Low-pressure N2 Adsorption Research Articles

Pore Properties of the Lacustrine Shale in the Upper Part of the Sha-4 Member of the Paleogene Shahejie Formation in the Dongying Depression in East China

Lacustrine shales hold a huge potential oil resource in China. Pore properties (pore volume, diameter, specific surface area, and fractal dimensions) and their relationships with geological factors (mineralogy, insoluble organic carbon, burial depth, and vitrinite reflectance) are critical for evaluating shale oil resource. However, the factors controlling pores for lacustrine shale oil remain unclear, as the relationships between pore properties of Soxhlet-extracted samples and geological factors have not been studied using multivariate analytical methods. In this paper, the samples from the lacustrine shale in the upper part of the Sha-4 Member of the Paleogene Shahejie Formation in the Dongying Depression were tested with a set of experiments including Soxhlet (solvent) extraction, X-ray diffraction mineral analysis, insoluble organic carbon, vitrinite reflectance, and low-pressure CO2 and N2 adsorption experiments. The micromesopore volume varies from 0.003 cm3/g to 0.045 cm3/g. The relationships of pore properties with geological factors were studied with partial least square regression analysis (PLSR analysis, a powerful multivariate regression analysis). The results of the PLSR analyses indicate that clay minerals and carbonates are two key factors affecting the pore properties of the lacustrine shale. Compared with marine shales, more clay minerals in the lacustrine shale make them become more important for controlling pores than organic matter. The PLSR results also illustrate that the shale with higher pore volume contains more clay minerals and fewer carbonates and thus is unfavorable for hydraulic fracturing. Therefore, the shale with high micromesopore volume may be unfavorable for shale oil production. The shale with the modest micromesopore volume (~0.036 cm3/g), relatively high content of brittle minerals (~71 wt%), and low clay mineral content (~29 wt%) is conducive to both oil storage and hydraulic fracturing for the development of the Es4U shale oil in the Dongying Depression in East China.

Insights into the pore structure and fractal characteristics of Permian Lucaogou Formation shale from the Jimusar sag, Junggar Basin

The Permian Lucaogou Formation in the Jimusar sag, Junggar Basin, northwestern China, is increasingly receiving attention for its shale oil potential, which is still in the exploration stage. In this work, core samples were obtained from two wells in the Jimusar sag, and their mineralogy, pore structure, and fractal characteristics were investigated by XRD (X-ray diffraction) analysis, LPNA (low-pressure N2 adsorption) analysis, FE-SEM (field emission scanning electron microscopy), CTS (casting thin sections), and fractal analysis. The results showed that the major mineral compositions were plagioclase (average 34.82%), quartz (average 17.07%), and dolomite (average 17.66%). Compared with well-known marine shales in China, the shales have higher feldspar (average 44.64%) and carbonate (average 26.03%), and lower quartz (average 17.07%). Based on the FE-SEM and CTS, organic matter, intergranular, interlayer, dissolved, intercrystalline pores, and microfractures were observed. The specific surface area ranged from 10.181 to 26.648 m2/g, and the total pore volume was between 0.03595 and 0.07102 cm3/g. The results of the DFT-PSD curves indicated that shales showed multi-modal distribution characteristics, whereas the results of the BJH-PSDs curves revealed that shales displayed unimodal distribution characteristics with a peak between 20 and 40 nm. Pore sizes of less than 5 nm had an important influence on the specific surface areas, whereas pore sizes ranging from 20 to 40 nm had an important influence on the total pore volume. The fractal dimension values ranged from 2.7564 to 2.8382. The fractal dimension decreased as the average pore size increased and increased as the total pore volume and specific surface area increased.

