Low-pressure Nitrogen Adsorption Research Articles

Structural and Textural Characteristics of Municipal Solid Waste Incineration Bottom Ash Subjected to Periodic Seasoning

The objective of this study was to determine the structural and textural description of municipal solid waste incineration (MSWI) bottom ash that was subjected to a six-month seasoning process. Bottom ash samples, with a particle size fraction of 0.063–0.1 mm, were seasoned in a closed landfill and collected for laboratory analyses at monthly intervals. The research focused on determining the structural parameters, using methods such as nuclear magnetic resonance spectroscopy (1H NMR), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-Vis), and the textural parameters, using low-pressure nitrogen adsorption (LPNA) at −196.15 °C. The analyses of the porous structure of the bottom ash samples revealed differences in texture of ASH 1 to ASH 6, specifically in the pore volume (micro- and mesopores), specific surface area, and pore size distribution. Changes in the structural and porous characteristics of the samples were attributed to the duration of the seasoning process. The results of the structural analysis of the bottom ash suggest its application in the concrete industry, potentially enhancing the long-term mechanical strength of concrete. The results of the textural analysis indicate the possible use of MSWI bottom ash in environmental applications, as the internal surface area could be further developed.

The Pore Structure Multifractal Evolution of Vibration-Affected Tectonic Coal and the Gas Diffusion Response Characteristics

Vibrations caused by downhole operations often induce coal and gas outburst accidents in tectonic zone coal seams. To clarify how vibration affects the pore structure, gas desorption, and diffusion capacity of tectonic coal, isothermal adsorption-desorption experiments under different vibration frequencies were carried out. In this study, high-pressure mercury intrusion experiments and low-pressure liquid nitrogen adsorption experiments were conducted to determine the pore structures of tectonic coal before and after vibration. The pore distribution of vibration-affected tectonic coal, including local concentration, heterogeneity, and connectivity, was analyzed using multifractal theory. Further, a correlation analysis was performed between the desorption diffusion characteristic parameters and the pore fractal characteristic parameters to derive the intrinsic relationship between the pore fractal evolution characteristics and the desorption diffusion characteristics. The results showed that the vibration increased the pore volume of the tectonic coal, and the pore volume increased as the vibration frequency increased in the 50 Hz range. The pore structure of the vibration-affected tectonic coal showed multifractal characteristics, and the multifractal parameters affected the gas desorption and diffusion capacity by reflecting the density, uniformity, and connectivity of the pore distribution in the coal. The increases in the desorption amount (Q), initial desorption velocity (V0), initial diffusion coefficient (D0), and initial effective diffusion coefficient (De) of the tectonic coal due to vibration indicated that the gas desorption and diffusion capacity of the tectonic coal were improved at the initial desorption stage. Q, V0, D0, and De had significant positive correlations with pore volume and the Hurst index, and V0, D0, and De had negative correlations with the Hausdorff dimension. To a certain extent, vibration reduced the local density regarding the pore distribution in the coal. As a result, the pore size distribution was more uniform, and the pore connectivity was improved, thereby enhancing the gas desorption and diffusion capacity of the coal.

Structure evolution law of coalbed methane reservoirs under different initial pressure CO2 phase change fracturing conditions

As a new coal bed fracturing technology, CO2 phase transition fracturing (CPTF) has received much attention due to its advantages of safety and high efficiency. However, little research has been conducted to investigate the fracture behavior of CPTF in coal seams under different CO2 release pressure conditions. To address this research gap, we performed fracture tests on coal rock under different initial pressure conditions using the developed CPTF experimental system. Then, scanning electron microscopy, low-pressure nitrogen adsorption, and mercury intrusion porosimetry testing techniques were used to investigate the fracture characteristics and microstructural evolution of coal rocks under different initial pressures of CPTF. The results show that with increasing initial pressure, the number of macroscopic fractures and the degree of fragmentation of the coal after CPTF fracturing increased significantly, the number of microscopic fractures and pores in the fractured coal samples increased, the N2 adsorption capacity and the amount of mercury intrusion of the coal samples increased to a greater extent, and the visible porosity increased from 52.47% of the raw coal to 63.88%, 64.31%, 68.48%, 63.64%, and 62.83%, and the proportion of macroporosity increased from 24.31% to 28.48%, 31.73%, 26.55%, and 34.38%. This research will contribute to a fuller understanding of the potential of CPTF as a technique for improving the pore and fracture structure of coalbed methane reservoirs.

