Pore Fissure Research Articles

Modification of pore structure of coal under hot steam injection as revealed by nuclear magnetic resonance

Heat injection provides a feasible approach for the extremely efficient extraction of coalbed gas. Injecting hot steam can effectively improve the pore structure of coal and increase the permeability of coal. To observe the changes in the pore structure of coal during hot steam injection, low magnetic field nuclear magnetic resonance technology is used to study the variations in the pore structure of coal under different heat injection durations. The results show that hot steam can promote the formation, growth, and expansion of coal pore fissures, thereby enhancing the gas permeability of the coal seam. At the same time, the analysis of relevant nuclear magnetic parameters indicates that when the heat injection duration is 15 minutes, hot steam has the best effect on coal modification. In the early stage of hot steam injection, hot steam stimulates the development of the porous structure. In the middle stage, some pore structures collapse and get blocked due to local thermal stress. In the late stage of hot steam injection, hot steam accelerates the conversion of micropore and mesopore structures into macropore or fissure structures, and hot steam has a significant modification effect on coal.

Numerical Simulation of Mechanical Properties of Deep Gray Sandstone with Holes and Radial Cracks

In order to study the effects of prefabricated pore-fracture on the mechanical properties, crack extension and energy evolution of rocks under uniaxial compression, the effects of rock samples containing pore-fracture deep grey sandstone on the mechanical properties, crack extension and energy evolution of rocks were analysed in detail through the indoor experimental inverse performance of the fine-scale mechanical parameters of the PFC3D model. The results show that the mechanical parameters of deep grey sandstone samples with pore fissures are significantly lower than those of intact samples, and the decrease is closely related to the diameter of the pore as well as the length and width of the fissure, with the increase of the diameter of the pore, the peak strength and the peak strain of deep grey sandstone with single pore are in the trend of attenuation, and the mechanical parameters of deep grey sandstone samples with pore fissures of different lengths and widths are in the trend of attenuation with the increase of fissure widths and lengths of different layers. The mechanical parameters of deep grey sandstone samples with different lengths and widths of fissures show a trend of attenuation with the increase of fissure width and length; based on the analysis of microscopic crack characteristics during loading of deep grey sandstone with pore fissures, it is concluded that the cracks produced by deep grey sandstone samples with pore fissures are mainly distributed at the two ends of the samples, and the middle area has fewer cracks and the number of the cracks produced shows a trend of attenuation with the increase in the diameter of the pore as well as the length and width of the fissure; Based on the energy evolution mechanism, it is concluded that as the pore size of the rock sample increases, the strain node at which the dissipated energy begins to appear becomes progressively earlier, the energy storage capacity of the rock appears to be drastically reduced, and the internal damage becomes more and more severe.

Microstructure evolution in bituminous-coal pyrolysis under in situ and stress-free conditions: a comparative study

A self-made triaxial testing machine with thermal–hydraulic–mechanical–chemical (THMC) coupling and a tubular heating furnace, combined with in situ (IS) micro-computed-tomography technology was utilized in this study. The evolution of pore-fissure (PF) structure parameters (porosity, PF scale distribution, effective PF volume ratio, and permeability) of bituminous coal under stress-free (SF) and IS conditions with temperature was investigated, and then the mechanism of experimental results was analyzed. Results showed that (1) under SF conditions, at 300–550 °C, the coal samples after pyrolysis are dominated by elongated large fissures, with PF structure parameters positively correlating with temperature. After 400 °C, the number of PFs increases, with most PFs having equivalent diameter (R) ≤ 100 μm. (2) Under IS conditions, coal sample fissures are dominated by elongated large fissures at 300–350 °C and by holes at 350–600 °C. (3) Under IS conditions at 300–600 °C, the PF structure parameters of coal samples initially decrease with temperature and subsequently increase. The number of PFs fluctuates within a certain range, and the PF scale distribution dynamically shifts with temperature. (4) After 300 °C, the PF structure parameters of bituminous coal under SF and IS conditions show a bipolar distribution with temperature. Therefore, the weakening effect of stress on the PF structure of coal samples should not be overlooked during IS pyrolysis mining of coal bodies.

