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The parameters of pore-fracture structure and permeability have a controlling effect on the behaviors of gas adsorption/desorption and transportation in coal reservoir. A mathematic model for coal seams is of great significance to evaluate the mass migration within the coal fracture-matrix system. In this paper, the hydraulic-mechanical coupling model considering both dual-porosity dual-permeability and anisotropy characteristics is first established by using the methods of theoretical analysis, nuclear magnetic resonance (NMR) test, and numerical simulation. Then, this model is applied to simulate the gas migration characteristics of No. 3 coal seam in Changping Mine, China. Results show that the pore structure of No. 3 coal seam is characterized by small radius of pores and pore throats, which is determined by the NMR test, verifying the dual-permeability dual-porosity of coal seams. Both matrix permeability and fracture permeability increase approximately linearly with the process of mine gas extraction. The increased magnitude of the matrix permeability is greater than that of the fracture permeability. The permeability is inversely proportional to the anisotropy coefficient. The pressure gradient within the coal matrix and fracture increases first and then decreases with the extraction time. This pressure gradient is proportional to the anisotropy coefficient at the early stage of extraction and is inversely proportional to the anisotropy coefficient at the later stage. The seepage and diffusion flux are proportional to the anisotropy coefficient. The proportion of matrix-to-fracture seepage flux to the total flux increases first and then decreases to a certain value. The proposed model provides a guide for accurate designation of gas extraction from coal seams. [ABSTRACT FROM AUTHOR]

Marine shales from the Niutitang Formation and marine–continental transitional shales from the Longtan Formation are two sets of extremely important hydrocarbon source rocks in South China. In order to quantitatively compare the pore complexity characteristics between marine and marine–continental transitional shales, the shale and kerogen of the Niutitang Formation and the Longtan Formation are taken as our research subjects. Based on organic petrology, geochemistry, and low-temperature gas adsorption analyses, the fractal dimension of their pores is calculated by the Frenkel–Halsey–Hill (FHH) and Sierpinski models, and the influences of total organic carbon (TOC), vitrinite reflectance (Ro), and mineral composition on the pore fractals of the shale and kerogen are discussed. Our results show the following: (1) Marine shale predominantly has wedge-shaped and slit pores, while marine–continental transitional shale has inkpot-shaped and slit pores. (2) Cylindrical pores are common in organic matter of both shale types, with marine shale having a greater gas storage space (CRV) from organic matter pores, while marine–continental transitional shale relies more on inorganic pores, especially interlayer clay mineral pores, for gas storage due to their large specific surface area and high adsorption capacity (CRA). (3) The fractal characteristics of marine and marine–continental transitional shale pores are influenced differently. In marine shale, TOC positively correlates with fractal dimensions, while in marine–continental shale, Ro and clay minerals have a stronger influence. Ro is the primary factor affecting organic matter pore complexity. (4) Our two pore fractal models show that the complexity of the shale in the Longtan Formation surpasses that of the shale in the Niutitang Formation, and type I kerogen has more complex organic matter pores than type III, aiding in evaluating pore connectivity and flow effectiveness in shale reservoirs. [ABSTRACT FROM AUTHOR]

The pore-fracture structure of deep coal reservoirs is highly important for evaluating, exploring, and developing coalbed methane (CBM) resources. This study considers three coal samples from the DJ57 well in the Benxi Formation in the Daning–Jixian block on the eastern margin of the Ordos Basin as the research object. Based on the coal quality parameters of the coal samples, field emission scanning electron microscopy (FE-SEM), gas adsorption experiments, high-pressure mercury intrusion porosimetry (MIP), and microcomputed tomography (micro-CT) scanning were used to quantitatively characterize the nanoscale pores and microscale fractures in deep coal reservoirs and to evaluate the pore-fracture structure at different scales. The results reveal that the pore types in the Benxi Formation coal samples are diverse and include mainly organic matter (OM) pores, inorganic pores (intraparticle and interparticle pores), and microfractures. The organic pores are diverse in shape and mainly exhibit round, oval, and wedge shapes, while the microfractures exhibit slender stripes or serrated curves. The multiscale quantitative characterization of deep coal reservoir pores and fractures is based on a variety of pore characterization methods, and the pore and fracture volume distributions are mainly U-shaped, revealing the coexistence of micropores and microfractures. The volumes of micropores (0.3–2 nm), mesopores (2–50 nm), macropores (50 nm to 10 μm), and microfractures (>10 μm) account for 78.00%, 6.78%, 2.08%, and 13.14%, respectively, of the total pore volume (PV). Based on a full-scale pore-fracture splicing calculation, the total permeability of the Benxi Formation coal samples ranges from 5.77 to 28.22 mD. The observation results indicate that the microfractures are connected to each other, forming a network structure with strong connectivity. The microfractures are mainly associated with pore diameters > 100 μm, accounting for approximately 95% of the total permeability. Moreover, micropores in deep coal reservoirs provide a large space for CBM adsorption, and microfractures enhance the seepage capacity of CBM. [ABSTRACT FROM AUTHOR]

Deep low-rank coalbed methane (CBM) resources are numerous and widely distributed in China, although their exploration remains in its infancy. In this work, gas adsorption (N2/CO2), mercury intrusion porosimetry, and 3D CT reconstruction were performed on five coal samples of deep and shallow low-rank coal from northeast China to analyze their pore structure. The impact of the features in the pore structure at full scale on the capacity for methane adsorption and seepage is discussed. The results indicate that there are significant differences between deep low-rank coal and shallow low-rank coal in terms of porosity, permeability, composition, and adsorption capacity. The full-scale pore distribution was dispersed over a broad range and exhibited a multi-peak distribution, with the majority of the peak concentrations occurring between 0.45–0.7 nm and 3–4 nm. Mesopores are prevalent in shallow coal rock, whereas micropores are the most numerous in deep coal rock. The primary contributors to the specific surface area of both deep and superficial coal rock are micropores. Three-dimensional CT reconstruction can characterize pores with pore size greater than 1 μm, while the dominating equivalent pore diameters (Deq) range from 1 to 10 μm. More mini-scale pores and fissures are observed in deep coal rock, while shallow coal rock processes greater total and connection porosity. Multifractal features are prevalent in the fractal qualities of all the numbered samples. An enhancement in pore structure heterogeneity occurs with increasing pore size. The pore structure of deep coal rock is more heterogeneous. Furthermore, methane adsorption capacity is favorably connected with D1 (0.4 nm < pore diameter ≤ 2 nm), D2 (2 nm < pore diameter ≤ 5 nm), micropore volume, and specific surface area and negatively correlated with D3 (5 nm < pore diameter ≤ 50 nm), showing that methane adsorption capability is primarily controlled by micropores and mesopores. Methane seepage capacity is favorably connected with the pore volume and connected porosity of macropores and negatively correlated with D4 (pore diameter > 50 nm), indicating that the macropores are the primary factor influencing methane seepage capacity. [ABSTRACT FROM AUTHOR]

