Pore Structure Of Coal Research Articles

Effects of Supercritical Carbon Dioxide Saturation Temperature on the Multiscale Pore Structure of Coal

Effects of Supercritical Carbon Dioxide Saturation Temperature on the Multiscale Pore Structure of Coal

Nano-scale pore distribution characterisation of coal using small angle X-ray scattering

In the process of coal seam fracturing with liquid nitrogen (LN2), the change of coal pore structure has an important influence on the efficiency of coalbed methane (CBM) extraction. The nano-scale pore size distribution (PSD) in coal particles before and after freezing with LN2 are experimentally studied in this work. Coal samples are collected from four coal mines, where coal and gas outburst accidents have occurred. Small angle X-ray scattering technology (SAXS) and scanning electron microscopy (SEM) are used to study the pore structure changes of coal samples quantitatively and qualitatively. It is found that the scattering intensity of coal samples increases after freezing. The PSD of all samples significantly changes in the range of 0.8–7 nm, showing new pore spaces in 0.8–4 nm and fewer pores in the 4–7 nm range. Both the pore fractal dimension and the radius of gyration of coal samples increase after freezing and are mainly affected by the changes in pores and the anisotropy of the coal matrix. Crack expansion and pore connections are observed in the surface structure of the coal sample using SEM. This study provides a better understanding of the nano-scale mechanism of coal seam fracturing with LN2 for the prevention of coal and gas outbursts.

Pore structure of low-permeability coal and its deformation characteristics during the adsorption–desorption of CH4/N2

The pore structure of coal plays a key role in controlling the storage and migration of CH4/N2. The pore structure of coal is an important indicator to measure the gas extraction capability and the gas displacement effect of N2 injection. The deformation characteristic of coal during adsorption–desorption of CH4/N2 is an important factor affecting CH4 pumpability and N2 injectability. The pore structure characteristics of low-permeability coal were obtained by fluid intrusion method and photoelectric radiation technology. The multistage and connectivity of coal pores were analyzed. Subsequently, a simultaneous test experiment of CH4/N2 adsorption–desorption and coal deformation was carried out. The deformation characteristics of coal were clarified and a coal strain model was constructed. Finally, the applicability of low-permeability coal to N2 injection for CH4 displacement technology was investigated. The results show that the micropores and transition pores of coal samples are relatively developed. The pore morphology of coal is dominated by semi-open pores. The pore structure of coal is highly complex and heterogeneous. Transition pores, mesopores and macropores of coal have good connectivity, while micropores have poor connectivity. Under constant triaxial stress, the adsorption capacity of the coal for CH4 is greater than that for N2, and the deformation capacity of the coal for CH4 adsorption is greater than that for N2 adsorption. The axial strain, circumferential strain, and volumetric strain during the entire process of CH4 and N2 adsorption/desorption in the coal can be divided into three stages. Coal adsorption–desorption deformation has the characteristics of anisotropy and gas-difference. A strain model for the adsorption–desorption of CH4/N2 from coal was established by considering the expansion stress of adsorbed gas on the coal matrix, the compression stress of free gas on the coal matrix, and the expansion stress of free gas on micropore fractures. N2 has good injectability in low-permeability coal seams and has the dual functions of improving coal seam permeability and enhancing gas flow, which can significantly improve the effectiveness of low-permeability coal seam gas control and promote the efficient utilization of gas resources.

Effect of Desorption Damage on the Kinetic Characteristics of Coal Particle Gas Desorption.

There is a positive feedback mechanism of desorption, pulverization, and redesorption in the process of coal and gas outburst. The coal pore structure changes to some extent in the process of coal pulverization and has a related impact on the dynamic parameters of coal particle gas desorption. To understand the impact of desorption damage on the dynamic characteristics of coal particle gas desorption, in this paper, a self-developed coal particle gas desorption test device is used to measure the amount of methane desorption of different coal samples repeatedly desorbed under the same adsorption equilibrium pressure. The results show that the methane desorption kinetic curves of the four coal samples based on desorption damage basically have the same trend. Nie's diffusion model can better describe the methane desorption characteristics of coal particles. After desorption, the methane desorption amount, initial desorption velocity, diffusion ability, and ultimate amount of methane desorption of the coal samples are greater than those before desorption. The desorption damage affects the relevant dynamic parameters of the coal particle gas diffusion model, and its impact on the outburst coal is greater than that on raw coal. In addition, the pore size distribution and change characteristics of the coal samples before and after desorption are analyzed quantitatively via a low-temperature liquid nitrogen adsorption test and fractal dimension-related theory. It was found that the pore volume peak area of the coal samples affected by desorption damage within each pore size range was significantly larger than that before desorption. Among them, micropores have the most significant impact on the desorption damage of coal samples, and the peak area of the pore volume of protruding coal is greater than that of raw coal, indicating that the internal pores of coal after desorption damage are more developed than those of raw coal.