Farmland hydrological cycle under agroforestry systems and efficient use of water resources in the karst desertification environment

In karst desertification (KD) regions, surface water (SW) easily enters underground through pore fissures and sinkholes despite the presence of abundant precipitation. Such regions have a typical distribution of “soil above and water below”, and, thus, the unique “karst drought” occurs. Hence, an urgent and primary problem in combating KD is to reach highly efficient utilization of water resources in these regions. We selected three karst research areas with different levels of karst desertification and different geomorphic types. By monitoring the storage and transformation of five types of water in the agroforestry system—precipitation, SW, groundwater (GW), soil water (SoW), plant water (PW), the following results were obtained: (1) In KD regions, a positive correlation was found among available precipitation, rainfall, and land evapotranspiration (LE), and LE was approximately equivalent to soil evaporation. (2) To varying degrees, agroforestry brings ecological benefits, including reducing surface runoff, increasing soil infiltration, lowering the transpiration rate, and reducing soil evaporation, thus achieving efficient use of water resources. (3) From 100 % rainfall, the transformation rates of SW, GW, PW, and SoW reached 0.14–12.71 %, 9.43–30.20 %, 9.79–49.97 %, and 40.72–82.58 %, respectively, and SoW showed a larger reserve than the other three types. (4) Drought stress contributes to the improvement of water use efficiency (WUE). Affected by drought stress, WUE was found to be the highest in a medium-intensity karst desertification environment. The transformation mechanisms of the five types of water observed in the agroforestry system provide a reference for efficient utilization of water resources in KD regions as well as theoretical support for addressing karst drought. They are also essential in helping to advance the ecological derivative industry, boosting the economy in karst mountainous areas, and controlling karst desertification.

Scanning Electron Microscope Micromorphology Characterization of Coal Samples from No.3 Coal Seam in Chengzhuang Mine Area

Matrix pores in coal reservoirs are important storage sites for adsorbed and free state CBM, and their adsorption amount is closely related to the development of pores and pore structure characteristics in coal. In this paper, coal samples from No. 3 coal seam of Chengzhuang coal mine were taken as the research object, and the microscopic morphology of coal matrix pore fissures was studied by scanning electron microscope. The results found that: in the coal samples of the study area, the pneumatic pores and dissolution pores are especially developed, and more fissures are developed at the same time due to the influence of tectonic action on the coal seam. The coal samples of Chengzhuang No.3 coal seam have many kinds of pore morphologies, mainly including round, wedge-shaped, triangular, slit-shaped and some irregular pores. The morphology, size and distribution of these pores have an important influence on the physical and chemical properties of the coal seam, which provides an important experimental basis for further research on the physical and chemical properties of coal seams and their roles in CBM adsorption, transportation and mining processes.

Application and Research of Multi-Scale Dynamic Diffusion Permeability Model in Different Coal Ranks

The spatial scale of coal mining and rock layer control is distributed between 10-10 and 107, spanning 17 orders of magnitude. There are multi-scale pore cracks from millimeter to micro and nanometers in the coal, and the pore size can reach one million times, which makes the coal permeability also show a multi-scale characteristics of millions and time. For the permeability of the coal seam, it is necessary to test and analyze the distribution of micro and nano pores and cracks in the coal [1]. CBM mining is accompanied by the dynamic change of adsorption pressure in coal and the complex deformation of multi-scale heterogeneous structure of coal, and the multi-scale deformation feature of pore fissure structure plays an important role in the transport of methane [2]. However, existing studies focus on treating coal as a homogeneous isotropic medium, without considering the influence of coal multiscale pore structure on permeability. The variation of heterogeneous degree of coal structure with structural scale and its influence on dynamic diffusion and permeability are rarely reported. Li Zhiqiang et al. [3] conducted an experimental-model-mechanism study of CBM gas micro-nano series multi-scale dynamic diffusion permeability. On this basis, the dynamic diffusion and permeability of different coal scales.

