Methane Sorption Research Articles

Comparison of Gravimetric Determination of Methane Sorption Capacities of Coals for Using Their Results in Assessing Outbursts in Mines

The gravimetric method for determining coal gas sorption has many advantages and limitations. The article presents the influence of various factors on the results of methane sorption on coal. In mining practice, in addition to sorption properties of coal, knowledge of methane sorption capacity and effective diffusion coefficient determined when assuming a unipore sorption/desorption model are crucial for predicting sudden releases of methane from coal seams to a mine ventilation environment. In Poland, determining sorption capacities of coals for methane is mandatory when starting mining operations in new parts of coal deposits threatened by outbursts. Traditionally, gravimetric microbalances, such as intelligent gravimetric analysis (IGA), are used to determine adsorption capacity and desorption rate. Recently, newer microbalances XEMIS have been introduced to the market. Two gas laboratories, AGH in Krakow and CLP-B in Jastrzebie-Zdroj, respectively, compared experimental adsorption isotherms using XEMIS microbalances with mutually exchanged coal samples. Both sorption capacity at the pressure of 1 bar (a1bar) and effective diffusion coefficient (De) were independently determined for the coal samples tested. The results obtained are comparable despite the use of different microbalance XEMIS models. The conducted studies and comparative evaluation of the results allowed for assessing procedures for determining sorption properties using XEMIS microbalances. The exchange of laboratory experiences also allowed for the identification of methodology factors crucial for the development of a uniform procedure for conducting similar studies with XEMIS microbalance. The proposed factors for testing the sorption behavior of methane in coal structures may be helpful in mining practice.

The occurrence of coalbed methane in the depocentre of the Upper Silesian Coal Basin in the light of the research from the Orzesze-1 deep exploratory well

The Orzesze-1 exploratory well with a depth of 3708 m (TVD) was drilled in 2019–2020 in the depocentre of the Upper Silesian Coal Basin (USCB). The methane content in the coal seams has been tested to a depth of 2840 m and the sorption capacity of the coal to a depth of 2576 m. These are the deepest measurements in the USCB so far. The vertical distribution of methane content in the borehole shows two depth zones of interest, the first at a depth 883 m to about 1300 m (maximum methane content about 12 m3/t coaldaf) and another in the range of 1500–2840 m, that is, to the maximum measurement depth, so the actual lower boundary depth of this zone is unknown. The maximum methane content here exceeds 18 m3/t coaldaf at a depth of >2800 m. Both zones are separated by an interval of reduced methane content of about 5 m3/t coaldaf at a depth of approximately 1400 m. The gas composition is dominated by methane (∼90%), and the content of carbon dioxide increases to approximately 15% at a depth of >2300 m. The methane-bearing zone at ∼900–1300 m corresponds to the zone of high- and medium-volatile bituminous coal (second coalification jump), while the highest methane content at a depth of >2800 m was determined in anthracite. The methane sorption capacity of the coal seams oscillates between 16 and 40 m3/t coaldaf with a maximum in anthracite at a depth of >2800 m, where the temperature of the rock approaches 100 °C and the deposit pressure exceeds 28 MPa. The highest sorption capacity in anthracite results from its inner structure characterised by the predominance of ordered aromatic lamellas and the dominance of vitrinite macerals (>70%), which contain coal micropores accumulating adsorbed methane. The comparison of the sorption capacity of the tested coal and the measured methane content displays undersaturation of 11–59%, however, due to significant gas content in the deep zone (depth > 1500 m), the drilling area can be considered as a prospect for further exploration and development of coalbed methane (CBM).

Influence of Grain Size and Gas Pressure on Diffusion Kinetics and CH4 Sorption Isotherm on Coal

The paper presents research on the influence of grain size of selected coals and their structural parameters on the diffusion coefficient and methane sorption isotherms. Two coals from Polish hard coal mines, differing in the coal rank, were tested. Sorption isotherms for methane were determined. An unconventional sequence of pressures 0→0.1→0→0.5→0→1.5 MPa was employed to assess the speed of achieving sorption equilibrium at different pressures. The studies of CH4 accumulation kinetics were performed on various grain classes of the tested coals. Both the sorption capacity of coal and the diffusion coefficient proved to be highly sensitive to the experimental methodology. Critical measurement parameters in terms of determining the diffusion coefficient concerning the assumptions of the Crank model were indicated. The influence of the equivalent radius of coal grain on the process kinetics was demonstrated. The stepwise pressure increase factor was examined in the context of minimising the impact of sorption isotherm non-linearity on the results. The importance of the width of the grain class of coals was determined to reduce their maceral inhomogeneities. These factors are the most common reason that makes it difficult to quantitatively compare diffusion coefficient values.