Experimental Study on Spontaneous Imbibition of Coal Samples of Different Ranks Based on the NMR Relaxation Spectrum.

The comprehension of the mechanisms governing spontaneous water imbibition in gas-water systems plays a significant role in the operation of hydraulic fracturing and the development of coalbed methane (CBM). In this study, nuclear magnetic resonance (NMR) techniques were used to investigate the pore structure and fluid behavior of different rank coal samples during spontaneous imbibition. Analyses were conducted on the 1D NMR T2 spectrum, 2D NMR T1-T2 spectrum, and layer division T2 spectra to achieve accurate and detailed information about the internal pore structure of coal and the characteristics of fluid transport during spontaneous imbibition. Low-rank coal exhibits good pore connectivity and favorable pore sorting characteristics, indicating favorable reservoir conditions. High-rank coal has larger pore spaces and highly developed micropores, which are highly beneficial for gas adsorption. However, its poor pore sorting characteristics and connectivity limit the migration and diffusion of fluids within the reservoir. The imbibition capacity follows a specific order of contribution, with small pores (10-50 nm) having the most significant role, followed by micropores (2-10 nm), ultramicropores (r < 2 nm), mesopores (50-1000 nm), macropores (1000-10,000 nm), and microfractures (r ≥ 10,000 nm). Low-rank coal stands out due to the restricted development of ultramicropores and small pores, leading to a different contribution of imbibition capacity compared to other samples, where macropores and microfractures dominate over all pore types. The coal reservoirs with favorable pore sorting characteristics and pore connectivity tend to exhibit a tendency toward rapid saturation and attainment of a prompt stable state during the hydraulic fracturing process. Finally, the mechanism of the late retreat effect of imbibition and the laws governing different coal ranks, pore structures, and fluid transport were discussed. This study offers comprehensive analyses of the mechanism of coal spontaneous imbibition and the characteristic laws of fluid seepage, providing insights into the optimization of CBM recovery and reservoir management.

Investigation of cross-scale characterization of porous structure and fluid index in bituminous coal via microwave-LN2 freeze-thaw cycles

Microwave-LN2 cyclic processing (MLCP) is a clean and high-efficiency anhydrous fracturing technology. The change in the porous structure of coal after freeze-thaw cycles significantly affects development efficiency of coalbed methane (CBM) in low porosity and permeability reservoirs, while there is a lack of in-depth macro-meso-micro studies. The damage mechanism of coal under microwave-LN2 cold and thermal shock and the evolution of pore fluid reservoir space is based on cross-scale pore characterisation. This paper evaluates the multi-scale porous structure variations of coal before and after MLCP was assessed by hydrogen nuclear magnetic resonance (1H NNR) and field emission scanning electron microscopy (FE-SEM). The evolutionary trends in coal pore fluid fugacity space under freeze-thaw cycles was indirectly characterised by T1-T2 spectra. Additionally, utilizing T2 distribution, peak area and pore throat changes reflect the evolution trend of coal pore space. The results indicate that as the cycles increased, the coal free water and total fluid reservoir space gradually expanded beyond the cycle threshold (n > 10), the free-fluid index rose to 25.68 %, and the free fluid saturation increased by 20.39 %. Additionally, the NMR permeability of the coal increased by 98.37–1471.65 %, and the pore throat distribution increased by 28.05 %. Comprehensive analysis showed that as the cycles progressed, the free water space in the pore space increased significantly, the proportion of bound water gradually decreased, and the internal pores and fluid space of the coal were improved. This study provides fundamental support for multidimensional evaluation of the porous structure of microwave-LN2 freeze-thaw cycles.