Pore characteristics and pore structure deformation evolution of ductile deformed shales in the Wufeng-Longmaxi Formation, southern China

Marine shales in southern China experiences multiple complex tectonic movements, which highly affects shale pore characteristics and pore evolution. The effects of tectonic deformation on nanopore structure and petrophysical properties of organic shales remain unclear. Three ductile deformed shale samples were collected and analyzed through an integrated experiment procedure for a comparative research against undeformed shale samples. Field emission-Scanning electron microscopy (FE-SEM), low pressure N2 and CO2 adsorption and mercury injection porosimetry (MIP) analyses were performed to characterize pore structure of the shales. The results reveal that the mineral compositions of these shale samples are controlled by sedimentary environment and source and exhibits no obvious relationship with tectonic deformation. With increasing deformation degree, the micropore proportion gradually decreases (from 17% to 6%), and the macropore proportion increases from 16% to more than 20%. Micropore volumes and proportion in the fold core samples are the smallest (4.2 × 10−3 cm3/g and 6.3%) and account for only one-third of the undeformed shale values, while the meso- and macropore values are larger-1.2 times those of the undeformed shale values. The evolution characteristics and deformation modes of organic matter (OM), interparticle (interP) and intraparticle (intraP) pores and microfractures under ductile deformation were examined. All pore types become interconnected during deformation and form microfractures ranging from a few nanometers to tens of microns, which is the main reason for the enhanced shale connectivity caused by deformation. Under the same porosity conditions, the deformed shale permeability is much higher than the undeformed shale permeability, and stronger deformation corresponds to higher permeability. The fold core permeability is generally higher than the limb permeability and exceeds 30 times. The different ductile deformation positions were divided into five levels, and the corresponding reservoir quality was evaluated. This study is of great significance to better understand the pore structure evolution process for shale exploration and development under tectonic deformation.

Characterization of nano-to-micron sized respirable coal dust: Particle surface alteration and the health impact

Chemical and physical properties of coal dust particles significantly influence the inhalation of respirable coal dust by miners, causing several lung diseases such as coal workers’ pneumoconiosis (CWP) and silicosis. Multiple experimental techniques, including proximate/ultimate analyses, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), laser diffraction, and low-pressure CO2 and N2 adsorption, were used to investigate the chemical and physical properties of micron-/nano-coal particles comprehensively. Compared to the micron-scale coal dust, the nano-coal dust (prepared by cryogenic ballmill) shows the increase of carbon content and aromaticity and a decrease of oxygen content along with the reduction of oxygen-containing functional groups. Pore volume and surface area estimated by low-pressure CO2 and N2 adsorption have more than five-time increase for the nano-coal dust. The reduction of oxygen functional groups suggests the dropped wetting behavior of coal nanoparticles. The significantly increased pore volume and surface area in coal nanoparticles could be caused by the enhanced pore interconnectivity on the particle surface and the alteration of coal macromolecules. Weaker wettability and the highly enhanced surface area suggest potentially more significant toxicity of nano-coal dust inhaled by coal miners.

Investigation of enhancing coal permeability with high-temperature treatment

Enhancing coal permeability with formation heat treatment (FHT) is a prospective method in coalbed methane (CBM) exploitation. Here, the mechanisms of coal permeability enhancement induce by heat treatment are investigated. Residual water coal samples were heated in a helium atmosphere at 50 °C, 100 °C, 200 °C, 400 °C, 600 °C, and 800 °C in sequence. The samples’ permeability and acoustic wave velocity were measured after each heat treatment, and Young’s modulus and shear modulus were calculated to determine the change in the mechanical properties. In addition, changes in the pore structure and surface properties were analyzed with low-pressure N2 adsorption and SEM. The results indicate that in the range of 50–200 °C, the bulk water and interlayer water were removed, which alleviated the aqueous phase trapping effect, enhanced gas flow channels and increased permeability by 4–14 times. Between 400 and 600 °C, the coal samples started to pyrolysis, weakening the mechanical properties of the coal, and fractures were initiated and extended, which caused permeability to increase by 79–1400 times. After 800 °C, the surface of the coal was completely pyrolyzed, samples were cracked, and the permeability increased by more than 20,000 times. Stimulation mechanisms of FHT in CBM reservoirs consist of aqueous phase trapping alleviation, coal fractures caused by mechanical properties weakening and combustion, and finally, the C–C bond and C–O bond of molecules are broken, which induced coal fracture to produce more gas flow channels. For field application of FHT, both advantages and disadvantages are analyzed.

