Nitrogen Adsorption Experiments Research Articles

Diabase Intrusion-Induced Changes in Shale Pore Structure in Subei Basin: Insights from Mono- and Multifractal Analysis of N2 Adsorption

Diabase intrusion is a common geological phenomenon in lacustrine shale formations in continental basins in China, which has important effects on the physical and chemical properties of shale oil reservoirs. In this paper, we systematically analyzed the pore structure of diabase-intruded lacustrine shale in the Gaoyou sag of the Subei Basin using geochemical tests, thin-section observation, argon ion polishing scanning electron microscopy (SEM), low-temperature nitrogen adsorption experiments (LTNA), and other methods combined with monofractal and multifractal theories. The results show that the intrusion metamorphic segments are a diabase zone, hornfels zone, slate zone, and normal shale zone from the intrusion center. The pores of hornfels and slate are mostly oriented and dissolution is obvious. Many microfractures and secondary minerals such as quartz and chlorite are observed. The pore volumes of diabase and hornfels are small, while those of slate and normal shale are larger. The monofractal dimensions D1 and D2 of the intrusion segment show a general trend of decreasing first and then increasing from the intrusion center to the shale zone. The multifractal parameters’ H index decreases gradually from the lower normal shale to the upper metamorphic zone hornfels, while Δα and Rd increase gradually. The total organic carbon (TOC) content of the intrusion zone has little effect on the pore structure, and the fractal characteristics fluctuate weakly, while the vitrinite reflectivity (Ro) value change has a significant impact on the monofractal characteristics of the shale pore. Pore volume also affects the pore heterogeneity; the larger the specific surface area (SSA) and total pore volume (TPV), the lower the pore heterogeneity and the higher the surface roughness and pore connectivity. The diabase intrusion caused three modification mechanisms of mechanical squeezing, the thermal effect, and chemical action on the shale surrounding rocks, resulting in different degrees of pore formation or change. The pore evolution model of the metamorphic belt with the combined action of “mechanical-thermal-chemical” is established, and the influence of diabase intrusion on the pore types and pore size distribution (PSD) of shale reservoirs is quantitatively described, providing a new perspective and method for understanding the impact of diabase intrusion on the characteristics and exploration potential of shale oil reservoirs.

Wettability and physical modification of coal under vacuum saturation invasion of SiO2 nanofluids

Coal seam water injection technology could improve the water content of coal seam, which is an effective technical measure to reduce dust generation in the mining process. Water-based silica nanofluids are a green wetting agent for coal seam water injection. To understand the wetting mode of nanofluids in coal, it is necessary to explore the physico-chemical properties of nanofluid-modified coals. First, a new idea of saturation and intrusion of nanofluid into coal was proposed by using vacuum pressurized saturation device. Then, the physical and chemical properties of the modified coal were studied by Fourier transform infrared spectrometer, low-temperature liquid nitrogen adsorption experiments, and uniaxial compression mechanics experiments. The results showed that the content of oxygen-containing functional groups in the modified coal increased, which was positively correlated with the concentration of nanofluids. The pore structure of the modified coal sample changed from complex to simple, and the nanoparticles blocked the micropores to make the coal surface smooth. The saturation invasion of SiO2 nanofluids changed the mechanical properties of coal samples, and the compressive resistance of coal was weakened, and the minimum strength of the coal invaded by 1.5 wt. % SiO2 nanofluid saturation was 13.68 MPa. The saturation intrusion of SiO2 nanofluids has a negative effect on the surface adsorption of coal samples and the blockage of microporous structures, and makes the coal seam easier to be wetted, which contributes to the application and development of nanofluids in the field of coal seam water injection.

