Middle-high Rank Coals Research Articles

Analysis and Characterization of Micro–Nano Pores in Coal Reservoirs of Different Coal Ranks

Coalbed methane represents a promising source of clean and efficient unconventional energy. The intricate network of micro–nano pores within coal serves as the primary adsorption space for gas, contributing to the complexity of gas migration channels. In this study, based on the box-counting method, three coal samples representing low, medium, and high ranks were subjected to high-precision micro-CT scanning and nano-CT scanning to generate three-dimensional (3D) pore network models using Avizo visualization software. This facilitated the accurate and quantitative characterization of the micro–nano pore structures within coal reservoirs. The results indicated that the face rate distribution range of each sample was large, indicating relatively strong heterogeneity in each sample. The volume fractal dimension of each sample, determined through micro–nano-CT scanning, was around 2.5, while the surface fractal dimension exhibited oscillatory characteristics with moderate uniformity. The pore equivalent radius and throat equivalent radius distributions were unimodal across all the samples, with the micro-CT scanning revealing a concentration primarily within the range of 100–400 μm for the pore equivalent radius and within 200 μm for the throat equivalent radius. Conversely, the nano-CT scanning exhibited concentrations primarily within the range of 500–2500 nm for the pore equivalent radius and within 2000 nm for the throat equivalent radius. The analysis of the 3D reconstruction structures indicated that the middle-rank coal exhibited more developed large–medium pores compared with the low-rank and high-rank coal, while the low-rank and high-rank coal exhibited relatively more micro–small pores. Furthermore, the low-rank coal exhibited the fewest number of pores but the largest average pore equivalent radius and throat radius. Additionally, the middle–high-rank coal exhibited a relatively larger number of pores. Despite the complex topological structures observed in each sample, a significant proportion indicated a coordination number of 0–20, indicating excellent connectivity within the coal samples. This study is conducive to the optimization of coalbed methane surface development blocks and the formulation of reasonable development plans.

Molecular structure characterization of middle-high rank coal via 13C NMR, XPS and FTIR spectroscopy

Elemental analysis, 13C nuclear magnetic resonance (NMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), combined with Grand Canonical ensemble Monte Carlo (GCMC) simulations and energy minimizations were used to establish the molecular structure model of middle-high coal rank accurately. The three-dimensional molecular structure models of middle-high coal rank (from the No. 6 Mine of Pingdingshan (PDS) Mining Area and the No. 4 Mine of Hebi (HB) Mining Area in Henan Province, Xiegou (XG) Mine, Changping (CP) Mine, and Sihe (SH) Mine in Shanxi Province, China) were optimized and constructed. The results showed that the structural arrangement is more orderly and compact with the increase of coal rank. The established molecular formulae for PDS, HB, XG, CP, and SH are respectively C69H43NO3, C166H93N3O8, C191H125N3O7, C169H105N3O7, and C221H123N3O7.

Study on the Applicability of Reservoir Fractal Characterization in Middle-High Rank Coals with NMR: Implications for Pore-Fracture Structure Evolution within the Coalification Process.