Controls of Distinct Mineral Compositions on Pore Structure in Over-Mature Shales: A Case Study of Lower Cambrian Niutitang Shales in South China

Investigating the impacts of rock composition on pore structure is of great significance to understand shale gas occurrence and gas accumulation mechanism. Shale samples from over-mature Niutitang formation of Lower Cambrian in south China were measured by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), low pressure N2 and CO2 adsorption to elucidate the controls of distinct mineral composition on pore development. Two distinct lithofacies, namely siliceous shale and argillaceous shale, were ascertained based on their mineral composition. Due to the variability of mineral composition in different lithofacies, pore structure characteristics are not uniform. Pores in siliceous shales are dominated by interparticle pores and organic matter (OM) pores, among which the interparticle pores are mainly developed between authigenic quartz. Furthermore, most of these interparticle pores and cleavage-sheet intraparticle pores within clay minerals are usually filled by amorphous organic matter that is host to OM pores. Due to the lack of rigid minerals, argillaceous shale was cemented densely, resulting in few interparticle pores, while cleavage-sheet intraparticle pores within clay minerals are common. Comparing siliceous shales with argillaceous shales, specific surface areas and pore volumes are higher on the former than on the latter. The content of total organic carbon (TOC) and authigenic quartz have a great influence on micropore structures, but less on mesopore structure for siliceous shales. The rigid framework structure formed by authigenic quartz is believed to be able to prevent primary interparticle pores from mechanical compaction and facilitate the formation of organic matter-associated pores. In terms of argillaceous shales, due to the lack of authigenic quartz, interparticle pores were rarely developed and its pore structure is mainly controlled by illite content.

Pore Size Distribution and Fractal Characteristics of the Nanopore Structure in Organic-Rich Shales Using the Neimark-Kiselev Fractal Approach.

Shale gas has been playing an increasingly important role in meeting global energy demands. The heterogeneity of the pore structure in organic-rich shales greatly affects the adsorption, desorption, diffusion and flow of gas. The pore size distribution (PSD) is a key parameter of the heterogeneity of the shale pore structure. In this study, the Neimark-Kiselev (N-K) fractal approach was applied to investigate the heterogeneity in the PSD of the lower Silurian organic-rich shales in South China using low-pressure N₂ adsorption, total organic carbon (TOC) content, maturity analysis, X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) measurements. The results show that (1) the fractal dimension DN-K obtained by N-K theory better represents the heterogeneity of the PSD in shale at an approximately 1-100 nm scale. The DN-K values range from 2.3801 to 2.9915, with a mean of 2.753. The stronger the PSD heterogeneity is, the higher the DN-K value in shale is. (2) The clay-rich samples display multimodal patterns at pore sizes greater than 20 nm, which strongly effect the PSD heterogeneity. Quartz-rich samples display major peaks at less than or equal to a 10 nm pore size, with a smaller effect on the PSD heterogeneity in most cases. In other brittle mineral-rich samples, there are no obvious major peaks, and a weak heterogeneity of the PSDs is displayed. (3) A greater TOC content, maturity, clay content and pore size can cause stronger heterogeneity of the PSD and higher fractal dimensions in the shale samples. This study helps to understand and compare the PSD and fractal characteristics from different samples and provides important theoretical guidance and a scientific basis for the exploration and development of shale gas resources.

Applicability Analysis of Determination Models for Nanopores in Coal Using Low-Pressure CO₂ and N₂ Adsorption Methods.