Origin and nature of pores in the Toolebuc Formation, a potential unconventional target in Australia

The Toolebuc Formation of Australia, a potential unconventional hydrocarbon resource, has limited studies on its pore structure and sorption characteristics. In this study, shale samples covering the lower mixed argillaceous mudstone (MAM) lithofacies, the middle interbedded calcareous mudstone and shelly thin beds (CM-STB) lithofacies, and the upper interbedded calcareous mudstone and shelly horizons (CM-SH) lithofacies of the Toolebuc Formation were collected. These samples were analysed for pore structure using a combination of scanning electron microscopy, X-ray diffraction, helium pycnometry, mercury intrusion porosimetry, and N2 physisorption techniques. Additionally, methane sorption isotherms were measured under in-situ conditions. The results reveal that most pores are mineral-related intraparticle (intraP) and interparticle pores, with slit, equant and elongated shapes. Organic matter (OM) pores are rare. Porosity, total pore volume and BET specific surface area (SSA) are 3.25–8.26%, 1.32–3.55 cm3/100 g, and 1.26–9.65 m2/g, respectively. Pore volume is dominated by mesopores and macropores while specific surface area is dominated by fine mesopores and micropores. The porosity of the organic matter is significantly low due to the rarity of OM pores in the stage of early oil-window thermal maturity; organic matter consequently occludes pore space and also negatively impacts the average porosity. Clay by contrast is positively correlated to the average porosity. Carbonate provides intraP pores in fecal pellets, but also fills in other pore spaces as occlusion. Methane isotherms exhibit linear shapes, suggesting that a portion of the gas is stored in solution. The lower MAM lithofacies, characterised by rich clay, high porosity and BET SSA, was measured to have an in-situ methane sorption capacity of 4.32 cm3/g; the middle CM-STB lithofacies has intermediate porosity, but exhibits excellent gas generation potential and high in-situ methane sorption capacity (4.12–5.5 cm3/g). Within the CM-STB lithofacies, porosity declines with depth. The upper CM-SH lithofacies is carbonate-rich, exhibiting the lowest porosity and in-situ methane sorption capacity (2.56 cm3/g), but may act as an intraformational seal. The combination and vertical stacking pattern of the three lithofacies provided a favourable setting for gas storage.

Effects of light hydrocarbons and extractable organic matter on the methane sorption capacity of shales

Effects of light hydrocarbons and extractable organic matter on the methane sorption capacity of shales

Modeling of methane and carbon dioxide sorption capacity in tight reservoirs using Machine learning techniques

The viability of enhanced coalbed methane recovery (ECBM) and enhanced shale gas recovery (ESGR) are abundantly explored in various studies since they present a solution for ongoing energy demand and environmental crisis. As a matter of challenge, the prediction of the gas sorption profile poses a significant obstacle in the development of such resources. In this regard, mathematical models, numerical simulations, empirical correlations, and experimental measurements suffer from over-simplification, computationally extensive calculations, time-consuming, and costly processes. This work examines different machine learning methods, from shallow to deep learning, to investigate their capability to model 3804 data points of methane (CH4) and/or carbon dioxide (CO2) sorption capacity in shale and coal at different thermodynamic conditions. Percentage of CO2 in injection gas, rock type, total organic carbon (TOC), moisture, temperature, and pressure were taken as input. Hyperparameter tuning was executed by Optuna optimizer. According to the results, the random forest (RF) was the most proficient predictor for both test subsets of CH4 (MAE = 0.0864, MSE = 0.0231, RMSE = 0.1520) and CO2 (MAE = 0.0529, MSE = 0.0533, RMSE = 0.2308) sorption capacity. Based on the trend prediction simulation, RF predictors were able to properly capture the single and binary sorption capacities. More importantly, they successfully predict the abnormal descending behavior of CO2 sorption in high pressures. In addition, a sensitivity analysis was carried out to acquire the importance of input features and model hyperparameters in the training process. Input feature impacts were consistent with technical expectations of geology and reservoir engineering aspects. Both models are capable of being applied to lab scale and further developed for their application in reservoir scale simulators.

