Adsorbed Phase Density Research Articles

Density and volume of adsorbed methane in key applications for in-situ gas content: Insights from molecular simulation

The evaluation of the in-situ gas content in deep coal seams, potentially containing free gas in saturated-oversaturated reservoirs, holds practical significance for exploration and production compared with shallow undersaturated-saturated reservoirs dominated by adsorbed gas. However, owing to the inherent limitations of volumetric and gravimetric methods, the accurate assessment of adsorbed and free gases faces significant challenges. In this study, based on molecular simulations, we focused on slit-shaped pores to explore the microscopic interaction mechanisms between coal and methane molecules under different pore sizes, temperatures, and pressures. By combining isothermal adsorption and pore structure experiments, the distribution characteristics of the adsorbed phase density (APD) and adsorbed phase volume (APV) were determined. The results showed that, in pores smaller than 1.5 nm, methane exhibited an aggregated state distribution (containing only adsorbed methane) characterized by micropore filling adsorption. In pores larger than 1.5 nm, methane showed a dispersed state distribution (coexistence of adsorbed and free methane) characterized by monolayer adsorption. The APD increased with pressure and decreased with temperature and pore size, whereas the APV remained independent of the pressure and temperature. In the in-situ gas content evaluation, three adsorption models (SL, SDR, and SL + SDR models) considering APV and APD were utilized to describe the methane adsorption behavior. The SDR model considering the APV was consistent with the actual physical properties and molecular simulation results. Traditional correction methods often lead to an overestimation of the adsorbed methane content, and neglecting the APV can result in an overestimation of the free methane content. In summary, the SDR model considering the APV can provide more rigorous scientific support for the refined study of methane occurrence states and gas content distribution.

Adsorption mechanism study of sour natural gas and sulfur in high-sulfur carbonate rock gas reservoirs

As the world pivots towards green and renewable energy sources, natural gas stands out as a clean and efficient alternative. In particular, there has been an upsurge in the exploration and development of gas fields rich in hydrogen sulfide (H2S). However, an escalating issue of sulfur deposition is notably pronounced in carbonate rock reservoirs predominantly comprised of dolomite, as exemplified in China's Yuan Ba gas field. This study examines sulfur adsorption in high-sulfur gas reservoirs and its effects on carbonate rock formations. Molecular simulation analysis reveals that the sour natural gas density in 20 Å dolomite nanopore structure experiences an average increase of 30% at the nanoscale. Notably, CO2 and H2S manifested strong adsorption tendencies in smaller pore sizes, with CO2's strong adsorption state layer thickness measuring approximately 2.93 Å, and H2S's at about 1.871 Å. Additionally, it was found that the adsorption energy of sour natural gas decreases with rising temperature and pore size, and falling pressure, yet the temperature's effect on excess adsorption density is minor above 40 MPa. As the pore size reaches 160 Å, the strong adsorption states of CO2 and H2S dissipate. Beyond 160 Å, the influence of nanoscale dimensions is negligible, with only a 2.23% increase in adsorbed phase density compared to the bulk phase. The introduction of S8 molecules amplified the adsorption proclivities of CO2 and H2S, urging them closer to the pore wall. The study reveals that CO2 or H2S reinjection into formations could effectively mitigate sulfur deposition in pores smaller than 160 Å, offering valuable insights for the development of high-sulfur gas fields.

Thermodynamic trends for evaluating hydrogen storage density and energy flow on Maxsorb-III, HKUST-1 and UiO-66 (Zr) MOFs