Comprehensive Comparison of Lacustrine Fine-Grained Sedimentary Rock Reservoirs, Organic Matter, and Palaeoenvironment: A Case Study of the Jurassic Ziliujing Formation and Xintiangou Formation in the Sichuan Basin

Lacustrine sedimentary formations potentially contain hydrocarbons. The lacustrine sedimentary rocks of the Ziliujung and Xintiangou Formations have been investigated for their hydrocarbon potential using low-pressure nitrogen adsorption (LP-N2A), low-field nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), total organic carbon (TOC), rock-eval pyrolysis (Rock-Eval), gas chromatography (GC), gas chromatography–mass spectrometry (GC–MS), X-ray fluorescence spectrometry (XRF), and inductively coupled plasma mass spectrometry (ICP-MS). The results show that the normalized difference of the pore parameters between the two formations is less than 10%, and the pores are mainly slit-like mesopores with high porosity. Macropores and micropores are often developed in the quartz skeleton, while mesopores often occur among organic matter, clay minerals, carbonate minerals, and pyrite particles. The organic matter abundance of the Ziliujing Formation is relatively high. Additionally, the organic matter types of the two formations are mainly type II and type III, and the sources of the organic matter are plankton and bacteria which have reached the mature gas production stage. The palaeoenvironmental differences between the depositional periods of the two formations lie within 10% of each other. The warm and humid climate promotes the development of quartz minerals to further enhance the proportion of both micropores and macropores, and the clay minerals, carbonate minerals, and pyrite carried in the terrigenous detritus are closely associated with the total organic carbon (TOC), which promotes the development of mesopores to enhance the porosity. The reservoir, organic matter, and palaeoenvironmental characteristics of fine-grained sedimentary rocks in the two formations are similar, and both of them have good potential for development. The above results provide a basic geological theoretical basis for unconventional oil and gas exploration in the northeastern margin of the Sichuan Basin.

The influence of hydrocarbon generation on the sealing capability of mudstone caprock rich in organic matter

To investigate the sealing capability of mudstone caprock during the evolution of organic matter (OM)-rich mudstone, a series of hydrous pyrolysis experiments were first conducted to examine the impact of hydrocarbon generation. The pore type, pore structure, porosity, and gas breakthrough pressure of pyrolytic residual samples were analyzed by field emission scanning electron microscopy, low pressure nitrogen adsorption measurements, porosimetry, and gas breakout core experiments. To model the environment at different depths, these six experiments on hydrous pyrolysis were performed at different temperatures, lithostatic pressures, and hydrodynamic pressures, while other experimental factors such as the original sample, heating time, and rate were kept constant. The results showed that during the thermal evolution process, hydrocarbons were generated from OM in mudstone, resulting in the formation of pores within the OM. Organic acids produced by hydrocarbon generation effectively dissolved minerals, leading to the creation of numerous dissolution pores. Changes in pore type led to changes in pore structure and porosity. The volume of micropores and macropores showed an increasing trend before reaching a Ro value of 1.41%. However, after passing this threshold, they began to decrease. The volume of mesopores showed a decreasing trend before reaching a Ro value of 1.32%. After 1.32%, they began to increase. The porosity was mainly affected by the pore volumes of the mesopores and macropores. The porosity exhibited two peaks: the first occurred at a Ro value of 0.72%, with a porosity level of 4.6%. The second occurred at a Ro value of 1.41% and a porosity level of 10.3%. The breakthrough pressure was a comprehensive reflection of these influences, and its trend exhibited a negative correlation with porosity (R2 = 0.886). For two high values of porosity, the breakthrough pressure corresponded to two low values. Smaller values of the breakthrough pressure indicated a poorer sealing capability of the mudstone caprock. Overall, hydrocarbon generation in the mudstone affected the sealing capability. The mudstone in the studied area exhibited good sealing at Ro below 1.32%. However, once above the 1.32% threshold, the fluctuations of the breakthrough pressure values exhibited considerable variability, requiring a comprehensive evaluation to assess its sealing capability.

