Methane Adsorption Capacity Research Articles

Modifying HKUST-1 Crystals for Selective Ethane Adsorption Using Ionic Liquids as Synthesis Media

Novel adsorbents for methane and ethane based on HKUST-1 metal-organic framework were synthesized by microwave (MW) assisted technique using ionic liquids (ILs) as synthesis media. It was found that the MW synthesis time remarkably impacts both the product yield and the physico-chemical characteristics of the produced HKUST-1 material. The crystalline phase purity, crystallite size/dispersion and textural properties of the synthesized HKUST-1 matrices determine their performance in methane and ethane adsorption. Therefore, the HKUST-1 material produced in MW fields for 3 min only shows the highest phase purity and the largest surface area (BET) and porosity, along with a rather small crystallite size (below ~300 nm), demonstrating high methane and ethane adsorption capacity in the pressure range 1–30 atm.

An NMR-Based Method for Multiphase Methane Characterization in Coals

Discriminating multiphase methane (adsorbed and free phases) in coals is crucial for evaluating the optimal gas recovery strategies of coalbed methane (CBM) reservoirs. However, the existing volumetric-based adsorption isotherm method only provides the final methane adsorption result, limiting real-time dynamic characterization of multiphase methane in the methane adsorption process. In this study, via self-designed nuclear magnetic resonance (NMR) isotherm adsorption experiments, we present a new method to evaluate the dynamic multiphase methane changes in coals. The results indicate that the T2 distributions of methane in coals involve three different peaks, labeled as P1 (T2 < 8 ms), P2 (T2 = 20–300 ms), and P3 (T2 > 300 ms) peaks, corresponding to the adsorbed phase methane, free phase methane between particles, and free phase methane in the sample cell, respectively. The methane adsorption Langmuir volumes calculated from the conventional volumetric-based method qualitatively agree with those obtained from the NMR method, within an allowable limit of approximately ~6.0%. Real-time dynamic characterizations of adsorbed methane show two different adsorption rates: an initial rapid adsorption of methane followed by a long stable state. It can be concluded that the NMR technique can be applied not only for methane adsorption capacity determination, but also for dynamic monitoring of multiphase methane in different experimental situations, such as methane adsorption/desorption and CO2-enhanced CBM.

Sensitivity analysis on the microwave irradiation enhancing coal seam gas recovery: A coupled electromagnetic-thermo-hydro-mechanical model

Microwave irradiation (MI) technology is a potential effective method to improve the efficiency of coal seam gas recovery. However, the research on how to increase coal seam gas production by microwave heating is rare. In this paper, microwave stimulation technology is introduced in coal seam gas extraction area for the first time. Based on the gas extraction technology with directional long borehole, a new coupled electromagnetic-thermo-hydro-mechanical model for coal seam gas recovery is established. The heat and gas transfer characteristics of coal seam under MI heating is analyzed. What's more, the impacts of key parameters on gas production, temperature distribution, gas pressure distribution and permeability ratio are discussed. Under the MI power 1000 W, numerical simulation results show that the cumulative gas production increased by 80%, which indicating the great effective of microwave heating for improving gas extraction rate. Furthermore, it reveals that the daily gas production will decrease rapidly with the attenuation coefficient λ. And with the greater of attenuation coefficient λ, the faster the permeability ratio decays. Comparing with the case of λ = 1 × 10−8, the permeability ratio decreases by 26% when λ = 5 × 10−8. And we also found that the initial permeability k0 is negatively correlated with the peak pressure in matrix. Overall, microwave heating can effectively reduce the methane adsorption capacity of the coal matrix and significantly increase the gas extraction production from coal seam.

Structural deformation and its pore-fracture system response of the Wufeng-Longmaxi shale in the Northeast Chongqing area, using FE-SEM, gas adsorption, and SAXS

