Pore Size Distribution Of Coal Research Articles

Coal pore size distribution and adsorption capacity controlled by the coalification in China

The coalification process significantly influences the composition of coal material and the development of pore size, ultimately affecting the adsorption capacity of coal seams and the extraction of coalbed methane. This project aimed to assess the pore structure and gas adsorption capacity of coal samples from nine key sedimentary basins in China. The N2 test results revealed that during the initial stage of coalification, micropore, transition pore, and mesopore dominate in order of magnitude. In the subsequent coalification stages, the development of mesopore is most significant during the second jump, while the transition pore becomes more prominent in the third jump. Additionally, the findings from nuclear magnetic resonance tests indicate that the proportion of adsorbed pore demonstrates a decreasing-then-increasing trend throughout the coalification process. The first and second coalification jumps primarily contribute to the development of adsorption pore and seepage pore, whereas the third jump mainly focuses on adsorption pore development. The gas adsorption capacity is influenced by various factors such as pore structure, material composition, and chemical structure. During the first coalification jump, gas adsorption mainly occurs in the adsorption pore. In the second jump, medium gas adsorption capacity is achieved due to the combined effects of enhanced adsorption pore development and reduced moisture and ash content. Finally, the third coalification jump exhibits highly developed adsorption pores, the lowest moisture and ash levels, and a sufficient aromatic structure in the chemical composition, resulting in high adsorption capacity. These findings contribute to a deeper understanding of the variations in coalbed methane enrichment across different basins in China, with coalification playing a crucial controlling role.

Re-Calibrating the Mercury-Intrusion-Porosimetry-Measured Pore Size Distribution of Coals: A Novel Method for Calculating the Matrix Compression Coefficient

Accurate measurement of the pore size distribution (PSD) in coals is crucial for guiding subsequent coalbed methane (CBM) engineering practice. Currently, mercury intrusion porosimetry (MIP) measurement has been widely used as a PSD testing method due to its effectiveness and convenience. Nevertheless, it is worth noting that the elevated pressure during the MIP experiments can lead to matrix compressibility, potentially causing inaccurate estimations of PSD in coals. Therefore, correction methods are used to modify the PSD in the high-pressure segment to improve the accuracy of MIP data. This study proposed a novel method with higher accuracy and convenience for calculating the matrix compressibility coefficient compared to the traditional calculation methods. Firstly, the matrix compressibility coefficients of six coal samples were calculated by using low-temperature nitrogen adsorption (LTNA) data. Subsequently, by utilizing the mathematical correlation between Kc (the compressibility coefficient of the coal matrix) and Ro,max (the maximum vitrinite reflectance) from prior research, a novel statistical method was designed to determine the matrix compressibility coefficient of the samples. Finally, the statistical matrix compressibility coefficient determination method was used to examine the fractal characteristics of the actual PSD. The results indicate that when the pressure exceeds 24 MPa, the volume obtained from mercury intrusion exceeds the pore volume measurement. The Kc calculated using the traditional correction method is in the range of 0.876–1.184 × 10−10 m2/N, while the Kc values of our proposed statistical correction method range from 0.898 × 10−10 to 1.233 × 10−10 m2/N, with a comparison error rate of ~0.11–5.25%. The MIP data greater than 24 MPa were effectively corrected using the statistical correction method, thus reducing the mercury intrusion volume error by 91.75–96.40%. Additionally, the corrected pore fractal dimension (D2) values fall within the range of 2.792 to 2.975, which are closer to the actual values than the pore fractal dimension range of 3.186 to 3.339.

An adsorption model for cylindrical pore and its method to calculate pore size distribution of coal by combining NMR