Multiscale characterization of carbonate-rich shale: From micro structure to macro mechanical properties, a case study in Hongxing Block, China

As a new growth point of shale gas production, carbonate-rich shale has recently attracted great interest. However, gas production from this reservoir was poor since the properties of the carbonate-rich shale have not been systematically studied. This is an urgent need to be addressed. Downhole cores collected from an exploration well were analyzed by micro-structure experiments (X-ray diffraction, mineral quantitative evaluation, low-pressure nitrogen adsorption, and mercury intrusion piezometry test) and macro-mechanical tests (triaxial compression, Brazilian splitting, and fracture toughness test) to obtain multiscale characterization of carbonate-rich shale. The intrinsic relationship between the microstructural and mechanical characteristics of this reservoir has been revealed. The results showed that (1) The content of carbonate minerals exceeds 30%, just slightly lower than that of quartz. Calcite has large particle sizes and is closely cemented with quartz to form a dense rock skeleton. (2) The maximum adsorption volume, specific surface area, total pore volume and average pore diameter values are poor, which is unfavorable for shale gas accumulation. Limited porosity and permeability and great tortuosity can be obstacles to the release of accumulated gas. (3) High peak deviatoric stress, Young's modulus, and Poisson's ratio were contributed by dense rock skeleton. (4) Narrow maximum main fracture width is not favorable to place proppants and further to create effective gas transport channels. (5) High tensile strength and fracture toughness enhance high breakdown and extension pressure. These comprehensive insights are of great significance for both the exploration and development of carbonate-rich shale reservoirs.

The main controlling factors of tensile strength in sight of shale reservoir under horizontal bedding: Example of the Lower Paleozoic Niutitang Formation shale from Micangshan, China

Understanding the mechanical characteristics of marine shale during fracturing is essential for shale gas development, and its core scientific problem is what factors in shale control its mechanical properties. The 12 shale samples from the Lower Paleozoic Niutitang Formation in Micangshan are tested for tensile strength and examined using X-ray diffraction, low-field nuclear magnetic resonance (NMR), EA2000 elemental analyzer, and scanning electron microscopy to explore the main controlling factors of shale tensile strength under horizontal bedding conditions. The findings are as follows. (1) The tensile strength of the shale is relatively high, ranging from 10.05 MPa to 20.34 MPa. Quartz is the largest proportion of the shale minerals, accounting for 53.2 wt%–59.0 wt%, followed by anorthose and clay minerals. Total organic carbon (TOC) concentration ranges from 1.7 wt% to 4.1 wt%. (2) NMR results indicate that the pore structure of shale is mainly mesoporous, accounting for 75.76%–88.03%, followed by macropores (12.57%–21.24%) and micropores (0.68%–4.91%). Low-pressure nitrogen adsorption and desorption results indicate that the average pore diameter of shale is 12.58–16.02 nm, which is basically consistent with NMR results. The negative correlation between fractal dimension D2 and tensile strength indicates that the higher the tensile strength of the shale, the lower the complexity of its seepage pores. (3) Micropores occur mainly in clay minerals, whereas quartz indicates positively correlation with mesoporous content. The higher the proportion of mesopores, the lower the tensile strength. This indicates that the mesopores are the main factor controlling the tensile strength, and the quartz content in minerals is a secondary factor restricting the tensile strength. TOC has little controlling action on the tensile strength. This contribution provides a theoretical basis for shale fracturing.