Tectonism caused strong deformation of the Wufeng-Longmaxi Formation in the periphery of the Sichuan Basin, which affected the pore-fracture of the organic-rich shale and migration of shale gas. Samples of structurally deformed shale from the anticline and detachment structure and undeformed shale were collected in the Northeast Chongqing area, China. The investigations of shale structural deformation and its pore-fracture system were performed using optical microscopy, field emission scanning electron microscopy (FE-SEM), low-pressure gas adsorption (LP-GA), and small-angle X-ray scattering (SAXS). The research shows that deformed samples with curved beddings and friction mirrors on the macro level, as well as the microscopically curved organic matter (OM) bands and OM folds, indicate the significant structural transformation and predominantly ductile deformation. The micro-fractures in deformed samples are more developed with different combinations in the two geological structures, showing brittle deformation. Compared with undeformed samples, the number of OM pores in deformed samples is reduced significantly, while the interparticle (interP) pores, intraparticle (intraP) pores, and micro-channels are more developed. The connected pore size distribution (PSD) of all samples is mainly from 0.3 nm to 10 nm. The pore volume (PV) of micropore in the deformed samples is between 0.100 and 0.500 cm3/100 g, and the specific surface area (SSA) is in the range of 2.175–13.554 m2/g. Undeformed samples have a larger PV and SSA than the deformed samples. TOC and the structural deformation control the structure of connected pore, and their PV and SSA are positively correlated with TOC content. Fractal dimension reveals that the surface of connected and closed pore (pore size 3.81–95.34 nm) becomes more complex, and the distribution of small pore (pore size 1.25–3.81 nm) becomes more uniform under the structural deformation of the shale. As the degree of shale deformation increases, the mesopore and macropore volume of connected pores decrease significantly, while the strongly structural deformation of samples results in a substantial increase in micropore volume and SSA. Besides, the PV of both closed and connected pores tends to decrease and the average pore size increases linearly. Furthermore, the shale deformation result in a weakening of the methane adsorption capacity. Shale gas can migrate through the fracture network to the central zone of the detachment structure or the core of the anticline, which has a significant effect on shale gas enrichment and preservation.

Methane adsorption capacity measurement in shale matrix nanopores at high pressure by low-field NMR and molecular simulation

Shale gas is an important unconventional gas resource because of its rich storage ability. The quantitative measurement of adsorption capacity is essential to shale gas exploration. However, traditional measurements, such as gravimetric and volumetric method, are influenced by excess adsorption effect and need to be corrected at high pressures. In this work, methane adsorption curves without excess adsorption effect were obtained through two ways: a low-field nuclear magnetic resonance (NMR) experimental measurement and the adsorption curves modification using density correction method by molecular simulation. Shale powder samples from the Longmaxi Formation, China, are used as representative sample in this work. In NMR experiments, the gas in packed beds, free spaces and nanopores is identified based on the multi-peaks in the NMR transverse relaxation spectrum, which guarantees the inexistence of excess adsorption effect. In simulations, considering the complex composition of the shale matrix and complicated pore shapes, the average density of methane in organic and inorganic nanopores with various size and shape was obtained using Grand Canonical Monte Carlo (GCMC) simulation, and the excess adsorption curve measured by gravimetric and volumetric method is corrected based on such density. Through comparing the NMR measured adsorption curve and corrected excess adsorption curve, it indicates that the adsorption curve measured via NMR was in good agreement with the modified adsorption curves by density correction, which validates the NMR experiment for measuring adsorption capacity of nanoporous media with complex structure, which is suitable for shale gas and hydrogen storage, as well as other adsorption processes.

The effect of oil extraction on porosity and methane adsorption for dry and moisture-equilibrated shales

The porosity and methane adsorption capacity of shale used to estimate gas in place (GIP) are affected both by moisture and oil residing in the pore structure, as well as oil naturally present from maturation, which can include contaminant drilling mud fluids used for drilling. To demonstrate the impact of extractable oil on methane adsorption capacity for both dry and moisture-equilibrated shales, two overmature shales from China (SH1, SH2) and two lower mature shales from UK (BS3, GH4) have been investigated. The oils extracted from the overmature shales (<0.5 wt% TOC) are in low yield and arise from oil-based drilling mud, while the much higher yields from the lower maturity UK shales (1.1–2.5 wt% TOC) is mainly from oil generated by maturation. After extraction, minimal changes (<5%) in total nanopore volume (<100 nm) were observed for the dry over mature shales, but significant increases (95 and 176%) were observed for the dry lower maturity shales. More than 60% of the extracted oil resides in micro and mesopores, and removal could unblock the micropore necks and enlarge the accessible meso and macropore volume. Moisture contents are lower for extracted shales, with reductions of 7–37% observed. Methane equilibrium adsorption capacities increased after oil extraction for both the dry and wet shales, especially for lower maturity shales, where increases were over 200% for the wet shales. Henry’s Law was used to show that there were not significant amounts of dissolved methane in oils for the dry shales. Extracting oil from shales prior to determining the porosity and methane adsorption capacity can lead to the GIP being over-estimated for moisture equilibrated shales, particularly for oil-window shales where an overestimation of 22% was obtained for the shale investigated in this study. These findings provide a better understanding of the combined impacts of residual oil and water on GIP and avoid its overestimation.