Ni alumina-based catalyst for sorption enhanced reforming - Effect of calcination temperature

Effect of calcination temperature (350 °C – 850 °C) on the physicochemical properties as well as catalytic performance of Ni-based catalyst for the hydrogen production via steam methane reforming (SMR) and sorption enhanced reforming (SER) has been investigated. Catalyst calcined at highest temperature (850 °C) shows formation of NiAl2O4 confirmed by XRD. Consequently, a much higher reduction temperature (875 °C) is required to reduce this Ni aluminate spinel to an active Ni catalyst. Catalyst calcined at highest temperature (850 °C) showed much higher CH4 conversion and H2 production in SMR compared to the catalysts calcined at lower temperatures. In addition, higher CH4 conversion and H2 production was observed for the 15%Ni/alumina_850 catalyst after aging (long exposure to steam and high temperature) compared to commercial reforming catalyst. Finally, a much better stability is observed for the 15%Ni/alumina_850 catalyst compared to the commercial reforming catalyst after 100 SER/regeneration cycles under relevant reaction conditions. The formation of NiAl2O4 during high temperature calcination plays a vital role in the robustness and stability of the Ni-based catalysts and can be useful synthesis procedure for increasing the catalyst lifetime.

Methane Sorption Capacity of a Carbon Material Based on Polymer Raw Materials

Methane Sorption Capacity of a Carbon Material Based on Polymer Raw Materials

Significance of organo-mineralogical constituents on pore distribution, fractals and gas sorption mechanism of Permian shale beds in Korba sub-basin of the Son-Mahanadi Valley, India

In this study, shale samples obtained from the Korbasub-basin of Central India have been evaluated for the significance of organo-inorganic constituents in terms of their source potential of gas genesis, pore-matrix setup, and gas adsorption and desorption. The samples were analysed for various properties, such as megascopic features, lithological variations, depositional facies, and sources of organic and inorganic sediments. Also, maceral content, mineralogical constituents, kerogen type, pore distribution, pore structure, fractal characteristics and gas storage capacity were examined. The analysis revealed a trend of alternating bands of carbonaceous matter, silts, intercalations, and fluvial bedding lithofacies patterns indicating that the sediments were deposited under fluvio-terrestrial-lacustrine depositional conditions. Ternary facies analysis of maceral composition and van Krevelen diagram suggested the evolutionary path of type IV kerogen and are thermally matured source material located within the dry hydrocarbon generation window. Evaluation of pore types and pore structures using BET sorption plots (hysteresis H3 and isotherm type IV) revealed the presence of cylindrical, slit and combined cylindrical-slit pores with two distinct fractal characteristics. The discrete macerals underwent partial decomposition during transportation and mixing with inorganic components (clay and minerals), which influenced organic matter preservation, gas genesis, pore fractals and storage mechanism. The chemical index of alteration (CIA – 88.36 to 91.97), weathering (CIW – 96.60 to 98.80) and compositional variation (ICV – 0.31 to 0.70) demonstrated the influence of partial decomposition and strong weathering patterns on the organo-inorganic matter. A significant correlation was observed between pore fractals and elemental composition, particularly related to the resistance of weathering elements such as MgO, Al2O3, Fe2O3, SiO2, P2O5, TiO2, etc., which partially contributed to the formation of pore rugged surfaces. Microscopic and XRD analyses were employed to determine the mineral type, grain shape and size, and their amount leading to the development of the shales brittleness index (MBI). The importance of brittle characteristics, governed by minerals and clays within the shale, is discussed in relation to the hydro-fracturing technique for permeability enhancement. The results of high-pressure methane sorption indicated a moderate gas storage potential. The variation in sorption capacity (VL – 6.65 to 8.25 cc/g) is described through relationships with the ICV, pore diameter, pore volume, liptinite and total maceral content. Fractal dimensions D1 (2.4571–2.5611) and D2 (2.5804–2.7529) exhibited a direct relationship with adsorption properties, indicating the presence of rugged surfaces associated with meso- and macro-scale pores, which primarily control gas storage in shale. This research offers detailed insights into shale gas, providing valuable information for CBM and petroleum operators engaged in methane gas exploration and production. While commercial CBM production has been ongoing in India for over a decade, the exploration and understanding of shale gas reservoir characteristics are still in progress. This investigation adds significant value and highlights the extensive opportunities for research and potentially commercially recovering shale gas in the Korba sub-basin.