Characteristics and controlling factors of pore structure of shale in the 7th member of Yanchang Formation in Huachi area, Ordos Basin, China

To clarify the influence of organic matter and mineral composition on the pore structure of shale reservoirs in the 7th member of the Yanchang Formation (Chang 7 Member) in the Ordos Basin, we characterized the pore structure of Chang 7 shale reservoirs using argon ion polishing-field emission scanning electron microscopy (FE-SEM) and low-pressure nitrogen adsorption (LP-N2A). This characterization was combined with whole-rock mineral composition and organic geochemical experiments to analyze the main controlling factors of the pore structure of Chang 7 Member shale. The results reveal the presence of various pore types in shale, including organic matter pores, intergranular pores, intercrystalline pores, dissolved pores, and micro-cracks. The LP-N2A isotherms of shale consistently exhibit type Ⅱ isotherms with H3 and H4 hysteresis loop characteristics, indicating the relatively developed nature of mesopores and a pore morphology characterized by parallel lamellar and “ink bottle” shapes. The primary determinants of shale pore structure are identified as organic matter, clay minerals, quartz, feldspar, and pyrite. Among these factors, clay mineral phase transformation generates a substantial number of micropores and mesopores within the mineral crystal layers, serving as the main source of shale pores in the study area. Additionally, liquid hydrocarbons generated, solid bitumen, and euhedral pyrite fill inorganic mineral pores, thereby reducing the pore space of Chang 7 shale to a certain extent. These results provide a new cognition into understanding the pore structure characteristics and controlling factors of Chang 7 shale.

Effects of Supercritical CO2 on the Pore Structure Complexity of High-Rank Coal with Water Participation and the Implications for CO2 ECBM.

To reveal how mineral changes affect a coal pore structure in the presence of water, an autoclave was used to carry out the supercritical CO2 (ScCO2)-H2O-coal interaction process. To reveal the changes in pore complexity, mercury intrusion capillary pressure (MICP), low-pressure nitrogen adsorption, CO2 adsorption, and field emission scanning electron microscopy (FESEM) experiments were combined with fractal theory. The experimental data of MICP show that the MICP data are meaningful only for the pore fractal dimension with pore sizes >150 nm. Therefore, the pores were classified into the classes >150, 2-150, and <2 nm. The results show that the pore volume and specific surface area of the coal increased significantly after the reaction. ScCO2-H2O can cause the formation of many new pores and fractures in the coal. The presence of H2O may increase the potential for the injection of CO2 into the coal seam. The complete dissolution of calcite surfaces caused a significant increase in the pore volume and specific surface area of the pores >150 nm. The morphologies of these pores are controlled by the morphologies of the complete dissolution carbonate particles. The pore morphologies were relatively uniform, and the fractal dimensions decreased. However, the incomplete dissolution of calcite leads to irregular variations in the morphologies for the pores in the 2-150 nm pore size range. The pore morphologies that are produced by incompletely dissolved calcite particles are more complex, which increases the fractal dimensions after the reaction. The fractal dimensions of the pores <2 nm decreased after the reaction, indicating that the newly generated micropores were more uniform and had regular pore morphologies.

Study on the Sedimentary Environments and Its Implications of Shale Reservoirs for Permian Longtan Formation in the Southeast Sichuan Basin

Marine–continental transitional shale is one of the most promising targets for shale gas exploration in the Lower Yangtze region. To investigate the sedimentary environments and the regularity of the enrichment of the Longtan shale, multiple techniques including core and thin-section observations, geochemical and elemental analyses, X-ray diffraction, scanning electron microscopy (SEM), and low-pressure nitrogen adsorption (LPNA) were used to analyze the sedimentology, mineralogy, and pore structure of the Longtan shale. The core descriptions and thin-section observations showed that the Longtan shale was deposited in marine–delta transitional environments including delta-front, shore swamp, mixed tidal flat and shallow shelf environments. The Sr/Cu, V/Cr, CIA, EF (Mo), EF (U), and other major and trace element results indicated warm and moist climates and water-reducing conditions in the Longtan period. Both the climate and water conditions were favorable for organic matter production and preservation. The geochemical results showed that the Longtan shale was in the overmature stage (Ro values ranging from 2.4% to 3.57%) and that the average total organic carbon (TOC) content was 5.76%. The pore system of the Longtan shale consisted of inorganic pores with a small number of organic pores and microfractures. The porosity and specific surface area were mainly affected by the TOC and clay mineral contents. An effective combination of brittle mineral particles, organic matter, and clay minerals provided the necessary conditions for pore preservation. The organic pores, intergranular pores in clay minerals, and brittle mineral pores formed the main network system for the Longtan shale. In summary, the lithological combinations, organic geochemistry, and pore structure system were all affected by the sedimentary environments.