Using Isothermal Adsorption/Desorption System Model 15-100 and Electrohydraulic Servo Rock System Model MT5815 as the main apparatuses and collecting samples from the major coal reservoirs in the south of Qinshui Basin, a hot point region of coalbed methane exploration, the paper carries out systematical comparisons of the isothermal adsorption experimental data for injection water coal samples, equilibrium moisture samples and dry coal samples, probes and establishes an experimental method of injection water coal sample preparation and isothermal experiment to simulate real reservoir conditions, and then summaries the experimental regulations and discusses the mechanism of liquid water influencing coal methane adsorption. Results of the experiment indicate that: The Langmuir volume of injection water coal samples is notably larger than that of equilibrium moisture samples, as well as larger than or equivalent to that of dry coal samples; the Langmuir pressure of injection water coal samples is the highest, the next is equilibrium moisture samples, while the dry samples is the lowest, of which the experimental results of injection water samples to simulate real reservoir conditions are more close to the fact. Under the conditions of in-position reservoirs, liquid water in coals has evident influence on methane adsorption ability of coal matrix, which can increase the adsorbability of coal and make the adsorption regulation fit to Langmuir model better. Its major reason is the increase of wetting coal matrix adsorbability. The above experimental results overthrow the conventional cognition that liquid water has no influence on coalbed methane adsorption, which may lead to an improvement of the coalbed methane isothermal adsorption experimental method and of the reliability of coalbed methane resource evaluation and prediction. [ABSTRACT FROM AUTHOR]

The interaction between shale and various fluids is crucial as it modifies pore structures, which govern the effective development of shale gas and the geological storage of carbon dioxide in shale formations. In this study, samples from the Longmaxi Formation shale in Sichuan Basin of China were exposed to different fluids, including 6 MPa CO2, 12 MPa CO2, 6 MPa CO2+brine, and 12 MPa CO2+brine, at 45 °C for 100 days. Various methods, including X-ray diffraction (XRD), X-ray fluorescence (XRF), field-emission scanning electron microscopy (FESEM), and the low-pressure gas adsorption (N2) test, were adopted to evaluate chemical and structural changes during the exposure process. After being treated with supercritical CO2+brine and subcritical CO2+brine, the shale underwent significant changes in its major element composition. The content of Ca, Al, and K in shale saturated with supercritical CO2+brine decreased from 13.00% to 10.34%, from 3.65% to 3.36%, and from 1.56% to 1.37%, respectively. Meanwhile, the content of Si and Na in the same shale increased slightly after saturation. The amount of quartz and dolomite increased, while the levels of clay and calcite slightly decreased. The surface of the shale sample became rougher and small bumps and cracks appeared after saturation with different fluids, as shown by the FESEM analysis results. Furthermore, the changes in both the total pore volume and pore size followed a similar pattern to the alterations in the specific surface areas. The highest level of variation occurred with the shale that was saturated with 12 MPa of CO2, indicating that gas pressure and CO2 phase state have a significant influence on the shale's pore structure. In addition, the distribution of pore sizes showed a bias towards larger sizes across all diameters; this suggests that the reaction resulted in a decrease in the number of micropores. This also highlights that the impact of varying fluid saturation was primarily focused on micropores and macropores. The results of this study provided experimental evidence to further test the mechanisms and permeability of geological storage of CO2 in organic-rich self-sourced shale. [ABSTRACT FROM AUTHOR]

Tectonic deformation of different intensities significantly controls shale pore structure, seepage channels, and gas content. The Longmaxi Formation shales in the southern Sichuan Basin have experienced multi-stage tectonic movements, resulting in a diverse fracture system and tectonic deformation. This study focuses on three representative tectonic morphologies: deeply buried strongly deformed (DBSD), deeply buried weakly deformed (DBWD) and shallowly buried weakly deformed (LBWD). We investigated the pore structure characteristics and heterogeneity of these shales under various tectonic conditions using total organic carbon (TOC) content, X-ray diffraction (XRD), scanning electron microscopy (SEM), a low-pressure N2/CO2 adsorption experiment (LP-N2/CO2 GA), and multi-scale fractal theory. The results reveal that strong tectonic compression and deformation conditions lead to the compression and flattening of organic pores by brittle minerals, resulting in long, oriented OM pores. Fracturing of brittle pore creates multiple internal fracture systems linked to dissolution pores, forming a complex micro-fracture–pore network. With intense tectonic deformation, mesopores tend to be compressed, increasing micropore pore volume (PV) and surface area (SA). The DBSD shale exhibits the highest micropore heterogeneity, while the LBWD shale shows the lowest heterogeneity. Fractal analysis indicates a significant decrease in micropore fractal dimension (Df) with increasing burial depth. In contrast, the surface and matrix fractal dimensions (Ds and Dm) of low-buried shale micropores and meso-macropores align vertically. Shale reservoirs in tectonically stable regions exhibit more favourable gas-bearing characteristics than strongly tectonically deformed areas. The LBWD has stable tectonic conditions that are favourable for shale gas preservation. Conversely, slip faults under deep burial conditions lead to extrusion and deformation of shale pore space, ultimately compromising the original reservoir capacity and hindering shale gas enrichment. These findings contribute significantly to our understanding of pore structure and heterogeneity in tectonically deformed shale reservoirs, providing invaluable guidance for the exploration, development, and prediction of shale gas resources. [ABSTRACT FROM AUTHOR]

Due to the insufficient accurate evaluation model of gas resources in abandoned mines, the development and utilization of abandoned mine gas resources in China are still in the preliminary exploration stage. To this end, in response to the problem of imprecise assessment of gas resource reserves in the adsorbed state of residual coal, the No. 3 coal seam in Jincheng mine was used as the research object, by analyzing the evolution of the ambient gas pressure and temperature of the residual coal in the abandoned mine, and simulating the pressure and temperature environment in which the residual coal is located, the gas adsorption experiment of the residual coal was carried out in this environment, and a gas storage model of the residual coal was constructed with the combination of the adsorption theory. The results show that pressure has a positive effect on gas adsorption by residual coal, while temperature has a negative effect on it. The adsorption potential gradually decreases with increasing pressure and increases with increasing temperature, while the adsorption space increases with increasing pressure and decreases with increasing temperature. The adsorption characteristic curves obtained by the two methods are temperature independent and polynomial in shape, and the fitted correlation coefficient of method 1 is generally higher than that of method 2. The adsorption characteristic curves of two methods are best fitted at k = 2.5 , so the value of k is most appropriately taken as 2.5 when calculating the virtual saturation vapour pressure. The average relative errors between the predicted and measured values of method 1 and 2 were 2.98% and 7.13%, respectively. The prediction effect of method 1 was significantly better than that of method 2 and met the requirements of engineering applications; therefore, method 1 was used to establish a model for the adsorbed gas storage capacity of residual coal in abandoned mines. [ABSTRACT FROM AUTHOR]