In order to evaluate the applicability of the pore-fracture structure fractal characterizations in coal reservoirs and confirm the internal relationships between the porosity, permeability, coal metamorphic grade, and pore-fracture structure, the pore-fracture features of 21 middle–high rank coal samples from Anhe, Jiaozuo, and Huaibei coalfields in northern China were investigated using a low-field nuclear magnetic resonance (NMR). All the coal samples are characterized by low moisture content (Mad), low and medium ash yield (Aad), and high vitrinite (V) in coal maceral. The adsorption space fractal dimension (DA) is positively correlated with the Langmuir volume (VL) under the three-peak transverse relaxation time (T2) spectrum. The fractal dimension of all effective T2 points under saturated water (DNMR) is positively correlated with VL and the adsorption pore volume, but negatively correlated with the volume ratio of seepage pores and fractures. The free flow space fractal dimension (DM) is negatively correlated with the porosity of full saturated water (ΦF) and the porosity of movable water (ΦM). There is a negative correlation between ΦF and the seepage space fractal dimension (DS) in the coal samples with one-peak and two-peak T2 spectra, but a positive correlation can be found with the three-peak T2 spectrum. Therefore, it is necessary to consider the types of T2 spectral peak as a prerequisite to analyze the correlations between pore-fracture parameters and NMR fractal dimensions. With the increase of coal rank, the adsorption pore content, ΦF, and bulk volume immovable (BVI) fraction first increase and then decrease, whereas the seepage pore content, fracture development, bulk volume movable (BVM) fraction, and BVM/BVI first decrease and then increase. The inflection points of these changes correspond to the maximum vitrinite reflectance (Ro,max) at 2.6–2.8%, which would be attributed to the third coalification jump. Generally, DA is the fractal dimension representing the coal pore surface, and DS and DM are closely related to the pore structure. Furthermore, DNMR not only represents the roughness of the pore surface but also the complexity of the pore structure.

Influence of coalification on methane diffusion dynamics in middle-high rank coals

ABSTRACT Understanding the effect of coalification on the diffusion of methane in middle-high rank coals (MHRC) is fundamental for optimizing the coalbed methane (CBM) drainage strategies. Safe coal mining relies critically on this approach. Hence, CH4 adsorption/desorption experiments and a new methane diffusion model were used to study methane diffusion behavior in five different metamorphic degree coals from China. The results indicate that, as the vitrinite reflectance (Ro) increase, the Langmuir volume (VL) shows a downward trend at first, then increases. Coalification especially for the third coalification jump (Ro = 1.26%) shows a turning effect on diffusion dynamics of MHRC. The desorption volume and initial diffusion coefficient (D0) both show a slight decreasing, then rapid increasing trend. The D0 of five coals jumps at Ro = 1.26%. VL increases alongside that of the micropores volume. With the decrease of the Raman parameter ID1/IG, the VL, methane desorption volume within 7200 s and D0 all increase. Results show that coalification changes the microporous structure and the macromolecular structure of the coal, which finally affects the diffusion capacity of coal. Coalification has a positive effect for the gas extraction and the development of CBM for the semianthracites.

Changes in Pore Structure and Permeability of Middle–High Rank Coal Subjected to Liquid Nitrogen Freeze–Thaw

The low permeability of coal reservoirs makes the exploitation and utilization of coalbed methane difficult. In recent years, the liquid nitrogen freeze–thaw (LN2-FT) fracturing technique has been ...

Influencing factor analysis of the coal matrix compressibility of middle-high rank coals

The pore structure and coal matrix compressibility evolution law and their influencing factors of middle-high rank coals was evaluated based on the combined application of high pressure mercury injection, low-temperature nitrogen gas adsorption and low-temperature carbon dioxide adsorption. The middle-high rank coals are dominated by macropores and micropores, along with lower developed mesopores. The porosity, total pore volume and macropores have an increasing trend with increasing coalification degree. Meanwhile, micropores had gradually become more dominant. Coal matrix compressibility of middle-high rank coals is between 0.55 × 10−10 and 1.42 × 10−10 N/m2, which had significant effects on mesopores, while its influence on macropores rise with the increases of coal rank. There was a negative correlation between coal matrix compressibility and coalification degree, resulting from the increasing peak intensity and elastic modulus during the coalification processes. The polarity of the water molecules tends to reduce the peak strength and elastic modulus of coal. As a result, the deformability becomes more enhanced resulting in increases in coal matrix compressibility. Organic compositions and inorganic minerals have opposite effects on coal matrix compressibility. Inorganic matter can effectively resist coal matrix compressibility, resulting from the higher intensities and elastic features and their roles in supporting cleats and pores while organic matter has the opposite effect. Pore structural features have important influences on coal matrix compressibility. Coal matrix compressibility has a trend of increasing with the increasing in the porosity, total pore volume and macropores or the decreasing in micropores. In the maceral composition, the vitrinite was positively correlated with the peak intensity and elastic modulus of medium rank coal, while inertinite tends to be the opposite. There is a positive correlation between coal matrix compressibility and vitrinite, but the opposite occurs for inertinite, resulting from differences in their mechanical strength and elastic properties.

