption. The methane adsorption is positively correlated to the specific surface area. The adsorption data was fitted to the Langmuir–Freundlich model to establish the method to calculate the adsorption of mixed minerals. The result showed that the parameters of the adsorption model of mixed clay minerals can be obtained by summing the parameters of the adsorption model of the pure minerals contained in the mixture according to their constituent mass ratio and the feasibility of this result was also verified by literature data. Overall, once the mass ratio in mixed clay minerals gets known, its theoretical adsorption can be calculated.

Natural gas storage properties of adsorbents synthesised from three different coal waste in South Africa

Adsorbed natural gas (ANG) storage has been identified as the most promising low-pressure alternative to storing natural gas. However, in addition to the thermal management of the system, successful implementation of this technology is largely constrained by the cost of adsorbents. The usefulness and cost of suitable adsorbent such as activated carbon are essential to ANG technology's productivity. In this study, activated carbons were prepared from three South Africa coal waste samples (run-of-mine fines, discard and flotation slurry) by KOH activation. The adsorption isotherm of methane on the synthesized activated carbons revealed that the activated carbon synthesized from run of mine fines (ACR) has the highest adsorbed amount compared to activated carbons synthesized from coal discard (ACD) and slurry (ACS). The maximum adsorbed amount of methane was attained at an average pressure of 37 bar with ACR having the largest volume of methane adsorbed at 162.71 cm3/g followed by ACD (157.58 cm3/g) and ACS (106.69 cm3/g) respectively. This sequence is consistent with the BET surface areas of the activated carbons; ACR (1925.34 m2/g) > ACD (1826.41 m2/g) > ACS (1484.96 m2/g). The methane adsorption data were validated with Langmuir, Toth, and D-A isotherm models for the three activated carbons. It was found that, for all three activated carbons, the D-A model was the best fit. Based on the requirements of suitable porous materials for ANG application, the study indicated that the produced activated carbons from South Africa coal wastes could well be used for natural gas storage.

Effect of additives in the nucleation and growth of methane hydrates confined in a high-surface area activated carbon material

A high-surface area activated carbon material (PPAC) prepared from a petroleum residue has been used as a host structure to promote the nucleation and growth of confined methane hydrates after the incorporation of additives. Sodium dodecyl sulfate (SDS), Leucine and Tetrahydrofuran (THF) have been evaluated either dissolved in the pre-impregnation media (ultrapure water) or incorporated in the carbon surface using a mechanochemical approach. High-pressure methane adsorption isotherms show that the mechanochemical approach does not improve the storage performance of the original carbon material due to the steric effects by the bulkier organic functionalities. Furthermore, the incorporation of the additives shifts the nucleation in narrow pores to higher pressures.On the contrary, when the additives are dissolved in water, high-pressure methane isotherms anticipate a modification in the kinetics and/or thermodynamics of the nucleation process. Whereas gas hydrates grown in large pores and/or the external surface exhibits a significant reduction in the induction time, the nucleation process in narrow micropores becomes more uniform and occurs in a narrower pressure window, slightly shifted to higher pressures. However, no changes in the total uptake at 10 MPa could be found. The situation is different when THF is dissolved in water. In this case, the nucleation takes place at pressures below the natural process (<3–4 MPa), the total amount of methane stored at 10 MPa being slightly limited. In-situ synchrotron X-ray diffraction studies anticipate a sI structure for the pure water methane hydrates, while a combination of sI and sII is identified for the THF-based system.

Characterisation of Surat Basin Walloon interburden and overlying Springbok Sandstone: a focus on methane adsorption isotherms, permeability and gas content

The Surat Basin in Queensland is one of the world’s premier producers of natural gas from coal seams. We investigate the potential for clay-rich Walloon Coal interburden and the overlying Springbok Sandstone to hold or produce gas. Seventeen core samples were analysed from two wells from intervals within the Walloon Upper Juandah Coal Measures interburden and the Springbok Sandstone. Samples were characterised using high-pressure methane adsorption isotherms, canister gas desorption tests, moisture contents, ash contents, carbon contents, scanning electron microscopy/energy dispersive X-ray spectrometry, X-ray quantitative clay analysis, permeability, helium pycnometry and mercury intrusion porosimetry. Methane adsorption was conducted at 30°C with up to 8 MPa pressure on dried crushed samples. The adsorption capacity of methane at 8 MPa varied from 3 cc/g (calcite-cemented) up to 25 cc/g (standard temperature and pressure equivalent) (coal). Clay-rich interburden samples adsorbed ~5–14 cc/g (dry). The measured isotherms and methane content from canister desorption tests show that appreciable volumes of gas are contained within some portions of interburden and in the overlying Springbok Sandstone. Gas within the interburden likely represents a large volumetric resource, albeit in low permeability rock that restricts direct productivity. The gas adsorption and gas content results for the Springbok Sandstone help to explain field observations of high gas content in some landholder water wells.

