Search

Hydraulic fracturing is an effective technology for coal reservoir stimulation. After fracturing operation and flowback, a fraction of fracturing fluid will be essentially remained in the formation which ultimately damages the flowability of the formation. In this study, we quantified the gel-based fracturing fluid induced damages on gas sorption for Illinois coal in US. We conducted the high-pressure methane and CO2 sorption experiments to investigate the sorption damage due to the gel residue. The infrared spectroscopy tests were used to analyze the evolution of the functional group of the coal during fracturing fluid treatment. The results show that there is no significant chemical reaction between the fracturing fluid and coal, and the damage of sorption is attributed to the physical blockage and interactions. As the concentration of fracturing fluid increases, the density of residues on the coal surface increases and the adhesion film becomes progressively denser. The adhesion film on coal can apparently reduce the number of adsorption sites for gas and lead to a decrease of gas sorption capacity. In addition, the gel residue can decrease the interconnectivity of pore structure of coal which can also limit the sorption capacity by isolating the gas from the potential sorption sites. For the low concentration of fracturing fluid, the Langmuir volume was reduced to less than one-half of that of raw coal. After the fracturing fluid invades, the desorption hysteresis of methane and CO2 in coal was found to be amplified. The impact on the methane desorption hysteresis is significantly higher than CO2 does. The reason for the increasing of hysteresis may be that the adsorption swelling caused by the residue adhered on the pore edge, or the pore blockage caused by the residue invasion under high gas pressure. The results of this study quantitatively confirm the fracturing fluid induced gas sorption damage on coal and provide a baseline assessment for coal fracturing fluid formulation and technology. [ABSTRACT FROM AUTHOR]

Coal-water interactions have profound influences on gas extraction from coal and coal utilization. Experimental measurements on three coals using X-ray photoelectron spectroscopy (XPS), low-temperature nitrogen adsorption and dynamic water vapor sorption (DVS) were conducted. A mechanism-based isotherm model was proposed to estimate the water vapor uptake at various relative humidities, which is well validated with the DVS data. The validated isotherm model of sorption was further used to derive the isosteric heat of water vapor sorption. The specific surface area of coal pores is not the determining parameter that controls water vapor sorption at least during the primary adsorption stage. Oxidation degree dominates the primary adsorption, and which togethering with the cumulative pore volume determine the secondary adsorption. Higher temperature has limited effects on primary adsorption process.The isosteric heat of water adsorption decreases as water vapor uptake increases, which is found to be close to the latent heat of bulk water condensation at higher relative humidity. The results confirmed that the primary adsorption is controlled by the stronger bonding energy while the interaction energy between water molecules during secondary adsorption stage is relatively weak. However, the thermodynamics of coal-water interactions are complicated since the internal bonding interactions within the coal are disrupted at the same time as new bonding interactions take place within water molecules. Coal has a shrinkage/swelling colloidal structure with moisture loss/gain and it may exhibit collapse behavior with some collapses irreversible as a function of relative humidity, which further plays a significant role in determining moisture retention. [ABSTRACT FROM AUTHOR]

Numerical simulation, a cost-effective technique used to describe and predict gas flow behavior in coal, can provide a reliable means of evaluating gas well performance, solving complex engineering problems, and determining how variations in reservoir properties and gas drainage practice affect wellbore performance at the field scale. This paper first presents a multi-mechanistic gas flow model using a modified dual-porosity model by incorporating the effects of multiscale transport mechanisms in coal, effective stress evaluation, and matrix shrinkage. Then, a numerical model and a simulator are derived using the finite difference method to solve the proposed model and are successfully tested against two sets of in situ gas extraction field data. Subsequently, the effect of parametric variations on gas transport behavior in dry coal seams is quantified through a series of simulations. The simulated results imply that (1) a greater initial pore pressure or a lower gas drainage pressure will lead to a higher gas flow rate; (2) the fracture permeability as well as matrix permeability plays significant roles in gas production profiles, and the former mainly influences the early initial depletion stage, while the latter controls gas flow rate and production decay rate at the late stage; (3) the mass transfer rate between matrix blocks and fractures varies with the distance from the matrix block to the free coal surface, and a shorter distance will result in a greater mass transfer rate; and (4) the decrease of the matrix radius increases the gas released from matrix and leads to a greater gas flow rate and a slower decay rate. The results also indicate that size and matrix permeability are two key parameters affecting the overall gas deliverability, and play a critical role in gas production at the late stage. [ABSTRACT FROM AUTHOR]