Effects of ultrathin igneous sill intrusion on the petrology, pore structure and ad/desorption properties of high volatile bituminous coal: Implications for the coal and gas outburst prevention

As the one of the most catastrophic mining hazards, coal and gas outburst seriously threatens the mining safety of collieries. Previous studies indicate that lots of outbursts are closely related to igneous intrusion. Thus, to further understand the influence of igneous intrusion on coal’s physical properties and outburst risk (especially the impacts from ultrathin igneous sill intrusion, which is hard to be detected underground), the combined analyses of petrographic, proximate, pore structure and methane ad/desorption are applied. Results indicate that with the approaching to the ultrathin igneous sill, physical properties of coal change significantly. Due to thermal evolution effect from intrusion, metamorphic grade of coal increases from the high volatile bituminous coal (Ro = 0.6949) of unaffected normal region to the low volatile bituminous coal (Ro = 1.6684), and the altered range of thermal evolution zone is larger than 3 times of the sill thickness but of which the range of strong thermal evolution zone only reaches 0.8 times of the sill thickness. Within the thermal evolution zone: moisture content of coal decrease while the ash content increase when approaching to the sill; micropore volume and specific surface area of the coal are nearly unchanged in the weak thermal evolution zone but reduced in the strong thermal evolution zone, however, methane adsorption capacities of the coal are generally enhanced when comparing to the normal region sample; the transition pores and mesopores pore of the sample close to distal boundary of thermal evolution zone is seriously suppressed (which are 49.21–68.32 % and 20.14–63.31 % of the normal sample, respectively). By comprehensively considering the impact of igneous intrusion on coal, coal locates near the distal boundary of strong thermal evolution zone (about 0.5 times of sill thickness away from the intrusion boundary) seems to have the most serious threat of outburst risk. Additionally, the moisture, ash contents and the gas desorption property of coal has the feasibility to be used as the signs for the hard-detectable small-scale igneous intrusion ahead during roadway tunneling process. Meanwhile, due to the correlation between igneous intrusion and outburst, which can also be regarded as the early warning indicator for outburst prevention.

Molecular insight into the diffusion/flow potential properties initiated by methane adsorption in coal matrix: taking the factor of moisture contents into account.

Coal seam permeability is one of the key parameters affecting coalbed methane (CBM), and plays an important role in resource evaluation and regional selection. To fully explore the diffusion/flow potential properties initiated by methane adsorption beneath diverse moisture contents (1-5%) in coal molecules. The pore size distribution and methane adsorption capacities were discussed based on Monte Carlo (MC) and molecular dynamics (MD) methods. The potential properties of diffusion/flow induced by methane adsorption were investigated using the maximum absolute adsorption capacities as benchmark. The variation patterns of the pore structure were analyzed using SEM scanning experiment to verify the results of simulation analysis. It is found that the free pores facilitate methane molecular adsorption and increase adsorption spaces; the skeleton pores restrict the flow and transport of water molecules. Reduction values in surface free energies increase at different temperatures, and released heat diffusion coefficients and permeabilities for methane molecules drop as moisture contents increase. Interestingly, however, enhancements in temperatures increase the methane molecular diffusion coefficients. The lower the activation energies, the easier they are to diffuse. Sufficiently, the optimum conditions for gas drainage of coal seam are at temperature of 293K and moisture content of 5%, indicating greater contributions to gas pressure relief for coal seam. By comparing the results of molecular simulation and SEM scanning, trend of change is basically the same. Moreover, it is explored that hydraulic measure was the most significant to the CBM stimulation technology through field engineering application. This research is expected to provide guidance for facilitating the effectiveness of gas extraction for coal seam.

Methane Adsorption Behavior and Energy Variations of Brittle Tectonically Deformed Coal under High Temperature and High Pressure.