Study on the methane adsorption characteristics of coal with heterogeneous potential wells

Due to the various components and the complex inner structures, the coal has the heterogeneous adsorption potential wells. The methane adsorption laws of the coal with heterogeneous potential wells are theoretically analysed and experimentally studied. It is concluded that the coverage of the potential wells with different depth obeys the law of the logistic (S) curve with the parameters of the adsorption pressure and the temperature. During the methane adsorption in coal, the Langmuir parameter a of coal has a fluctuate growth with the adsorption pressure increase, which is indicated that the number of the potential wells in coal with different depth tends to increase with the depth decrease. The Langmuir parameter b shows decrease trend with the adsorption pressure increase as the methane molecules in coal tend to be adsorbed in the deep potential wells. The heterogeneous potential wells method (HPWM) for the methane isothermal adsorption amount calculation is put up. From the experiment, the high positive errors are shown in low adsorption pressures based on the Langmuir's equation while the high negative errors are shown in high adsorption pressures. The methane adsorption amount calculation based on the HPWM is much accurate than that based on the Langmuir equation. [Received: December 9, 2017; Accepted: April 19, 2018]

Thermal maturity and reservoir quality of the Velkerri Formation, Beetaloo Sub-basin, Northern Territory

The Proterozoic (~1.43 Ga) Velkerri Formation (McArthur Basin, Northern Territory) is believed to host one of the world’s oldest petroleum systems. Drilling and production tests demonstrated the encouraging potential of this unconventional gas resource and current efforts are aimed at better defining the extent of the gas and liquid-rich portions of this shale play. In this context, we conducted a comprehensive characterisation of specimens from the Amungee Member of the Velkerri Formation (previously referred to as the Middle Velkerri) sampled along a thermal maturity gradient from wells drilled across the Beetaloo Sub-basin. The scope of our analysis was to shed light on how the organic matter in the Velkerri Formation is affected by thermal maturity and how this affects the pore structure of the host rock. To this end, we integrated light and electron microscopy imaging with Raman spectroscopy and pyrolysis techniques to define the maturity of the sediments and visualise the shale pore network. In concert with the direct imaging, bulk samples were experimentally tested to measure their gas permeability and gas adsorption capacity. The results indicate that porosity is mostly organic-hosted with pore size dominated by meso- and micro-pores (i.e. &amp;lt;50 nm). Methane adsorption isotherms measured at temperatures up to 80°C also show that organic content is one of the main factors controlling the amount of adsorbed gas in the Velkerri shale.

Recognitions on the flow mechanism of shale during the plug pulse decay measurement

When using the plug pulse decay method to measure shale permeability, a comprehensive analysis of various factors influencing the measurement and analysis precision is conducive to improving the accuracy of the test result. A great number of nano-scale pores are developed in shale, so the lower the test pressure is, the more easily the non-Darcy flow tends to emerge. In order to ensure the gas is in the form of Darcy flow in the process of measurement, we put forward the lower pressure limit of Darcy flow in pores according to the definition of Knudsen number. Then, the gas desorption-flow coupling model for the gas flow process during the plug pulse decay measurement was established by considering the effects of gas on the adsorption in organic pores. After the partial differential equation set was solved and derived, the corresponding permeability calculation method was proposed. Finally, after the plug pulse decay measurement, the supporting nitrogen isothermal adsorption test and methane isothermal adsorption test were conducted on shale samples. And the following research results were obtained. First, during the plug pulse decay measurement, the flow of gas in rock samples is one-dimensional linear, so when establishing the flow equation, the gas pseudopressure can be replaced with the gas pressure to simplify the calculation. Second, the adsorption of nitrogen in shale is much less than that of methane in shale, so the influence of nitrogen desorption on the flow is negligible when the pressure difference (i.e. <5%) of upstream and downstream changes within 5% of the initial pore pressure. In conclusion, the available industrial standards don't take into consideration the influence of gas adsorption, but the shale permeability measurement can still satisfy the accuracy requirement.

Effective removal of methane using nano-sized zeolite 4A synthesized from kaolin

The purpose of this study was to synthesize nano-sized zeolite 4A from kaolin obtained from Guizhou New Materials Technology Co., Ltd. The properties of the nano-sized zeolite 4A were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), 29Si magic-angle spinning (MAS)-nuclear magnetic resonance (NMR) and 27Al MAS-NMR, X-ray fluorescence (XRF), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET), and thermogravimetry (TG)-differential thermal analysis (DTA). According to SEM studies, it can be concluded that the nano-sized zeolite 4A has a typical cubic morphology. TEM images validate that the nano-sized zeolite 4A is of 100–150 nm in size. From the selected-area electron diffraction (SAED) patterns, it is confirmed that these nano-sized zeolite 4A particles have a pure body-center-cubic structure. The textural properties of the nano-sized zeolite 4A were studied by nitrogen adsorption technique, and it can be concluded that the BET surface area of the nano-sized zeolite 4A is 25.131 m2·g−1. In addition, it is found that the Langmuir model is best suited for the adsorption isotherms of methane on nano-sized zeolite 4A. Meanwhile, the methane adsorption analysis confirmed that the maximum adsorption amount of the nano-sized zeolite 4A to methane increases with the increase of adsorption temperature in the temperature range of 288–318 K.