Hydrochemical and microbial community characteristics and the sources of inorganic nitrogen in groundwater from different aquifers in Zhanjiang, Guangdong Province, China

Groundwater from different aquifers in the Zhanjiang area suffers from different degrees of nitrogen pollution, which poses a serious threat to the health of urban and rural residents as well as the surrounding aquatic ecological environment. However, neither the water chemistry and microbial community characteristics in different aquifer media nor the sources of inorganic nitrogen pollution have been extensively studied. This study integrated water quality parameters, dual isotopes (δ15N–NO3− and δ18O–NO3−), and 16S rRNA data to clarify the hydrochemical and microbial characteristics of loose rock pore water (LRPW), layered bedrock fissure water (LBFW), and volcanic rock pore fissure water (VRPFW) in the Zhanjiang area and to determine inorganic nitrogen pollution and sources. The results show that the hydrochemistry of groundwater in different aquifers is complex and diverse, which is mainly affected by rock weathering and atmospheric precipitation, and the cation exchange is strong. High NO3− concentration reduces the richness of the microbial community (VRPFW). There are a large number of bacteria related to nitrogen (N) cycle in groundwater and nitrification dominated the N transformation. A quarter of the samples exceeded the relevant inorganic nitrogen index limits specified in the drinking water standard for China. The NO3− content is highest in VRPFW and the NH4+ content is highest in shallow loose rock pore water (SLRPW). In general, NO3−/Cl−, dual isotope (δ15N–NO3− and δ18O–NO3−) data and MixSIAR quantitative results indicate manure and sewage (M&S) and soil organic nitrogen (SON) are the main sources of NO3−. In LRPW, as the depth increases, the contribution rate of M&S gradually decreases, and the contribution rate of SON gradually increases. The results of uncertainty analysis show that the UI90 values of SON and M&S are higher. This study provides a scientific basis for local relevant departments to address inorganic nitrogen pollution in groundwater.

Characteristics and dynamic mechanism of rill erosion driven by extreme rainfall on karst plateau slopes, SW China

Driven by climate change and increased human activities, the frequency and intensity of extreme rainfall events are increasing, leading to increased risk of water erosion on slopes in karst rocky desertification (KRD) areas. However, the driving factors (rainfall intensity, slope gradient, and the degree of underground fissure pore), characteristics, and dynamic mechanisms of rill erosion on KRD slopes under extreme rainfall have received limited attention. A rainfall simulation experiment was performed to investigate the driving factors of rill erosion on KRD slopes under extreme rainfall events. The experiments were conducted using an erosion flume (4 m long, 1.5 m wide and 0.35 m deep) with five rainfall intensities (50, 105, 115, 125 and 145 mm·h−1), three-level slope gradients (5°, 15° and 25°), and three typical degrees of underground pore fissure (1 %, 3 % and 5 %). The results showed that extreme rainfall significantly stimulated the formation and development of rill erosion on KRD slopes. In particular, rainfall intensity and slope gradient significantly affected rill erosion (p < 0.05), whereas the degree of underground pore fissure did not have any distinct effect. Increasing rainfall intensity intensified rill erosion by significantly (p < 0.05) promoting the rill density, total rill surface area, and longest of rill length. With increasing slope gradient, the rill density, rill distribution density, and average rill depth significantly increased (p < 0.05). Moreover, extreme rainfall events exacerbated surface soil erosion by enhancing the hydrodynamic characteristics of slope runoff. Rill erosion was most sensitive to the shear stress of runoff, and rill density was found to be the most suitable rill index for describing surface soil erosion on the slope under extreme rainfall. The findings provide valuable insights into the characteristics and mechanisms of soil erosion in karst regions under future climate warming.