Laboratory investigation on gas slippage phenomenon in coal sample and its research significance

The gas slippage effect (GSE) of coal is associated with the coalbed methane (CBM) wells' productivity. Through the helium permeability experiment of five coal samples with different vitrinite reflectance, characteristics of gas permeability variations in coal samples with different coalification degrees are analyzed, the influence of coalification degrees, coal pore structures, and different gases on GSE is revealed, and the impact of GSE on the CBM wells productivity is discussed. It indicatess that the gas permeability of coal varies with the pore pressure in a negative power function and a gas slippage coefficient of coals declines with the rise of Klinkenberg permeability by a negative power function. With the ascend of vitrinite reflectance of coal, there is an asymmetric “U-shaped” change trend in GSE. The GSE in coal with different vitrinite reflectance is mostly govern by the pore diameter distribution of coal. The slippage parameter of gas is consistent with the evolution law of coal pore structures during coal metamorphism. The slippage coefficient of different gases is obtained. On this basis, the prediction model of CBM well production considering GSE is established, and the impact degree of GSE on CBM well productivity is revealed.

Real-time pore structure evolution difference between deep and shallow coal during gas adsorption: A study based on synchrotron radiation small-angle X-ray scattering

Coal seam CO2 sequestration is an important option to address global warming. A better knowledge on coal pore structure evolution during gas adsorption can provide guidance for coal seams CO2 sequestration. However, few investigations on the pore structure evolution differences between the deep and shallow coal were conducted during gas adsorption. In this study, based on the real-time synchrotron radiation small-angle X-ray scattering (SAXS) observation, the average pore diameter and pore surface fractal dimension evolution differences between deep and shallow coal were investigated from the aspects of coal compositions and stress history. Two types of coal deformation (inner-swelling and outer-swelling) coexist during gas adsorption. Coal compositions have significant impact on the dominance of deformation type. The dominance of inner-swelling in deep coal is induced by the higher ash contents, and there is the decrease of average pore diameter during gas adsorption. The impact of stress-history (burial depth) on adsorption-induced deformation is more prominent than that of gas adsorption capacity. In deep coal, the surface fractal dimension evolution presents a negative correlation with the evolution of pore diameters. In shallow coal, the surface fractal dimension evolution presents a Langmuir-type correlation with the adsorption time.

Analysis of the Pore Structure and Fractal Characteristics of Coal and Gas Outburst Coal Seams Based on Matrix Compression Correction

The comparative analysis of coal samples under different gas occurrence conditions systematically reveals the pore structure characteristics of coal and gas outburst coal seams. The functional relationship between R0,max and Kc was clarified using mathematical statistical methods, and the pore structure and fractal characteristics of coal and gas outburst coal seams were analyzed on the basis of modified mercury pressure data and fractal analysis. The results show that the functional relationship between R0,max and Kc is consistent with y = 1.59x−0.48, and when the mercury inlet pressure is 10~120 MPa, the coal sample is the most affected by the matrix compression effect. The average pore volume and average specific surface area of the outburst coal samples were 41.71% and 23.09%, which is greater than those of the non-outburst coal samples, respectively, and the specific surface area and pore volume provided by the outburst mining micropores were 46.24% and 81.67%, respectively. The fractal dimension, D, of the coal seam increased with the increase in metamorphism, and compared with low gas mines, the fractal dimension of coal samples in the coal and gas outburst mines was higher, the influence of the matrix compression effect was more obvious, and the heterogeneity was stronger.