The development characteristics of nanopores (with pore sizes <200 nm) in coal are a key factor affecting the accumulation and migration of coalbed methane (CBM). Thus, an appropriate determination method and calculation model are essential for accurate nanopore representation. Based on the experiments of low-pressure CO₂ adsorption (LP-CO₂GA) at 273 K and low-pressure N₂ adsorption (LP-N₂GA) at 77 K on four coals with different ranks, the abilities of different models (e.g., Langmuir, Dubinin-Radushkevich (D-R), Dubinin-Astakhov (D-A), Brunauer-Emmett-Teller (BET) and nonlocal density functional theory (NLDFT)) to accurately predict the pore parameters were analyzed. The results showed that (1) for LP-N₂GA, the Langmuir model is only suitable for gas adsorptions at low relative pressure conditions (P/P0 < 0.01), and its error value increased with the relative adsorption pressure. The fitting results of the D-R model showed good agreement with the D-A model under low relative pressure of LP-CO₂GA (P/P0 < 0.01), and the D-A model had more accurate fitting results. The BET model is more accurate than the other models (φ = -1.2733%) only in the interval of LP-N₂GA with 0.05 < P/P0 < 0.35. The data also showed that the NLDFT model can maintain a higher fitting accuracy for LPCO₂/N₂GA processes at relative adsorption pressures from 0.001-0.9996. (2) Using LP-CO₂GA with the Langmuir, D-R, D-A, and NLDFT models, the micropore specific surface area (SSA; 66.9570-248.6736 m²/g) and pore volume (0.0201- 0.0997 cm³/g) were obtained, while the values of meso-/macropore SSA (0.0007-2.3398 m²/g) and pore volume (0.0036-0.04 cm³/g) were calculated by LP-N₂GA with the BET and NLDFT models. The results showed that the fitting accuracy in descending order was the D-R, D-A, Langmuir and NLDFT models. (3) In combination with the applicable model range, LP-CO₂GA with the NLDFT model was recommended for micropore analysis of the coal pore sizes from 0.36-1.1 nm, while LP-N₂GA combined with the NLDFT model was recommended for nanopore analysis of pore sizes from 1.1-200 nm. (4) The characteristics of pore development in the Beiloutian coal were analyzed using LP-CO₂/N₂GA combined with the NLDFT model. It was found that a pore volume and SSA less than 1.0 nm accounted for 88.82% of the total pore volume and 98.05% of the total SSA, indicating that micropores in coal are the main space for CBM storage and are key physical factors for the occurrence and migration of coalbed methane. The conclusions of this article will provide a basis for the accurate calculation of nanopores in coal.

Influence of Supercritical CO &lt;sub&gt;2&lt;/sub&gt; Exposure on Water Wettability of Marine and Continental Shale

To investigate the influence of supercritical CO2 (ScCO2) exposure on water wettability of shale, a systematic contact angle experiments considering ScCO2 exposure time and pressure were conducted on shale samples collected from Sichuan Basin (marine) and Ordos Basin (continental), respectively. In addition, X-ray diffraction analysis and low-pressure N2 adsorption were performed to evaluate the variations of mineral compositions and pore structure of shale. Results indicate that the tested shale samples are both hydrophilic rocks, and the shale-water contact angles generally increased with increasing ScCO2 exposure time and pressure, which is primarily caused by the the decrease of clay minerals and carbonates in shale. The water wettability of shale weakened after ScCO2 exposure, indicating that the interaction between shale and water molecules decreased, which may reduce the resistance of water to flow in pores and fractures of shale. Accordingly, it can be inferred that the alteration of shale water wettability after ScCO2 injection is beneficial to the seepage of gas in shale pore channels, but may exist a negative influence on the stability of CO2 geological sequestration. This study provides a theoretical reference for CO2 sequestration and CO2-enhanced shale gas recovery (CS-ESGR).

Variations in pore structure of marine shale from the same horizon of the Longmaxi Formation with changing position in a small-scale anticline: Implications for the influence of structural deformation

Nanopores are well developed in shale, and are influenced by many rock properties, such as mineral composition, TOC (Total Organic Carbon) and thermal maturity. However, the influence of structural deformation on pore properties of shale is still not well understood. In this study, 19 samples from the same horizon of the lower Silurian Longmaxi shale, but from different positions in a small-scale anticline of Wuping Village, Chongqing (China), were collected. Low pressure N2 and CO2 adsorption experiments were performed to study the influence of structural deformation on the pore structure. The results show that the mineral composition, TOC and thermal maturity of all samples are similar, but the specific surface areas and pore volumes exhibit great differences in different positions of this anticline. The mean N2-BJH pore volumes, the mean N2-BET specific surface areas, the mean CO2 D-R pore volumes, as well as the mean CO2 D-R specific surface areas exhibit decreasing trends for the samples from the core part to the sub-core part, and to the limb part of the studied anticline, respectively. Structural deformation has shown a considerable influence on the micropores and mesopores, especially for the pores lower than 10 nm because that tectonic stress increases from the core part to the sub-core part and the limb part of the anticline. Moreover, the structural curving degree characterized using curvature shows good correlation with the pore volume and specific surface area. Compared to the limb part, the core part of the anticline with high curvature is located in the tension environment, and it is beneficial for pore development and gas accumulation. Our study implies that moderate structural deformation can provide more specific surface area for absorbed gas and pore volume for free gas at the core part of the structure on a larger scale.