Precursor Signal Identification and Acoustic Emission Characteristics of Coal Fracture Process Subjected to Uniaxial Loading

In deep underground mine engineering, the critical warning signals before the sudden failure of coal are crucial to predict coal or rock dynamic catastrophes and to help the coal industry grow sustainably. Therefore, with the objective of accurately identifying the precursor signals of coal fracture, a uniaxial compression test was adopted. Tests were performed on multiple sets of raw coal samples, and acoustic emission (AE) technology was used to capture the deformation and destruction courses of the coal samples. Furthermore, the signal intensity of AE energy was discussed. Based on the critical slowing down theory, the AE energy sequence was processed. The results indicate that there are significant discrepancies in the strength of coal affected by initial pore fissures. During the whole loading process, the AE energy signals showed obvious stage characteristics, and there was a high risk of rapid coal energy storage during the unstable rupture development (URD) stage, which predicted the imminent destruction of the coal. The variance mutation point that was not affected by the lag step selection was easier to identify than that of the autocorrelation coefficient, and the precursor points were all in the URD stage, which is more accurate than using the AE cumulative energy curve slope.

Experimental study on the effects of pre-oxidation on the pore structure and fractal characteristics of coal

ABSTRACT To assess the effects of pre-oxidation on pore structures and fractal features of coal samples, pre-oxidized coals at various oxidation temperatures were prepared and compared. Pore structure parameters of coal samples were analyzed qualitatively and quantitatively using electron scanning microscopy (SEM) and N2 adsorption. The pore volume of coal samples increased first and then decreased with the increase of oxidation temperature. Among them, the pre-oxidized coal at 150°C is the highest, which is 27.92% higher than the raw coal. The fractal features of coal samples were calculated by fractal theory. It was found that pre-oxidation makes pore fissures of coal samples develop with obvious pore expansion effect, which is more obvious at the higher pre-oxidation temperature. In addition, fractal dimension D 2 of coal samples was increased by pre-oxidation, indicating that pore structures of pre-oxidized coal are more complex than those of raw coal.

Analysis of the wetting and exothermic properties of preoxidized coal and the microscopic mechanism

In the processes of coal mining and transportation, low-temperature oxidation occurs continuously, and it is accompanied by multiple wetting. The effect of water on coal spontaneous combustion (CSC) cannot be ignored. To study the wetting and exothermic properties of preoxidized coal and the mechanism of its effect on CSC, molecular dynamics simulations, microcalorimetry, scanning electron microscopy (SEM) and fourier transform infrared spectrometer (FTIR) are used to analyze the molecular dynamics mechanism, heat of wetting (Hw), surface structure and microscopic group changes. Experimental results show that the surface potential distribution of coal is uneven, which leads to the adsorption of polar water molecules; the total energy of the system continues to decrease during the adsorption process. The preoxidation treatment further develops pore fissures, increases the percentage of hydrophilic groups and evaporates the original water. Therefore, in the coal low-temperature oxidation stage, the higher the degree of preoxidation is, the greater the Hw value. After preoxidation and wetting, the loose and porous characteristics of coal are further reflected; the oxygen percolation and adsorption are enhanced, which accelerates the system heat accumulation and thus intensifies the risk of CSC. The experimental results provide an important theoretical basis for the prevention of CSC.

Research on Energy and Instability Criterion of Sandstone Hydraulic Fracturing in the Three Gorges Reservoir Area, Chongqing, China

At present, the understanding of structural failure and energy analysis of sandstone under hydrofracture is still insufficient. This time, we selected the sandstone in Chongqing Three Gorges Reservoir Area as the main analysis object to discuss and compare the characteristics of conventional uniaxial (triaxial) compression strength test and uniaxial (triaxial) compression hydraulic fracturing strength test and conduct instability analysis from the perspective of energy. Based on the mechanical characteristics and parameters about the uniaxial (triaxial) compressive strength test and uniaxial (triaxial) compressive hydrofracture strength test, we focus on the analysis of the evolution rules of the hydraulic pressure-strain curve, analyzing the criteria of crack expanding and instability by using energy principle. According to the viewpoint of fracture mechanics, the fracture morphology and failure type of sandstone under hydrofracture were discussed. The test results show that the deformation evolution rules of rock hydrofracture can be divided into four stages, including the characteristics of pore fissure water injection stage (OA), the elastic deformation stage (AB), the volume expansion stage (BC), and global rupture stage (CD). Using the P/C modulus (the ratio of hydraulic pressure and cohesive force, abbreviated as P/C), the ability of hydraulic pressure overcoming cohesive force can be evaluated during hydrofracture. Using energy variables (expressed by E k ) about crack expanding from beginning to end, the unexpanded state, critical state, unstable state, and unstable failure of crack can be estimated. There is an interaction between tensile deformation and shear deformation from the crack initial stage to crack expanding stage.