Variations in the pore structure and fluid mobility under anionic surfactant assisted matrix acidification of coal based on nuclear magnetic resonance T1-T2 spectra

Matrix acidification is a potential method for increasing the permeability of coalbed methane reservoirs. However, current research on the acid corrosion process is still unclear, and acid solutions can cause water blocking damage to reservoirs. Therefore, this article obtained a T1-T2 joint spectrum through nuclear magnetic resonance (NMR) test and conduct contact angle (CA) test to analyze the pore structure and fluid mobility evolution of bituminous and anthracite coal under sole acidification and Sodium secondary alkyl sulfate (SAS) surfactant assisted acidification. The results show that the acidification process exhibits two distinct stages. Stage 1: acidification mainly expand the pore space of the free fluid, while stage 2: acidification significantly increases both the bound fluid and free fluid space. After sole acidising, the porosity of bituminous and anthracite coal increased by 4.44% and 31.31%, while increased by 18.28% and 38.35% under SAS assisted acidification, respectively. The adding of SAS resulted in better connectivity of coal samples, with pore throat (>0.1 μm) of bituminous and anthracite increasing by 56.02% and 351%, respectively. Under SAS assisted acidification, the maximum decrease of T2cutoff value of bituminous and anthracite coal is 62.79% and 20.28% respectively. Surfactants can effectively alleviate the negative impact of acidification on coal surface wettability. The results can provide references for field application of matrix acidification in coalbed methane reservoir.

Characterization of the pore structure in Chinese anthracite coal using FIB-SEM tomography and deep learning-based segmentation

Coalbed methane (CBM) is an unconventional natural gas that possesses significant impacts on energy supply, mining safety, and environmental conservation. CBM is primarily stored within the pores of coal, highlighting the significance of pore structures for both methane storage and migration. However, comprehensively understanding the intricate pore structures in coal pose challenges. This study employed focused ion beam-scanning electron microscopy (FIB-SEM) tomography and deep learning-based segmentation to characterize the pore structures within a Chinese anthracite sample. The obtained pore structures exhibited a considerable degree of disconnection, comprising numerous separate pore components. Isolated pores prevailed in number, while connected pores dominated in surface area and pore volume. Mesopores (100–1000 nm) contributed the most to pore number, surface area, and pore volume. Pore size distribution analysis revealed distinct patterns among different pore structure properties, with pore number exhibiting an intensive distribution while surface area and pore volume displaying dispersed distributions. Pore structure connectivities exhibited a hierarchical nature and held distinct meanings at the levels of pore, pore component, and pore network. The pore structure characteristics observed in this study have implications for primary CBM recovery, emphasizing the necessity to improve connectivity between pore components and fractures to enhance production rates and recoverability.

Study on the Response Law of Coal Pore Structure and Permeability Affected by Acidification Time.

In order to study the pore alteration and permeability of coal affected by acid liquor, samples were collected from the Yuwu coal mine in the Qinshui basin, Shanxi Province, and taken as the study object. Hydrochloric acid (HCl) with a 10% volume concentration was used to treat these coal samples. The pore structure characteristics and gas permeability of this coal with different treatment times were measured by laboratory experiments. The fractal theory was introduced to analyze the complexity of the sample pore and the changing rules of coal permeability. The research results show that acid liquor has a significant influence on the pore structure of YW coal. Under the effect of acid treatment, the micropore volume of the coal sample is reduced, but both the mesopore and macropore volume are increased. The fractal dimension of acidulated coal samples changes in the range of 2.57-2.81, and it is exponentially decreased with the increase of the treatment time. This indicates that the acid treatment can make the YW coal surface become smooth, and the pore structure tends to be smooth. The acidification method is able to effectively enhance the coal permeability with a maximum increment of about 9.43 times. After 30 h of acid treatment, the total pore volume, fractal dimension, and gas permeability of this coal tend to be stable. The acid treatment time should be controlled at around 30 h.

Experimental study on the effect of magma intrusion and temperature on the pore structure of coal

The rich and intricate pore structure that coal possesses has a big impact on CBM engineering. We characterized the pore structure of coal samples at various distances from the magma intrusion point and at various temperatures using MIP, low-temperature nitrogen adsorption, and low-temperature carbon dioxide adsorption to study the thermal effect of temperature and magma intrusion on the pore structure of coal seams to better understand the properties of coal affected by temperature and thermal effect of magma. The results of the study prove that there are similar and different pore structures of coal samples at different distances from the magma intrusion point and at different temperatures. Heating and magma thermal effects have similar effects on coal samples in macroporous and micropore stages. The pore volume of coal samples increased with increased temperature or magma intrusion effect in the macroporous stage but decreased with increased temperature or magma intrusion effect in the microporous stage, with the magma thermal effect being more pronounced. While in the medium pore stage, the pore volume of coal samples increases and then decreases with the enhancement of magmatic intrusion influence, but the trend of decreasing development with the increase of temperature. This result makes a theoretical contribution to the selection of coal-bed methane utilization technology in the magmatic rock-influenced region.