The accuracy of adsorbed gas content under actual in situ reservoir conditions is crucial for the evaluation of shale gas reserves. In this study, the characteristics of methane adsorption on the Wufeng–Longmaxi shale were investigated under a wide range of pressure (0–51 MPa) and actual in situ water saturation. Methane–shale adsorption exhibits the Gibbs excess adsorption phenomenon at high pressure. The excess adsorption amount needs be corrected to absolute adsorption amount, otherwise it will be seriously underestimated. The optimal supercritical methane adsorption model was determined by the corrected Akaike's Information Criterion method. The methane adsorption amount of shale samples ranged from 1.521 to 4.079 m3/t. Adsorption capacity was dominated by the total organic carbon content as well as micropore volume and total specific surface area. Additionally, pore volume and specific surface area were contributed mainly by abundant micropores associated with organic matter. Thermodynamic parameters revealed that the adsorption of methane on shale was an exothermic process. As the temperature increased from 40 to 80 °C, the methane adsorption capacity decreased from 4.27 to 2.99 m3/t, a 30% decrease. The actual in situ water saturation correlated primarily with clay content, regardless of clay types, and increased with clay content. The formation of an adsorbed water film and the blockage of pores for gas adsorption by clusters of water molecules significantly lowered the adsorption capacity. The relative difference in the adsorption capacity increased with water saturation, with the adsorption capacity of water-bearing shales decreasing by 21–84% at a water saturation of 30–71% compared to dry shales. [ABSTRACT FROM AUTHOR]

The accurate description of micro-/nanopores in deep coal reservoirs plays an important role in evaluating the reservoir properties and gas production capacity of coalbed methane (CBM). This work studies nine continuous samples of high–rank coal from the Daning–Jixian area of the Ordos Basin. Maceral analysis, proximate analysis, field emission scanning electron microscopy (FE-SEM), low-pressure CO2 adsorption (LPA), low-temperature N2 adsorption (LTA) and high-pressure methane adsorption (HPMA) experiments were conducted for each sample. The fractal dimension (D) of the LPA data was calculated by using the micropore fractal model. The characteristics of the deep coal reservoir pore structure, proximate analysis, relationship between maceral and fractal dimensions, and gas adsorption capacity of the micropores are discussed. The results showed that the combination of LPA with nonlocalized density functional theory (NLDFT) models and LTA with NLDFT models can more accurately determine the pore size distribution of the micropores. The pore volume (PV) and specific surface area (SSA) of the coals were distributed in the ranges of 0.059~0.086 cm3/g and 204.38~282.42 m2/g, respectively. Although the degree of micropore development varies greatly among different coal samples, the pore distribution characteristics are basically the same, and the PV and SSA are the most developed in the pore size range of 0.4–0.7 nm. Ash content (Ad) and mineral composition are two major factors affecting micropore structure, but they have different impacts on the fractal dimension. The higher the vitrinite content, moisture content (Mad) and Ad are, the larger the micropore fractal dimension (D) and the stronger the heterogeneity of the pore structure. Micropores account for 99% of the total SSA in coal, and most methane can be adsorbed in micropores. The fractal dimension of micropores can be used to evaluate the pore structure characteristics. The larger the fractal dimension, the smaller the micro-SSA and micro-PV of the coal sample. Fractal analysis is helpful to better understand the pore structure and adsorption capacity of CBM reservoirs. [ABSTRACT FROM AUTHOR]

Using salt caverns for an underground strategic petroleum reserve (SPR) is considered as an ideal approach due to the excellent characteristics of low porosity, low permeability, self-healing of damage, and strong plastic deformation ability of rock salt. Salt deposits in China are mostly layered rock salt structures, with the characteristics of many interlayers, bringing great challenges for the construction of SPR facilities. Studying the microscopic pore characteristics of the rock surrounding SPR salt caverns in different environments (with brine and crude oil erosion) is necessary because the essence of mechanical and permeability characteristics is the macroscopic embodiment of the microscopic pore structure. In this paper, XRD tests and SEM tests are carried out to determine the physical properties of storage media and surrounding rock. Gas adsorption tests and mercury intrusion tests are carried out to analyze the microscopic pore structure, specific surface area variation and total aperture distribution characteristics of SPR salt cavern host rock. Results show that: (1) Large numbers of cores in interlayer and caprock may provide favorable channels for the leakage of high-pressure crude oil and brine. (2) The blockage of pores by macromolecular organic matter (colloid and asphaltene) in crude oil will not significantly change the structural characteristics of the rock skeleton, which is beneficial to the long-term operation of the SPR salt cavern. (3) The water–rock interaction will bring obvious changes in the micro-pore structure of rock and increase the leakage risk of the storage medium. The results can provide theoretical bases and methods for the tightness analysis of China's first underground SPR salt cavern. [ABSTRACT FROM AUTHOR]

The pore structure and microfracture evolution characteristics of coal in high gas mine are worthy of studying to make clear the mechanism of coal and gas outburst. Samples were collected from 2# coal seam (high gas coal seam) in Dongpang Coal Mine, China. Two groups of samples were divided: one was cut from the intact coal blocks which was roughly a cube with the particle size less than 1 cm, and the other was the pulverized coal drilled from the working face. Mercury intrusion porosimetry (MIP) and gas absorption were used to investigate the pore size distributions (PSDs) of coal samples. The experimental results display that the compressibility of coal matrix has a significant influence on the pore volume of coal specimens especially when the pressure is greater than 20 MPa. Macropores in both intact and pulverized coal samples account for the largest proportion but the average micropore volume for the pulverized coal is 2.67 times that of the intact coal. The greater pore tortuosity of pulverized coal indicates that it has worse pore connectivity overall. Multi-fractal analysis result shows the PSDs of pulverized coal are more homogeneous than intact coal. During the process of mining, the vertical loading and horizontal unloading cause the opening and propagation of microfractures and increase the difference of the PSDs between the intact and pulverized coal. It results in the not smooth of gas flow from pulverized coal area. Gas might be prone to accumulate in the area with more pulverized coal. [ABSTRACT FROM AUTHOR]

In order to explore the influences of effective stress change on gas adsorption–desorption behaviors, primary undeformed coal (PUC) and tectonically deformed coal (TDC) from the same coal seam were used for adsorption–desorption experiments under different effective stress conditions. Experimental results showed that gas adsorption and desorption behaviors were controlled by the coal core structure and the pore-fissure connectivity under effective stress. The coal matrixes and fissures were compressed together under effective stress to reduce connectivity, and it was difficult for gas to absorb and desorb as the stress increased in primary undeformed coal. The loose structure of tectonically deformed coal cores can help gas to fully contact with the coal matrix, resulting in higher adsorption gas volumes. The support of coal particles in tectonically deformed coal cores weakens the compression of intergranular pores when effective stress increases, which in this study manifested in the fact that while the volumetric strain of the coal matrix change rapidly under low effective stress, but the adsorbed gas volume did not decrease significantly. The reduction in effective stress induced the rapid elastic recovery of the coal matrix and the expansion of cracks, and increased desorption gas volumes. The stress reduction significantly increased the initial gas volume of the tectonically deformed coal, while promoting slow and continuous gas desorption in primary undeformed coal. Therefore, the promotion effect of the reservoir pressure reduction on gas desorption and coal connectivity enhancement can help to improve coalbed methane recovery in primary undeformed coal and tectonically deformed coal reservoirs. [ABSTRACT FROM AUTHOR]