Permeability Response Characteristics of Primary Undeformed Coal and Tectonically Deformed Coal under Loading-Unloading Conditions in Huainan Coalfield, China.

Reservoir pressure relief is a practical method to enhance permeability for coalbed methane (CBM) extraction in tectonically deformed coal (TDC) reservoirs. To explore the coal permeability response to stress changes, the primary undeformed coal (PUC) and TDC from the same coal seam were sampled for the pore–fissure structure analysis, mechanical property test, and permeability experiments under different stress loading–unloading methods in this study. The experimental results demonstrated that the coal permeability is more sensitive to the changes in confining pressure (perpendicular to airflow) than axial stress (parallel to airflow). Coal permeability decreases negatively exponentially as the confining pressure increases, and its change process with increased axial pressure can be divided into five stages in this study. The pore structures and mechanical properties of coal samples affected their permeability response to stress changes. Under the stress loading condition, the coal matrix and fractures of PUC samples were compressed simultaneously, and the permeability was regulated by the pore–fissure structures in the coal matrix. Due to the deformation and displacement of coal particles, the permeability of the TDC sample is predominantly dependent on changes in intergranular pores. At the initial stress unloading stage, the fissure recovery and expansion lead to a rapid increase in permeability, but the permeability cannot rereach the original value when the stress is fully released. Furthermore, the influencing factors of coal permeability in response to stress loading–unloading also include confining pressure conditions and coal matrix adsorption swelling. Research on the permeability response characteristics of the stress loading–unloading process can provide some clarifications for the reservoir depressurization and permeability enhancement of CBM extraction in the TDC reservoir.

Failure and deformation characteristics of shale subject to true triaxial stress loading and unloading under water retention and seepage.

A multi-functional true triaxial fluid-structure coupling system was used to conduct water retention and seepage tests of shale under true triaxial loading and unloading stress paths. The stress–strain evolution of shale specimens under different experimental conditions was obtained, and the corresponding deformation and strength were analysed. The evolution and failure characteristics of cracks in shale were obtained by CT scanning images before and after the experiment. The results show that the volumetric strain of shale specimen increases first, then decreases and finally continues to increase with an increase in deviatoric stress under water retention, indicating that the volumetric change has experienced a compaction-expansion-compacting. The σ-ε1 curve of the sample increases first and then decreases, while the deformation in the σ2 direction shows the repeated compression and expansion. In the seepage test, the permeability–strain curve can be divided into two parts before and after fracture according to the σ-ε1 curve. Before fracture, the compression velocity of the specimen in the loading direction exceeds the expansion velocity in the unloading direction, resulting in a decrease in volume and a decrease in permeability. With an increase in deviatoric stress, fractures occur inside the particles and continue to spread from the tip until the fractures break through the shale specimen. The pore fissure area increases and the permeability of the sample increases rapidly. In terms of fracture evolution, for the water retention test, dense tensile and shear cracks appear on the failure plane perpendicular to the σ1 and σ3 directions, and complex shear fracture network appears on the failure plane perpendicular to the σ2 direction. For the seepage test, heavy shear failure occurs throughout the original fracture of the sample. With an increase in the penetration depth, the fracture shape on the failure surface perpendicular to the σ2 direction gradually changes from single to complex.