Wettability alteration and dynamic wetting behavior of coal during geologic CO2 sequestration using LF-NMR technology

As CO2 has a strong affinity to coal surface, coal seams are considered as an attractive geological formation for CO2 sequestration. Naturally occurring coal seams are often saturated with free water in the cleats as well as moisture. Hence, the wetting behavior of coal by water is an important aspect in the study of coal properties. However, carbon dioxide, which replaces methane on the surface of coal, interacts with water and coal for a long time. Geochemical reactions of CO2-H2O-coal system can change the surface properties and pore structure of coal. Studies on wettability of coal altered by CO2-H2O-coal interaction are rare. This paper explored the wettability alteration and wetting dynamic behavior of two bituminous coals in Huainan and Huaibei coalfield using low-field nuclear magnetic resonance (LF-NMR) combined with CO2-H2O-coal interaction experiments. The results show that dynamic wetting of water in the coal mainly depends on the pores structure. The geochemical reaction changed the pore structure of coal reservoir, further promoted the hydrophobicity of coal surface, and thereby affected the dynamic wetting process of water. Moreover, the wetting of large pores in coal is not significantly affected by the weakening of hydrophilicity, whereas wetting in small pores is obviously affected by the enhanced hydrophobicity, which may be beneficial to the adsorption and storage of CO2 in the coal.

Evolution Characteristics of the Microstructure with Different Fracturing Fluids: An Experimental Study of Guizhou Bituminous Coal.

Fracturing fluid is a key factor affecting the hydraulic fracture morphology and coal microstructure, which plays a key role in the hydraulic fracturing effect. To compare the effect of clean water, clean fracturing fluid, and acid fracturing fluid on the pore structure of coal, this paper used high-pressure mercury injection (MIP), low-temperature N2 adsorption (LT-N2A), and scanning electron microscopy (SEM) to determine the pore structure of Guizhou bituminous coal before and after the action of fracturing fluid. The results show that clean water can cause mineral expansion and reduce pore volume by about 6% and clean fracturing fluid and acid fracturing fluid can increase pore volume by 3 and 12%, respectively, due to different degrees of acidity. The MIP data show that the pore structure of coal samples is more complex after the action of different fracturing fluids, and acidic fracturing fluids can increase the fractal dimension of the pore by about 7%. The LT-N2A data showed that the fractal dimension of micropores and transition pores decreased after the action of different fracturing fluids. In general, acid fracturing fluid has the best effect on the coal microstructure, followed by clean fracturing fluid, and the least effect on clean water.

Analysis of char structure and composition from microwave and conventional pyrolysis/gasification of low and middle rank coals

The structure and composition of residual coal char after pyrolysis dictates its reactivity towards gasification. The nature of the char is a result of the parent coal composition and structure as well as the reaction conditions during pyrolysis. Further, due to the unique characteristics of selective dielectric heating, char development during microwave CO2 gasification may differ from conventional thermal gasification, and little is known about the effect of microwave heating on the char structure development. In this work the heating method used is compared (microwave vs conventional thermal) to generate char samples from four different coal types. These coal chars generated by pyrolysis and gasification were characterized by ultimate analysis, surface area analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and dielectric characterization to investigate the chemical and structural differences between microwave- and conventionally-generated chars. The results reveal notable chemical and structural differences between the chars from microwave and conventional pyrolysis and gasification. In general, the conventional chars had greater pore structure development as seen by higher specific surface areas compared to microwave chars, possibly due to rapid thermal collapse of coal pore structure. Microwave pyrolysis chars had greater ordering of the carbon crystal structure for low-ash coals, while graphitization was inhibited for coals with high ash percentage yield. The observations from this study highlight the differences in char characteristics from microwave and conventional pyrolysis and gasification, which can aid microwave gasifier design, implementation, and integration for efficient syngas production and coal ash utilization.