Coalbed methane (CBM) adsorption is closely related to pore size distribution in coal, the study of its adsorption state and pore characteristics of coal are of great significance to its exploitation and storage. In this paper, an extended simplified local density adsorption (SLD) model is proposed, which can be applied to cylindrical pores. Combining with the nuclear magnetic resonance (NMR) tests, the extended SLD model can predict the adsorption amount and pore size distribution of the whole sample without destroying its original pore structure. Three coal samples collected from different areas are selected for NMR test to measure T2 distribution and Magnetic Resonance Imaging (MRI) image. Then, the adsorption amount of the samples are obtained under constant temperature and different pressures. The fitting coefficients of the extend SLD model and experimental data of the three samples are all greater than 0.99, indicating that the proposed model matches the experimental results well. The two parameters obtained by fitting, transverse surface relaxivity (K) and the total number of pore (N), combined with T2, can get the radius and number of each pore in the sample. The inversion results show that the number summation of micropores and mesopores of the three samples are all exceeds 99%. However, due to the small pore size of micropores and mesopores, the pore volume summation of the three coal samples occupied by fractures and macropores are all over 27%. Further observation of cylindrical pore size and fluid molecular position shows that the distance between fluid-solid molecules is the main factor affecting the adsorption. The results of this paper are beneficial to predict and calculate the adsorption amount and pore size distribution of coal.

Effects of inhibitors on the sequestration of flue gas in goaf and pore structures of coal

Injecting power plant flue gas into goaf for sequestration can not only reduce the emission of CO2, but can also effectively inhibit the spontaneous combustion of coal left in the goaf. To analyze the effect of the chemical environment in the goaf on the sequestration of CO2 in a coal seam, a home-made adsorption experiment device and ASAP 2020 specific surface analyzer were used to study the regularity of CO2 adsorption in coal and the pore structure of coal before and after inhibition. The results indicate that the adsorption capacity of CO2 in coal increased by 14.05%–21.04% after inhibition. The inhibitors can significantly increase the adsorption capacity of CO2 in coal, enhancing the sequestration of flue gas in goaf.The pore volume and specific surface area of micropores in the coal samples after inhibition are larger compared to those in the raw coal, whereas those of mesopores and macropores are smaller, but those of the micropores are 4.3 and 205 times those of mesopores and macropores, respectively. Thus, the total pore volume and specific surface area of coal increased significantly after inhibition, which is the reason for the increase of CO2 adsorption capacity. The inhibitor does not fundamentally change the pore size distribution of coal, most of the pores in coal are in the pore size range of 0.4–0.6, 0.8–0.9, 1.5–75, and 120–170 nm, which greatly contribute to its pore volume and specific surface area. The pores with a pore size of <7.5 nm are the main adsorption pores in coal, and the inhibitors can significantly increase the number of micropores and mesopores with a pore size of <7.5 nm. The fractal dimension of the coal samples was calculated using the FHH model, which indicated that the intrusion of inhibitors can remove the impurities in pores and reduce the surface roughness of coal. Conversely, the macropores in the coal samples after inhibition are broken and new micropores are generated. This study is of great significance for increasing the sequestration capacity of CO2 in goaf.

Simulation of coal microstructure characteristics under temperature-pressure coupling based on micro-computer tomography

The distribution of pore-fracture structure in coal directly affects the storage and migration characteristics of fluids. Simultaneously, under the condition of high buried depth, the coal structure is further changed by the temperature-pressure coupling. To study the pore size distribution of coal and the deformation characteristics of different pore-fracture structures under temperature-pressure coupling conditions, six different coal structure models are constructed using three-dimensional computer tomography reconstruction in this work. The box-counting algorithm is used to calculate the fractal dimensions of the pore-fracture structure and pore clusters in different pore sizes of coal. In the meantime, the fractal characteristics of the pore structure in different pore sizes are analyzed. Based on the coal structure model, a linear elastic deformation model under the temperature-pressure coupling is constructed. Moreover, the relationship of porosity and fractal dimension with the deformation of models is studied. The research results show that the porosity positively correlated with fractal dimension. The changes of pore volume fraction and fractal dimension in each pore size distribution range are consistent. Under the temperature-pressure coupling condition, both the model porosity and fractal dimension exhibit a quadratic function relationship with deformation. The deformation analysis of the three-dimensional models and two-dimensional cross-sections show that as the porosity and fractal dimension increase, the coal body deformation exhibits a trend of first decreasing and subsequent increasing. Low fractal dimension and high porosity could increase the model deformation. The research results can afford a theoretical basis for the study of coal structure characteristics under complex conditions.