A Comparative Investigation of the Adsorption Characteristics of CO2, O2 and N2 in Different Ranks of Coal

The adsorption mechanism of carbon dioxide, oxygen and nitrogen in coal is important for preventing and controlling coal spontaneous combustion and for understanding the technology of CO2 storage in goaf. Adsorption amount and adsorption heat are key adsorption parameters that are required to understand the material and energy conversions during adsorption in coal. In this study, we studied the factors that influence the adsorption amounts and adsorption heat values of carbon dioxide, oxygen and nitrogen in coal by testing four different coal samples using conventional coal quality analysis, low-pressure nitrogen and carbon dioxide adsorption, Fourier transform infrared spectroscopy and three gas adsorption experiments at different temperatures. Then, we analyzed the relationships between the structural parameters of the coal samples and the adsorption amounts and the adsorption heat values of carbon dioxide, oxygen and nitrogen. The results show that the adsorption isotherms of carbon dioxide conform to the Langmuir equation, and the adsorption isotherms of oxygen and nitrogen conform to Henry’s law between 0 and 110 kPa. The adsorption amounts of carbon dioxide, oxygen and nitrogen decreased with an increase in temperature, and the change in the rate of the adsorption amount with temperature was determined by the adsorption heat. The results of the pore structure show that the pores of the coal samples are composed of mesopores and micropores; the micropores contribute to the main specific surface area. The micropore and mesopore structures are the main determinants of the adsorption amounts of carbon dioxide, oxygen and nitrogen in coal. The gas adsorption heat is affected by the pore structure and the chemical composition of coal. The adsorption heat of nitrogen correlates positively with the pore structure of the coal. The adsorption heat of oxygen correlates positively with the ash, elemental nitrogen, elemental sulfur and mineral contents of the coal. The adsorption heat of carbon dioxide correlates positively with the elemental sulfur content of the coal.

Experimental and molecular investigation of water adsorption controls in marine and lacustrine shale reservoirs

The restriction of hydrocarbon migration within shale reservoirs by pre-adsorbed water necessitates an investigation into water adsorption control mechanisms within shales. In this study, water vapor adsorption isotherms obtained from marine and lacustrine shale samples along with isotherms from their organic matter and experimentally verified kerogen molecular models were used to evaluate water adsorption mechanisms, with simulated wet kerogen-quartz nanocomposites. Furthermore, these were also used to investigate organic matter-inorganic interaction controls on water adsorption. Pore size distribution (PSD) controls on water distribution in organic matter were obtained via water vapor adsorption and low-pressure nitrogen adsorption analysis, along with small angle neutron scattering (SANS) analysis. The results reveal water adsorption is linked to changes in mesopore volume within organic matter and porosity in kerogen models. PSD obtained from the shales only reveal mesopore controls at high RH, with water adsorption mainly controlled by oxygen functional groups in organic matter pores and swelling clays. Inorganic controls on water adsorption are also observed in adsorbed water not tightly bound to the shale surface due to a high montmorillonite content in clay interlayers, and organic matter interaction with quartz with a negative relationship between the adsorbed water and the quartz content. Collectively, these findings indicate that CO2 migration in shale reservoirs could be inhibited by water that is distributed in potential CO2 adsorption sites within organic matter pores and montmorillonite.

Multi-scale pore structure transformation of shale under mixed acid acidification method

Most of the acid fluids used in the existing acidfrac process have the shortcomings of plugging pores, polluting formation and poor effect. The method of adding nitric acid in a specific proportion to conventional acid can avoid the above problems to a certain extent. A mixture of HCl: HF: HNO3 (3:1:1) was used to acidize the shale of the Longmaxi Formation in Sichuan and the Piyuancun Formation in Jiangxi for 24 h. The mineral fractions and pore structures of the samples were then analyzed using scanning electron microscopy-energy dispersive spectrometry, X-ray diffraction, low-pressure nitrogen adsorption, and nuclear magnetic resonance. The results showed that by reducing the mineral constituents of shale, the 2–15-nm sized pores decreased, 1–10-μm sized pores increased, and fractal dimensions decreased. The mixed acid solution caused cross-scale porosity expansion in the shale and increased gas circulation within the matrix. The process of acid mixing in the shale pore structure was divided into four stages: surface dissolution, macroporous dissolution, meso–microporous dissolution, and feedback dissolution.