The composite of a nanocrystalline SnO2 thick film deposited on an Al-doped ZnO ceramic substrate was firstly proposed. This study also provided a simple, fast and cost effective method to prepare SnO2 thick film and Al-doped ZnO ceramic as well as the final composite. The crystal structure, morphology, composition, pore size distribution and gas sensitivity of the composite were investigated by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy dispersive spectroscopy, Barrett-Joyner-Halenda analysis and gas sensitive measurement system. Results indicated that the composite was fully porous consisted of SnO2, ZnO and ZnAl2O4 crystal phases. The macrosized pores generated in the composite could enhance the gas infiltration into the sensing layers effectively. In this way, combining a high gas-transporting-capability and a nanocrystalline SnO2 thick film, the composite showed very impressive performance. The gas sensitivity of the composite was high enough for ethanol vapor with different concentrations, which was comparable to other kinds of reported SnO2 gas sensors, while showing two straight lines with a turning point at 1000ppm. Finally, the gas sensitive mechanism was proposed based on the microstructure and composition of the composite. [ABSTRACT FROM AUTHOR]

This paper proposed a conceptual model of broken rock mass compaction based on elastic theory by simplifying the compression process. This conceptual model assumes that the contact connection between two adjacent rock particles is similar to a cubic mass. With this simplification, the stress-strain constitutive law is established. The change in the secant modulus in the mechanical model derived from the variation of the connection coefficient agrees well with the reported experimental results. The permeability of compacted rock mass evolution was modeled based on the cubic law. The mechanical compression model was coupled with the permeability evolution model. The modeled permeability evolution is consistent with reported simulation and experimental results. The modeled permeability results were validated using Karacan's data with broken shale rock properties. Compared to an intact rock mass, we found that the stress-strain curve of a compacted rock mass takes a longer compression path to reach linearity due to the void space compaction resulting from friction slipping and the re-arrangement of particles. It was also found that the particle elastic modulus does not contribute to the overall bulk compaction and permeability reduction at the initial compaction stage. However, the particle elastic modulus controls the permeability evolution for a fully compacted gob, where the gob can be treated as an intact rock mass. The proposed conceptual models will potentially lay the foundation for future permeability and caving behavior characterizations using numerical simulations for complex gob areas. [ABSTRACT FROM AUTHOR]

Gas diffusion in coal is controlled by nano-structure of the pores. The interconnectivity of pores not only determines the dynamics of gas transport in the coal matrix but also influences the mechanical strength. In this study, small angle neutron scattering (SANS) was employed to quantify pore accessibility for two coal samples, one of sub-bituminous rank and the other of anthracite rank. A theoretical pore accessibility model was proposed based on scattering intensities under both vacuum and zero average contrast (ZAC) conditions. The results show that scattering intensity decreases with increasing gas pressure using deuterated methane (CD 4 ) at low Q values for both coals. Pores smaller than 40 nm in radius are less accessible for anthracite than sub-bituminous coal. On the contrary, when the pore radius is larger than 40 nm, the pore accessibility of anthracite becomes larger than that of sub-bituminous coal. Only 20% of pores are accessible to CD 4 for anthracite and 37% for sub-bituminous coal, where the pore radius is 16 nm. For these two coals, pore accessibility and pore radius follows a power-law relationship. [ABSTRACT FROM AUTHOR]