Pore size distribution of high volatile bituminous coal of the southern Junggar Basin: a full-scale characterization applying multiple methods

Studying on the pore size distribution of coal is vital for determining reasonable coalbed methane development strategies. The coalbed methane project is in progress in the southern Junggar Basin of northwestern China, where high volatile bituminous coal is reserved. In this study, with the purpose of accurately characterizing the full-scale pore size distribution of the high volatile bituminous coal of the southern Junggar Basin, two grouped coal samples were applied for mercury intrusion porosimetry, low-temperature nitrogen adsorption, low-field nuclear magnetic resonance, rate-controlled mercury penetration, scanning electron microscopy, and nano-CT measurements. A comprehensive pore size distribution was proposed by combining the corrected mercury intrusion porosimetry data and low-temperature nitrogen adsorption data. The relationship between transverse relaxation time (T2, ms) and the pore diameter was determined by comparing the T2 spectrum with the comprehensive pore size distribution. The macro-pore and throat size distributions derived from nano-CT and rate-controlled mercury penetration were distinguishingly analyzed. The results showed that: 1) comprehensive pore size distribution analysis can be regarded as an accurate method to characterize the pore size distribution of high volatile bituminous coal; 2) for the high volatile bituminous coal of the southern Junggar Basin, the meso-pore volume was the greatest, followed by the transition pore volume or macro-pore volume, and the micro-pore volume was the lowest; 3) the relationship between T2 and the pore diameter varied for different samples, even for samples with close maturities; 4) the throat size distribution derived from nano-CT was close to that derived from rate-controlled mercury penetration, while the macro-pore size distributions derived from those two methods were very different. This work can deepen the knowledge of the pore size distribution characterization techniques of coal and provide new insight for accurate pore size distribution characterization of high volatile bituminous coal.

Pore Size Distribution Characteristics of High Rank Coal with Various Grain Sizes.

Particle void filling effects (Pf) under low pressure and coal matrix compressibility effects (Pc) at high pressure should not be ignored when using mercury intrusion porosimetry (MIP) to study the pore size distribution of coal. In this study, two coal samples (FX and HF) collected from western Guizhou were crushed into three different grain sizes; then, the subsamples were analyzed by MIP and low-pressure nitrogen adsorption to study the pore size distribution characteristics. The micro- and transition pore volumes contribute to the total pore volume of the FX and HF subsamples. With decreasing subsample grain sizes, the macropore volume of FX subsamples tends to increase, while mesopore volume decreases; the volumes of micropores and transition pores first increase and then decrease. In regard to the HF subsamples, the volumes of macropores and mesopores do not reveal any distinctive changes, while the 40–60 mesh subsample contains the greatest volume of micropores and transition pores. Fractal theory was introduced to determine Pf and Pc. Pf barely changed as grain size decreased; it ranged from 0.1 to 0.15 MPa. However, Pc increased with reduced coal grain sizes. The coal matrix compressibility coefficients of the subsamples were calculated from the cumulative mercury volume curve, and the true pore volume was also modified. The modified volume of macropores does not change markedly, while the volumes of mesopores and transition pores decrease significantly, clearly indicating the coal matrix compressibility under high mercury injection pressure. The modified pore volume shows that the pore (<10,000 nm) still harbors fractal characteristics.

Experimental study on modification of physicochemical characteristics of acidified coal by surfactants and ionic liquids

To study the modification of the physicochemical characteristics of acidified coal by surfactants and ionic liquids, N2 adsorption, scanning electron microscopy and so on, combined with fractal theory were used. The results show that when the coal sample is treated with HNO3, a large number of corrosion pores can be generated, and the aromaticity of the coal can be improved. [Bmim]Cl (1-Butyl-3-methylimidazolium chloride) can reduce the proportion of oxygen-containing functional groups. Both HNO3 and [Bmim]Cl can effectively reduce the mineral content of coal. Neither HNO3 nor [Bmim]Cl is conducive to the wettability of coal, they increase the surface tension by 10.79 mN/m and 2.66 mN/m, respectively; SLS (Sodium N-lauroyl sarcosinate) reduces it by 20.79 mN/m. So SLS is added to improve the wettability. The combined action of HNO3 and SLS not only decreases the surface tension to 22.16 mN/m, which is 39.16% of that of HNO3 and can improve the wettability of the coal; but also, greatly increase the injectable water porosity of coal to 9.28%, which is 2.15 times of that of coal sample #1 and beneficial to coal seam water injection. HNO3, SLS and [Bmim]Cl have a strong synergistic effect, which severely breaks the pore size distribution of coal but can enlarge the average pore size of the coal and increase the porosity by 1.58 times. This approach can provide a new way to improve the efficiency of coalbed methane mining.