Correlation between surface wettability and fluid physics characteristics of lignite under compound solution regulation

The specific composition and ratio of the composite wetting agent will affect its dust reduction effect. Therefore, in order to study the effect of the composite wetting agent on the wettability and physical structure of lignite, the anionic surfactant sodium dodecyl sulfate was selected and mixed with different concentrations of sodium chloride to obtain different concentrations of the composite wetting agent. The correlation between the fluid physical properties of these composite wetting agents and the wetting of lignite surfaces was comprehensively explored by contact angle measurement, surface tension analysis, and low-temperature liquid nitrogen adsorption experiments, and the potential effects on pore structure were investigated. The results showed that as the sodium chloride concentration increased, the surface tension, contact angle, adhesion work, and surface free energy of the composite solution decreased, while the diffusion coefficient increased. There was a high linear correlation between the pore diameter of coal and the contact angle (R2 = 0.955), which revealed the close relationship between the permeability behavior of the fluid in the coal pores and the physical structure of the coal body. In addition, composite wetting agents with high sodium chloride concentrations exhibited a specific pore expansion effect, which helped optimize the coal's pore distribution and thus promoted effective coal wetting. This study not only deepens our understanding of the role of fluid physics principles in dust removal technology but also provides an important theoretical basis for selecting suitable composite wetting agents and their optimal ratios.

Study on physical and chemical structure modification law and mechanism of coal based on organic acidification

Deep coal seams in China have the characteristics of low permeability, low porosity, and poor gas permeability, which makes the extraction of coalbed methane very difficult. To study the modification effect of acidic solutions on coal pore structure and mineral components, this study used three different concentrations (5%, 10%, and 15%) of organic acids (HCOOH and CH3COOH) to treat coal. Through industrial/elemental analysis, low-temperature nitrogen adsorption experiments, and x-ray fluorescence spectroscopy detection experiments, the pore size distribution, mineral elements, and oxides of the acidified coal were analyzed. The strength of the acid solution's modification effect on coal was analyzed, as well as the relationship between different mineral elements and changes in pore structure. The results show that the two acids have a pore expanding effect on coal, with an increase in average pore size; the pore size distribution is between 2 and 50 nm, belonging to mesopores; acetic acid has a stronger overall dissolution efficiency on inorganic minerals, while formic acid has a better effect on the removal of Ca and P elements, but none of them can effectively remove silicoaluminates; the content of Ca element in coal is positively correlated with the specific surface area provided by mesopores; and iron dolomite is not sensitive to the reaction with acetic acid, resulting in an opposite trend in Fe content and mesoporous specific surface area. Organic acid solution changes the pore and fracture distribution structure of coal through chemical dissolution by dissolving the mineral components inside the coal, thereby affecting its internal structural characteristics.

Modified gum ghatti based hybrid hydrogel nanocomposite as adsorbent material for dye removal from wastewater

A hybrid hydrogel nanocomposite based on the polysaccharide-gum ghatti, has been made and evaluated as an adsorbent material for wastewater treatment. The nanocomposite is composed of a network of gum ghatti-graft-poly(2-acrylamido-2-methylpropane sulfonic acid) with magnetite nanoparticles embedded within. This functional material Ggh-g-PAMPS/Fe3O4 has been characterized by FTIR, TGA, SEM, EDS, XRD, BET and VSM techniques. The presence of magnetite nanoparticles imparted superparamagnetic property to the adsorbent material enabling its easy separation after use with an external magnet. The characterization data indicated mesoporous nature of the nanocomposite adsorbent with mean pore diameter of 4.9 nm. The nanoparticles imparted high surface area to the adsorbent material the value being 4.7 m2g−1 based on nitrogen adsorption experiments. The suitability of the material as an adsorbent for removal of cationic dyes from water was checked with two cationic dyes namely, rhodamine 6G and methylene blue. The maximum adsorption capacity of the nanocomposite Ggh-g-PAMPS/Fe3O4 towards rhodamine 6G and methylene blue were observed to be 403.2 and 427.8 mg g−1 respectively from their individual solutions of concentration 500 mg L−1. The pH dependence on swelling and surface charge indicated medium of pH of 7.0 as the ideal condition for effective adsorption. Freundlich isotherm model and pseudo second order kinetic model are observed to be the most befitting models to describe adsorption. Further, the thermodynamic studies revealed the adsorption to be a spontaneous and endothermic process. The desorption study portrayed the reusability of the adsorbent material. The excellent adsorption performance and magnetic nature of the developed nanocomposite suggests its potential application as an adsorbent material in wastewater treatment.