Magmatism can significantly alter the pore structure of shale formation due to the thermal and pressure effects, thus the gas enrichment mechanism is naturally affected. The uncertainty of shale gas development has been raised in the petrogeology community due to the magma intrusion. This study investigates pore evolution, pore alteration, and gas enrichment of gas shale in the Permian Gufeng Formation (Fm.) in the Lower Yangtze, Liqiao district. In this work, multiple techniques were used for shale characterization including total organic carbon (TOC) content quantification, reflectivity microscope tests, X-ray diffraction, high-pressure mercury injection, low-pressure nitrogen adsorption, and carbon dioxide adsorption measurements. The results show that the closer the shale approaches the intrusion, the higher the vitrinite reflectance (R o) and quartz content, the lower the TOC and clay mineral content, and metamorphic minerals for instance augite and anatase appear in the affected shale formation. As shale is closer to the intrusion, the type of pores changes from small pores (10～100 nm) to super-large pores (1000～100,000 nm), and the super-small pores (6～10 nm) almost disappeared. In addition, a decrease in the fractal dimension of the pores was observed indicating that the heterogeneity and complexity of the pore structure are reduced. Within the contact metamorphism halo, the maximum adsorption capacity, pore volume, and specific surface area decrease with increasing proximity to the intrusion, which reduces the potential for shale gas enrichment. In basin-scale, a huge contact metamorphic range and plugging effect of the intrusion in sill intrusion generate enormous amounts of hydrocarbon and block the escape of the shale gas. Due to these factors, the intrusion can be conducive for shale gas enrichment. However, the dike that penetrates the shale reservoir is not conductive to shale gas enrichment because the fracture channels created in the contact metamorphism halo accelerate the escape of the shale gas. This study proposed critical theories of shale gas enrichment and could increase the success rate of shale gas exploration. • Magmatism heavily impact on pore of shale reservoir in the Lower Yangtze region. • The magmatism has a prominent effect on geomaterials and pore evolution. • Contact metamorphism halo is unfavorable for the enrichment of shale gas. • The gas enrichment of three invasion patterns are different on the basin scale. [ABSTRACT FROM AUTHOR]

A correction to this paper has been published: https://doi.org/10.1007/s00603-021-02492-1 [ABSTRACT FROM AUTHOR]

For the significant impact of pore structure on gas storage and transport in coal seams, research on coal pore structure characterization has been a hotspot. Benefited from the high spatial resolution synchrotron-based nano-CT instrument, pore structure characterization of coal is investigated in nano scale. Image alignment and 3D reconstruction were completed at the platform designed by National Synchrotron Radiation Laboratory and Chinese Academy of Sciences. The segmentation of the unimodal grey-scale value histograms is solved by Between-class Variance Maximisation (BCVM) algorithm and the nano-CT images are segmented into three components, pore, organic components and mineral components. Based on the voxel number, components fraction is computed. Pore size distribution (PSD) presents bimodality. Pores with equivalent radius less than 60 nm account for 84% of the total pore number. Throats with equivalent radius less than 60 nm account for 89% of the total throat number. Throats with length less than 100 nm account for 58% of the total throat number and throats with length less than 400 nm account for 84% of the total throat number. Pore number decreases with the increase of coordination number. There are over 50% of pores without coordination pore and pore connectivity was analysed. Nanopore structure-based computational fluid dynamics (CFD) simulation was explored. The permeability in three coordinate axes directions presents anisotropy. [ABSTRACT FROM AUTHOR]