Abnormal adsorption and desorption of nitrogen at 77 K on coals: Study of causes and improved experimental method

Low-pressure nitrogen adsorption at 77 K is a widely-used technique for determining surface area, pore structure, and pore size distribution of coal. However, some high-rank coal samples show inaccurate results when investigated by the traditional experimental method due to non-negligible amount of helium being captured in the extremely narrow micropores (< 0.7 nm) at 77 K, which affects measurement of the void volume. In this study, an improved experimental method, based on calibrating the free space value of the sample rather than using the measured value directly, was adopted to re-test the high-rank coal samples. The results showed that the adsorption/desorption isotherm of the high-rank coal samples was accurately obtained by the new experimental method, which was further used to calculate the Brunauer–Emmett–Teller specific surface area and the Barrett–Joyner–Halenda pore volume.

Construction of high-pressure adsorption isotherm: A tool for predicting coalbed methane recovery from Jharia coalfield, India

Adsorption isotherm relates the gas storage capacity as a function of pressure at constant temperature. In this paper, adsorption isotherm of two dry borehole samples was constructed in the laboratory using the manometric method. Isotherm was measured for two gases, i.e., CH4 and CO2 to pressure up to 8.4 MPa. Before the construction of sorption isotherm, coal was characterized by proximate, ultimate and petrographic analysis. Coalbed gas content of these two samples was found 2.29 m3/t and 2.75 m3/t. SEM images were obtained for the pore size distribution of coal using pore image analysis. Prediction of coalbed methane recovery from CH4 adsorption isotherm showed that these coalbeds are under saturated. CO2 isotherm was constructed to estimate enhanced coalbed methane (ECBM) recovery. Volume wise CO2/CH4 sorption ratio was found 2.09 times to 2.75 times respectively. This paper presents the interpretation of isotherm data to find the recovery factor of methane production from Jharia coalfield.

Evaluation of coal and shale reservoir in Permian coal-bearing strata for development potential: A case study from well LC-1# in the northern Guizhou, China

Indentifying reservoir characteristics of coals and their associated shales is very important in understanding the co-exploration and co-production potential of unconventional gases in Guizhou, China. Accordingly, comprehensive experimental results of 12 core samples from well LC-1# in the northern Guizhou were used and analyzed in this paper to better understand their vertical reservoir study. Coal and coal measured shale, in Longtan Formation, are rich in organic matter, with postmature stage of approximately 3.5% and shales of type III kerogen with dry gas generation. All-scale pore size analysis indicates that the pore size distribution of coal and shale pores is mainly less than 20 nm and 100 nm, respectively. Pore volume and area of coal samples influenced total gas content as well as desorbed gas and lost gas content. Obvious relationships were observed between residual gas and BET specific surface area and BJH total pore volume (determined by nitrogen adsorption). For shale, it is especially clear that the desorbed gas content is negatively correlated with BET specific surface area, BJH total pore volume and clay minerals. However, the relationships between desorbed gas and TOC (total organic carbon) as well as siderite are all well positive. The coals and shales were shown to have similar anoxic conditions with terrestrial organic input, which is beneficial to development of potential source rocks for gas. However, it may be better to use a low gas potential assessment for shales in coal-bearing formation because of their low S1+S2 values and high thermal evolution. Nevertheless, the coalbed methane content is at least 10 times greater than the shale gas content with low desorbed gases, indicating that the main development unconventional natural gas should be coalbed methane, or mainly coalbed methane with supplemented shale gas.