Pore structure characterization of coal particles via MIP, N2 and CO2 adsorption: Effect of coalification on nanopores evolution

To study the pore structure characteristics of middle-high rank coals (MHRC), five different kinds of coal samples from northern China were selected and pulverized, and the pore volume, pore size distribution, and fractal dimension were analyzed using N2/CO2 adsorption and mercury intrusion porosimetry. The results show that the nanopores size of MHRC tends to decrease with the increase of coal rank, especially the ultra-micropores dominate in semi-anthracite coal. In addition, the fractal dimensions of the adsorption pore (pores >0.5 nm and pores <0.5 nm) are calculated, respectively. Both of these fractal dimension-fitting curves exhibit a U-shaped parabolic relationship with the Ro. It can be found that the nanopores is affected by the coalification jumps by analyzing the relationship between the pore structure parameters and Ro. The third coalification jump has a turning effect on the micropores development, and the fifth coalification jump significantly promoted the mesopores volume.

Molecular structure characterization of middle-high rank coal via XRD, Raman and FTIR spectroscopy: Implications for coalification

In recent years, the major coal producing areas in China have entered the period of deep mining, with coal rank changing from low-rank coal (LRC) to middle-high rank coal (MHRC). Even though material properties of MHRC, such as pore size and shape, have attracted increasing attentions, its molecular structures still remains unclear. This study probes molecular structural properties of five MHRC samples using X-ray diffraction (XRD), Raman and Fourier transform infrared (FTIR) spectroscopy. Test results show that with the deepening of coalification, the aromatic rings of MHRC increase gradually. The aliphatic side chains between the aromatic rings continuously fall off. The ordered structure of coal continues to enhance, resulting in an increase in the degree of graphitization of coal. The average lateral sizes (La), stacking heights (Lc) and interlayer spacing (d002) of the crystallite structures of MHRC samples derived from the XRD range from 20.65 to 31.68, 10.62 to 19.21 and 3.42 to 3.66 Å, respectively. The La values derived from the Raman spectra using the classical linear relationship between 1/La and the ID1/IG band ratio are higher (36.36–59.91 Å) than the values obtained from XRD. The FTIR spectra reveals that MHRC samples contain aliphatic functional groups such as CH2 and CH3, aromatic functional groups such as CC and aromatic ring CH, and oxygen-containing functional groups such as OH, CO and CO. The aromaticity (fa) of the coal samples determined by FTIR ranges from 0.66 to 0.98, and compared with the fa valued from XRD (0.63 to 0.93), which shows that the MHRC contains abundant of aliphatic components. On the basis of these results, the structural parameters obtained by different spectral techniques are also used to comparatively study the effects of coalification on the molecular structure of coals. The structural parameters of coal derived by these methods reflect the coalification jumps at Ro = 0.5, 0.7–0.9, 1.2, 2.6, 3.3, 3.5–4.0 and 4.2–4.8%. Among them, the jump at Ro = 4.2–4.8% is unreported in previous studies and should be further studied in subsequent research. The application range of structure parameters used to evaluate the coal rank such as d002, La (XRD), fa and G-D1 should be considered because of coalification jumps. The parameters such as 'C', ID1/IG and La (Raman) are better choices for evaluating coal rank for their better correlation with Ro. It is worth pointing out that these parameters have implications for coalification and should be concerned in further study.