For CO2-free power generation in power plants or vehicles, hydrogen should be implemented as H2 possesses not only the highest energy density (120 MJ/kg) but also can be generated from the environment and reactions. However, H2 is difficult to store due to its low density at ambient conditions. To understand the efficacy of hydrogen interactions in porous materials including the randomness of hydrogen adsorption in terms of storage density, energy flow and dissipation, a thermodynamic study is required. This paper at first focuses on the study of H2 adsorption on various porous adsorbents such as activated carbons (AC types Maxsorb-III) and metal–organic frameworks (types UiO-66 (Zr) and HKUST-1 MOFs) and the results are presented in temperature-density co-ordinate system. Employing experimentally confirmed isotherms data of AC/MOFs + H2 systems, the density of H2 adsorbed phase, entropy flow and generation are evaluated. These results indicate the feasibility of hydrogen adsorption on functional adsorbents at various temperature and pressure. The simulation results show that the adsorbed phase densities of H2 on Maxsorb-III and HKUST-1 increase up to 18 kg/m3 and 20 kg/m3 at 5 bar and 77 K. The temperature-entropy diagram shows a close loop for the charging and discharging of hydrogen and ensures the storage of internal energy with less entropy dissipation under cryo-adsorption conditions.

High-Pressure adsorption of supercritical methane and carbon dioxide on Coal: Analysis of adsorbed phase density

The occurrence of supercritical CH4 and CO2 in coal is significant for both coalbed methane recovery and CO2 storage. Adsorption isotherms of CH4 and CO2 on two coals have been measured at three temperatures up to pressures of 25 MPa. Three modified supercritical adsorption models (SL-V, SBET-V, and SDR-V) were used to describe the excess adsorption behavior, with the pressure-independent adsorbed phase volume being considered. The adsorbed phase density (APD) and absolute adsorption amount were calculated based on the modified models and compared with those estimated by previous models. The results indicate that SDR-V is the most suitable for CH4 adsorption due to the dominant role of filling of micropores by CH4, while SBET-V is the most appropriate model for CO2 adsorption because of multilayer adsorption of CO2 in both micropores and meso-macropores. The APD increases with pressure until reaching a maximum value but decreases as temperature increases following a Langmuir-style trend. The previous models, which use a fixed APD, seriously underestimate the absolute adsorption, the extent of which increases with increasing pressure and temperature, leading to significant errors in deep formations with high temperature and pressure conditions. Furthermore, as burial depth increases, the adsorption process can be divided into three stages based on changes in APD and free phase density (FPD). These findings contribute to a comprehensive understanding of the behavior and occurrence of CH4 and CO2 within deep coal seams, enhancing the applicability of CO2-ECBM (CO2 Geological Storage Enhanced Coalbed Methane Recovery).

Modeling carbon dioxide and methane adsorption on illite and calcite: enhancing the simplified local density model through crystal structure modifications.

The simplified local density (SLD) model has been frequently utilized to describe gas adsorption in porous media. The common assumptions associated with the SLD model are the graphene slit and uniform distribution of atoms. However, these fail to depict the heterogeneous surface of minerals. In this study, the original SLD model was modified by substituting mineral crystal structures for the homogeneous carbon layer when building the slit. The modified model may capture the heterogeneity-induced variation of the fluid-pore interaction potential and adsorbed phase density near the mineral surface. The calculated adsorption isotherms of methane and carbon dioxide on illite and calcite surfaces at 330.15 K were compared with literature experiment data to validate the modified SLD model. For the simulation of gas adsorption isotherms, the modified model predictions agree reasonably well with the previously reported experiment results using the gravimetric method. The calculated density profile in the slit indicates the monolayer adsorption behavior of CH4 and CO2. Based on more precise interaction potentials, the number of regression parameters was reduced to two, and the physical meaning of the model parameters was clarified. Therefore, for estimating hydrocarbon storage in reservoirs, the modified SLD model could be used as an efficient alternative to time-consuming molecular simulation methods.

Adsorption equilibrium of methane on activated carbon and typical metal organic frameworks

To develop adsorbents suitable for the storage of natural gas by adsorption, activated carbon SAC-02, HKUST-1 and MIL-101(Cr) were synthesized and characterized in terms of structural morphology observation, nitrogen physisorption at 77.15 K, and methane adsorption at 293.15–313.15 K and 0–4 MPa. The methane adsorption isotherms were comparatively correlated with the Toth, D-A and Ono-Kondo equations and the performances of the adsorbent samples were evaluated in terms of the isosteric adsorption heat and the adsorbed phase density. The results indicate that, in comparison with the D-A and Ono-Kondo equations, the Toth equation displays much smaller relative errors in correlating the methane adsorption data and is more suitable for the adsorption equilibrium analysis on the adsorbed natural gas (ANG) system. MIL-101(Cr) exhibits the largest mean isosteric heat for methane adsorption and the density of the adsorbed phase of methane is smaller than that of the liquid methane but increases with the equilibrium pressure; overall, MIL-101(Cr) with the highest adsorption capacity is more suitable for methane adsorption than activated carbon and HKUST-1.