This paper aims to reveal the methane adsorption characteristics of brittle tectonically deformed coal (TDC) under high temperature and high equilibrium pressure and the surface free energy changes during methane adsorption. Taking the anthracite of the Yangquan mining area of China as the research object, methane adsorption tests of high temperature and high pressure were conducted to discuss the influences of coal deformation, temperature, and pressure on the gas adsorption behaviors of coal under the in situ conditions of deep coal seams. Results indicated that coal deformation and pressure had positive effects on the methane adsorption capacity of coal, whereas temperature showed negative effects. Meanwhile, the negative influences of temperature rise on gas adsorption gradually increased with the equilibrium pressure and the enhancement of coal deformation. In a given adsorption system, the adsorption potential decreased with increasing pressure while increased with temperature. In contrast, the adsorption space increased with increasing pressure and decreased with temperature rise. Meanwhile, adsorption potential decreased with the increase in adsorption space. In the process of methane adsorption, with increasing adsorption pressure, the cumulative reduction of surface free energy gradually increased, whereas the surface free energy reduction at each pressure point decreased. The cumulative and incremental reduction of surface free energy at each pressure point gradually decreased with the increase in temperature. The reduction of adsorption space and cumulative surface free energy and the changes in surface free energy at each pressure point of brittle TDCs were higher than those of primary structure coal, and the changes in the strongly deformed coal (granulitic and mortar coal) were more significant than those of the weakly deformed coal (cataclastic coal). Based on the adsorption potential theory, we drew the adsorption characteristic curves and established adsorption capacity prediction models of primary structure coal and brittle TDCs.

Water adsorption characteristic and its impact on pore structure and methane adsorption of various rank coals.

Coalbed methane not only is a new clean energy source, but also has potential damage to ecological environment. Water and methane coexist in coal reservoir; understanding the adsorption of water on coal and its impact on pore structure and methane adsorption of coal is vital to evaluate the reserves and productivity of coalbed methane. In the paper, water adsorption characteristics of various rank coals are firstly investigated by ten mathematical models. The modified Dent model provides a best fit, followed by GAB and Dent models. For GAB model, the primary site adsorption is more difficult to reach saturation, and the contribution rate of the secondary site adsorption is surprisingly high at P/P0 approaching 0, which can be attributed to the possible overestimation of GAB monolayer adsorption capacity and secondary site adsorption. Besides, the low-rank coal sample YZG2 exhibits more prominent hysteresis than middle- to high-rank coals. The low-pressure hysteresis can be attributed to the water-water interactions over the primary site and the strengthened binding forces of water molecules in the water desorption process. In contrast, the high-pressure hysteresis largely depends on pore structure of coal such as ink-bottle pores, especially for the studied sample YZG2. Besides, pore analyses by low-temperature nitrogen adsorption method show that the pre-adsorbed water has remarkable influence on micropores smaller than 10 nm, and the micropores smaller than 4 nm almost disappear for water-equilibrated coals, which is closely related to the formed water clusters and capillary water in pore throats. This finding reveals that more methane gas can only be adsorbed in the larger pores of moist coal, and provides an explanation for water weakening methane adsorption capacity.

Modeling of methane adsorption capacity in shale gas formations using white-box supervised machine learning techniques

Energy demand is increasing worldwide and shale gas formations have gained increasing attention and have become crucial energy sources. Therefore, accurate determination of shale gas-in-place (GIP) is vital for a successful production plan. Since most of the gas in shale formation is in the form of adsorbed gas, the determination of methane adsorption capacity is a very important task. In this study, two rigorous data-driven techniques, namely gene expression programming (GEP) and group method of data handling (GMDH), were utilized to provide accurate and reliable explicit mathematical expressions for predicting methane adsorption. For this purpose, a comprehensive database involving 352 data points was gathered from the literature. Pressure, temperature, moisture, and total organic carbon (TOC) were employed as input variables for the implemented correlations. Results indicate that both correlations can provide accurate predictions. However, the GEP-based correlation exhibits more reliable predictions for methane adsorption with a correlation coefficient of 0.9837. Moreover, it was shown that GEP-based correlation can accurately predict the variation of shale gas capacity for the alteration of inputs. Further, it was revealed that methane adsorption is highly dependent on moisture value, while temperature, TOC, and pressure are the most influential variables after moisture. The results of this study shed light on the power of GMDH and GEP modeling approaches, and show that these models can be employed to provide accurate and simple-to-use correlations for estimating methane adsorption in shale gas formations. • Methane adsorption capacity in shale gas formations was modeled using GMDH and GEP. • Extensive database was used for the development of the correlations. • The proposed correlations are reliable and user-friendly. • The proposed GEP-based correlation is deemed the best paradigm.