Characteristics and Governing Factors of Pore Structure and Methane Sorption in Deep-Marine Shales: A Case Study of the Wufeng–Longmaxi Formations, Weirong Shale Gas Field, Sichuan Basin

Characteristics and Governing Factors of Pore Structure and Methane Sorption in Deep-Marine Shales: A Case Study of the Wufeng–Longmaxi Formations, Weirong Shale Gas Field, Sichuan Basin

Anisotropic Evolution of Effective Stress and Pore Pressure during Coalbed Methane Drainage

Summary The anisotropy and dynamic variation in permeability of gas-adsorbing coals have a significant influence on fluid flow behavior in the cleat system. The assumption of a constant anisotropy coefficient (the ratio between permeability components in orthogonal directions) has been traditionally made to simplify the seepage-stress coupling analytical model. In this approach, the pressure drop of the coalbed is separated into desorption and nondesorption areas. To evaluate the effective stress, pore pressure, permeability distribution, and variable anisotropy coefficient more accurately, analytical formulas were developed that consider elastic mechanics and methane sorption. The results show that the anisotropy coefficient can be dynamic when cleat compressibility anisotropy exists. Pressure contours are a set of ellipses that increase in eccentricity from the near-wellbore area to the pressure drop boundary, leading to corresponding anisotropy changes in effective stress and permeability. The gas desorption-related matrix shrinkage effect causes a discontinuous pressure drop gradient at the boundary between desorption and nondesorption areas, resulting in nonsmooth pressure drop curves. The pressure gradient difference changes with the radius of the desorption area and is nonisotropic, with the high-permeability direction showing a greater difference than the low-permeability direction. These results indicate that the dynamic anisotropy coefficient has a significant impact on coalbed drainage and extraction. Compared to previous mathematical models, which assumed permeability isotropy or constant anisotropy coefficient in cleat systems, the proposed model provides a more accurate method to evaluate pressure and permeability distribution.

Investigation of methane mass transfer and sorption in Marcellus shale under variable net-stress

Natural gas in shale exists as free and adsorbed gas, subject to prevailing pore pressures and stress conditions. Accordingly, to accurately estimate/predict the shale gas recovery potential, a central requirement is to represent gas transport and sorption behavior under varying stress conditions. The objective of this work is to facilitate the interpretation of laboratory-scale experiments, at relevant conditions, in an attempt to bridge the gap in scales between laboratory- and field-scale observations.We have conducted a series of high-pressure experiments on a full-diameter core sample from the Marcellus shale. These include gas loading (pressure-decay) and depletion (production) experiments with pure methane (CH4) at variable stress conditions to characterize transport and sorption behavior under reservoir-relevant conditions. We have formulated and applied a novel integral model for mass transfer and storage in multi-porosity shale systems that allows us to effectively investigate transport and sorption phenomena: We delineate gas transport by interpreting helium (He) pressure-decay experiments and demonstrate how to use the information gained to calculate the relevant transport coefficients of CH4 and other gases. A separate measurement of the CH4 sorption isotherm on a smaller sample (a shale cube) was interpreted and combined with the transport description to predict the production behavior of CH4 from the experiments with the full-diameter core.Our experiments demonstrate that the representation of sorption hysteresis is crucial for predicting and guiding shale gas production: At the end of both gas production experiments, approximately 20% of the initial gas in place remained in the core. Without accounting for sorption hysteresis, our modeling demonstrates that the CH4 production could be overestimated by 10%. We demonstrate that our integral, triple-porosity model provides an effective approach for the interpretation and prediction of gas transport and sorption behavior during loading and production experiments on shale cores under variable net-stress conditions.In summary, our work combines measurements and modeling of mass transfer and sorption in shales at different scales to validate a characterization approach that facilitates an improved understanding of shale gas production. Furthermore, the triple-porosity model utilized in our work defines a potential pathway for the translation of laboratory-scale experimentation to larger-scale applications.