The Triassic Zhangjiatan shale in the Ordos basin, recognized as one of most promising shale oil and gas resources in China, is one example of medium-low-maturity lacustrine shale reservoirs wherein the pore network has been proven to be different from those of both conventional and high-maturity marine shale reservoirs. Here, we examine the quantitative characterization of pore size and movable fluid distribution of the Zhangjiatan shale reservoir. Eight shale core plugs were measured in a series of nuclear magnetic resonance (NMR) experiments to determine the pore structure and pore fluid transport during the processes of centrifuging and heating. The complete pore size distribution was obtained from the T 2 spectrum by integrating NMR, high-pressure mercury intrusion, and nitrogen gas adsorption. The results show that the movable fluid saturation of Zhangjiatan shale is 19.47–33.02% and the capillary-bound fluid saturation is 14.53–27.62%, whereas the unrecoverable fluid saturation was 43.51–65.26%. The movable fluids are mainly detected in macropores and mesopores with a minimum pore size of 50.5–121.2 nm, while capillary-bound fluids are mainly found in small pores and some mesopores with a minimum size of 25.8–67.5 nm. Almost all unrecoverable fluids are in micropores and small pores. In contrast to high-maturity shale reservoirs, the Zhangjiatan shale shows a poor correlation between total organic carbon content and unrecoverable fluid saturation but a good correlation between clay minerals and unrecoverable fluid saturation, showing that most micropores are associated with clay minerals and the organic matter pores are less developed. This study provides an accurate determination of the pore size distribution and pore fluid typing of Zhangjiatan shale, which is of great significance for the development of high-quality but medium-low-maturity shale reservoirs in China. [ABSTRACT FROM AUTHOR]

Fine dissection of microscopic pore structure variations between the Niutitang Formation and the Wufeng-Longmaxi Formation will help to improve the understanding of the underlying geological theory of shale gas in northern Guizhou Province. The stratigraphic, geochemical, physical, and tectonic properties of the two formations vary greatly, resulting in differential development of the microscopic pore structure among reservoirs and, as a result, major variances in gas concentration. To explore the mechanism of differential pore evolution, experimental techniques and instruments such as gas adsorption, liquid intrusion, SEM, XRD, and organic geochemical tests were utilized. The results indicate that the Wufeng-Longmaxi Formation is in a high-maturity stage, while the Niutitang Formation is in an over-mature stage. The latter has a higher TOC content. Both petrographic phases are siliceous shale petrographic phases, and the former has more developed dissolution pores with better pore volume, throat radius, and macropore pore diameters than the latter, as well as organic matter pores, intergranular pores, and microfracture structural parameters, whereas the specific surface area is the opposite. The differences in reservoir pore formation between the two formations were analyzed, and the results showed that the petrographic type, thermal evolution, and tectonic preservation conditions were the primary controlling elements of differential shale gas reservoir formation. A differential reservoir-forming model of the Wufeng-Longmaxi Formation and the Niutitang Formation was constructed, providing a geological and theoretical basis for shale gas geological exploration in northern Guizhou Province. [ABSTRACT FROM AUTHOR]

This paper describes the effects of igneous intrusions on pore structure and adsorption capacity of the Permian coals in the Huaibei Coalfield, China. Twelve coal samples were obtained at different distances from a ~120m extremely thick sill. Comparisons were made between unaltered and heat-affected coals using geochemical data, pore-fracture characteristics, and adsorption properties. Thermal alteration occurs down to ~1.3 × sill thickness. Approaching the sill, the vitrinite reflectance (R°) increased from2.30% to 2.78%, forming devolatilization vacuoles and a fine mosaic texture. Volatile matter (VM) decreased from 17.6% to 10.0% and the moisture decreased from 3.0% to 1.6%. With decreasing distance to the sill, the micropore volumes initially increased from 0.0054 cm³/g to a maximum of 0.0146 cm³/g and then decreased to 0.0079 cm³/g.The results show that the thermal evolution of the sill obviously changed the coal geochemistry and increased the micropore volume and adsorption capacity of heat-affected coal (60-160m from the sill) compared with the unaltered coals. The trap effect of the sill prevented the high-pressure gas from being released, forming gas pocket.Mining activities near the sill created a low pressure zone leading to the rapid accumulation of methane and gas outbursts in the Haizi Mine. [ABSTRACT FROM AUTHOR]

Gas injection into coal seams is gaining increasing attention for improving coalbed methane (CBM) production. This technical development requires detailed and reliable information on gas adsorption/desorption-induced deformation and gas occurrence mechanisms. Hence, in this paper, high volatile bituminous coal was taken as the experiment samples from northern Shannxi Province, China, and adsorption-induced swelling experiments with CO 2 and N 2 were carried out under the conditions of one-time injection and step-by-step injection. Then, based on adsorption-induced swelling characteristics, a thermodynamic model was further established to disclose the adsorption/desorption-induced deformation mechanism of confined coals. Results showed that the swelling/shrinkage strain curves of CO 2 and N 2 adsorption/desorption in the coal matrix were generally divided into four stages, including pressurization-deformation, adsorption-induced swelling, depressurization-deformation, and finally desorption-induced shrinkage. It can be seen that the coal swelling effect caused by N 2 adsorption was far less than that caused by CO 2 adsorption. However, both CO 2 and N 2 adsorption displayed that the apparent volumetric swelling weakened with increasing strain when compared with the same adsorption pressure under different strain conditions. In contrast, the absolute volumetric swelling of the confined coal was a coupling result from adsorption-induced swelling and pore pressure effects. Further, based on the thermodynamic model, it was verified that the adsorbate type, the chemical potential of the adsorbate molecule and stress conditions are key factors affecting adsorption-induced swelling of coal under a certain temperature and pressure. In addition, a desorption lag phenomenon during the desorption-induced shrinkage stage was observed, due to incomplete gas desorption and gas retention in the coal macromolecular network. It was also found that there was a shrinkage phenomenon after the peak strain during gas adsorption. That is to say, when fixed adsorption sites on the coal pore surface reached the adsorption saturation state at the peak strain, gas molecules would be stored in an absorption state in the coal macromolecular network under continuous adsorption. These phenomena further explained coal swelling and gas occurrence mechanisms. • Swelling/shrinkage strain curve can be generally divided into four stages. • Absolute volumetric swelling is controlled by adsorption swelling and pore pressure. • Thermodynamic model reflects Gibbs free energy of gas adsorption in coal decreases. • Adsorption-induced swelling effect tends to be dropped with increasing stress. • Shrinkage effect after peak strain and desorption lag are occurred in whole process. [ABSTRACT FROM AUTHOR]