Pore structure characterization and its effect on methane adsorption in shale kerogen

Pore structure characterization and its effect on methane adsorption on shale kerogen are crucial to understanding the fundamental mechanisms of gas storage, transport, and reserves evaluation. In this study, we use 3D scanning confocal microscopy, scanning electron microscopy (SEM), X-ray nano-computed tomography (nano-CT), and low-pressure N2 adsorption analysis to analyze the pore structures of the shale. Additionally, the adsorption behavior of methane on shales with different pore structures is investigated by molecular simulations. The results show that the SEM image of the shale sample obviously displays four different pore shapes, including slit pore, square pore, triangle pore, and circle pore. The average coordination number is 4.21 and the distribution of coordination numbers demonstrates that pores in the shale have high connectivity. Compared with the adsorption capacity of methane on triangle pores, the adsorption capacity on slit pore, square pore, and circle pore are reduced by 9.86%, 8.55%, and 6.12%, respectively. With increasing pressure, these acute wedges fill in a manner different from the right or obtuse angles in the other pores. This study offers a quantitative understanding of the effect of pore structure on methane adsorption in the shale and provides better insight into the evaluation of gas storage in geologic shale reservoirs.

Characteristics of nanopores in terrestrial low-maturity shale based on organic-inorganic interactions: A case study on the Shahejie Formation shale in the southern Bohai Bay Basin, Eastern China

In this study, the influence of organic matter and inorganic matter on the pore structure of the Shahejie Formation (E2-3S) shale in the Bohai Bay Basin, was analysed. The pore characteristics, including the BET surface area, BJH adsorption cumulative volume and average pore diameter of the original samples, extracted samples, H2O2-treated samples and kerogen samples, were analysed with low-pressure N2 adsorption experiment. The study showed that the soluble organic matter in low-maturity shale blocked a certain number of pores, which mainly had pore sizes below 5 nm. The solid organic matter mainly formed pore diameters of 1-2 nm and pore diameters larger than 10 nm. A large amount inorganic pore diameter in the ranges of less than 0.8 nm, 1-2 nm, and 2.3-20 nm was blocked by solid organic matter. The micropores formed by minerals were much more abundant than those formed by organic matter in the terrestrial low-maturity shale. The symbiotic relationship between the minerals and the organic matter led to a decrease in the specific surface area and pore volume of the terrestrial low-maturity shale.

Meso- and microporosity characteristics of Miocene lignite and subbituminous coals in the Kınık coalfield (Soma Basin, W. Turkey)

Three Miocene-age coal seams - a lower seam (kM2, subbituminous), a middle seam (kM3, subbituminous), and an upper seam (kP1, lignite) - in the Kınık coalfield, located in the southwesternmost part of the Soma Basin in western Turkey, and factors controlling mesoporosity and microporosity characteristics, were investigated using low-pressure N2 and CO2 adsorption techniques, respectively. In addition, this study is also aimed to compare controlling factors on meso- and microporosity characteristics of Soma Basin and Late Carboniferous Zonguldak, and Late Miocene-Pliocene Dombayova Basins. It is found that distinct differences with regard to meso- and microporosity exist among the three seams (kM2, kM3 and kP1) investigated in the Kınık coalfield. Specifically, samples from kP1 have greater mesopore surface areas and volumes than those from kM3. In addition, samples from the upper part of kM2 have significantly lower mesopore surface area and volume than from the lower part of this seam and from kM3 and kP1. Samples from the lower part of kM2 have higher meso- and micropore surface area and volume values than kM3 and kP1 samples. The increase in mesopore surface area and volume, along with the increase in ash yield toward the lower part of kM2, suggests that mesoporosity characteristics in this seam appear to be controlled mainly by mineral matter content. The results of this study show that meso- and micropore surface area and volume of Miocene subbituminous coal (kM2) in Kınık and Eynez coalfields of the Soma Basin have higher values than lignite from the Dombayova Basin and bituminous coal from the Zonguldak Basin. The differences in mesopore characteristics between the Soma and the Zonguldak Basins could be related to differences in rank. In addition, the mesoporosity of coal seams in the Dombayova Basin is more similar to seams in the Zonguldak Basin. These conflicting data suggest that rank might not be a controlling factor on the meso- and micropore characteristics of the basins investigated in this paper. Nevertheless, the common presence of structured huminite macerals in the Soma Basin seems to have caused relatively higher microporosity surface area and volume values, whereas the common presence of illite, particularly in the lower parts of kM2, resulted in higher values of mesopore surface area and volume in the Soma Basin samples. Consequently, maceral and mineralogical compositions of the studied Turkish coals seem to exert major controls on meso- and micropore characteristics.