Surface Properties and Pore Structure of Anthracite, Bituminous Coal and Lignite

Properties of coal surface and pore structure are important aspects to be investigated in coal preparation and utilization. In order to investigate the limits of different probe methods, a comprehensive approach was comparatively used to probe surface properties and pore structure of anthracite, bituminous coal and lignite. Surface morphology of the three coal samples was analyzed by scanning electron microscopy (SEM). Combining mercury intrusion porosimetry (MIP), physisorption method with carbon dioxide (CO2) at 273 K and nitrogen (N2) at 77 K was used to quantify a broad pore size distribution of coals, while FT-IR and water vapor sorption methods were used to study the coal surface properties. The results show that wedge-shaped pores develop with the increase of coal rank due to compression effect. The determined specific surface area (SSA) and pore volume of N2 decrease with the increase of coal rank, while CO2 SSA and pore volume are of a kind of U-shaped function of coal rank. MIP results indicate that that the pore size of 10–100 nm accounted for 70.7–97.5% of the total volume in the macropore range. Comparison of different methods indicates that micropores cannot be fully covered by the standard probes. CO2 adsorption technique can only probe micropores in the range of 0.5 nm to 0.9 nm. Water vapor is not an effective probe to detect the micropores in coals, due to that the water clusters is mainly filled in mesopores and macropores. The results also show that both water vapor adsorption and FT-IR analysis can provide qualitative information of coal surface, rather than qualification of functional groups.

Porosity changes in progressively pulverized anthracite subsamples: Implications for the study of closed pore distribution in coals

Coal samples for low-pressure nitrogen (N2) adsorption measurement in previous work cover a large particle size range (from 0.075 to 4.75 mm). However, minimal attention has been paid to the effect of coal particle size on pore structure using gas adsorption methods. Anthracite coal collected from the Zhina Coalfield, China, was crushed, subsampled, and sieved to eight particle size ranges: 1–2 mesh (8000–25400 µm), 40–50 mesh (270–380 µm), 50–70 mesh (212–270 µm), 70–90 mesh (160–212 µm), 90–160 mesh (96–160 µm), 160–200 mesh (75–96 µm), 200–300 mesh (48–75 µm), and >300 mesh (<48 µm). The adsorption–desorption isotherms of each subsample were measured using N2 at 77.35 K to compare differences in pore structure characteristics. The results of the N2 adsorption tests show that particle size has a significant effect on pore volume, specific surface area, and pore size distribution of coal. Specifically, decreasing coal particle size results in continuous increase in macro- and mesopore volumes and specific surface areas. This can be attributed to the fact that smaller-sized coal particles open more of the previously closed pores, which are then accessible to adsorping gas. The contribution of closed pores to the total pore volume is 94.94% in the pore aperture range of 3.1–370 nm. The volume of closed macropores varies from 48.96 to 84.69% of the total closed pore volume. According to optical microscope and SEM observations of the Zhina Coalfield subsamples, massive gas pores exist in an isolated form with poor connectivity; some plant tissue pores are filled by pyrites and clay minerals, and may be totally occluded. Thus, gas pores contribute the dominant amount of the closed pore volume. In addition, different Zhina Coalfield subsamples show varied hysteresis loop shapes, indicating that closed pores in coal possess a variety of pore morphologies and sizes. To improve the accuracy and comparability of the pore structure of coal, we propose >300 mesh as the preferred particle size of coal for all low-pressure N2 adsorption measurement in future work. Furthermore, caution must be used in evaluating coal bed methane resource recovery potential as coal possesses high closed porosity; failure to account for this will result in an overestimation of the amount of gas that can be recovered from coal seams during production.

Experimental study on the petrophysical variation of different rank coals with microwave treatment

To maximize coalbed methane recovery, the reservoir is often stimulated because of its low permeability. An exploratory study on improving coal porosity and permeability by microwave treatment was proposed. Pore size distribution of four unconstrained coals (lignite, subbituminous, bituminous, and anthracite), before and after microwave treatment, were evaluated using the nuclear magnetic resonance. With continuous exposure to microwaves, the pore size distribution of coals (especially the lignite coal) extends, and the pore volume and connectivity increase. In addition, coal porosity and permeability evaluated based on the Schlumberger Doll Research model increase by 33–72% and 73–181%, respectively. The mechanism was revealed by combining P-wave tests, thermal imaging and X-ray computed tomography scanner. The moisture and minerals bounded in pores are selectively heated and then detached, and, as a result, the pore structure is opened. Continued exposure to microwaves rapidly converts the mobilized moisture into super-heated steam. Under the steam pressure, pores and fractures generally expand. Borrowing from microwave-assist oil recovery, we presented a conceptual design of a CBM extraction borehole with microwave irradiation.