Stress sensitivity characterization and heterogeneous variation of the pore-fracture system in middle-high rank coals reservoir based on NMR experiments

Five groups of middle to high rank coal samples (Ro, max value between 1.87 and 3.01%) were tested under different confine pressures using nuclear magnetic resonance (NMR) technology. Stress sensitivities of adsorption pore, seepage pore and fracture were calculated to investigate the relationship between the compressibility of pore-fracture system and its effective stress. The heterogeneous variation of pore and fracture caused by the effect of stress was discussed based on the T2 spectrum by using the NMR method. The conclusions are as follows. 1) The overall compressibility (OC) of the middle rank coal samples is mainly controlled by seepage pore and fracture. Adsorption pore are generally well developed in high-rank coal samples and the pores are primarily responsible for the stress sensitivity of these kinds of samples. 2) The compressibility shows a trend of decrease with increase of coal rank and there exists a good logarithmic relationship between OC and effective stress. 3) With the decrease of OC, the pore and fracture heterogeneity tends to become more complex. The variation of the heterogeneity of middle rank coal sample is higher than that of a high rank coal sample, and the heterogeneity variation of adsorption pore is greater than that of seepage pore and fracture. 4) The effect of stress on the heterogeneity of pore-fracture system has a two-stage characteristic. At the initial stage that the confine pressure is less than 9 MPa, the heterogeneity of pore-fracture system becomes more complicated. And when the stress is higher than 9 MPa, it moves to the stable stage that the heterogeneity tends to be stable. 5) Compared to the middle rank coal samples, the OC of high rank coal reveals a significant negative linear correlation with the (total fractal dimension) DR, indicating the simultaneity of pore structure and strain variation under the action of stress. The above results can provide theoretical guidance for increased understanding of the changes in a coal reservoir’s permeability during coalbed methane drainage.

Structural and fractal characterization of adsorption pores of middle–high rank coal reservoirs in western Yunnan and eastern Guizhou: An experimental study of coals from the Panguan syncline and Laochang anticline

To better understand the structural characteristic of adsorption pores (pore diameter &lt; 100 nm) of coal reservoirs around the coalbed methane production areas of western Yunnan and eastern Guizhou, we analyzed the structural and fractal characteristics of pore size range of 0.40–2.0 nm and 2–100 nm in middle–high rank coals ( Ro,max = 0.93–3.20%) by combining low-temperature N2/CO2 adsorption tests and surface/volume fractal theory. The results show that the coal reservoirs can be divided into three categories: type A ( Ro,max &lt; 2.15%), type B (2.15% &lt; Ro,max &lt;2.50%), and type C ( Ro,max &gt; 2.15%). The structural parameters of pores in the range from 2 to 100 nm are influenced by the degree of coal metamorphism and the compositional parameters (e.g., ash and volatile matter). The dominant diameters of the specific surface areas are 10–50 nm, 2–50 nm, and 2–10 nm, respectively. The pores in the range from &lt;2 nm provide the largest proportion of total specific surface area (97.22%–99.96%) of the coal reservoir, and the CO2-specific surface area and CO2-total pore volume relationships show a positive linear correlation. The metamorphic degree has a much greater control on the pores (pore diameter less than 2 nm) structural parameters than those of the pore diameter ranges from 2 to 100 nm. Dv1 and Dv2 can characterize the structure of 2–100 nm adsorption pores, and Dv1 (volume heterogeneity) has a positive correlation with the pore structural parameters such as N2-specific surface area and N2-total pore volume. This parameter can be used to characterize volume heterogeneity of 2–10 nm pores. Dv2 (surface heterogeneity) showed type A &gt; type B &gt; type C and was mainly affected by the metamorphism degree. Ds2 can be used to characterize the pore surface heterogeneity of micropores in the range of 0.62–1.50 nm. This parameter has a good correlation with the pore parameters (CO2-total pore volume, CO2-specific surface area, and average pore size) and is expressed as type C &lt; type B &lt; type A. In conclusion, the heterogeneity of the micropores is less than that of the meso- and macropores (2–100 nm). Dv1, Dv2, and Ds2 can be used as effective parameters to characterize the pore structure of adsorption pores. This result can provide a theoretical basis for studying the pore structure compatibility of coal reservoirs in the region.