Xenon storage density and its energy flow through adsorption on metal–organic frameworks

The global demand for Xe storage and separation increases significantly over the coming decades for various applications ranging from ion propulsion to nuclear power plants. Due to very low amount of Xe in the earth's atmosphere and the complexity of cryogenic storage facilities, the storage and recovery of Xe in porous adsorbents such as MOFs (metal–organic frameworks) are an important research area. The research activities on adsorption-assisted Xe storage include (i) the synthesis of adsorbents with greater micropore distribution and large micropore volume and (ii) the Xe storage capability in the form of isotherms and kinetics. But the thermodynamic properties such as enthalpy, internal energy, entropy and density can judicially be applied to simulate the energetic and storage performances of Xe gas in structural porous adsorbents. Therefore, this manuscript presents the thermodynamic formulations of entropy flow, generation and adsorbed phase density of MOFs + Xe systems. Secondly, these parameters are evaluated for Xe adsorption on MIL-100 (Fe), MIL-101(Cr), UiO-66(Zr) and FMOF(Zn) MOFs at temperatures changing from 233 K to 303 K and pressures up to 100 kPa. A deeper understanding of Xe storage capacity on MOFs is presented by temperature–density maps. Furthermore, the entropy and temperature maps are plotted to identify the internal energy storage capacity of Xe in MOFs storage cells under charging and discharging conditions of Xe.

Adsorption of methane onto mudstones under supercritical conditions: Mechanisms, physical properties and thermodynamic parameters

Since the mechanisms of methane-mudstone interactions are important for estimating shale gas reserves, methane adsorption under supercritical conditions of 30 MPa pressure and 303.15, 333.15, 363.15 K temperatures was studied to measure the excess methane adsorption in two mudstone samples from Yanchang Formation, Ordos Basin. Excess adsorption features inflection points where the amount of adsorbed gas changes from increasing to decreasing concentrations. Three methods (fixed, slope, and freely fitted density) were applied to calculate the adsorbed-phase density ( ρ ad ), which was then used to fit the measured excess adsorption. Two criteria, the goodness-of-fit and whether the fitting can obtain reasonable absolute adsorption, were applied to determine the most accurate model. Results indicated that the supercritical Dubinin-Radushkevich ( SDR ) model with freely fitted ρ ad was the most reasonable model. The volume of adsorbed methane at 363.15 K is close to the micropore ( d < 2 nm) volume of the corresponding mudstone. Considering the actual geological conditions, the adsorbed gas should be predominantly stored in micropores. Thermodynamic parameters reveal that the methane adsorption on mudstone is a physisorption process that is jointly controlled by the heterogeneity of, and interaction forces between the methane molecule and, the rock surface.

Examining Supercritical Methane Adsorption on Nanoporous Shales Using Constant and Varying Adsorbed Phase Density Approaches

Supercritical adsorption studies are crucial, as adsorbed methane has a significant contribution to the total gas-in-place along with the free gas-in-shale reservoirs. Due to the low specific adsorption of shales, Gibbs excess isotherms that decrease after a certain pressure are being reported. Corrections to the excess adsorption amount can be made using models that predict absolute adsorption through empirically estimated adsorbed phase density. However, due to the inherent heterogeneity of shale and uncertainties in the adsorbed phase density estimated from different models, the quantitative characterization of adsorption is becoming increasingly challenging. The assumption of constant adsorbed phase density is being challenged with varying adsorbed phase density approaches, which are theoretically correct; however, their applicability has not been analyzed distinctively. In this work, supercritical adsorption models like supercritical BET (SBET), supercritical Dubinin–Radushkevich (SDR), Ono–Kondo mono- and multilayer, fixed volume Ono–Kondo (FV-OK1), and variable density Langmuir (VD-Langmuir) are revisited, analyzed, and compared for their applicability in shale reservoirs. For this, methane adsorption isotherms at two different temperatures (45 and 90 °C) are generated for four samples with varying geochemical and petrophysical properties selected from the Cambay and Krishna–Godavari (KG) basins of India. Through this analysis, it has been found that fixed-density approaches better explain the isotherms in comparison to varying density approaches. The average adsorbed phase density estimated using SBET and SDR models was closer to 0.2 g/cm3, which is very low in comparison to the theoretical limit. The SBET and Ono–Kondo theories have displayed that the probability of adsorption after the third layer is low. The VD-Langmuir and FV-OK1 could fit perfectly to highly specific adsorbing solids; however, they failed to explain adsorption in low-adsorbing solids. The mathematical drawbacks of variable density approaches have not been analyzed previously and are explained in this work.