Coal pores not only serve as the storage space for coalbed methane but also provide channels for gas migration. The accurate characterization of coal pore structures is of significance to study the methane adsorption behaviors. In this work, the quantitative relationship between gas adsorption and pore characteristics was investigated in depth for bituminous coals. Results of scanning electron microscopy showed that the surface morphological characteristics of these samples differ greatly. Some typical pore types including cylindrical pores and conical pores were found in these samples. The remarkable hysteresis loop was observed, which is attributed to the bottle-shaped pores with poor connectivity. Fractal theory was introduced to quantitatively evaluate the surface roughness of coal. Pore fractal dimensions, D1 and D2, were calculated using low-pressure N2 gas adsorption data, and their values were in the range of 2.125–2.721 and 2.084–2.461, respectively. D1 was larger than the corresponding D2 for the same sample, suggesting that micropore structures in coal were more complex when compared with mesopores and transition pores. Both D1 and D2 were enhanced with increase in micropore specific surface area, but they were reduced with increase in mesopore specific surface area. Gas adsorption in coal was estimated from the perspective of fractal dimension. Judging from the fitting degree, the influence of D1 on adsorption capacity of coal was remarkably greater than that of D2. D1 is expected to be used as one of the major adsorption indicators in the future study. [ABSTRACT FROM AUTHOR]

To understand the pore structure and heterogeneity of pore size distribution (PSD) is essential for revealing fluid mechanics and evaluating the utilization of unconventional resources. In this study, there are multiple shale examples collected from the Chang 7 section in the Ordos Basin for the investigation was conducted on the basis of various experiments on total organic carbon (TOC), X-ray diffraction (XRD), and nitrogen gas adsorption, through scanning electron microscopy (SEM) and multifractal method. The multifractal characteristic parameters, including the width of singularity spectra (Δ α), Hurst exponent (H), D 1 / D 0 , and nitrogen gas adsorption, were used to find out about the characteristics of pore development and to quantify the complexity and heterogeneity of pore structure. Depending on the exact mineral composition, the Yanchang Formation of Chang 7 shales is classified into either silty mudstone (SM) or muddy siltstone lithofacies (MS). According to the investigative results, the Chang 7 lacustrine shale features a complex pore system with the pores ranging from 1.5 to 10 nm in diameter. Besides, mesopores contribute significantly to the total pore volume (TPV) and total surface area (TSA). As for TPV and TSA of the SM lithofacies in the samples under investigation, they are nearly 1.09–1.78 and 0.80–1.72 times greater as compared to the MS lithofacies samples. The dominant types of reservoir spaces include organic matter (OM) pore and interparticle pore which are related to inorganic minerals. The value of Δ α is higher for MS lithofacies than for SM lithofacies, indicating a greater heterogeneity of PSD in the MS lithofacies. The pore structure of MS lithofacies is determined mainly by TOC and siliceous mineral content, whereas the influencing factors for SM lithofacies are TOC and clay mineral content. There is a significant relationship between multifractal parameters and pore structure parameters for both SM and MS lithofacies. The TOC of SM and MS lithofacies exhibits a close correlation with Δ α , suggesting that the pores in organic matter are dominated by those nanopores with a complex and heterogeneous pore structure. The rock composition of the lithofacies can affect Δ α to a varying extent, which means that the minerals have an evident impact on the heterogeneity of MS and SM lithofacies. [ABSTRACT FROM AUTHOR]

The storage of CO2 in deep unminable coal seams can mitigate greenhouse gas emissions. However, CO2 storage in deep anthracite coal is complex with some uncertainties in the estimation of CO2 storage capacity. Based on isothermal adsorption experiments and gas solubility experiments under high temperature and pressure conditions, the total storage capacity of CO2 in anthracite coal is discussed. The results show that the absolute adsorption amount is over 44 cm3/g at temperatures of 318.15, 335.65, and 353.15 K as well as adsorption equilibrium pressures of 10 MPa. The storage capacity of adsorbed and free gas is 35–70 cm3/g and 5–8 cm3/g, respectively, within a depth range of 1000–2000 m. The soluble gas can be ignored for its low content between 0.22 cm3/g and 0.28 cm3/g with a proportion of less than 1%. The storage capacity of CO2 may be estimated inaccurately because of the heterogeneity and uncertainty of the macroscopic geological conditions and coal reservoir parameters. Taking the No. 3 coal seam in Zhengzhuang block as an example, the storage priority area was divided into supercritical area and subcritical area with five sub-areas according to storage conditions, and the storage capacity was calculated, showing a relatively good storage potential. [ABSTRACT FROM AUTHOR]

To investigate pore characteristics and the factors controlling lacustrine shales, geochemical, mineralogical and petrophysical experiments were performed on 23 shale samples from the Qingshankou Formation of the Songliao Basin, China. A comparison of mercury injection capillary pressure (MICP) and low‐temperature N2 adsorption pore‐size distribution showed that MICP has a higher pore‐size distribution (PSD) line in its overlapping pore diameter range, which may be elevated by the higher pressure of MICP. Therefore, in the overlapping range, low‐temperature N2 adsorption data were preferred in pore characterization. Negative correlations were observed between pore volumes and TOC content, indicating organic matter pores are not well‐developed in the studied samples. This may be related to their low grade of maturity and type I kerogens. There existed negative relationships between pore volumes and S1, which illustrated that liquid hydrocarbons occupied some pore space. Micropore volume had a better correlation with S1 than mesopore and macropore volumes, which suggests that liquid hydrocarbons preferentially occur in micropores. No obvious relationships between pore volumes and quartz or feldspar were observed, while pore volumes increased with the increasing clay mineral content. These relationships indicate that intraparticle pores in clay minerals represent the principal pore type. [ABSTRACT FROM AUTHOR]

Organic‐rich shales, deposited in marine‐continental transitional environments, are widely distributed in southern China. The pore evolution of the Late Permian Longtan Formation shale (Guizhou Province) during its diagenesis and organic matter (OM) evolution was quantitatively and qualitatively investigated through thermal simulation, mercury intrusion capillary pressure, gas adsorption, fractal dimension, and field emission‐scanning electron microscopy observation. Diagenesis and OM evolution stage were subdivided on the basis of X‐ray diffraction, rock pyrolysis, and vitrinite reflectance test; moreover, the main controlling factors of pore structure during evolution were also discussed. Shales were heated to different temperatures with their vitrinite reflectance ranged between 1.23% and 3.12%, indicating that organic matter had evolved from a low‐ to a post‐mature stage. According to the changes in clay mineral composition, hydrocarbon generation, and Tmax, we subdivided diagenesis into four parts, each of which has a good correspondence with OM evolution. Pore volume (PV) varied between 0.012162 and 0.033482 cm3/g, while the specific surface area (SSA) varied between 13.3693 and 23.0094 m2/g. Mesopores were the main contributors to the total pore volume, while mesopores and micropores were the main contributors to the total specific surface area. In this study, the evolution of pore structure was not monotonous, but intermittent: The PV and SSA of shale samples first decreased and then increased. Maturity was the most important factor affecting the evolution of pore structure. The abundance of pores in OM, associated with hydrocarbon generation, resulted in large micro‐PV and micro‐SSA; moreover, the composition of clay minerals also influenced the pore structure evolution. The transformation of kaolinite into illite increased the content of illite and illite/smectite mixed layer, hence affecting the overall meso‐PV and meso‐SSA. [ABSTRACT FROM AUTHOR]