Probing the Pore Structure of the Berea Sandstone by Using X-ray Micro-CT in Combination with ImageJ Software

During diagenesis, the transformation of unconsolidated sediments into a sandstone is usually accompanied by compaction, water expulsion, cementation and dissolution, which fundamentally control the extent, connectivity and complexity of the pore structure in sandstone. As the pore structure is intimately related to fluid flow in porous media, it is of great importance to characterize the pore structure of a hydrocarbon-bearing sandstone in a comprehensive way. Although conventional petrophysical methods such as mercury injection porosimetry, low-pressure nitrogen or carbon dioxide adsorption are widely used to characterize the pore structure of rocks, these evaluations are based on idealized pore geometry assumptions, and the results lack direct information on the pore geometry, connectivity and tortuosity of pore channels. In view of the problems, X-ray micro-CT was combined with ImageJ software (version 1.8.0) to quantitatively characterize the pore structure of Berea Sandstone. Based on its powerful image processing function, a series of treatments such as contrast enhancement, noise reduction and threshold segmentation, were first carried out on the micro-CT images of the sandstone via ImageJ. Pores with sizes down to 2.25 μm were accurately identified. Geometric parameters such as pore area, perimeter and circularity could thus be extracted from the segmented pores. According to our evaluations, pores identified in this study are mostly in the range of 30–180 μm and can be classified into irregular, high-circularity and slit-shaped pores. An irregular pore is the most abundant type, with an area fraction of 72.74%. The average porosity obtained in the image analysis was 19.10%, which is fairly close to the experimental result determined by a helium pycnometer on the same sample. According to the functional relationship between tortuosity and permeability, the tortuosity values of the pore network were estimated to be in the range of 4–6 to match the laboratory permeability data.

Selection Effect of Liquid Nitrogen Freeze-Thaw Cycles on Full Pore Size Distribution of Different Rank Coals.

In China, the capacity to produce coalbed methane and extract underground gas is restricted by the prevalence of low-permeability coal seams. Liquid nitrogen fracturing is a new low temperature-high-pressure anhydrous fracturing technology that uses low temperature and high frost heave forces to increase coal permeability. To better understand the liquid nitrogen fracturing effect on coal, we conduct the liquid nitrogen freeze-thaw cycle (LNCFT) experiments on different rank coals from Qinghai, Shanxi, and Shaanxi provinces. We combined the low-pressure nitrogen and carbon dioxide adsorption experiment with the non-local density functional theory model and mercury injection porosimetry with compressibility corrections to examine the full pore size distributions of untreated and water-saturated samples before and after LNCFT. The results found that LNCFT can effectively increase the pore volume (PV) and specific surface area of the water-saturated coal sample. Compared with the raw coal, the increased ratio of the full pore size PV is 70.41-100.17%. However, the scale-selective transformation effect on pores during liquid nitrogen fracturing is noticeable. Under the same conditions, LNCFT can significantly increase the pore volume of micropores (>2 nm) and macropores (>50 nm), and the increase ratios are 24.40-44.16 and 103.55-327.93%, respectively. The PV of mesopores (2-50 nm) shows a slightly increasing trend with the increase in metamorphic degree, and the increase ratio is between 8.7 and 56%. Comparing the full pore size distribution curves before and after LNCFT, it is found that the alteration of high-volatile bituminous coal (BLT coal) and anthracite (SH coal) has more significance in the range of less than 2 and 50-20,000 nm, while middle-volatile bituminous coal (YJL coal) varies between 50 and 2000 nm. Meanwhile, the ratio of micropore and mesopore PV to the total decreased gradually before and after LNCFT, while the proportion of macropores increased, indicating that small-scale pores would intersect and connect to form larger-scale pores during the fracturing. The combined effects of temperature gradient, water-ice phase transition, and heat transmission rate are the key factors that determine the impact of LNCFT on pore size distribution. Our results provide new information for enhancing the permeability of low-permeability coal seams of different ranks.