Insight into difference in high-pressure adsorption-desorption of CO2 and CH4 from low permeability coal seam of Huainan-Huaibei coalfield, China

This study aimed to investigate the differential characteristics of high-pressure adsorption-desorption of supercritical CO2 and CH4 at main temperatures of 36 ℃, 42 ℃, and 48 ℃ and the pressure of 0–20 MPa in low permeability coal, a case from the tectonic coal area of Huainan-Huaibei coalfield, eastern China. Combining the variation characteristics of adsorption capacity with pressure, gas free phase density, the prediction of CO2 adsorbed phase density, the correction method of gas adsorption capacity, the sequestration capacity of CO2 in low permeability coal, and the adsorption hysteresis index of adsorbed gas were assessed. The controlling factors of gas adsorption capacity were revealed based on the general discussion of the relation between the gas adsorption capacity and the conditions such as temperature, pressure, coal maceral, coal quality, and pore structure. The results show that the lowest equilibrium pressure associated with the CH4 excess adsorption peak is 9 MPa, which is larger than most of the experimental pressures for the CH4 isothermal adsorption test. CO2 excess adsorption capacity of the different coal samples has characteristic of an obvious peak at the experimental pressure near 6 MPa under different experimental temperatures, all CO2 excess adsorption capacity changes with CO2 free phase density in a two-step pattern, CH4 or CO2 adsorbed phase density can be predicted by the truncation method. Desorption of CH4 or CO2 in coal has an obvious "hysteresis loop", the cumulative adsorption hysteresis index (CAHI) and the incremental adsorption hysteresis index (IAHI) of CH4 based on the calculation of CH4 excess adsorption capacity are controlled by the temperature and pressure, it has the obvious rise during the stage of low pressure (less than 4 MPa), whether IAHI or CAHI of CO2, it shows characteristics of complex changes. Variations in CAHI and IAHI for CH4 or CO2 are governed by the development of pore structure in coal, and the temperature. Coal maceral, coal quality, and coal grade have an effect on gas adsorption in coal by affecting the growth, filling, and deformation of pores in coal; the adsorption capacity of the gas depends both on the surface area of the pores and the pore volume of the pores in the coal. The differential mechanism for the gas adsorption capacity from the low permeability coal requires further understanding.

Modified Flowing Material Balance Equation for Shale Gas Reservoirs

To determine original gas-in-place, this study establishes a flowing material balance equation based on the improved material balance equation for shale gas reservoirs. The method considers the free gas in the matrix and fracture, the dissolved gas in kerogen, and the pore volume occupied by adsorbed phase simultaneously, overcoming the problem of incomplete consideration in the earlier models. It also integrates the material balance method with the flowing material balance method to obtain the average formation pressure, eliminating the problem with the previous method where shutting down of wells was needed to monitor the formation pressure. The volume of the adsorbed gas on the ground is converted into volume of the adsorbed phase in the formation using the volume conservation method to characterize the pore volume occupied by the adsorbed phase, which solves the problem of the previous model that the adsorbed phase was neglected in the pore volume. The model proposed in this study is applied to the Fuling Shale Gas Field in southwest China and compared with other flowing material balance equations, and the results show that the single-well control area calculated by the model proposed in this study is closer to the real value, indicating that the calculations in this study are more accurate. Furthermore, the calculations show that the dissolved gas takes up a large fraction of the total reserves and cannot be ignored. The sensitivity analyses of critical parameters demonstrate that (a) the greater the porosity of the fracture, the greater the free gas storage; (b) the values of Langmuir volume and TOC can significantly affect the results of the reservoir calculation; and (c) the adsorbed phase occupies a smaller pore volume when the Langmuir volume is smaller, the Langmuir pressure is higher, or the adsorbed phase density is higher. The findings of this study can provide better understanding of the necessity to take into account the dissolved gas in the kerogen, the pore volume occupied by the adsorbed phase, and the fracture porosity when evaluating reserves. The method could be applied to the calculation of pressure, recovery of free gas phase and adsorbed phase, original gas-in-place, and production predictions, which could help for better guidance of reserve potential estimations and development strategies of shale gas reservoirs.