Study on Multifractal of Coal Pore Structure Based on Low Pressure Argon Adsorption and Its Effect on Gas Adsorption/Desorption

Study on Multifractal of Coal Pore Structure Based on Low Pressure Argon Adsorption and Its Effect on Gas Adsorption/Desorption

Experimental study on erosion mechanism and pore structure evolution of bituminous and anthracite coal under matrix acidification and its significance to coalbed methane recovery

Acid fracturing is a potential method to resolve the mineralization restriction around wellbore and enhance the permeability in coalbed methane extraction. The previous study mainly focused on exploring the effect on permeability enhancement for oil reservoir. However, the performance of acid fracturing fluid in coal reservoir is still unclear. In this paper, the surface morphology and internal pore structure of coal were characterized by erosion rate test, scanning electron microscope-energy dispersive spectrum (SEM-EDS) and low-temperature nitrogen adsorption test (LT-N2GA), and the relationship between erosion rate and pore structure index of bituminous and anthracite coal under matrix acidification was obtained. The results show that the bituminous coal presented higher acid sensitivity than anthracite from no matter aspects of erosion rate, surface morphology and internal pore structure variation. The increment of erosion rate and pore structure index is increase with the coal ranks under matrix acidification, which can provide reference for the selection of acid pad fluids for acid fracturing in different coal seams.

Experimental study on the effects of ultrasonic excitation on pore structure of water-bearing coal

Coal seam water injection is one of the main coal mine dust control measures. To study the effects of ultrasonic excitation on the pore structure of water-bearing coal in depth, ultrasonic excitation experiments were conducted with varied variables including ultrasonic excitation time, ultrasonic power, and fluid pressure on coal. The effects of sodium dodecyl sulfate (SDS) treatment and ultrasonic excitation were also compared. The coal samples treated under different conditions are characterized by low-temperature nitrogen adsorption and scanning electron microscopy (SEM) respectively for the quantitative analysis of pore volume and specific surface area and qualitative analysis of surface pore and fracture structure. The results show that ultrasonic excitation can significantly increase the pore volume and specific surface area of coal and damages the inner wall surfaces of the pore to different extents, making the pore inner surfaces rougher. However, the effect of ultrasonic excitation becomes weaker after the pore volume increases to a certain extent, resulting in more uniform pore size and volume distributions. Ultrasonic excitation time affects the pores with smaller size more significantly, and there is a time threshold effect. The volume and specific surface area of large pores and fractures show limited responses (or increases) to as the excitation time increased. The energy is mainly used for the fracturing and expanding the pores with smaller size. The effect of ultrasonic excitation on pore expansion is more obvious under the fluid pressures lower than 4 MPa. High fluid pressures weaken the fracturing and expanding effect of ultrasonic excitation on the pore structure of water-bearing coal. The ultrasonic power threshold effect is also observed. Once exceeding a certain value, further increasing the ultrasonic power does not dramatically affect the pore structure of water-containing coal. Ultrasonic excitation can enlarge the primary pores and fracture and generate secondary pores and fracture in coal. In contrast, the 0.85% SDS treatment is only able to clean the primary small pores, and no new pore structures are formed.

Characterizing water vapor adsorption on coal by nuclear magnetic resonance: Influence of coal pore structure and surface properties

Steam injection is a novel and promising technology for enhancing coalbed methane (CBM) recovery. Accurate descriptions of the migration and adsorption of water vapor on coal is crucial for determining the occurrence state of the CBM and improving CBM production. This study quantitatively characterized the dynamic transport and adsorption processes of water vapor in coal pores and fractures using nuclear magnetic resonance (NMR). The results demonstrated that water vapor was preferentially adsorbed in the coal micropores, which was mainly related to capillary condensation, whereas liquid water filled a wide range of pores and fractures. Due to the differences in the microstructure and surface properties among different coals, the water vapor adsorption behaviors varied significantly. Among the coals used in this study, water vapor was most easily adsorbed on the bitumite with the biggest specific surface area, pore volume, pore area, the highest content of oxygen-containing functional groups, and the strongest wettability. Additionally, the coal pore structure had a greater impact on the water vapor adsorption capacity than the oxygen functional groups and the wettability. This research is of great significance for understanding the flow behaviors of water vapor in porous media and the interactions between coal and water.