Modelling of methane sorption enhanced reforming for blue hydrogen production in an adiabatic fixed bed reactor: unravelling the role of the reactor's thermal behavior

Methane sorption enhanced reforming (SER) is investigated in this work as a promising route for blue H2 production. A 1-D dynamic heterogeneous model is developed to evaluate the thermal behavior of a fixed bed reactor under adiabatic conditions. The heterogeneous model allows to decouple the feed gas temperature from the initial solid one in order to investigate the behavior of the reforming step in a temperature swing reforming/regeneration process. The effects of the feed gas temperature, the initial bed temperature, and the bed thermal capacity are studied by evaluating the global impact of each parameter through a set of key performance indices (CH4 conversion, H2 yield and purity, carbon capture ratio) calculated as integrals over the duration of the reforming step. The results highlight the minor effect of the initial bed temperature on the process performances showing the potential of minimizing the extent of a cooling step between regeneration and reforming stages. Besides, due to the endothermic nature of the methane sorption enhanced reforming process at high temperatures, thermal energy must be provided to the SER process to achieve high CH4 conversion and high carbon capture ratio. This can be made either in the form of high feed temperature or by utilizing the energy stored in the bed benefiting from the bed thermal capacity.

Temperature Dependence of Light Hydrocarbons Sorption and Transport in Dense Membranes Based on Tetradecyl Substituted Silicone Rubber

Solubility-selective polymer membranes are promising materials for C3+ hydrocarbons removal from methane and other permanent gas streams. To this end, a dense solubility-selective membrane based on crosslinked poly(tetradecyl methyl siloxane) was synthesized. Sorption of methane, ethane, and n-butane in the polymer was measured in the temperature range of 5-35 °C. An abnormal temperature dependence of sorption was detected, contradicting the generally accepted view of sorption as an exothermic process. In particular, methane shows minimal sorption at 5 °C. The abnormal temperature behavior was found to be related to crystallization of the alkyl side chains at temperatures below ~10 °C. Gas permeability determined by sorption and permeation methods are in reasonable agreement with each other and decrease in the order n-C4H10 > C2H6 > CH4. The solubility of these alkanes changes in the same order indicating that poly(tetradecyl methyl siloxane) is indeed the sorption-selective membrane. The diffusivities and permeabilities of studied alkanes declined with decreasing temperature, whereas the n-C4H10/CH4 permselectivity increases with decreasing temperature, reaching a value of 23 at 5 °C.

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.

Elemental composition and mineralogy of NW Venezuelan Guasare coals: Provenance study and role of illite in methane sorption capacity

A series of coal seams (groups 4 to 11) of Paleocene age in the Marcelina Formation have been open pit exploited in two mines (Paso Diablo and Mina Norte) in the Guasare Coalfield (NW Venezuela). An investigation has been conducted on the elemental composition, mineralogy, and coalbed methane (CBM) in Guasare coals. For this study, 46 core coal samples of 16 seams (groups 2 to 7) were collected from 16 exploratory boreholes that were drilled in both mines. It was also considered unpublished proximate-ultimate and gross calorific data of other 66 coals of 31 seams intersected in most of these wells. The coals under study showed low ash yields (3.4 wt % on average) and were classified as high volatile bituminous A. Mean values of trace element and lanthanide (REE) contents in Marcelina coals generally indicate depletion compared to average values for world hard coals. The statistical evaluations indicate that elements (except Se) show inorganic affinity or intermediate association and mixed mode of occurrence. La/Sc, Th/Sc, La/Co, and Th/Cr ratios support a felsic or metamorphic felsic source for the mineral matter present in Marcelina coals. Most coal samples exhibited similar normalized REE patterns, showing light REE enriched, as well as negative Ce and Eu anomalies. The comparison of REE patterns to those of some nearby granitoids suggests that Marcelina coals received a supply of sediments from theses granitic source rocks. The mineralogical compositions of representative coal samples are notably similar, kaolinite and quartz were noted as the major mineral species in all coal seams. The measured gas volumes available for CBM production from Marcelina seams vary approximately between 5 and 11 cm3/g. Variation in the illite content has an appreciable positive effect on gas adsorption capacity of Guasare coals. Lastly, a numerical model of the gas generative potential of the coals in the Marcelina Formation was also developed.