Improved analytic methods for coal surface area and pore size distribution determination using 77 K nitrogen adsorption experiment

77K nitrogen adsorption was the most widely used technique for determining surface area and pore size distribution of coal. Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) model are commonly used analytic methods for adsorption/desorption isotherm. A Chinese anthracite coal is tested in this study using an improved experimental method and adsorption isotherm analyzed by three adsorption mechanisms at different relative pressure stages. The result shows that the micropore filling adsorption predominates at the relative pressure stage from 6.8E−7 to 9E−3. Theoretically, BET and BJH model are not appropriate for analyzing coal samples which contain micropores. Two new analytic procedures for coal surface area and pore size distribution calculation are developed in this work. The results show that BET model underestimates surface area, and micropores smaller than 1.751nm account for 35.5% of the total pore volume and 74.2% of the total surface area. The investigation of surface area and pore size distribution by incorporating the influence of micropore is significant for understanding adsorption mechanism of methane and carbon dioxide in coal.

Molecular simulation and experimental characterization of the nanoporous structures of coal and gas shale

Characterization of coal and shale is required to obtain pore size distribution (PSD) in order to create realistic models to design efficient strategies for carbon capture and sequestration (CCS) at full scale. Proton nuclear magnetic resonance (NMR) cryoporometry and low-pressure gas adsorption isothermal experiments, conducted with N2 at 77K over a P/P0 range of 10−7 to 0.995, were carried out to determine the PSD and total pore volumes to provide insight into the development of realistic simulation models for the organic matter comprising coal and gas shale rock. The PSDs determined on the reference materials (SiliaFlash F60 and Vycor 7930) show a reasonable agreement between low-pressure gas adsorption and NMR cryoporometry showing complementarity of the two independent techniques. The PSDs of coal and shale samples were determined with low-pressure gas adsorption isothermal experiments, but were unable to be measured by NMR cryoporometry. This is likely due to a combined size and pore surface chemistry effect that prevents the water from condensing in the pores, such that when the sample is heated there is no distinction based upon melting or phase change. Molecular modeling is carried out to create the pore structure network in which the transport and adsorption predictions are based. The three-dimensional (3D) pore network, representative of porous carbon-based materials, has been generated atomistically using the Voronoi tessellation method. A comparison of the computed PSD using this method was made to the measured PSD using isothermal low-pressure gas adsorption isothermal experiments on coal and gas shale samples. Applications of this work will lead to the development of more realistic 3-D models from which enhanced understanding of gas adsorption and transport for enhanced methane recovery and CO2 storage applications can be developed.

Pore size distribution and accessible pore size distribution in bituminous coals

The porosity and pore size distribution of coals determine many of their properties, from gas release to their behavior on carbonization, and yet most methods of determining pore size distribution can only examine a restricted size range. Even then, only accessible pores can be investigated with these methods. Small-angle neutron scattering (SANS) and ultra small-angle neutron scattering (USANS) are increasingly used to characterize the size distribution of all of the pores non-destructively. Here we have used USANS/SANS to examine 24 well-characterized bituminous and subbituminous coals: three from the eastern US, two from Poland, one from New Zealand and the rest from the Sydney and Bowen Basins in Eastern Australia, and determined the relationships of the scattering intensity corresponding to different pore sizes with other coal properties. The range of pore radii examinable with these techniques is 2.5 nm to 7 μm. We confirm that there is a wide range of pore sizes in coal. The pore size distribution was found to be strongly affected by both rank and type (expressed as either hydrogen or vitrinite content) in the size range 250 nm to 7 μm and 5 to 10 nm, but weakly in intermediate regions. The results suggest that different mechanisms control coal porosity on different scales. Contrast-matching USANS and SANS were also used to determine the size distribution of the fraction of the pores in these coals that are inaccessible to deuterated methane, CD 4 , at ambient temperature. In some coals most of the small (~ 10 nm) pores were found to be inaccessible to CD 4 on the time scale of the measurement (~ 30 min–16 h). This inaccessibility suggests that in these coals a considerable fraction of inherent methane may be trapped for extended periods of time, thus reducing the effectiveness of methane release from (or sorption by) these coals. Although the number of small pores was less in higher rank coals, the fraction of total pores that was inaccessible was not rank dependent. In the Australian coals, at the 10 nm to 50 nm size scales the pores in inertinites appeared to be completely accessible to CD 4 , whereas the pores in the vitrinite were about 75% inaccessible. Unlike the results for total porosity that showed no regional effects on relationships between porosity and coal properties, clear regional differences in the relationships between fraction of closed porosity and coal properties were found. The 10 to 50 nm-sized pores of inertinites of the US and Polish coals examined appeared less accessible to methane than those of the inertinites of Australian coals. This difference in pore accessibility in inertinites may explain why empirical relationships between fluidity and coking properties developed using Carboniferous coals do not apply to Australian coals. Fraction of 8 nm radius pores in coals inaccessible to methane versus maceral composition. X, European and American coals; diamonds, Australian and New Zealand coals. ► The fraction of fine pores in coals decreases with increasing rank. ► The fraction of fine closed pores in bituminous coals is not affected by rank. ► Inertinite in Australian coals has less closed pores than that in Laurasian coals.