Influence of CO2 injection on characterization of microscopic pore throat structure in shale reservoirs

CO2 injection to displace oil in shale reservoirs will form asphaltene precipitation and harm the microscopic pore-throat structure, influencing the displacement efficiency. However, asphaltene deposition harmed reservoir characteristics and the damage mechanism to microscopic pore-throat structure is not clear. In this study, on the basis of CO2 displacement experiments, nuclear magnetic resonance (NMR) scanning and low-temperature nitrogen adsorption (LTNA) experiments were used to investigate the damage characteristics of asphaltene deposition on physical properties, wettability, oil recovery and micro-structure of three types of reservoirs under different injection pressures, and reveal the microscopic damage mechanism of asphaltene precipitation on the pore-throat structure. The results show that after CO2 displacement, the permeability and porosity damage rate is positively correlated with injection pressure, the maximum damage rate is 45.84 % and 42.73 %, respectively. And the asphaltene precipitation results in wettability reversal of pore throats. In addition, the blockage rate of nano-scale pore throats in the three types of reservoirs is higher, with the highest blockage rates of 45.43 %, 52.41 % and 76.45 %, respectively. The worse the reservoir physical properties, the higher the pore throat blockage rate, and the greater damage to the micro-structure by asphaltene precipitation. The amplitude of the LTNA adsorption–desorption curves all decreased, and the adsorption amount of the core samples decreases. The contribution of micropore specific surface area of Type I and II reservoirs decreases, while the contribution of Type III reservoir increases up to 90.76 %. Asphaltene precipitation resulting in a decrease in total pore volume. Asphaltene particles are adsorbed on the pore surface, and the pore diameter decreases with the maximum decrease of 3.36%. The contribution of macropores to the total pore volume and specific surface area is significantly reduced compared to that of micropores and mesopores.

Investigation of the pore structure characteristics and fluid components of Quaternary mudstone biogas reservoirs: a case study of the Qaidam Basin in China

Quaternary mudstone biogas reservoirs in the Qaidam Basin have shown great potential. However, complex pore structures with high clay contents and high heterogeneity limit the understanding of the storage and migration principles of these reservoirs. In this paper, HPMI and nitrogen adsorption experiments, in combination with NMR experiments under water saturation, centrifugation, various drying temperatures and other conditions, were adopted to determine the pore structure characteristics. Specifically, the reservoir space types and pore radius distribution characteristics were clarified. The cutoff values for different types of pores were identified based on the water-saturated mudstone NMR T2 spectra for full aperture distribution scales jointly characterized by mercury injection and nitrogen adsorption experiments. Furthermore, the three pore components and the saturation of different fluids were obtained. The research results indicate that the mudstone biogas reservoir has developed various reservoir spaces, and the pore size is primarily in the micronanometer range. The average total porosity reaches 27.28%, but the proportion of movable water pores is only 9.23% with poor fluid mobility, and the fluids in the pores are mostly capillary-bound water and clay-bound water. Among the different lithologies, argillaceous sand is more likely to become a good production layer.

Characteristics and main controlling factors of the Sangonghe tight sandstone reservoirs in the Shengbei Sub-sag of the Turpan-Hami basin, China

The Sangonghe tight sandstone hydrocarbon reservoirs in the Shengbei Sub-sag of the Turpan-Hami Basin are key areas for hydrocarbon exploration. However, previous research has mostly focused on petrological characteristics, physical properties, and pore types. There is a lack of understanding regarding diagenesis, microscopic pore-throat features, and the impact of overpressure on the reservoir. This gap in knowledge poses a disadvantage for future studies and hydrocarbon exploration. This study examines the Sangonghe tight sandstone hydrocarbon reservoirs in the Shengbei Sub-sag of the Turpan-Hami Basin. It combines cast thin sections, physical property tests, high-pressure mercury injection experiments, low-temperature nitrogen adsorption experiments, and well logging overpressure identification to investigate the basic characteristics, diagenesis, and microscopic pore-throat features of these reservoirs. Based on this analysis, the study identifies the main controlling factors of the reservoir properties and presents three key findings. First, the reservoir rocks are primarily composed of lithic sandstone and feldspathic lithic sandstone with porosity ranging from 4% to 10% and permeability ranging between 0.1 and 10 mD, classifying it as a low-porosity, low-permeability tight sandstone reservoir. Second, the reservoir has undergone several diagenetic processes, with significant compaction and common occurrences of cementation and metasomatism. Acidic dissolution is the main dissolution process, but it is relatively weak. The pore-throat system exhibits poor and concentrated sorting, with nanometer-sized pores being predominant. These include 'ink-bottle' pores with narrow necks and wide bodies, and parallel plate-structured fissure pores. Finally, the development of the reservoirs is mainly influenced by sedimentation, dissolution, and overpressure. The parent rock provenance features a high content of stable heavy minerals, larger grain sizes, and thicker sand body layers. These factors enhanced the reservoir's resistance to compaction and promoted the development of primary intergranular pores. Additionally, organic acid fluids caused extensive dissolution of feldspar and lithic fragments, resulting in the formation of numerous secondary dissolution pores. Additionally, under-compaction overpressure helped preserve primary pores, while hydrocarbon generation overpressure extended the hydrocarbon generation period. This extended period provided the driving force for fluid migration, enhanced the transformation of reservoir space, and aided in hydrocarbon accumulation.