Gas occurrence in coal seams shows zonation, and the outburst risk prediction index is significantly affected by coal seam gas. For different coal mine, the sensitivity of prediction index of outburst risk is inconsistent, and the different area of the same coal mine can present different sensitivities to the same index. In this study, we measured the industrial analysis of coals and determined the degree of outburst risk of each coal seam using coal samples with the particle size of 1 ∼ 3 mm. The relationships between the equilibrium gas pressure and gas desorption index of drill cuttings, between the desorption index of drill cuttings and the adsorption pressure, and between the critical values of the indexes of drill cuttings were also analyzed using the fuzzy clustering method. Based on the analysis of the particle size distribution of coal samples, the influence of grain size on the critical value of the drill cuttings desorption index was studied. The results showed that the critical values of the desorption index of drill cuttings were different under different adsorption equilibrium gas pressures, and those critical values increased with increasing adsorption equilibrium gas pressure. The desorption index of drill cuttings and adsorption equilibrium gas pressure had a linear relationship, and the higher the degree of outburst risk was, the greater the slope of the fitting function and the smaller the intercept; under the same gas pressure of adsorption equilibrium, there was a linear relationship between Δh2 and K 1, and the higher the outburst risk of coal seam was, the smaller the slope of the fitting function and the greater the intercept. The drill cuttings desorption index Δh2 was more sensitive than the index K 1. Under the same test pressure, the smaller the particle size of the coal samples was, the greater the value of desorption index of drill cuttings and the higher the outburst risk of coal seam. The research results have significant theoretical importance and practical value for the prediction and prevention of coal and gas outbursts in coal mines with similar occurrence conditions of coal seams. [ABSTRACT FROM AUTHOR]

Based on the experiments of nitrogen gas adsorption (N2GA) and nuclear magnetic resonance (NMR), the multifractal characteristics of pore structures in shale and tight sandstone from the Chang 7 member of Triassic Yanchang Formation in Ordos Basin, NW China, are investigated. The multifractal spectra obtained from N2GA and NMR are analyzed with pore throat structure parameters. The results show that the pore size distributions obtained from N2GA and NMR are different, and the obtained multifractal characteristics vary from each other. The specific surface and total pore volume obtained by N2GA experiment have correlations with multifractal characteristics. For the core samples with the similar specific surface, the value of the deviation of multifractal spectra Rd increases with the increase in the proportion of large pores. When the proportion of macropores is small, the Rd value will increase with the increase in specific surface. The multifractal characteristics of pore structures are influenced by specific surface area, average pore size and adsorption volume measured from N2GA experiment. The multifractal characteristic parameters of tight sandstone measured from NMR spectra are larger than those of shale, which may be caused by the differences in pore size distribution and porosity of shale and tight sandstone. [ABSTRACT FROM AUTHOR]

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According to the emission characteristics of volatile organic compounds (VOCs) in the petrochemical plants of China, the storage stability of VOCs for two different bags, polyester aluminum (PEA) and polyvinyl fluoride (PVF), was investigated in this study by comparing the adsorption of gas samples. A series of experiments were carried out to study the impact of different factors of sampling in the petrochemical industry. The results showed that the C2∼C3 substances can be adsorbed by the Tedlar bag, and after being refilled with pure nitrogen, the VOCs adsorbed previously by the bag material can be released. The aromatic hydrocarbon VOCs with larger molecular weight had a relatively lower recovery rate than the smaller molecular weights. And the average recovery of PEA airbags was significantly better than that of PVF airbags for storing stationary VOCs in the refinery of China. More kinds of substances can be detected in the airbags that had been added with helium protective gas, and it had a higher recovery rate for both kinds of simple bags after 24 hours of storage time, which indicated that the airbags without protective gas had adsorbed these substances. [ABSTRACT FROM AUTHOR]

Tectonism is one of the major controlling factors of shale gas accumulation and enrichment in China. To explore the relationship between tectonism and composition and pore characteristics of shale reservoirs, this research carried out mineralogy tests, organic geochemistry tests, field emission scanning electron microscopy (FE-SEM) experiments, and low-pressure gas adsorption (LPGA, N2 and CO2) experiments on the shale samples of various deformation intensities from Southwestern China. Based on the FE-SEM image analyses, it can be found that there are large differences in pore characteristics in shale samples with different deformation intensities. The samples with strong deformation have more organic pores, mainly related to the clay-organic aggregates and rigid grains. Tectonism can cause organic matter (OM) and clay minerals to be mixed or OM to fill in the clay layers, resulting in the retention of some organic pores. It is the presence of pressure shadows around the rigid grains that can resist tectonic extrusion and protect some organic pores. LPGA experiment results also show that micropore-specific surface areas and pore volumes of the samples with strong deformation are larger than those with weak deformation. The shale samples with strong deformation also have more microchannels and microfractures. Tectonism can also cause some micropores to become macropores; for example, tectonism can cause the rigid grains to slide and rotate, enlarging the dissolution pores at the edges of rigid grains. Shale samples with strong deformation have a smaller mesopore volume; but due to the presence of organic-clay aggregates, a larger mesopore-specific surface area embarks on these samples. According to fractal dimension calculations, it is found that in strong deformed shale, more multiple dimensions of the pore system tend to represent rougher pore surfaces and more irregular shapes. Besides, rougher pore surfaces are eager to provide more adsorption sites and enhance the adsorption capacity of the deformed shale. This study investigates the relationship between tectonism and composition and pore characteristics of shale reservoirs and may promote understanding of the accumulation of shale gas in highly deformed areas. [ABSTRACT FROM AUTHOR]