Structural Evolution Characteristics of Middle–High Rank Coal Samples Subjected to High-Voltage Electrical Pulse

High-voltage electrical pulse (HVEP) technology has been proposed to increase the gas production of low-permeability coal reservoirs in recent years. In this study, we investigated the variation characteristics of the pore structure of coal samples by combining scanning electrical microscopy with mercury intrusion porosimetry analysis, to better understand the structural evolution characteristics of middle–high rank coal subjected to HVEP. Furthermore, changes in the chemical structure of the coal samples before and after HVEP treatment were investigated by Fourier transform infrared spectroscopy analysis. The results show that, under the action of HVEP, both anthracite and bituminous coal samples can be crushed into many small pieces. Because the conductivity of anthracite coal samples is better than that of bituminous coal samples, the average breakdown voltage of anthracite coal samples is lower than that of bituminous coal samples. It was found that the greater the breakdown voltage, the more the numb...

Effect of pore characteristics on coalbed methane adsorption in middle-high rank coals

Gas hazards are still one of the most severe disasters restricting mine safety, and the occurrence of them is heavily dependent on the storage and transportation of methane in coal seams. In order to investigate the influence of pore structure characteristics of middle-high rank coals (V daf < 25 %) on coalbed methane adsorption, six coal samples of different metamorphism were studied with regard to their surface chemical structure and pore morphological features using Fourier transform infrared (FTIR) spectroscopy, low-pressure nitrogen gas adsorption (LP-N2GA) and scanning electron microscopy (SEM), and their coalbed methane adsorption capacities were also tested. Based on the Langmuir equation, the Langmuir volume and Langmuir pressure were obtained to characterize the adsorption capacity, and the impact of structural parameters of coal samples on coalbed methane adsorption was analyzed. The results indicate that the pore shape varies a lot between coal samples, suggesting the significant heterogeneity on coal surface. The micropores (<10 nm) in coal samples are well-developed, and the pore size distributions from adsorption analysis are multi-modal. Pore characteristics in coal samples is affected by coalification to a large extent. The adsorption volume of gas mainly concentrates in micropores, and the adsorption capacities of different coal samples display remarkable difference. The Langmuir volume (V L) is closely related to micropores, but shows little relationship with mesopores, while the Langmuir pressure (P L) is remarkably affected by both micropores and mesopores. The research results are of great importance for the coalbed methane storage and the accurate prediction of gas emission.

Fractal Analysis on Heterogeneity of Pore–Fractures in Middle–High Rank Coals with NMR

Because of the complex nature of coal reservoirs, no determined method can clearly assess their pore–fracture structure. This paper combines a fractal method with the transverse relaxation time (T2) of nuclear magnetic resonance (NMR) to study pore–fractures in a coal reservoir. Based on 10 coal samples with vitrinite reflectance (Ro,m) in the range 1.32–2.43%, the pore size/volume distribution from mercury intrusion porosity (MIP) and NMR was compared, revealing that a strong correspondence exists, and that the correspondence for pore diameter of >100 nm is better than that for pore diameter of 100 ms of T2 values, respectively...