Sorption of Deep Shale Gas on Minerals and Organic Matter from Molecular Simulation

Deep shale gas is one of the key targets of China’s natural gas exploitation in the future. Abnormally high reservoir pressures cause an unclear occurrence state of shale gas. In this paper, molecular modeling techniques are first used to establish slit-pore models of illite, quartz, and kerogen with different widths. Monte Carlo and molecular dynamics methods are second applied to simulate the sorption of methane under the geological conditions of deep reservoirs. Then, the gas distributions in the pores and changes of energies in sorption are analyzed. The applicability of the Langmuir model is also verified. The results show that (1) shale gas is approximately adsorbed as strong monolayers in a pore; the density of the adsorbed gas in mesopores is not sensitive to the pore size; (2) the Langmuir equation is applicable for the sorption of shale gas over a wide range of pressures; the surface excesses and maximum adsorbed amounts for the mesopores follow the order of illite > quartz > kerogen, which results from the different actual surface areas depending on the roughness and holes on the surfaces; (3) in a mesopore, total gas energy reduces more in adsorption under higher pressure; van der Waals force dominates gas adsorption, while electrostatic force affects weakly; under high pressure, the total gas energy for kerogen matrix decreases less when pressure increases, which is related to the extremely dense absorbed gas and nonmonotonic variation of van der Waals force with the distance between molecules; the interactions between shale gas and different types of pores follow the order of kerogen matrix > kerogen mesopore > illite > quartz. This study clarifies the sorbing behaviors of deep shale gas as well as the applicability of the Langmuir model, which can be a reference for developing the deep shale gas.

Development of a High-Accuracy Statistical Model to Identify the Key Parameter for Methane Adsorption in Metal-Organic Frameworks

The geometrical and topological features of metal-organic frameworks (MOFs) play an important role in determining their ability to capture and store methane (CH4). Methane is a greenhouse gas that has been shown to be more dangerous in terms of contributing to global warming than carbon dioxide (CO2), especially in the first 20 years of its release into the atmosphere. Its accelerated emission increases the rate of global temperature increase and needs to be addressed immediately. Adsorption processes have been shown to be effective and efficient in mitigating methane emissions from the atmosphere by providing an enormous surface area for methane storage. Among all the adsorbents, MOFs were shown to be the best adsorbents for methane adsorption due to their higher favorable steric interactions, the presence of binding sites such as open metal sites, and hydrophobic pockets. These features may not necessarily be present in carbonaceous materials and zeolites. Although many studies have suggested that the main reason for the increased storage efficiencies in terms of methane in the MOFs is the high surface area, there was some evidence in certain research works that methane storage performance, as measured by uptakes and deliveries in gravimetric and volumetric units, was higher for certain MOFs with a lower surface area. This prompted us to find out the most significant property of the MOF, whether it be material-based or pore-based, that has the maximum influence on methane uptake and delivery, using a comprehensive statistical approach that has not previously been employed in the methane storage literature. The approach in our study employed various chemometric techniques, including simple and multiple linear regression (SLR and MLR), combined with different types of multicollinearity diagnostics, partial correlations, standardized coefficients, and changes in regression coefficient estimates and their standard errors, applied to both the SLR and MLR models. The main advantages of this statistical approach are that it is quicker, provides a deeper insight into experimental data, and highlights a single, most important, parameter for MOF design and tuning that can predict and maximize the output storage and capture performance. The significance of our approach is that it was modeled purely based on experimental data, which will capture the real system, as opposed to the molecular simulations employed previously in the literature. Our model included data from ~80 MOFs and eight properties related to the material, pore, and thermodynamics (isosteric adsorption energy). Successful attempts to model the methane sorption process have previously been conducted using thermodynamic approaches and by developing adsorption performance indicators, but these are either too complex or time-consuming and their data covers fewer than 10 MOFs and a maximum of three MOF properties. By comparing the statistical metrics between the models, the most important and statistically significant property of the MOF was determined, which will be crucial when designing MOFs for use in storing and delivering methane.