In China, one or several ultrathin soft coal sublayers are widely developed in the coal seam. Therefore, a method of using hydraulic flushing in soft sublayer to enhance the gas extraction in these particular coal seams is developed in this work. We first established a new fully coupled gas extraction model by combing gas diffusion, gas flow, and a permeability model that considers the effect of stress change and plastic failure. By adopting this model, the gas extraction enhancement mechanism and its main influence factors were studied using the numerical simulation method based on the engineering and geological background in the Yangquan No.5 coalmine. Thereafter, a hydraulic flushing equipment, which could move freely in the underground coalmine, was developed to apply the hydraulic flushing method in soft coal sublayers used in the 8402 working face. Its gas extraction effect was systematically investigated. Our simulation results match well with the field results, suggesting that our model is feasible. Meanwhile, after adopting this method, the gas extraction condition in this coalmine improves significantly. The borehole number decreases by 66.7%, while the gas extraction rate and gas extraction concentration increase by 1.33 times and 3 times, respectively. Moreover, during the coal mining process, the gas adsorption index of drilling cuttings, the quantity of drilling cuttings, and the CH4 concentration also decrease dramatically. [ABSTRACT FROM AUTHOR]

Microscopic pore structure and water distribution are important and fundamental parameters for coalbed methane reservoir characterisation. These are closely related to the calculation/interpretation of other critical parameters, such as permeability and flow capacity. In this study, scanning electron microscopy, low-pressure nitrogen gas adsorption, nuclear magnetic resonance spectroscopy, and theoretical calculations were used to study the pore structure characteristics and water distribution of Zhaozhuang coal mine in the southeastern Qinshui Basin, PR China. The results show that the pore radius ranges from 2.066 to 594.045 nm, mainly classified as micropores and transitional pores. The micropores significantly contribute to the total pore volume. The adsorption pores (micropores and transitional pores) contribute the most to the total specific surface area. The T2 spectrum distributions of saturated water samples show two peaks. The first T2 spectrum peak is related to adsorption pores, whereas the second T2 spectrum peak is related to seepage pores. The seepage pores were not well developed as the adsorption pores. Most adsorption pores are saturated with irreducible water that could not be discharged by centrifugation, whereas the seepage pores are saturated with movable water that could be completely discharged by centrifugation. The T2 cutoff method was used to calculate the irreducible water saturation, and the irreducible water saturation of the sample was ≥90%. The irreducible water was mainly distributed in the micropores, and some of the irreducible water was distributed in the transitional pores. The irreducible water that remains in the pores can cause reservoir damage. [ABSTRACT FROM AUTHOR]

Based on 10 shale samples collected from 4 wells in Qinshui Basin, we investigate the full‐sized pore structure and fractal characteristics of Marine‐Continental transitional shale by performing organic geochemistry, mineralogical composition, Nitrogen gas adsorption (N2 adsorption) and Nuclear Magnetic Resonance (NMR) measurements and fractal analysis. Results show that the TOC content of the shale samples is relatively high, with an average value of 2.44wt%, and the thermal evolution is during the mature‐over mature stage. The NMR T2 spectrum can be used to characterize the full‐sized pore structure characteristics of shale. By combining N2 adsorption pore structure parameters and NMR T2 spectrums, the surface relaxivity of samples are calculated to be between 1.7877 um/s and 5.2272 um/s. On this basis, the T2 spectrums are converted to full‐sized pore volume and surface area distribution curves. The statistics show that the pore volume is mainly provided by mesopore, followed by micropore, and the average percentages are 65.04% and 30.83% respectively; the surface area is mainly provided by micropore, followed by mesopore, and the average percentages are 60.8004% and 39.137% respectively; macropore contributes little to pore volume and surface area. The pore structure characteristics of shale have no relationship with TOC, but strong relationships with clay minerals content. NMR fractal dimensions Dmicro and Dmeso have strong positive relationships with the N2 adsorption fractal dimensions D1 and D2 respectively, indicating that Dmicro can be used to characterize the fractal characteristics of pore surface, and Dmeso can be used to characterize the fractal characteristics of pore structure. The shale surface relaxivity is controlled by multiple factors. The increasing of clay mineral content, pore surface area, pore surface fractal dimension and the decreasing of average pore size, will all lead to the decreasing of shale surface relaxivity. [ABSTRACT FROM AUTHOR]

To evaluate pore networks of the lower Cambrian Niutitang Shale in the Cen-gong Block, multiple techniques, including geochemical analysis, low-pressure gas (CO2 and N2) adsorption, mercury intrusion capillary porosimetry (MICP) and field-emission scanning electron microscope (FE-SEM), were performed to investigate pore structure, including pore-size distribution (PSD), pore volume and surface area of black organic-rich shales from two wells to better understand the mechanism of shale-gas accumulation. Geochemical analysis shows that equivalent vitrinite reflectance ranges from 2.1 to 3.5% and are overmature. Surface areas and pore volumes of micropores calculated from CO2 adsorption, which vary from 8.23 to 37.39 m2/g and from 3.06  10−3 to 10.89  10−3 cm3/g, respectively, can provide large amounts of adsorptive sites for shale-gas accumulation. N2 adsorption shows reversed S-shaped isotherms, which are classified into type H2 and H3 hysteresis loops, indicative of ink-bottle pores and slit-or plate-like pores. N2 adsorption and MICP analysis indicates that the inorganic-associated pores in shales may play a significant role in the storage and production of hydrocarbons. High-resolution FE-SEM analysis suggests that OM pores (OMPs) are common in Niutitang samples with a dominant diameter less 50 nm, but a sample with the highest total organic carbon (TOC) of 10.01 wt% has few OMPs, whereas a sample with the TOC of 7.51 wt% has more OMPs. The percentage of OMP area vs total organic matter area (Øs) ranges from 1.70 to 7.92%. The pore structure characteristics of TX1 well, calculated by multiple techniques, are better than samples from TM1 well. In addition, pore types are dominated by interparticle, intraparticle and organic matter pores, and the minor amounts of OMPs in TM1 well indicates that most of the organic matter may have lost the potential to generate hydrocarbons owing to the release of formation pressure with some graphitisation and high-angle fractures larger in the TX1 well. TOC content, mineral composition and tectonically related thermal maturity may be the main controlling factors of the pore structure and the lower Cambrian Niutitang Shale demonstrates the greatest potential for shale-gas production in South China. [ABSTRACT FROM AUTHOR]

Shale has been considered as good gas reservoir due to its abundant interior nanoscale pores. Thus, the study of the pore structure of shale is of great significance for the evaluation and development of shale oil and gas. To date, the most widely used approaches for studying the shale pore structure include image analysis, radiation and fluid invasion methods. The detailed pore structures can be studied intuitively by image analysis and radiation methods, but the results obtained are quite sensitive to sample preparation, equipment performance and experimental operation. In contrast, the fluid invasion method can be used to obtain information on pore size distribution and pore structure, but the relative simple parameters derived cannot be used to evaluate the pore structure of shale comprehensively and quantitatively. To characterize the nanoscale pore structure of shale reservoir more effectively and expand the current research techniques, we proposed a new method based on gas adsorption experimental data and the method of moments to describe the pore structure parameters of shale reservoir. Combined with the geological mixture empirical distribution and the method of moments estimation principle, the new method calculates the characteristic parameters of shale, including the mean pore size (), standard deviation (σ), skewness (Sk) and variation coefficient (c). These values are found by reconstructing the grouping intervals of observation values and optimizing algorithms for eigenvalues. This approach assures a more effective description of the characteristics of nanoscale pore structures. Finally, the new method has been applied to analyze the Yanchang shale in the Ordos Basin (China) and Longmaxi shale from the Sichuan Basin (China). The results obtained well reveal the pore characteristics of shale, indicating the feasibility of this new method in the study of the pore structure of shale reservoir. [ABSTRACT FROM AUTHOR]

The article presents a study which describes desorption behavior and anisotropy of coalbed gas in high rank coal reservoir in the Zhengzhuang Block of the Southern Qinshui Basin, China. Primary measurements and methods used to investigate the 93 core from the 13 wells were offered. Results indicate the factors influencing the adsorption time including gas content, critical desorption pressure, and total densities and determine a strong anisotropy within coal reservoir.