Strategies for the development of CBM gas industry in China

Since the environment for the CBM development in China has been changing in recent years, it is necessary to re-consider the relevant strategies. Through investigations, surveys, geologic assessment, strategic decision-making and other techniques, the strategies for CBM development in China were discussed in respect to present situations, opportunities, challenges, proved reserves, producing reserves, strategic principles, strategic countermeasures, time-spatial allocation of strategies, risk assessments, and elimination of relevant risks. Some research results were obtained. Firstly, still in its initial development stage with fast growth, the CBM industry in China has made outstanding achievements in six aspects and also faces challenges in six aspects. Secondly, strategic focus can be summarized as constructing six CBM production bases in Qinshui, Eastern margin of Ordos Basin, Southwest China, Changqing, Northwest China, and Northeast China, respectively, according to the principles of “steadily developing middle-high rank coals, accelerating the development of low-rank coals and strengthening the comprehensive utilization of mining gas wells”. It is expected that the producing reserves and peak-production rate will be 3–4 trillion m3 and 35–45 billion m3/a, respectively. Thirdly, major strategic risks in CBM development in China include low productivities of individual wells, improper understandings of geologic conditions, decline in investments and lack of technical reserves. To eliminate these risks, it is necessary to reinforce work in the following five aspects, namely, strengthening comprehensive exploration and development of coal-bearing formations, creating favorable environments for the development of private oil companies, expanding spaces for the growth of technical service companies, conducting more researches for the development of innovative technologies in more areas and intensifying law enforcement.

Influence of Coalification on the Pore Characteristics of Middle–High Rank Coal

The pore size distribution, pore shape and connectivity, and fractal characteristics are investigated to determine the pore characteristics of three different samples of middle–high rank coal. Pores of more than and less than 10 nm were measured using mercury intrusion porosimetry (MIP) and gas adsorption, respectively. The pore size distribution was verified with the initial methane diffusion rate and CH4 desorption. Fractal dimensions of seepage pores and adsorption pores were counted using the results from MIP and gas adsorption, respectively. First, the results show that micropores and transition pores occupy the most volume and specific surface area. Micropores and transition pores, as well as porosity, gradually increase as coal rank increases. Second, the fractal dimensions of seepage pores and adsorption pores gradually increase with increasing coal rank, which shows that coalification makes pore structure more complex and pore surface rougher. Additionally, the fractal dimensions of bigger pores ...

Variation in Pore Structure Characteristics with Composition in the High Volatile Bituminous Coal of Binchang Area

Pore structure characteristics and the effect of lithotype and maceral on pore for three types of high-volatile bituminous coals from Binchang area were investigated by combined low-temperature nitrogen adsorption/desorption, nuclear magnetic resonance (NMR), scanning electron microscope (SEM) and maceral analysis. The low temperature N2 adsorption/desorption test results show that: micropores are more abundant than transitional pores with high BET surface area; two types of pore structures can be identified by adsorption/desorption isotherms; Pore morphology is mainly represented by well-connected, ink-bottled, cylindrical and parallel plate pores. NMR T2 distributions at full saturated condition are apparent or less obvious trimodal and three types of T2 distributions are identified; Seepage pores are better developed when compared with the middle-high rank coal. Further research found that the three coal lithotypes are featured by remarkably different pore structure characteristics and maceral contents of coal are linearly correlated to some of pore structure parameters.

Heat flow and its coalbed gas effects in the central-south area of the Huaibei coalfield, eastern China

Based on an analysis of the present geo-temperature field and the thermal conductivity (K) of 62 samples from the central-south area of the Huaibei coalfield in eastern China, we calculated the heat flow and plotted its distribution map. The results show that the average heat flow in the research area is about 60 mW/m 2 . It is different from other major energy basins in the North China Plate, but has close relationship with the regional geology and the deep geological setting. The heat flow is comparatively higher in the southeastern, central, and northwestern areas than in the northeastern and southwestern areas. The geo-temperature distribution map of the bottom interface of the Permian coal measure was drawn by calculating its embedding depth and geo-temperature gradients. Finally, the present gas generation condition of the Permian coal measure is discussed by associating with the temperature condition, the vitrinite reflectance (Ro), the metamorphism of coal and tectonic-burial evolution. The study indicates all present characters of the Permian coal measure, such as lower present temperature, higher Ro value, middle-high rank coals, and uplift and extension events after the coal measure sediment, are favorable for the generation of secondary biogenic gas, but not thermogenic gas or primary biogenic gas.