Molecular simulation of methane adsorption in nanoscale rough slits

There are large amounts of nanoscale rough pores in shale, which provides the sites for methane adsorption. Proper understanding of the mechanisms of methane adsorption in nanoscale rough slits is important for estimating the shale gas reserves. In this study, graphene was used instead of shale to construct nanoscale slits with different widths and roughness, and the effects of roughness, slit width and pressure on the methane adsorption characteristics are investigated by using the GCMC and molecular dynamics methods. The results showed that the methane adsorption configurations are influenced by the slit width and surface roughness. The adsorption layers in rough slits are discontinuous and weak second adsorption layers are formed in large slits at high pressures. The inaccessible volume increases with the roughness resulting in the reduction of accessible volume. Therefore, both the excess adsorption and absolute adsorption decrease with the roughness, and they increase with the slit width in the microscopic scale due to the increased adsorption. While decrease slightly with the slit width in the mesoscopic scale due to the reduction of adsorbed phase density. The excess adsorption isotherms are well fitted by the Langmuir-Freundlich model. The adsorption is found more stable in smaller and rougher slits due to the superposition of adsorption potentials through the analysis of adsorbate potential energy and adsorption heat.

Reconstructions of supercritical CO2 adsorption isotherms and absolute adsorption estimation in nanoporous coals considering volumetric effects and varying adsorbed phase densities

• Adsorbed phase density and fugacity facilitate identifying the adsorption mechanisms. • SLD-PR model is efficient in obtaining adsorbed phase density at various pore sizes. • Excess adsorption isotherms are effectively rebuilt considering volumetric effects. • Absolute adsorption amounts are calculated accurately and reasonably considering PSD. • Empirical adsorbed densities for engineering practice are proposed through iteration. Understanding supercritical CO 2 (ScCO 2 ) adsorption behavior in nanoporous coals is crucial for long-term CO 2 geological storage. Four coal samples with a broad spectrum in rank were selected to obtain CO 2 excess adsorption isotherms at 35 ℃ and pressure up to 14 MPa using the volumetric method. As conversion from excess adsorption to absolute adsorption requires accurate capture of adsorbed phase density (APD), the simplified local density (SLD) theory and Peng-Robinson (PR) equation of state (EOS) were tailored to calculate the APD considering both pore size distribution (PSD) and pressure. Besides, volumetric effects, including void volume overestimation and coal matrix shrinkage/swelling, were quantificationally calibrated to reconstruct excess adsorption isotherms. Then, we proposed a novel conversion method based on real adsorption space distribution and adsorption mechanism. Results show that micropores in the range of 0.30–1.50 nm dominate the total pore space. APD and fugacity profiles reveal distinct CO 2 adsorption behaviors in micropores and mesopores. Fitted expansion coefficients indicate that coal matrix undergoes a superimposed effect of shrinkage and swelling during adsorption and the reconstructed isotherms could amend the large deviations around 8 MPa caused by the drastic changes of CO 2 bulk density and compressibility factor. Compared with conventional fixed APDs, the converted absolute adsorption isotherms generated by the varied ones exhibit slight fluctuations in the supercritical state and could plateau at high pressures. Continual iterations suggest that “averaged” APD in the range of 33–36 mol/L could be utilized for conversion in engineering practice only when the original excess adsorption isotherms are correctly reconstructed.