Investigation of the Impact of Moisture in Various States on the Adsorption of CH4, N2, and CO2 by Coal.

To investigate the impact of moisture in various states on the adsorption of CH4, N2, and CO2 by coal, this study conducted isothermal adsorption experiments on dry coal for moisture and on coal with varying moisture content for CH4, N2, and CO2. The results revealed that coal moisture adsorption progresses through four stages: monolayer adsorption, multilayer adsorption, cluster formation, and condensation into free moisture filling pores. The presence of moisture in coal varies across these stages, with its impact on CH4, N2, and CO2 adsorption decreasing as moisture content increases. As the moisture content rises, the adsorption constants (a and b values) progressively decline and eventually level off. Based on differences in coal moisture states, delineated by a relative humidity of 0.50 (corresponding to a moisture content of 2.39% in experimental samples), adsorption potential theory identified low moisture content stages where the monolayer and multilayer adsorption potentials for moisture (E1 = 4.99 kJ/mol, E2 = 39.71 kJ/mol) exceeded those for CH4, N2, and CO2, suggesting competitive adsorption effects. In the high-water content stage, the mesoporous structural fractal dimension D2 of the coal samples was found to be 2.77, nearly reaching 3, as determined through liquid nitrogen adsorption experiments and fractal dimension theory. This suggests that the intricate pore structure leads to water condensation, free water formation, and pore blockage, making it the primary factor influencing the adsorption of CH4, N2, and CO2 in coals, which is inherently linked to the characteristics of coal.

Study on the Thermal Effect of Spontaneous Combustion of Coal-Based Activated Carbon By-Products Oxidized and Carbonized Powders

ABSTRACT To investigate the spontaneous combustion thermal effect of the by-products oxidized powder (OP) and carbonized powder (CP) produced during the preparation of coal-based activated carbon (CBAC). First, the changes in composition and pore structure of OP and CP compared with raw coal (RC) were measured by coal quality analysis and nitrogen adsorption experiments. The ability of OP and CP to physically adsorb oxygen was determined. Then, differential scanning calorimetry (DSC) was used to study the whole process thermal effect of the spontaneous combustion of OP and CP. Finally, the thermal effect of OP and CP during low-temperature oxidation process was further explored by the C80 micro-calorimeter, and the thermodynamic characteristics were also probed. The results show that the content of fixed carbon and carbon elements in OP and CP are significantly greater than that in RC, which makes the pore structure of both complicated. In addition, the total pore volume, average pore size, and specific surface area of OP and CP are greater than that of RC, which is more conducive to the physical adsorption of oxygen by both. No transition platform is caused by the decomposition of intermediate products on the DSC curve of CP. The characteristic temperatures of CP (except T D10) are lower than that of OP, indicating that the spontaneous combustion reaction of CP started earlier than that of OP. However, part of the combustible substance of CP was consumed in the dry distillation process, leading to the thermal release and maximum thermal release rate being smaller than that of OP. The apparent activation energy (E a) of stages H2 and H3 in the low-temperature oxidation process of OP is 75.04 and 11.42 kJ mol−1, while that of CP is 83.11 and 30.91 kJ mol−1. The threshold of CP entering stages H2 and H3 is higher than that of OP. Nevertheless, from the pre-exponential factor (A), the low-temperature oxidation process of CP is more rapid.