Molecular insights into supercritical methane sorption and self-diffusion in monospecific and composite nanopores of deep shale

• Montmorillonite, kerogen and MMT-kerogen composite slit nanopores with desirable pore sizes are constructed; • GCMC and EMD simulations are conducted to study the sorption and self-diffusion behaviors of methane in various nanopores; • Pore surface roughness can impede the diffusion of gases, especially in smaller pores. Up to now, a large number of studies have utilized various representative materials to approximate the replacement of shale and investigate the gas adsorption and diffusion behavior in various shapes of nanopores by means of molecular simulations. However, the study of methane sorption and self-diffusion behavior under high temperature and pressure reservoir conditions in deep shale is not clear. In this study, we first established montmorillonite (MMT), kerogen and MMT-kerogen composite slit nanopores using representative shale components of MMT and kerogen. On this basis, we investigated the methane sorption and self-diffusion characteristics employing the grand canonical Monte Carlo (GCMC) and equilibrium molecular dynamics (EMD) simulations. The results show that the differences in the molecular composition and structure of MMT and kerogen surfaces lead to stronger adsorption performance of MMT surface than kerogen in a single nanopore for the same conditions, which results in the inhomogeneity of spatial distribution of methane molecules in the pores. While the sorption capacity of the kerogen matrix is more potent than MMT due to its larger specific surface area (SSA). The smaller pores have stronger adsorption capacity compared to the larger pores. At the same time, the methane density profiles indicate that the adsorption of the MMT and kerogen in smaller pores is less differentiated. The methane sorption energy distribution and isosteric heat of sorption also mutually corroborate the more stable methane sorption at a higher pressure and smaller pores. The self-diffusion coefficient of methane in nanopores gradually decreases with increasing pressure, with a more pronounced variation at lower pressure. Pore surface roughness will hinder the diffusion of gas, and the closer the distance is, the more noticeable the effect will be. This study sheds light on the sorption and self-diffusion phenomena and difference of gas in monospecific and composite nanopores, which provides insights into the reserve evaluation and development of the deep shale gas reservoirs, and is further expected to furnish ideas for exploring the occurrence and transport characteristics of supercritical fluids on other composite nanomaterials.

Methane sorption behavior on tectonic coal under the influence of moisture

Water is widely distributed in coal under the in-situ reservoir conditions, which has a great impact on gas adsorption and migration within coal. In this work, methane sorption behavior on tectonic coal under the influence of moisture was quantitatively evaluated using the combination of high-pressure volumetric method, X-ray photoelectron spectrometer (XPS) and mercury intrusion porosimetry (MIP). Comparative analysis of the differences in microstructures and methane adsorption capacity was also made between primary coal and tectonic coal. The results show that a large number of micropores are contained in tectonic coal, leading to the increasing complexity of pore structure. Primary coal has less pores but the largest number of oxygen-containing functional groups. Methane sorption properties are affected by moisture content in varying degrees for primary coal and tectonic coal. Combined with XPS and MIP results, the influencing mechanism of water content on methane adsorption was revealed. CH4 adsorption in primary coal is more sensitive to water, and only 2% of moisture content leads to the 67% of reduction in the CH4 adsorption amount, which is attributed to the stronger hydrophilia and less pore space. Owing to the developed pore networks, the increase of water content only results in a slight reduction of adsorption capacity in mylonitic coal when moisture content less than 2.5%. The further increase of moisture content results in the pore-blocking effect, inhibiting the access of other gas adsorbing molecules to the pores, hence, methane adsorption capacity is greatly reduced in mylonitic coal. This work is of great significance for better understanding the adsorption/desorption processes of tectonic coal.