Methane Storage Capacity of Permian Shales with Type III Kerogen in the Lower Yangtze Area, Eastern China

Marine–terrestrial transitional Permian shales occur throughout South China and have suitable geological and geochemical conditions for shale gas accumulation. However, the Permian shales have not made commercial exploitation, which causes uncertainly for future exploration. In this study, high-pressure methane (CH4) adsorption experiments were carried out on the Permian shales in the Lower Yangtze area, and the influences of total organic carbon (TOC) content and temperature on adsorption parameters were investigated. The characteristics and main controlling factors of methane storage capacity (MSC) of the Permian shales are discussed. The results show that the maximum adsorption and the adsorbed phase density of these Permian samples are positively correlated with TOC contents but negatively correlated with temperatures. The pores of organic matter in shale, especially a large number of micropores and mesopores, can provide important sites for methane storage. Due to underdeveloped pore structure and poor connectivity, the methane adsorption capacities of the Permian shales are significantly lower than those of marine shales. Compared with the Longmaxi shales, the lower porosity and lower methane adsorption of the Permian shales are reasonable explanations for their lower gas-in-place (GIP) contents. It is not suitable to apply the index system of marine shales to the evaluation of marine–terrestrial transitional shales. The further exploration of Permian shales in the study area should be extended to overpressure stable reservoirs with high TOC contents (e.g., &gt;5%), high porosity (e.g., &gt;3%), and deep burial (e.g., &gt;2000 m).

Molecular Structure of Adsorbed Water Phases in Silica Nanopores

The adsorption of water vapor in silica nanopores with different pore morphologies and surface hydrophilicities was studied to quantify the densities and thicknesses of the water sorption layers and deduce their molecular structures. Water adsorption to surface hydroxyls is described by a multilayer sorption model. At low pressure, the water adsorption isotherms are largely independent of pore size and the adsorbed amounts scale with the surface hydroxyl density. Adsorbed-phase densities corresponding to the adsorption of two water molecules per surface hydroxyl group are found in the first adsorbed water layer for a wide range of surface hydroxyl densities. The densities and layer thickness values found in narrow pores indicate that patchy adsorbed layers form if not enough water molecules exist for a full layer, which coexist with dry pore surface regions. This behavior indicates cooperative adsorption effects, i.e., a preference for the formation of hydrogen bonds between water molecules bound to surface hydroxyls. In narrow pores, pore condensation limits further growth of the sorption layer, and in larger pores and at the planar quartz surface, a second adsorbed water layer is formed, which can hold up to approximately 4 additional water molecules per surface hydroxyl group. The water molecules in these thicker adsorption layers arrange such that the sorption layer density is similar to the bulk water density. Pore confinement limitations on the sorption layers are observed in pores with radii of as large as 8 nm. Molecular dynamics modeling reveals two preferential orientations for water molecules adsorbing to surface hydroxyl groups and suggests an intralayer structuring in the adsorbed monolayer. Adsorbed water molecules in the sorption layer are bonded to the surface hydroxyl group via the donation and acceptance of hydrogen bonds.

Absolute adsorption and adsorbed volume modeling for supercritical methane adsorption on shale

Adsorbed methane significantly affects shale gas reservoir estimates and shale gas transport in shale formations. Hence, a practical model for accurately representing methane adsorption behavior at high-pressure and high-temperature in shale is imperative. In this study, a reliable mathematical framework that estimates the absolute adsorption directly from low-pressure excess adsorption data is applied to describe the excess methane adsorption data in literature. This method provides detailed information on the volume and density of adsorbed methane. The obtained results indicate that the extensively used supercritical Dubinin-Radushkevich model with constant adsorbed phase density underestimates absolute adsorption at high pressure. The adsorbed methane volume increases both the pressure and expands with the temperature. The adsorbed methane density reduces above 10 MPa, and approaches a steady value at high pressure. This study provides a novel method for estimating adsorbed shale gas, which is expected improve the prediction of shale gas in place and gas production.