Multi-scale pore fractal characteristics of differently ranked coal and its impact on gas adsorption

Well-developed pores and cracks in coal reservoirs are the main venues for gas storage and migration. To investigate the multi-scale pore fractal characteristics, six coal samples of different rankings were studied using high-pressure mercury injection (HPMI), low-pressure nitrogen adsorption (LPGA-N2), and scanning electron microscopy (SEM) test methods. Based on the Frankel, Halsey and Hill (FHH) fractal theory, the Menger sponge model, Pores and Cracks Analysis System (PCAS), pore volume complexity (Dv), coal surface irregularity (Ds) and pore distribution heterogeneity (Dp) were studied and evaluated, respectively. The effect of three fractal dimensions on the gas adsorption ability was also analyzed with high-pressure isothermal gas adsorption experiments. Results show that pore structures within these coal samples have obvious fractal characteristics. A noticeable segmentation effect appears in the Dv1 and Dv2 fitting process, with the boundary size ranging from 36.00 to 182.95 nm, which helps differentiate diffusion pores and seepage fractures. The D values show an asymmetric U-shaped trend as the coal metamorphism increases, demonstrating that coalification greatly affects the pore fractal dimensions. The three fractal dimensions can characterize the difference in coal microstructure and reflect their influence on gas adsorption ability. Langmuir volume (VL) has an evident and positive correlation with Ds values, whereas Langmuir pressure (PL) is mainly affected by the combined action of Dv and Dp. This study will provide valuable knowledge for the appraisal of coal seam gas reservoirs of differently ranked coals.

Multiple-experimental investigation on the physicochemical structures alternation during coal biogasification

The aim of microbially-enhanced coalbed methane (MECBM) generation is to replicate the natural process of microbial methane production in coal seams and to increase coalbed methane production. However, a systematic evaluation of the response of coal structure during biogasification is still lacking. Here, alternations of the pore and molecular structures were investigated during the bioconversion of coal to methane using proximate/ultimate analysis, scanning electron microscope, low-field nuclear magnetic resonance (LNMR), low-pressure nitrogen adsorption (LPNA), micro-Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) techniques. The results showed that MECBM is a process of carbon enrichment, nitrogen/sulfur fixation and dehydrogenation/deoxygenation. Moreover, microbial modify coal pore structure by destroying aliphatic side chains, dissociating small molecular clusters and reducing the degree of aromaticity. These changes in the coal pore structure result in the “loose” of the coal reservoir spatial structure. We found that micropores (<10 nm) became transition pores (10–100 nm) and mesopores (100–1000 nm) after microbial degradation, which then led to the increase of the average pore width and the reduction of the specific surface area. This process benefits the desorption and transportation of coalbed methane. In addition, the surface fractal dimension decreased, and the volume fractal dimension increased according to the fitting date of LPNA. The fractal dimension of transition pores and mesopores increased of most coal after biotreatment based on NMR. These results led us to draw the hypothesis that the organic compounds formed by biodegradation may adhere to the pores, thus blocking the pores, making the structure of transition pores and mesopores complex, as demonstrated by the SEM images. This phenomenon is thought to be improved by continuous degradation or water movement.

Experimental Study of the Pore Structure and Gas Desorption Characteristics of a Low-Rank Coal: Impact of Moisture.