Designing highly porous three-dimensional triazine-bearing covalent organic polymers as prominent adsorbents of carbon dioxide and iodine

A new class of triazine-containing covalent organic porous polymers TCOP1-3 were made using a versatile one-pot [3 + 2] cyclocondensation reaction revealing remarkable robustness as proven by their thermal stability which disclosed a 10 % weight loss temperature ≥400 °C. Investigation of the intrinsic microporosity properties of TCOP1-3 using nitrogen adsorption experiments disclosed salient Brunauer–Emmett–Teller (BET) surface areas ranging from 725 to 932 m2 g−1 with average pore volumes in the range of 0.52–0.65 cm3 g−1. TCOP1-3 exhibited high CO2 uptakes attaining 120 mg g−1 at 273K and low pressure whereas iodine capture tests demonstrated significant results achieving a maximum adsorption of 4.02 g g−1. Recycling experiments confirmed the potential for these polymers to be restored even after a multitude of concurrent iodine adsorption-desorption cycles.

Insight into influence of degradation metabolism of Firmicutes on the microstructure of anthracite coal

Promoting the permeability of deep, low-permeability coal seams through biological means is currently a research hotspot for enhancing the efficiency of coalbed methane extraction. In this study, we selected anthracite coal from Sihe Mine for microbial anaerobic degradation culture experiments. The effects of adding functional microorganisms on the microstructure of anthracite coal were analyzed by high-throughput sequencing of samples before and after the cultivation and microcharacterization experiments of coal samples. The results showed that the maximum methane migration during the period of biodegradation reached 0.640 mL/g coal, and the cumulative migration of the whole cycle was as high as 1.318 mL/g coal. 16S rRNA high-throughput sequencing results indicated that the bacterial community structure had undergone significant succession after the biodegradation experiments, and that the Firmicutes represented by Bacillus(82.41% of the total) occupied the dominant niche. Metabolic pathway analysis based on Kyoto Encyclopedia of Genes and Genomes (KEGG) database showed that the degradation of aromatic compounds by microorganisms appeared to be significantly enhanced by the addition of nitrogen sources. Also, the relative abundance of a number of key metabolic enzyme genes capable of catalyzing the oxygen-containing functional groups into the structure of the coal molecule and the de-cyclization reaction were increased. Fourier Transform Infrared Spectrometer (FTIR) experiments revealed that biodegradation stimulated by nitrogen source reduced the aromaticity of coal by 59.62% and enhanced the hydroxyl functional group content by 1.822 times. Mercury pressure and low-temperature nitrogen adsorption experiments showed that the micropore pore volume of the treated coal decreased by 34.09%, and the macropore pore volume accounted for an increased 168.28%, with an average pore size increment of 60.72 nm. Therefore, the nitrogen source can stimulate Firmicutes on the degradation of polycyclic aromatic hydrocarbon and increase the content of oxygen-containing functional groups, which might promote the development of pores in coal and make the difficult-to-desorption methane migrates rapidly. This study investigated the effect of nitrogen source on the degradation of coal by Firmicutes. This will help improve the efficiency of gas extraction and ensure the safety of coal mine production.

Polymer‐Sorbent Direct Air Capture Contactors with Complex Geometries 3D‐Printed via Templated Phase Inversion

AbstractLow‐cost direct air capture (DAC) systems require efficient gas–solid contactors. These contactors provide rapid rates of heat and mass transport with minimal pressure drops. Triply periodic minimal surfaces (TPMS) are a class of geometries that are shown to have heat transfer properties that exceed those of other geometries, and these benefits are expected to manifest in mass transfer as well due to the analogies between heat and mass transfer. However, creating TPMS contactors is difficult using conventional manufacturing techniques such as injection molding. Non‐solvent‐induced phase separation (NIPS) of a polymeric ink containing the adsorbent is one way to construct adsorption contactors with excellent mass transport rates, but contactors have to date only been fabricated in simple geometries (e.g., films, fibers). This study demonstrates that NIPS of polymeric inks occurs within a simultaneously‐dissolving, water‐soluble, 3D‐printed template. This templated phase inversion (TPI) technique can create seemingly unlimited macroscopic architectures, including TPMS contactors, using a variety of potential DAC adsorbents, including zeolite, silica, activated carbon, and metal–organic frameworks. SEM, micro‐computed tomography, and nitrogen adsorption experiments reveal the microscopic pore structures and macroscopic geometries of the resulting sorbent contactors. TPMS DAC contactors with poly(ethyleneimine)/silica adsorbents are shown to have CO2 capture performances that rival or exceed other contactor geometries.