Effect of particle size and adsorption equilibrium time on pore structure characterization in low pressure N2 adsorption of coal: An experimental study

This study examined the influence of particle sizes and adsorption equilibrium time (AET) on pore structure characterization using low pressure N2 adsorption (LPGA-N2) method. The results demonstrate that pore structures change with progressive crushing. The increase in pore volume in the minipore and mesopore with decreasing coal particle sizes was caused by creating additional large pores during the crushing. The tendency of decreasing at first and then increasing was observed for both the specific surface area (SSA) and micropore volume, demonstrating that the micropore structure was primarily damaged and then followed by additional micropores being created when the sample was broken down smaller than 80 mesh. It is difficult to choose a suitable coal particle size for LPGA-N2 measurement to yield pore structure parameters closest to the “real” values due to the difficulty in evaluating the compound effect of crushing on pore structures. To make testing results closer to the coal structure under in situ conditions, we propose 1∼3 mm as the preferred coal particle size. Additionally, many replicate experiments were performed by adjusting the AET to ensure LPGA-N2 isotherms in equilibrium states. The results indicate that a decrease in particle size did not reduce the time for reaching N2 adsorption equilibrium. Comparison of pore size distributions under different AETs shows that the AETs' increase has a little influence on the measurement of minipore and mesopore but has a positive correlation with micropore, revealing that N2 adsorption equilibrium is mainly influenced by the micropore whose volume is underestimated at small AETs. The optimal AET for LPGA-N2 measurement was 8 min and its yielded data can more accurately characterize the pore structure parameters.

Study on the characterization of pore structure and main controlling factors of pore development in gas shale

Shale has a complex pore structure, which leads to complex petrophysical characteristics. The porosity, pore type, pore size, pore volume, TOC, and other petrophysical parameters of shale all affect the shale gas content. Therefore, using multiple methods to jointly characterize the pore structure of shale with different gas content and study on the main controlling factors of pore development have an important guiding role in the exploration and development of shale gas. In this study, we carry out gas shale samples in different areas. The samples collected from wells in the Sichuan Basin have high gas content, collected from wells near the Pengshui, and Tongren areas have medium gas content and collected from wells near the Guiyang area have low gas content. Shale in these areas with different gas production has been characterized, by the integration of geophysical and petrophysical data, in terms of pore type and pore connectivity, including X-ray diffraction, low-field Nuclear Magnetic Resonance (NMR), low-pressure N2 and CO2 adsorption isotherms, porosity, permeability, thin-section observations, scanning electronic microscopy (SEM) and mercury injection capillary pressure (MICP) measurement. The results show that the pore types are clay mineral intercrystalline pores and intergranular pores, rock skeleton mineral pores, organic pores, and micro-fractures. The high gas-bearing shale has a small pore size, a large specific surface area, and pore volume, and the T2 distribution is bimodal, with a peak value around 1 ms. It shows that micropores and mesopores are widely developed in high gas-bearing shale, which provides the main place for adsorbing gas, especially shale with high TOC content. During the hydrocarbon generation process, a large number of organic pores favorable for gas adsorption will be produced. The low gas-bearing shale has a large pore size, a small specific surface area, and pore volume, and the T2 distribution presents a bimodal shape with a wider peak and the lowest spectral value. This shows that low gas-bearing shale mainly develops macropores and micro-fractures, and the pore connectivity is good, especially the interconnection between small pores and fractures, which leads to gas escape. In the high gas-bearing samples, bone needles and radiolaria are distributed in large quantities, and the silicon content of biological sources is high, which provides a lot of organic matter. In the low gas-bearing samples, the horizontal layer is not developed and the mineral particles are evenly distributed.