Origin and isotopic fractionation of shale gas from the Shanxi Formation in the southeastern margin of Ordos Basin

The Ordos Basin is abundant in natural gas resources, and the Shanxi Formation with the transitional facies is considered as a potential stratum for shale gas reservoir. However, the origin and geochemical characteristics of the Shanxi Formation shale gas remains unclear. In this study, 22 shale gas samples were collected from the SX-1 well in the southeastern margin of Ordos Basin, and tested for gas components and compositions of stable carbon isotope. Moreover, five rock samples were measured for total organic carbon (TOC), the vitrinite reflectance (Ro), kerogen type, porosity, permeability, and methane adsorption capacity. The shale gas in the Shanxi Formation has a high proportion of methane which is consistent with the high maturity of source rocks. The contradictory relationship between the carbon isotopes of ethane (δ13C2) from the Shanxi Formation shale gas and kerogen types of rock samples may be caused by the complex sedimentary environment, multiple provenance systems, and high maturity. The carbon isotopes of methane (δ13C1), δ13C2 and alkane gas components indicate that the shale gas near coal seams is coal-derived gas, the gas at the bottom of the Shanxi Formation is oil-type gas and the gas in mudstones among the sandstone layer at the top of the Shanxi Formation may be mixed with natural gas migrating from other layers or mixed with gas generated in the later period. Meanwhile, the carbon isotopes of carbon dioxide (δ13CCO2) suggest that the CO2 generated with hydrocarbons in the coal-measures is mainly of organic origin. The carbon isotopic fractionation of methane is commonly observed in shale gas samples. This fractionation is mainly controlled by the diffusion effect which can be promoted by high porosity and permeability. Furthermore, the positive correlation between the TOC values and the degree of fractionations may be attributed to the high TOC values in the samples, which can improve the development of organic pores and increase the content of adsorbed gas.

Quantitative Characterization of Pore Connectivity of Organic Matter and Clay Minerals in Shale and its Control on the Methane Adsorption Capacity of Continental Ziliujing Formation Shale from the Northeastern Sichuan Basin in China

Quantitative Characterization of Pore Connectivity of Organic Matter and Clay Minerals in Shale and its Control on the Methane Adsorption Capacity of Continental Ziliujing Formation Shale from the Northeastern Sichuan Basin in China

Pore structure characterization and its significance for gas adsorption in coals: A comprehensive review

The coal reservoir has multifaceted and hierarchical characteristics influenced by heterogeneity of the pore and fracture networks. The coal pore including micropore, mesopores and macropores determines the critical migrate processes of gas molecules, especially for adsorption. Normally, coal pore structure refers to the pore shape, size distribution and interconnection of variable pores. Together with pore volume, pore size distribution, specific surface area and porosity. For coalbed methane, pore structure is the storage place, which largely controls the methane adsorption capacity of coal. Fluid injection, microscope observation and X-ray and spectroscopic analysis can be used to explore coal pore structure. Understanding the pore structure in varied rank coals is very challenging mostly due to the complex pore system leading to ambiguous petrophysical properties and gas performance. Among several important factors controlling the performance of coalbed methane, adsorption is the most crucial one, which affected by the pore aspects including pore volume, specific surface area, pore size distribution, pore shape and porosity. Therefore, accurate characterization of pore structure is coupled with advanced technologies, which resulted in the development of a wide range of methods. In this work, all the necessary technologies from imaging, segmentation, and modeling of the pores to various methods of gas adsorption evaluation are reviewed. This paper presents a critical review of all of the existing relevant and significant techniques and compares their performances together with their limitation with special reference to pore structure characterization. And the impacts of pore structure are thus reviewed. Finally, this review work proposes the current challenges and possible future research. • The coal pore classification scheme is systematically reviewed. • The pore characterizing techniques are summarized. • The influence of coalification on pore structure and its impact on CH 4 adsorption is analyzed. • The challenges and prospects on coal pore research are proposed.

Reasons of Low Formaldehyde Adsorption Capacity on Activated Carbon: Multi-Scale Simulation of Dynamic Interaction between Pore Size and Functional Groups

Reasons of Low Formaldehyde Adsorption Capacity on Activated Carbon: Multi-Scale Simulation of Dynamic Interaction between Pore Size and Functional Groups

Response of Molecular Structures and Methane Adsorption Behaviors in Coals Subjected to Cyclical Microwave Exposure.