Effect of pre-oxidation and cooling process on characteristics and mechanism of the coal re-ignition

Based on the engineering background that the sealed fire area is easy to reignite, the reignition characteristics of pre-oxidized coals (POC) after cooling are studied. Predecessors mainly explain this problem from the perspective of pre-oxidation affecting coals pore structure. This paper analyzes the effects of different pre-oxidation temperatures and cooling atmospheres on coal physical and chemical structure. The changes of coals during oxidation-cooling-reoxidation were studied by means of a programmed heating device, low temperature nitrogen adsorption experiment, FTIR and quantum chemical calculation. The results show that the average pore size of the coal cooled in nitrogen at the same pre-oxidation temperature is smaller than that of the coal cooled in dry air, while the spontaneous combustion characteristics of the coal cooled in nitrogen are stronger than those of raw coal, and spontaneous combustion characteristics of coal cooled in air are the opposite. The oxygen-containing functional groups of the coal cooled in nitrogen are pyrolyzed to produce active sites which can exist stably in nitrogen. Then, the rationality of “The active sites tend to be alkyl radical” is analyzed and speculated by molecular orbital theory, and it's found that alkyl radicals can release lots of heat at 30 °C.

Geological Conditions of Shale Gas Accumulation in Coal Measures

The shale of different potential layers is studied by using rock pyrolysis analysis, total organic carbon determination (TOC), kerogen microscopic component identification, mineral X-ray diffraction, scanning electron microscopy, and low-temperature nitrogen adsorption experiments. The results are as follows: (1) Shishui Formation of the Lower Carboniferous and Longtan Formation of the Upper Permian are the two most important shale gas reservoirs in the Chenlei Depression. The sedimentary environment of the target shale is a marine land interaction facies coastal bay lagoon swamp sedimentary system. Two sedimentary facies of tidal flat facies, subtidal zone, and lagoon swamp facies are developed. (2) The organic matter types of shale are Type III and II2, with TOC content greater than 1%. The maturity of shale samples is relatively higher (Ro,max is above 2%), which means they have entered the stage of large-scale gas generation. The overall brittle mineral content of the target shale sample is relatively higher (above 40%), which is conducive to artificial fracturing and fracture formation in the later stage, while an appropriate amount of clay minerals (generally stable at 40%) is conducive to gas adsorption. (3) The overall pore structure of the water measurement group and Longtan group is good, with a higher specific surface area and total pore volume (average specific surface area is 12.21 and 8.36 m2/g, respectively), which is conducive to the occurrence of shale gas and has good adsorption and storage potential. The gas content of the water measurement group and the Longtan Formation varies from 0.42 to 5 cm3/g, with an average of 2.1 cm3/g. It indicates that the water measurement group and the Longtan Formation shale gas in the study area have good resource potential.

Evolution of pore structure and diagenetic stages of late Permian shales in the Lower Yangtze Region, South China

This study examines the evolution characteristics of shale pores in the Late Permian Longtan Formation located in the Lower Yangtze region of South China. The complete process of thermal evolution of the samples was achieved through thermal simulation experiments while identifying qualitatively the pore types and their development characteristics using field emission-scanning electron microscope (FE-SEM). Quantitative analysis of pore size distribution was conducted through mercury intrusion capillary pressure (MICP), nitrogen (N2) and carbon dioxide (CO2) adsorption experiments. The paper comprehensively analyzed the pore evolution characteristics and controlling factors of shale in the study area, with an analysis of diagenetic differences. Findings reveal that during the pore evolution process, both morphology and pore size are influenced by thermal maturity. The pore volume is dominated by mesopores and macropores, while the specific surface area is mainly dominated by mesopores and micropores. Thermal evolution promotes the formation of micropores and mesopores but hinders the development of macropores. Moreover, clay minerals transformation and mineral dissolution make certain contributions to the development of micropores. The diagenesis in the study area is controlled primarily by the pyrolysis of organic matter. Pyrite and clay minerals are the first to dissolve, followed by calcite and quartz. Five stages of evolution characterization have been identified from low-mature stage (Ro = 0.88 %) to overmature stage (Ro = 3.35 %) in combination with diagenesis. The development of pore structure and its influencing factors vary across different stages. The influencing factors mainly include hydrocarbon generation from organic matter, compaction, mineral transformation, and dissolution. The process of hydrocarbon generation in organic matter occurs throughout the entire pore evolution process, resulting in the development of numerous micropores and mesopores. Compaction primarily impacts pore development during the early diagenetic stage, causing a substantial transition of primary mineral pores from macropores to mesopores. Mineral transformation and dissolution take place during and after the middle diagenetic stage. The former governs the development of mesopore-sized clay interlayer pores, while the latter primarily generates micropores.