Lower Cambrian shale and lower Silurian shale are both typical of oil-prone kerogen and siliceous composition, but different in thermal maturities. Porosity differences were determined in marine shales between the two shales. Measurements were utilized including organic geochemistry, XRD, scanning electron microscopy (SEM) and N2 gas adsorption. Pore volume (PV) of lower Silurian shale was approximately 1.5 times higher than that of lower Cambrian shale, and pore surface area (PSA) of lower Silurian shale was almost 2.5 times higher than that of lower Cambrian shale. Lower Cambrian shale and lower Silurian shale possess similar materials, but distinctive thermal degrees. Evolution mechanisms of different types of pores, especially organic matter (OM)-hosted pores, may trigger this different pore features. Pores of rigid framework are the residue of primary interparticle pores during the burial history. Pores associated with clay flakes can be preserved well adjacent to rigid grains or secondary minerals acting as rigid frameworks or grain supporters. Dissolved pores in both lower Cambrian shale and lower Silurian shale barely contribute to the total porosity and mean little to the permeability. Both excessive OM content and over thermal maturity are detrimental to development of OM-hosted pores. Rigid particles, clay flakes, and OM commonly co-exist within shale matrix. Rigid grains act as supporters, clay flakes confine ample space, and OM first migrates into and provides secondary OM-hosted pores. In this condition, pores can be preserved owing to associating matrix with good mechanic and chemical stability. The significant differences of structural settings result into various hydrocarbon explosion efficiency and different pressure circumstance, which consequently leads to the different pore features between the two shales. For lower Cambrian shale, overpressure circumstance diminish if hydrocarbon expels outside of the shale system, and OM-hosted pores destroy through compaction. Sustaining overpressure and abundant residue hydrocarbon (migrated OM) make positive contributions to the pore properties, in terms of numbers, diameters and connectivity of the lower Silurian shale samples. [ABSTRACT FROM AUTHOR]

Different gas adsorption isotherm experiments were carried out on the coal samples from Jincheng district, including N2, CH4, CO2, as well as the binary-component gas of CH4-N2 and CH4-CO2 of three different concentrations. In the binary-component gas adsorption isotherm experiments, the gas component with higher adsorption ability is adsorbed firstly. Thus the result in its relative concentration in the free phase shows a trend of decrease first and then increase, whereas the relative concentration of the gas component with lower adsorption ability shows a trend of increase first and then decrease. In the adsorption phrase, the relative concentration of the gas component with higher adsorption ability increases gradually, and the relative concentration of the gas component with lower adsorption ability decreases gradually. In the adsorption competition of the binary-component gas, the adsorption rate of the gas component with higher adsorption ability shows a trend of rapidity at first then slowness, but the adsorption rate of the gas component with lower adsorption ability shows a trend of slowness at first then rapidity. The component fractionation effect in the binary-component gas adsorption isotherm experiments is caused by the difference of the adsorption ability of coal of different gas components. [ABSTRACT FROM AUTHOR]

The effect of clay minerals on the methane adsorption capacity of shales is a basic issue that needs to be clarified and is of great significance for understanding the adsorption characteristics and mechanisms of shale gas. In this study, a variety of experimental methods, including XRD, LTNA, HPMA experiments, were conducted on 82 marine shale samples from the Wufeng–Longmaxi Formation of 10 evaluation wells in the southern Sichuan Basin of China. The controlling factors of adsorption capacities were determined through a correlation analysis with pore characteristics and mineral composition. In terms of mineral composition, organic matter (OM) is the most key methane adsorbent in marine shale, and clay minerals have little effect on methane adsorption. The ultra-low adsorption capacity of illite and chlorite and the hydrophilicity and water absorption ability of clay minerals are the main reasons for their limited effect on gas adsorption in marine shales. From the perspective of the pore structure, the micropore and mesopore specific surface areas (SSAs) control the methane adsorption capacity of marine shales, which are mainly provided by OM. Clay minerals have no relationship with SSAs, regardless of mesopores or micropores. In the competitive adsorption process of OM and clay minerals, OM has an absolute advantage. Clay minerals become carriers for water absorption, due to their interlayer polarity and water wettability. Based on the analysis of a large number of experimental datasets, this study clarified the key problem of whether clay minerals in marine shales control methane adsorption. [ABSTRACT FROM AUTHOR]

Ordos Basin is an important continental shale gas exploration site in China. The micropore structure of the shale reservoir is of great importance for shale gas evaluation. The Taiyuan Formation of the lower Permian is the main exploration interval for this area. To examine the nanometer pore structures in the Taiyuan Formation shale reservoirs in the Lin-Xing area, Northern Shaanxi, the microscopic pore structure characteristics were analyzed via nitrogen adsorption experiments. The pore structure parameters, such as specific surface area, pore volume, and aperture distribution, of shale were calculated; the significance of the pore structure for shale gas storage was analyzed; and the main controlling factors of pore development were assessed. The results indicated the surface area and hole volume of the shale sample to be 0.141–2.188 m2/g and 0.001398–0.008718 cm3/g, respectively. According to the IUPAC (International Union of Pure and Applied Chemistry) classification, mesopores and macropores were dominant in the pore structure, with the presence of a certain number of micropores. The adsorption curves were similar to the standard IV (a)-type isotherm line, and the hysteresis loop type was mainly similar to H3 and H4 types, indicating that most pores are dominated by open type pores, such as parallel plate-shaped pores and wedge-shaped slit pores. The micropores and mesopores provide the vast majority of the specific surface area, functioning as the main area for the adsorption of gas in the shale. The mesopores and macropores provide the vast majority of the pore volume, functioning as the main storage areas for the gas in the shale. Total organic carbon had no notable linear correlation with the total pore volume and the specific surface area. Vitrinite reflectance (Ro) had no notable correlation with the specific surface area, but did have a low "U" curve correlation with the total pore volume. There was no relationship between the quartz content and specific surface area and total pore volume. In addition, there was no notable correlation between the clay mineral content and total specific surface area and total pore volume. [ABSTRACT FROM AUTHOR]