Enhanced coalbed methane (ECBM) achieved by injecting liquid carbon dioxide (LCO 2 ) has been proposed and applied in industrial production for decades and has been demonstrated to be an applicable method to boost CBM production. Most of the studies have concentrated on the gas bursting and flooding effect and have rarely focused on the accompanying “freeze–thaw” phenomenon, and the temperature effect of cyclic LCO 2 injection on the pore variation of different coals has been partly investigated. In this paper, the influence of cycle parameters, such as cycle number and cycle time, on the pore variation was studied. Infrared thermal imagery (ITI) and low-field nuclear magnetic resonance (NMR) were used to measure the temperature and pore size distribution (PSD) change, respectively. The results show the following: (1) The gas pressure displayed square cyclicity with different cycle time, the temperature of gasified CO 2 was almost 248.15  K , and the end and lateral surface temperatures of a core were in the range from 259.35 to 261.85  K , which could cause the water within the pores to freeze with a 9% volume increase, and the fracturing formula was deduced; (2) The relaxation time spectra obtained by different cycle parameters expressed changeable PSD of cores with increasing cycle parameters, and the magnified proportion of bulk water and capillary water, as well the diminished proportion of adsorbed water, all indicated that the increased number of macropores and mesopores formed a larger free volume; (3) The increased total porosity φ t and the decreased T 2cutoff of six cores with the increasing cycle parameters meant that the larger cycle number could enhance the porosity due to amount of damage accumulation, and the larger cycle time might make the water freeze completely with larger ice swelling stress; (4) There is a polynomial fitting between relative increase ratio Rφ and cycle time, and the fitting coefficients were all higher than 0.99, and the larger the cycle time was, the greater the Rφ (e/t) increment and Rφ (r/t) decrement were. The interval increase ratio Iφ e was positively correlated to cycle time without obvious increase behavior; however, the Iφ r variation expressed that the greater the cycle number was, the lesser the Iφ r with the increasing cycle time was, which indicates that the increasing cycle parameters might help the proportion of connected pores to increase and provide more pathways for permeable fluid; (5) The NMR permeability k SDR of a core increased as the cycle number increased, and the longer cycle time was superior in terms of permeability enhancement. [ABSTRACT FROM AUTHOR]

The aims of this research are to quantitatively evaluate the complexity of the pore structure in coal frozen with liquid nitrogen (LN 2 ) and then study the influence of the modified pore system on coalbed methane (CBM) extraction. To do this, nuclear magnetic resonance (NMR) and fractal dimension theory were used to determine the properties of the coal's pore system after samples of low rank coal had been frozen and then thawed. The fractal dimensions of pores in frozen-thawed coal samples were divided into five types according to pore size and the state of the fluid in the coal pores. The results showed that the fractal dimension D A of adsorption pores was less than two, indicating that these pores did not exhibit fractal characteristics. The fractal dimensions D ir and D T representing closed pores and total pores presented low fitting precision, so the closed pores showed insignificant fractal characteristics. However, the fractal dimensions D F and D S representing open pores and seepage pores had high fitting precision, suggesting that open and gas seepage pores exhibited a favorable fractal characteristic. Correlation analysis revealed that D F and Ds were negatively correlated with LN 2 freezing time and the number of freeze-thaw cycles. After being frozen and thawed, coal porosity and permeability showed a strong negative correlation with fractal dimension and this relationship allowed predictive models for permeability and fractal dimensions (D F and D S ) to be constructed. The models showed that the smaller the fractal dimension, the more uniformly the pores were distributed and the higher their degree of connection. These properties favor the production of CBM. This study also showed that compared with single LN 2 freezing events, repeated cyclic freezing with LN 2 followed by thawing is more favorable for CBM production. [ABSTRACT FROM AUTHOR]