Coal–water interactions have a prominent impact on the prediction of coal mine gas disasters and coalbed methane extraction. The change of characteristics in the microscopic pores of coal caused by the existence of water is an important factor affecting the diffusion and migration of gas in coal. The low-pressure nitrogen adsorption experiments and gas desorption experiments of a low-rank coal with different equilibrium moisture contents were conducted. The results show that both the specific surface area and pore volume decrease significantly as the moisture content increases, and the micropores (pore diameter <10 nm) are most affected by the water adsorbed by coal. In particular, for a water-equilibrated coal sample at 98% relative humidity, micropores with pore sizes smaller than 4 nm as determined by the density functional theory model almost disappear, probably due to the blocking effects of water clusters and capillary water. In this case, micropores with a diameter less than 10 nm still contribute most of the specific surface area for gas adsorption in coal. Furthermore, the fractal dimensions at relative pressures of 0–0.5 (D1) and 0.5–1 (D2) calculated by the Frenkel–Halsey–Hill model indicate that when the moisture content is less than 4.74%, D1 decreases rapidly, whereas D2 shows a slight reduction as the moisture content increased. In contrast, when the moisture content exceeds 4.74%, further increases in the moisture content cause D2 to decrease significantly, while there is nearly no change for D1. The correlation analyses show that the ultimate desorption volume and initial desorption rate are closely related to the fractal dimension D1, while the desorption constant (Kt) mainly depends on the fractal dimension D2. Therefore, the gas desorption performances of coal have a close association with the pore properties of coal under water-containing conditions, which indicate that the fluctuation in moisture content should be carefully considered in the evaluation of gas diffusion and migration performances of in situ coal seams.

Fractal Characteristics of Deep Shales in Southern China by Small-Angle Neutron Scattering and Low-Pressure Nitrogen Adsorption

The occurrence and flow of shale gas are substantially impacted by nanopore structures. The fractal dimension provides a new way to explore the pore structures of shale reservoirs. In this study, eight deep shale samples from Longmaxi Formation to Wufeng Formation in Southern Sichuan were selected to perform a series of analysis tests, which consisted of small-angle neutron scattering, low-pressure nitrogen adsorption, XRD diffraction, and large-scale scanning electron microscopy splicing. The elements that influence the shale fractal dimension were discussed from two levels of mineral composition and pore structures, and the relationship between the mass fractal dimension and surface fractal dimension was focused on during a comparative analysis. The results revealed that the deep shale samples both had mass fractal characteristics and surface fractal characteristics. The mass fractal dimension ranged from 2.499 to 2.991, whereas the surface fractal dimension ranged from 2.814 to 2.831. The mass fractal dimension was negatively correlated with the surface fractal dimension. The mass fractal dimension and the surface fractal dimension are controlled by organic matter pores, and their development degree significantly affects the fractal dimension. The mass fractal dimension increases with the decrease of a specific surface area and pore volume and increases with the increase of the average pore diameter. The permeability and surface fractal dimension are negatively correlated, but no significant correlation exists between the permeability and mass fractal dimension, and the internal reason is the dual control effect of organic matter on shale pores. This study comprehensively analyses the mass fractal characteristics and surface fractal characteristics, which helps in a better understanding of the pore structure and development characteristics of shale gas reservoirs.

Main controlling factors of marine shale compressive strength: A case study on the cambrian Niutitang Formation in Dabashan Mountain

Lower Paleozoic shale is an important hydrocarbon source rock and natural gas reservoir in the Upper Yangtze Plate. In this study, compressive strength + acoustic emission (AE), low-pressure nitrogen adsorption and desorption (LP-N2A), low-field nuclear magnetic resonance (NMR), total organic carbon (TOC), X-ray fluorescence spectroscopy (XRF), scanning electron microscopy (SEM) and other experiments were carried out on the marine shale of the Cambrian Niutitang Formation in Fucheng Town, Dabashan Mountain. The results showed that: 1) The compressive strength of shale with an average of 58.38 MPa, and its rupture process can be divided into four stages; 2) The high proportion of mesopores in shale is significantly negatively correlated with compressive strength; 3) TOC in shale averaging 2.37 wt% was positively correlated with compressive strength; 4) The main element was SiO2 averaging 68.47 wt%, which is positively correlated with compressive strength. Meanwhile, Al2O3, Fe2O3 and CaO + MgO content were negatively correlated with compressive strength. To sum up, the organic and quartz content in shale jointly increase the compressive strength of shale, while the mesopores proportion has a constraining effect on the compressive strength. This study provides geological information and a theoretical basis for shale fracture development.