Differences in CH4 and C2H6 carbon isotopic compositions from open and closed pores in coal: Implications for understanding the two-stage δ13C shift during canister desorption

Canister desorption of freshly drilled coal cores is routinely employed for exploratory assessments of coalbed gas. Coals are molecular sieves containing both open and closed pores. However, little attention has been paid to the differences between the carbon isotopic compositions of CH4 and C2H6 from open and closed pores in coal and their influence on isotopic fractionation during desorption. In this study, canister desorption and pulverization–degassing experiments were conducted to study the carbon isotopic shifts of CH4 and C2H6 during desorption. The δ13C values of CH4 and C2H6 desorbed from three coal samples from the Zhina Coalfield in southwest China displayed a two-stage shift over time during canister desorption (that is, an isotopic decrease followed by an increase), ranging from −38.4 to −30.6‰ and − 32.1 to −28.5‰, respectively. The δ13C values of CH4 and C2H6 desorbed from the coal samples during pulverization ranged from −29.3 to −24.1‰ and − 31.5 to −27.2‰, respectively, approaching the isotopic ranges of coal-derived gas. Low-pressure N2 adsorption measurements showed that many closed pores in coal were opened during progressive pulverization. Gases trapped in closed pores may be considered to be of their original nature, unaffected by subsequent geological processes (i.e., mixing of secondary biogenic gas). Thus, CH4 and C2H6 were more depleted in 13C in open pores than in closed pores. Some open pores on the surface of the coal core were previously closed when in situ but were opened during drilling. Therefore, the initial gas released from the cores was a mixture of gases desorbed from the open and closed pores. The 13C-enriched CH4 and C2H6 desorbed from the pores on the coal core surfaces may migrate into canister headspaces preferentially, thereby providing insight into the two-stage δ13C shift observed during canister desorption.

A novel approach to identify accessible and inaccessible pores in gas shales using combined low-pressure sorption and SAXS/SANS analysis

Resolving the pore attributes of gas bearing shale formations is key to precise estimation of the gas-storing capacity of a reservoir. Decades of research and development on the characteristics of shale pores have proven that both organic matter and clay minerals are capable of storing hydrocarbons in their pores by virtue of adsorption; however, very few studies have been able to quantitatively characterize the individual adsorption capacity of clay minerals and organic matter. The volume of the gas that is produced depends on the accessibility to the pores. Due to the very low permeability of shale, there is a significant difference between the accessible pore volume and total pore volume. By general convention, we often term the accessible pore volume as the total pore volume, whereas the total pore volume is usually quite higher than the accessible pore volume.This study attempts to address these two research gaps using Permian age Barakar Formation shales that were collected from two separate sites in the eastern part of India, from a depth range of 221 to 701 m. The shales had a moderately high content of clay (ranging between 25% and 70%) and an abundance of Type III kerogen. The mesopore size distributions were calculated from N2 adsorption isotherms using a combination of N2 upon silicate and N2 upon carbon density functional theory (DFT) model. It shows a 66–100% increase in the cumulative mesopore volume when compared to the conventional N2 upon carbon model. Micropore size distribution was calculated from CO2 adsorption isotherms using CO2 upon the carbon grand canonical Monte Carlo model, which shows a better fitting than the conventional DFT model. Further, a comparison between small-angle X-ray scattering and low-pressure N2 adsorption indicates a 3% to 15% higher fractal dimension of total pores than accessible pores. The ‘degree of connectivity’ of the shale pores was calculated based on the accessible and total pore volumes, which was found to decreasesteadily with depth. Finally, a correlation between the binding potential of the inorganic-organic constituents, and the surface fractal dimension was established using Monte Carlo simulation. We confirmed that the fractal dimension depends on the binding potential of the inorganic-organic matter, and the difference in the fractal dimension of accessible and inaccessible pores could be explained by the smoothening of accessible pores due to pore fluid invasion. This new combined and comprehensive approach will lead to a better understanding of CH4 and CO2 dynamics in shale pores.