Liquid CO 2 (LCO 2 ) enhanced coalbed methane recovery had been studied in laboratory experiments and field applications, supporting many improvements and achievements. Previous studies primarily investigated the gas bursting, flooding effect and adsorption effect; however, the freezing-thawing phenomenon (drikold formation and gasification) that commonly occurs during the LCO 2 injection process was insufficiently studied. The freezing-thawing phenomenon might enhance the pore volume and change the permeability evolution of the coalbed; thus, cyclical LCO 2 injection was proposed to exploit the phenomenon, and the influence of multiple freezing-thawing cycles on the coal pores was investigated in this paper. Nuclear magnetic resonance (NMR) and infrared thermal imagery (ITI) were used to monitor the pore variation and surface temperature distribution, respectively. Low temperatures could make the saturated water in the pores freeze and undergo a 9% volume increase. The three coals used in this experiment displayed different crack intensities and forms with ITI. After cyclical LCO 2 injection, the NMR amplitude increased, and the T 2 range was widened under a saturation condition, while the cores under a centrifuge state had lower amplitudes and a narrower T 2 range; this difference indicated that the pore structure could be altered by multiple freezing-thawing cycles of LCO 2 . The more freezing-thawing cycles the cores experienced, the greater the change in pore structure was. The total porosity φ t and effective porosity φ e increased while the residual porosity φ r and T 2cutoff values decreased with more freezing-thawing cycles. However, the variations with coal rank were observed; with higher coal ranking, φ t and φ e increased less, and the φ r and T 2cutoff values decreased less, which suggests that lower ranking coals could be most easily affected by the LCO 2 enhanced recovery method and have the most improved pore connectivity. Moreover, the enhancement ratio of φ t and φ e increased for all three coals tested, which could be fit with quadratic functions with fit coefficients greater than 0.99. The increasing relative ratio D e / t of anthracite was fit with a linear function, while the lignite and bitumite were fit with quadratic functions. These functions all indicate that the multiple freezing-thawing cycles of LCO 2 injection had a positive impact on the enhancement efficiency of pore porosity. Finally, a potential field application of cyclical LCO 2 injection was also discussed to improve the fracturing effect. [ABSTRACT FROM AUTHOR]

Liquid carbon dioxide (LCO 2 ) fracturing technology has been applied in the enhanced coalbed methane recovery (ECBM), and it has made some progress in the physical experiments and field applications. However, the freeze phenomenon during the injection process might induce coal matrix shrinkage, hindering the fracturing efficiency. A multiple cycle LCO 2 fracturing technology is proposed, and the feasibility of the cryogenic effect from LCO 2 on the crack evolution of five different coal cores under the loading state was investigated by using an innovative cryogenic loading experimental system. Nuclear magnetic resonance (NMR) and infrared thermal imaging (ITI) were used to measure the pore changes and temperature distribution, respectively. After 25 injection cycles, some cracks on the side and lateral surfaces of five cores were generated, and a low temperature distribution was formed. The temperature values were almost less than −18 °C, which could cause the saturated water to freeze into ice with a 9% volume increase; thus, the stress analysis diagram during one cycle injection was analyzed, and the initiation criterion was deduced. The T 2 spectra variation showed that the various pore sizes changed with the increased number of cycles. The peaks increased in amplitude and shifted to the right under saturated conditions, while they decreased and shifted to the left under centrifuged conditions, causing the amplitude increment ΔA in the post-test stage to be greater than that in the pre-test stage, which indicated that the cryogenic effect of LCO 2 could significantly improve the connectivity of pores. The total porosity φ t and effective porosity φ e of all five cores increased with the number of cycles. A quadratic function described the relationship between incremental ratio of φ e ( D c ) and cycle number, the fitting coefficients for which all exceeded 0.99, which indicated that the cryogenic effect of LCO 2 could improve the permeability observably. [ABSTRACT FROM AUTHOR]

Liquid nitrogen (LN 2 ), a non-aqueous medium, has attracted attention in recent years as a fluid for fracturing in the petroleum/energy industry. This study proposes a freeze-thaw method using LN 2 to improve coal permeability for the production of coal bed methane. Experiments were conducted using nuclear magnetic resonance (NMR) to explore the physical properties of frozen-thawed coal. The coal samples were subjected to different LN 2 freezing times and to freeze-thaw cycles and coals of different rank and with different moisture contents were tested. Changes in these four freeze-thaw variables changed the petrophysical properties of the frozen-thawed coal samples; the pore structure, porosity, and permeability of the coals were modified. Of these variables, the number of freeze-thaw cycles had the most substantial effect on modifying the coal’s petrophysical properties. The degree of modification on the coals of different rank was affected by the coal’s initial porosity. In general, lignites were modified the most, anthracite coal was modified less, and bituminous coal was modified the least. The study analyzed three of the classic NMR transforms for determining permeability and found that the Schlumberger-Doll Research (SDR) model matched the measured gas permeabilities most consistently. Based on this SDR permeability model, equations suitable for predicting the permeability of frozen-thawed low-rank coals were derived. In addition, results from scanning electron microscope studies showed that a fracture network with fracture widths of as much as 32.3 μm was formed in the coal after 30 freeze-thaw cycles. Additionally, micron-size particles falling from the coal surface gradually increased as the number of freeze-thaw cycles increased, indicating that freeze-thaw using LN 2 materially modified the physical properties of the coal. [ABSTRACT FROM AUTHOR]