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.

Study on preparation and properties of steel slag based composite gel for mine fire prevention and extinguishing

A novel fire-preventing composite gel, mainly made from steel slag (SS) and silica fume (SF), was created for a coal mine’s spontaneous combustion. The gelation mechanism of the steel slag-based composite gel (SSG) was investigated using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and field emission scanning electron microscopy (SEM). The findings suggest that SSG is generated through the processes of hydration and geopolymerization involving SS and SF. And in the alkaline milieu of a 1.5 M water glass solution, SSG manifests enhanced strength and water retention capacities. Moreover, the fire prevention and extinguishing performance of the SSG was analyzed and verified using low-temperature oxidation, thermogravimetry, and low temperature nitrogen adsorption experiment (LTNA). The SSG has proven highly effective in suppressing spontaneous coal combustion by inhibiting CO production, raising the coal oxidation temperature, and reducing the contact area between oxygen and the coal body.

Influence of methane-oxygen coupled gas field on the transformation mechanism of microporosity and active structure of cryogenic pre-oxidized residual coal

Due to the impact of coal seam mining processes and the release of adsorbed gases in residual coal, a mixed gas field of methane and oxygen is formed in the goaf, which affects the oxidation process of the residual coal over a long period. In this study, to explore the low-temperature oxidation mechanism of residual coal in a hypoxic methane-containing environment, Low temperature nitrogen adsorption (LTNA) experiments and Fourier transform infrared spectroscopy (FTIR) were utilized to examine the pore development properties of residual coal and the transformation mechanisms of reactive micro-groups. The results indicate that increases in temperature and methane proportion promote the transition of various pores to larger scales, with 3 % methane inducing the formation of fine micropores, thereby increasing the specific surface area of the coal. High temperatures (80℃，100℃) and increased proportions of oxygen and methane synergistically elevate the fractal dimensions. Methane inhibits the consumption of hydroxyl groups, and a low oxygen concentration (5 %) impedes the gas production from coal oxidation. The presence of both oxygen and methane exhibits a compound effect; the strongest tendency towards spontaneous combustion occur at 1 % methane and 9 % oxygen. The findings provide valuable insights for controlling spontaneous combustion disasters in goafs.

Effect of liquid CO2 phase change on pores and fractures in coal: An experimental study

AbstractThe evolution characteristics of pores and fractures in coal after liquid carbon dioxide (CO2) phase change are important factors that determine the permeability increase effect. Therefore, it is critical to correctly understand the influences of liquid CO2 phase change on pores and fractures in coal. The changes of adsorption and desorption isotherm, pore size, pore volume, and specific surface area of fractured coal and fractured coal were compared by low temperature liquid nitrogen adsorption experiment. In addition, a scanning electron microscope was adopted to observe fracture characteristics of fractured and unfractured coal samples and analyze changes in the connectivity and fracture development. Experimental results show that the fractured coal samples exhibit better hysteresis loops and a larger proportion of gas desorption than the unfractured ones. Fractured coal samples contain more developed pores and fractures compared with unfractured ones, and their fragmentation degree, pore diameter, fracture width, and connectivity of pores and fractures are also better. Besides, the closer the samples from the fracturing boreholes are, the better the fracturing effect. This indicates that liquid CO2 phase change can effectively enhance the gas transport capacity in pores and fractures in coal. The research results provide a solid basis for the better application of liquid CO2 phase‐change fracturing to the prevention of coal and gas outburst disasters and the realization of efficient gas extraction in deep coal seams.