Experimental and Numerical Simulation of Water Adsorption and Diffusion in Coals with Inorganic Minerals

The study on the adsorption and micropore filling of water vapor in coal is significant for predicting coalbed methane content in coal seams. The primary purpose of this study is to explain the effects of coal pore structure and its surface chemistry on water vapor monolayer adsorption, micropore filling, and diffusion coefficient. First, X-ray diffraction (XRD) and mercury intrusion porosimetry (MIP) analyzed inorganic mineral components of two kinds of coal samples and pore fissures structures. Then, we divide pores and fissures according to the theory of fractal dimensions. Furthermore, we carried out the water vapor adsorption and desorption experiments on two kinds of coal; in particular, we set 14 points of relative pressure between 0 and 0.2. Guggenheim–Anderson–de Boer (GAB), Frenkel–Halsey–Hill (FHH), and Freundlich models were used to analyze the data of water vapor adsorption to obtain the boundary pressure points of the monolayer, multilayer adsorption, and capillary condensation. Finally, the parameters of the models were obtained by fitting the adsorption data of water vapor according to the combined GAB, Freundlich, DA, and bidisperse adsorption (BDA) models to analyze the interaction mechanism between coal and water. We explain why the strongly adsorbed water minerals, such as pyrite, illite, and nacrite coal, can improve water vapor’s adsorption and diffusion capacity in coal pore fissures.

Water vapor sorption properties on coals affected by hydrophilic inorganic minerals and pore fissures

It is essential to understand the law by which coal adsorbs water to prevent coal and rock disasters in the mining process. However, the hydration of coal containing inorganic minerals is seldom studied. In this study, X-ray diffraction (XRD), low-pressure nitrogen adsorption (LPNA), and low-temperature carbon dioxide adsorption (TCA) experiments were performed on coal samples to obtain inorganic mineral compositions and pore structure characteristics. Then, vacuum vapor sorption analyzer (VVS) experiments were carried out on coal samples. GAB and Freundlich models were used to classify the strong adsorption region, weak adsorption region, and critical humidity for monolayer adsorption. The relationship between the amount of adsorbed water and the coal's pore structure and surface chemical properties was discussed. Then, the reason for the difference in specific surface area between N2 and H2O adsorbents was elucidated. Finally, we discuss the significance of coal-water adsorption by pyrite and clay minerals in preventing coal and dynamic rock disasters.

The effect of tillage depth and traffic management on soil properties and root development during two growth stages of winter wheat (&amp;lt;i&amp;gt;Triticum aestivum&amp;lt;/i&amp;gt; L.)

Abstract. The management of agricultural soils during crop establishment can affect root development due to changes in the soil structure. This paper assesses the influence of tillage depth (250 mm, 100 mm, and zero tillage) and traffic management (conventional tyre pressure, low tyre pressure, and no traffic) on wheat root system architecture during winter wheat (Triticum aestivum L.) tillering and flowering growth stages (GS) at a long-term tillage trial site. The study revealed that zero-tillage systems increased crop yield through significantly greater root biomass (P&lt;0.001), root length density, and deeper seminal rooting analysed using X-ray computed tomography (CT) (P&lt;0.001) compared with trafficked treatments. In general, conventional-pressure traffic had a significant negative influence on the crop yield (P&lt;0.01), root development (0.001), bulk density (P&lt;0.05), and total soil porosity (P&lt;0.05) of deep- and shallow-tillage conventional-pressure systems compared with no-traffic zero- and deep-tillage systems. Visual improvements in soil structure under zero-tillage conditions may have improved crop rooting in zero-tillage treatments through vertical pore fissures (biopores), enhancing water uptake during the crop flowering period. This study highlights the increasing implications of soil structural damage on root system architecture created by machinery traffic in crop production. Although the tillage method was less important, the constricted root systems were more pronounced in conventional-pressure shallow-tillage and deep-tillage systems, emphasizing the importance of using controlled-traffic farming methods to improve soil management and reduce the trafficked areas of agricultural fields.

Experimental research of the surfactant effect on seepage law in coal seam water injection