• Coal structure alterations induced by ultrasound probed using NMR and LTLN techniques. • Ultrasound increases the N 2 accessible, specific surface area and pore volume in coal. • Pore dilation induced by ultrasound can convert micropores into the meso-/macro-pores. • High-power ultrasound has better performance on promoting gas desorption from coal. Effective reservoir stimulation is the key for enhancing coalbed methane (CBM) recovery. Ultrasound treatment provides a novel alternative to traditional hydraulic fracturing for gas drainage from coal formations. An improved understanding of the ultrasound effect on coal structure and gas desorption and transport behavior through coal is essential for the potential field implementation. This study aims to investigate the pore/fracture structure alteration with ultrasound treatment for coal and its implication on gas desorption and diffusion behaviors. We used multiple techniques, including nuclear magnetic resonance (NMR), low pressure nitrogen adsorption (LPNA) and gas desorption/diffusion experiments, to characterize the pore structure alterations and its implication on the gas desorption and diffusion behaviors of coal. The NMR tests show that the pore volume and their overall interconnectivity in coal increase with ultrasound treatment. With the ultrasonic treatment (25 kHz, 18 kw, 2 h), the porosity increases by 9.6%–11.9%, and the permeability increases by 21.5%–40.7% for the tested coal. Based on the experimental data, ultrasonic treatment can effectively modify the coal pores within size range of 1 nm–100 nm, and the greater power of ultrasound leads to the greater effect on the pore alteration in coal. After being treated with ultrasound (25 kHz, 3–18 kw, 2 h), the average pore diameters of the tested coal increased by 5.05%–61.81%. It was found that the micropore dilation due to mechanical vibration and cavitation effects can effectively convert micropores into the meso-/macro-pores during the ultrasonic treatment. The results demonstrate that ultrasound treatment changes the coal structure and significantly increases the N 2 accessible specific surface area and pore volume in coal. These results can jointly make positive impact on the gas transport during the gas drainage and CBM production. [ABSTRACT FROM AUTHOR]

• The nanopores expansion effect during the metamorphism-deformation process of tectonic coal is demonstrated through organic solvent extraction experiments. • The impact of small molecular structures on coal pore recombination and their consequent inhibition of gas adsorption is analyzed. • The evolution pattern and driving mechanism of small molecular structures during the deformation process of tectonic coal are revealed. This work aimed to investigate the impact of the evolution of small molecular structures during the deformation of coal on the constraints imposed on nanoscale pores and adsorption behaviors. Both undeformed and tectonically deformed coal (TDC) from the Huaibei mining area in China were collected for analysis. The pyridine was employed for the extraction of small molecules from coal, which was further complemented by Fourier-transform infrared spectroscopy (FTIR) and low-pressure CO 2 and N 2 adsorption experiments (LP-CO 2 /N 2 AD). The study yielded the following results: (1) Intense tectonic stresses facilitate the removal and recombination of small molecular side chains within the basic structural units (BSU) of coal. As deformation occurs, hydroxyl groups, aliphatic hydrocarbons, C=O, C=C, aromatic hydrocarbons, and 'C' structures tend to accumulate, resulting in an enhanced extraction rate. (2) The deformation process of the coal demonstrates a significant pore-expansion effect, with the most substantial increase observed in micropores (surface area increased by over 70 %), followed by mesopores and macropores. This enlargement facilitates the aggregation of small molecules and gas storage. (3) Intense deformation stages involving shear slip, ductile deformation, and maceral fragmentation result in small molecular fractures and an increased number of micropores. The removal and recombination of I, DOC, and CH 2 /CH 3 within BSUs lead to the formation of larger interconnected pores accompanied by the aggregation of hydroxyl groups (OH–), C=O, C=C, aliphatic hydrocarbons, and 'C' structures. (4) The adsorption capacity of residual coal is lower than that of the raw coal before extraction, indicating that small molecular structures contribute to increased adsorption. This likely plays a significant role in the higher gas content observed in structurally deformed coal. The removal of small molecular structures effectively diminishes the adsorption capacity, providing novel perspectives for coalbed methane development and gas extraction in TDC areas. [ABSTRACT FROM AUTHOR]