Petrophysical characterization of coals by low-field nuclear magnetic resonance (NMR)

Nuclear magnetic resonance (NMR) has been widely used in petrophysical characterization of sandstones and carbonates, but little attention has been paid in the use of this technique to study petrophysical properties of coals, which is essential for evaluating coalbed methane reservoir. In this study, two sets of NMR experiments were designed to study the pore types, pore structures, porosity and permeability of coals. Results show that NMR transverse relaxation ( T 2 ) distributions strongly relate to the coal pore structure and coal rank. Three T 2 spectrum peaks identified by the relaxation time at 0.5–2.5 ms, 20–50 ms and >100 ms correspond to pores of <0.1 μm, >0.1 μm and cleats, respectively, which is consistent with results from computed tomography scan and mercury intrusion porosimetry. Based on calculated producible and irreducible porosities through a T 2 cutoff time method, we propose a new NMR-based permeability model that better estimates the permeability of coals. In combination with mercury intrusion porosimetry, we also propose a NMR-based pore structure model that efficiently estimates the pore size distribution of coals. The new experiments and modeling prove the applicability of NMR in petrophysical characterization of intact coal samples, which has potential applications for NMR well logging in coalbed methane exploration.

Stability of the Bituminous Coal Microstructure upon Exposure to High Pressures of Helium

Small-angle neutron scattering (SANS) and ultra-small-angle neutron scattering (USANS) measurements of the structure of two Australian bituminous coals (particle size of 1−0.5 mm) before, during, and after exposure to 155 bar of helium were made to identify any effects of pressure alone on the pore size distribution of coal and any irreversible effects upon exposure to high pressures of helium in the pore size range from 3 nm to 10 μm. No irreversible effects upon exposure were identified for any pore size. No effects of pressure on pore size distribution were observed, except for a small effect at a pore size of about 2 μm for one coal. This study provides a convenient baseline for SANS and USANS investigations on sorption of gases at elevated pressures on coals, by distinguishing between the effect of pressure alone on coal pore size distribution and against the effect of the gas to be investigated.

95/00331 Pore size distribution of coals and chars from western Canada

A backscattered electron image detector has been used in a scanning electron microscope to study simultaneously the image intensity If of thin sputtered films of copper-aluminium alloys on single-crystal silicon substrates and the image intensity Ir of an adjacent, bulk copper reference specimen. The contrast ratio k = If/Ir for a given film and the reference sample depends on the energy E of the incident electrons. A graph of k vs. 1/E is used to obtain a characteristic energy Ei called the intercept energy which, in agreement with previous work, depends on the thickness of the film but not on its composition. Another feature of a curve of k vs.1E is that k tends to a limit as 1/E increases. This limit depends on the composition of the film to which the curve applies, and thus both thickness and composition can be obtained from a given curve. The present results which cover the low range of atomic numbers (Z = 13−29) are compared with the results obtained for Cu-W films which cover the atomic number range from Z = 29 to Z = 74. It is confirmed that the thicknesses of the light Cu-Al alloys studied in the present experiments and of Cu-W films can be obtained from a single curve of ln(thickness) vs. ln(Ei) and therefore that the method of using electron image contrast to measure thickness is applicable over a wide range of atomic numbers.