Coal seam water injection is one of the important means of mine disaster prevention, which helps to restrain the production of dust and reduce gas concentration. Ionic surfactant can change the capillary force of water in coal pore fissures, so as to enhance the wetting effect of coal seam water injection. However, the research on the effect of surfactant on the seepage process is still relatively weak. In order to explore the influence of surfactant on the seepage of coal seam water injection, the bituminous coal of Daliuta Coal Mine was used as the experimental coal sample to carry out wettability test and triaxial seepage experiment. The influence mechanism of capillary effect in the wetting and seepage stage of coal seam water injection is analyzed. The results show that the water injection process of cylindrical coal core goes through the unsaturated seepage stage and the steady seepage stage. In the process of unsaturated seepage, the increase rate of cumulative flow at the entrance of coal sample changes from fast to slow. And the instantaneous flow rate of surfactant is higher than that of water with a range of 0.1∼4 mL/min in the unsaturated seepage stage. The cumulative flow rate of water injection increased by 40%–60%. The instantaneous flow rate of water injection increases with the increase of capillary rising factor, and the correlation coefficients is 0.977. While in the stable seepage stage, the cumulative flow basically shows a linear increasing trend, and the increasing percentage of cumulative flow of water injection is lower than that of unsaturated seepage. It indicates that the surfactant can significantly improve the wetting rate at the stage of unsaturated seepage, and surfactants enhance the ability of liquid-solid interface to replace gas-solid interface.

Experimental Study of 3D Micro-CT on Meso-Structure Evolution of Coal Samples with Different Coal Grades under the Action of Temperature

In the three-dimensional micro-CT experiment system, the room temperature is set to 300°C when different coal samples (lignite, anthracite, lean coal and gas coal) are observed for mesoscopic observation. The evolution regularity of mesoscopic structure is analyzed according to the CT scan of coal samples under different temperatures and three sections of scanning images, and by ImageJ image processing software, image processing, and analysis of the characteristics of the profile, the following conclusions are obtained: (1) Coal specimen will have an overall expansion deformation along with the rise of temperature. The sample expansion can be divided into two types: outward expansion and inward expansion. Outward expansion means that the expansion of the skeleton extends outward from the adjacent pores, while inward expansion means that the solid skeleton intrudes into the adjacent pores. When the temperature rises, the outward expansion and inward expansion occur simultaneously. The dominant expansion mode is influenced by the type of coal sample and the temperature value. (2) With the increase of temperature, coal and anthracite coal specimen pore fissure structure shows an expansion tendency before contraction, while gas coal and lean coal show reverse patterns; in addition to the above the reason of the difference vitrinite differences, one must also consider selected specimen original porosity and mechanical physical properties, such as a combination of other factors. (3) In the temperature range of 100–200°C, when the temperature increases at the same rate, lignite porosity increases the most, followed by gas coal, lean coal, and anthracite. (4) There are certain differences in the variation trends of the pixel proportions of the three sections of the coal specimen, and the temperature values of the three curves at the maximum pixel point are also different, which indicates that the expansion of each point in the coal specimen with the change of temperature is not completely synchronous, and the physical and mechanical properties of the sample are heterogeneous.

Investigations on the mechanism of the microstructural evolution of different coal ranks under liquid nitrogen cold soaking

ABSTRACT Liquid nitrogen (LN2) is an important means to improve the permeability of coal reservoirs. The LN2 soaking method has a wide range of application prospects. However, not enough studies exist to highlight the effect of increasing the permeability of different coal samples using LN2. Hence, this investigation was carried out to examine the effect of cold soaking in LN2 using different coal ranks such as fat coal, lean coal and anthracite coal, on their pore structures and fractal characteristics. Frenkel-Halsey-Hill (FHH) fractal theory was used to understand the fractal dimension characteristics of pores in these different coals before and after cold soaking in LN2. Furthermore, the relationship between the fractal dimensions and pore parameters was also studied. The results show that low-temperature nitrogen adsorption curves of fat coal, lean coal and anthracite before and after cold soaking in LN2 are “S” type. An increase in the total pore volume and pore surface area of coal after LN2 cold soaking is noted. LN2 cold soaking is found to transform the internal pore structure of different coals effectively. Moreover, the heterogeneity of the porous structure of coal is enhanced after LN2 cold soaking along with an increase in the fractal dimension. The growth also decreases with an increase in the metamorphism. The uneven shrinkage of coal occurred during LN2 cold soaking, and the thermal stress generated is greater than the tensile strength of coal, which promotes the development of the porous structure of coal. These results are beneficial in revealing the macroscopic and microscopic pore fissure space expansion and connectivity law of coal seam during the process of LN2 cracking.