Quantification of the influences of radiolarian fossils on the pore structure of Wufeng-Lungmachi gas shales (Ordovician-Silurian) in the Sichuan Basin, South China

Understanding the differences among shale pore systems is crucial to exploring the enrichment mechanism of shale gas. In this study, we found that radiolarian micro-fossils are widely distributed in the overmature marine organic-rich Wufeng-Lungmachi formations surrounding the Sichuan Basin, South China. This enables an investigation on the full-scale pore structure characteristics of the two formations by using a combination of qualitative methods (field emission scanning electron microscopy (FE-SEM)) and quantitative methods (low-pressure N2 and CO2 adsorption and high-pressure mercury intrusion porosimetry (MIP)). The results show that micropores (accounting for 75.81%) and mesopores (accounting for 63.56%) are mainly responsible for the total surface area and the total pore volume, respectively. Furthermore, organic matter significantly contributes to the shale surface area, and the intercrystalline pores of clay minerals make an important contribution to the pore volume of shale. Abundant biological pores can be discovered in radiolarian fossils, and these biological pores belong to mesopores and macropores. The investigation of gas adsorption shows that radiolarian-poor shale reflects the presence of slit-shaped pores, while radiolarian-rich shale indicates the presence of ink-bottle-shaped pores. In addition, despite similar TOC contents, clay minerals and brittle minerals, radiolarian-rich layers have larger pore volumes than radiolarian-poor layers. The results calculated using the threshold method with ImageJ software show that the average surface porosities of the organic matter, mineral matrix, and radiolarians are 15.69%, 1.84%, and 5.34%, respectively. Given that the quantitative calculation results in a greater contribution by the pores developed in radiolarians, the biological pores developed in radiolarians are beneficial for the enrichment of free gas.

Effect of supercritical CO2 saturation pressures and temperatures on the methane adsorption behaviours of Longmaxi shale

Abstract To investigate the effects of supercritical CO2 (SC–CO2) saturation pressures and temperatures on the methane adsorption capacity of shale, total organic carbon (TOC) analysis, X-ray diffraction (XRD) analysis, scanning electron microscope and energy dispersive spectrometer (SEM/EDS) analysis, low-pressure N2 adsorption (LP-NA), and high-pressure methane adsorption (HP-MA) were conducted on raw and SC–CO2–saturated (8, 12, 16 MPa; 40, 60, 80 °C) shale collected from the Sichuan Basin. Results indicate that the excess adsorption isotherm of shale to methane clearly exhibited excess adsorption characteristics with increasing adsorption pressures. This excess methane adsorption capacity decreased after SC-CO2 saturation and is mainly attributable to decreases in the TOC content, clay minerals and specific surface area (SSA) in the SC–CO2–saturated shale. The density of adsorbed phase methane was considered at different adsorption models, and the results suggest that the Langmuir-Freundlich (LF) and Dubinin-Astakhov (DA) models better describe methane adsorption behaviours than the Langmuir and Ono-kondo models. The methane adsorption capacity of shale is closely related to the solubility of SC-CO2, which decreased significantly with increasing SC-CO2 saturation time and pressure, while the influence of SC-CO2 saturation temperatures was not evident at low-pressure (8 MPa). This study provides a theoretical reference for CO2 enhanced shale gas recovery.

Isothermal characteristics of methane adsorption and changes in the pore structure before and after methane adsorption with high-rank coal

The pore structure is an essential factor that influences the isothermal characteristics of methane adsorption of coal, and the pore structure is altered after methane adsorption. In this study, a high-rank coal sample was investigated via methane adsorption isothermal measurement, and changes in the pore structure were studied using low-pressure N2 adsorption and low-pressure CO2 adsorption before and after the methane adsorption. The excess adsorption capacity exhibits a rapid increase at low pressure, reaching a maximum when the test pressure is approximately 8 MPa. Following that, the excess adsorption capacity of the high-rank coal tends to decrease. After the methane adsorption, the pore volume and specific surface area of the micro-, meso-, and macropores increase as compared to those before the methane adsorption, especially for micropores with apertures greater than 0.8 nm and mesopores with apertures below 10 nm. This is mainly caused by high pressure in the methane adsorption, indicating a pressure effect on the pore structure after the methane adsorption. After the methane adsorption, the ratio of pores with various sizes in the high-rank coal is enhanced, but the connectivity for meso- and macropores presents a slight decrease.