The porous structure of coal directly determines its gas transport property. The fluid flow behavior of coal is one of the key science questions that will influence the coal energy industry. In this study, the influence of real coal macropore structure on the fluid flow through coal was studied through 3-D coal structure reconstruction by the CT images. Based on the reconstructed coal structure, the micron-scale structure parameters were quantitatively analyzed. A newly programmed Matlab code was established to find the volume fractal dimension, obtain the relationship between porosity/permeability of coal and volume fractal dimension, and estimate the tortuosity fractal dimension by using the 3-D box dimension algorithm. The results show that the volume fractal dimensions of 6 coal samples range from 2.25 to 2.79 and the tortuosity fractal dimensions of capillaries range from 2.15 to 2.73. The 3-D coal structure cannot only quantitatively estimate the real porosity of the coal, but it can be used to characterize the complexity of coal's porous structure through mean deviation of surface porosity. It can be clearly seen from the reconstructed coal that coal specimen-C3 is highly heterogeneous because it has complex pore structure as well as wider pore size distribution and the highest mean deviation of the surface porosity. The volume fractal dimension can be used to quantitatively define the complexity of pores. The larger the porosity of coal, the greater the fractal dimension. The permeability and porosity of coal are negatively correlated with the volume fractal dimension. The tortuosity fractal dimension can effectively characterize coal permeability, but it weakly correlates with coal porosity. The outcome of this study helps to understand the structure-based flow characterization and gas transport behavior in heterogenous coal which will have the implication of the gas extraction from coalbed methane reservoirs and coal mine gas drainage. [ABSTRACT FROM AUTHOR]

Abstract The permeability of coalbed methane (CBM) reservoirs is typically very low and it is challenging and essential to effectively increase coal permeability to maximize CBM recovery. An exploratory study on enhancing coal porosity/permeability using freeze−thaw cycling with liquid nitrogen (LN 2) was conducted. The changes of fracture and porosity in coal with the freeze−thaw treatment using LN 2 were evaluated using nuclear magnetic resonance (NMR). After freeze−thaw treatment, the coal pore size tended to increase and new pores/fissures were generated. The growth rate of the pore size was positively correlated with the LN 2 freezing duration. The effective porosity had a positive correlation with the freezing duration, but the correlation for residual porosity was negative. This means that the volume of irreducible fluid in the coal decreased while the amount of free fluid increased. Scanning electron micrograph studies indicated that the maximum fracture width in the coal samples grew from 5.56 μm at a T freezing = 1 min to 100 μm for T freezing = 60 min, matching the NMR findings. This study provides a scientific basis and guidance for engineering application of freeze−thaw using liquid nitrogen to enhance coalbed methane extraction. Highlights • NMR was used to conduct the evolution of pore structure in coal samples. • Coal pore size and effective porosity increase with increasing LN 2 freezing time. • A method of LN 2 frozen-thawed fracturing for CBM extraction was proposed. [ABSTRACT FROM AUTHOR]