To better understand the methane adsorption behavior after microwave exposure, the importance of quantitatively characterizing the effect of cyclical microwave exposure on the molecular structures of coals cannot be overemphasized, with implications for enhancing coalbed methane (CBM) extraction. Thus, cyclical microwave exposure experiments of three different metamorphic coals were conducted, and the methane adsorption capacity before and after each microwave exposure (10 in total) for 120 s was evaluated. Fourier transform infrared spectroscopy analysis and peak fitting technology were applied to quantitatively characterize the changes in the structural parameters of coal molecules. The results showed that after modification, the structural parameters like aromatic carbon fraction (fa–F), aromaticity (I1 and I2), degree of condensation (DOC1 and DOC2), and the maturity of organic matter (“C”) gradually increased with increasing exposure times, while the length of the aliphatic chain or its branching degree (CH2/CH3) and the hydrocarbon generating capacity (“A”) showed a decreasing trend. The Langmuir volume (VL) of three different rank coal samples decreased from 29.2, 32.8, and 40.4 mL/g to 25.7, 29.3, and 35.7 mL/g, respectively; the Langmuir pressure (PL) increased from 0.588, 0.844, and 0.942 MPa to 0.626, 1.007, and 1.139 MPa, respectively. The modification mechanism was investigated by analyzing the relationship between the methane adsorption behaviors and molecular structures in coals. The release of alkane side chains and the oxidation of oxygen-containing functional groups caused by microwave exposure decreased the number of methane adsorption sites. As a result, the methane adsorption capability decreased. In addition, the decomposition of minerals affects methane adsorption behaviors in coals. This work provides a basis for microwave modification of coal as well as in situ enhancement of CBM extraction using microwave exposure.

Fractal Characteristics and Significance of Different Pore Types of the Wufeng-Longmaxi Formation, Southern Sichuan Basin, China, Based on N2 Adsorption and Image Analysis.

Shale gas is an important unconventional natural gas resource, and its reservoirs have pores with strong heterogeneity, which have an important effect on the adsorption and migration of shale gas, but the specific mechanism is still unclear. To further clarify the pore structure characteristics of shale gas reservoirs and the mechanism of their influence on CH4 adsorption capacity, marine shale samples from the Wufeng–Longmaxi formation of wells N1, N3, and N10 in Changning block, southern Sichuan Basin, China, were selected for total organic carbon (TOC), X-ray diffraction (XRD), N2 gas adsorption (N2-GA), CH4 gas adsorption (CH4-GA), and field emission scanning electron microscopy (FE-SEM). The Frenkel–Halsey–Hill (FHH) model and Slit Island Analysis (SIA) were used to calculate the fractal dimension of the pore system and different types of pores, and their relationship and influence on CH4 adsorption capacity were also discussed. The results show that the fractal dimension could reflect the complexity and heterogeneity of pores. According to the FHH model, fractal dimensions of the surface and structure of the pore system (D1 and D2, D1 < D2) were obtained, and the pore structure was more complex than the pore surface. According to SIA, the surface fractal dimensions of four types of reservoir space (DDP, DOP, DIP, and DMF) decrease progressively, and their main body is 2.60–2.80, 2.40–2.65, 2.20–2.40, and 2.05–2.30. Organic pores and intergranular pores are the most abundant, and so D1 is mainly related to DOP and DIP. In high-TOC samples, D1 is close to DOP, while in low-TOC samples, D1 is close to DIP. The complexity of the pore surface, D1, and specific surface area have a positive correlation, and with the increase of pore surface complexity, methane adsorption capacity could be significantly improved. Therefore, D1 may be used as a characterization parameter of CH4 adsorption capacity, which could provide some evidence to further clarify the adsorption mechanism of shale gas.