Improved Methane Adsorption Model in Shale by Considering Variable Adsorbed Phase Density

Numerous models have been used to describe the isotherm adsorption of supercritical methane in porous media. Many models assume that the adsorbed phase density does not change with pressure during ...

Measurement of gas contents in shale reservoirs – impact of gas density and implications for gas resource estimates

Gas shale reservoirs pose unique measurement challenges due to their ultra-low petrophysical properties and complicated pore structures. A small variation in an experimental parameter, under high-pressure conditions, may result in huge discrepancies in gas contents and the resource estimates derived from such data. This study illustrates the impact of the equation of state on the gas content determined for a shale sample. The gas content was determined from laboratory-measured high-pressure methane adsorption isotherms and theoretically described by a hybrid type model. The modelling involved the use of the Dubinin–Radushkevich isotherm to obtain the adsorbed phase density followed by the Langmuir isotherm to describe the resultant absolute adsorptions. Significant variations were observed in measured adsorption isotherms due to the variations in gas densities calculated from different equations of states. The model parameters and the gas in-place volumes estimated from those parameters also varied significantly.

Experimental investigation and mathematical modelling of shale gas adsorption and desorption hysteresis

To quantify shale gas contents and hysteresis behaviour of the Ordovician Goldwyer shales as functions of temperature and rock properties, methane adsorption and desorption isotherms were measured at four temperatures. Early pore saturation was observed for all eight samples tested and consequently, the measured sorption isotherms could not be directly modelled by Langmuir isotherm or its modified version. Thus, a hybrid-modelling approach comprising the use of Dubinin–Radushkevich (D-R) model to obtain the adsorbed phase density (ρads) needed to convert excess amounts to absolute followed by Langmuir model to describe the resultant absolute isotherms. Significant hysteresis observed resulted in lower Langmuir parameters for the desorption isotherms compared to the adsorption isotherms. For both processes, Langmuir volumes positively correlated with TOC contents but showed no significant correlations with total clay contents. To capture the effect of clay contents, the ratio of total clay to TOC (subsequently referred to as clay-to-organic-carbon, COC) was applied as a single predictor instead. The results showed that Langmuir volumes, and the size of the hysteresis showed significant correlations with TOC contents and COC. Lastly, analytical models were developed for the temperature-dependence of Langmuir volume as functions of each of TOC contents and COC for both adsorption and for the first time, desorption isotherms. The models showed significant match with experimental results and as such, can be adopted for conditions beyond those investigated in this study.

Effect of adsorbed phase density on the correction of methane excess adsorption to absolute adsorption in shale

In order to evaluate the accuracy of different methods for the methane excess adsorption correction, the Polanyi adsorption potential theory was introduced to analyze the absolute methane adsorption amount obtained by correction. One shale sample from the Lower Silurian Longmaxi Formation in the southern Sichuan Basin, China was selected to conduct high-pressure methane adsorption experiments at 40.6 °C, 60.6 °C, 75.6 °C, and 95.6 °C at pressures up to 52 MPa. Based on the experimental results at different temperatures, four densities (the liquid density of methane at boiling temperature and ambient pressure, the van der Waals density, the adsorbed phase density based on the falling segment fitting of excess adsorption isotherm, and the adsorbed phase density obtained by fitting excess adsorption isotherm using modified Langmuir equation) were used for the correction of excess adsorption to absolute adsorption. At the same time, in order to enhance the reliability and accuracy of the conclusions in this study, the results of methane adsorption experiments on shale in the reference were used for a comparative study to further support our conclusions. The results indicate that the greater the constant adsorbed phase density used for the excess adsorption correction, the lower the degree of overlap of the adsorption characteristic curves at different temperatures. The adsorbed phase density used for methane excess adsorption correction may be in the vicinity of 0.30 g/cm3 to 0.40 g/cm3. Particularly, it should not be greater than the liquid density of methane at boiling temperature and ambient pressure, which is 0.421 g/cm3. Although the adsorbed phase density differs across different temperatures and pressures, the current results show that 0.373 g/cm3 can be used as the adsorbed phase density for the excess adsorption correction, which may be a convenient method and can yield results close to the actual adsorption.