Pore structural characteristics and methane adsorption capacity of transitional shale of different depositional and burial processes: a case study of shale from Taiyuan formation of the Southern North China basin

Pore structural characteristics and methane adsorption capacity are two significant aspects affecting shale gas potential, but the impact of deposition and burial processes on these two aspects is not clear. Hence, the shale samples of Taiyuan Formation deposited continuously and experienced multi-stage tectonic uplift in Fuyang-Bozhou area of Southern North China Basin were collected in this study. Based on the total organic carbon content analysis, mineral composition determination, low-pressure CO2 and N2 adsorption, high-pressure methane adsorption and argon ion polishing-field emission scanning electron microscope observation. The impact of depositional and burial processes variation on shale reservoir physical properties and adsorption performance is studied. The results display that the pore types of shale samples which were continues deposited and experienced multi-stage tectonic uplift have no obvious differences, while the pore volume as well as specific surface area (SSA) of micropores and mesopores of shale samples under multi-stage tectonic uplift are larger significantly. Meanwhile, the roughness of shale pores increases also. The decrease of loading pressure caused by multi-stage tectonic uplift may be the main factor for the pore structure changes of shale sample. Compared with the continuous deposited samples, the shale samples under multi-stage tectonic uplift have stronger methane adsorption capacity, which is relevant to the greater SSA of micropores as well as mesopores. This study provides an example and new revelation for the influence of depositional and burial processes on shale pore structure and methane adsorption capacity.

Quantitative characterization of pore network and influencing factors of methane adsorption capacity of transitional shale from the southern North China Basin

Quantitative characterization of pore structure and analysis of influencing factors of methane adsorption are important segments in shale gas reservoir and resources evaluation and have not been systematically carried out in marine–continental shale series. A series of integrated methods, including total organic carbon (TOC) contents, Rock-Eval pyrolysis, mineral composition analysis, pore structure measurement, high-pressure CH4 adsorption analysis and FE-SEM observation, were conducted on 12 transitional shale samples of well WBC-1 in the southern North China Basin (SNCB). The results indicate that TOC contents of the transitional shales range from 1.03 to 8.06% with an average of 2.39%. The transitional shale consists chiefly of quartz, white mica and clay minerals. Interparticle pore, intraparticle pore, dissolution pore and microfracture were observed in the FE-SEM images. The specific surface area (SSA) of BET for the samples ranges from 3.3612 to 12.1217 m2/g (average: 6.9320 m2/g), whereas the DR SSA for the samples ranges from 12.9844 to 35.4267 m2/g (average: 19.67 m2/g). The Langmuir volume (VL) ranges from 2.05 to 4.75 cm3/g (average = 2.43 cm3/g). There is unobvious correction between BET and DR SSA with TOC contents, which means inorganic pores are the main component of pore space in the transitional shale from the SNCB. The relationship of SSA and pore volume shows that micropore has a greater impact on the CH4 adsorption capacity than mesopore–macropore in the transitional shale. Different from shales in other petroliferous basin, clay minerals are the primary factor affecting adsorption capacity of CH4 for transitional shale in this study. The pore structure of the transitional shale for this study is characterized by higher fractal dimension and more heterogeneous pore structure compared to shale in other petroliferous basin. This study provides an example and new revelation for the influencing factors of pore structure and methane adsorption capacity of marine–continental transitional shale.

Evolution mechanism of methane adsorption capacity in vitrinite-rich coal during coalification

In this study, the evolution of methane adsorption capacity was investigated by analyzing both naturally matured coal samples and samples that matured by pyrolysis to different ranks. Methane adsorption isotherms, mercury injection, and small-angle X-ray scattering experiments were employed to characterize the methane adsorption capacity and pore structures. Simulation methods were used to study the methane adsorption capacities of different functional groups in the coal matrix by constructing slit pores whose surfaces were modified by carboxyl, hydroxyl, aromatic ring, or methyl groups. In the simulations, the methane adsorption capacity per surface at 10 MPa were very similar: 0.31 cm3/m2, 0.29 cm3/m2, 0.28 cm3/m2 and 0.26 cm3/m2, respectively, indicating different functional groups do not significantly affect the variability of methane adsorption capacities. The evolution of methane adsorption capacity was similar to that of micropore volume, implying methane molecules were mainly adsorbed in micropores in coals. It was also observed that the evolution of methane adsorption capacity in artificially matured samples was similar to that of the naturally matured coals, implying differences in the coalification pathways had minimal influence on the evolution of methane adsorption capacity. With Rr increasing from 0.5% to 4.2%, the methane adsorption capacity evolved in four stages: 0.5%–1.4% (decreasing adsorption), 1.4%–2.0% (significantly increasing adsorption), 2.0%–3.7% (slightly increasing adsorption) and 3.7%–4.2% (decreasing adsorption). The results of this study show that during the coalification process, the molecular structure essentially determines the evolution of the methane adsorption capacity by controlling evolution of micropores.