Pore Structure Of Coal Research Articles

Comparative Study of Pore Structure Characteristics between Mudstone and Coal under Different Particle Size Conditions

In order to investigate the difference of pore structure characteristics between mudstone and coal under different particle size conditions, samples acquired from Henan province were smashed and screened into three different particle sizes (20–40, 80–100, and >200 mesh) to conduct the experiments, using the high-pressure mercury intrusion porosimetry (MIP) and low-temperature N2 adsorption (LT-N2A) techniques. The results demonstrated that the proportion of open pores or semi-enclosed pores increased, and the pores became preferable contacted each other for both mudstone and coal during the crushing process. These variations of pore structure characteristics in the coal were beneficial to methane storage and migration. The total specific surface areas and pore volumes all showed a tendency of increasing continually for both mudstone and coal, as the particle sizes decreased from the LT-N2A test. The mudstone and coal were non-rigid aggregates with micropores, plate-shaped pores, and slit-shaped pores developed inside. The effect of the crushing process on the pore shape for the mudstone and coal was inappreciable. Moreover, the influence of the particle sizes on the mesopore was the most significant, followed by the macropore; and on the micropore, the influence was negligible for both mudstone and coal. The crushing process only had a significant impact on the pore structure of mudstone with a particle size of less than 100 mesh, while it could still alter the pore structure of coal with a particle size of larger than 100 mesh. It is believed that this work has a significant meaning to explore the diffusion and migration rules of coal-bed methane in coal.

Coal Macrolithotype Distribution and Its Genetic Analyses in the Deep Jiaozuo Coalfield Using Geophysical Logging Data

Coal macrolithotypes are closely correlated with coal macerals and pore–fracture structures, which greatly influence the changes in gas content and the coal structure. Traditional macrolithotype identification in coalbed methane (CBM) wells mostly depends on core drilling observation, which is expensive, time-consuming, and difficult for broken core extraction. Geophysical logging is a quick and effective method to address this issue. We obtained coal cores from 75 wells in the deep regions of the Jiaozuo Coalfield, northern China, quantitatively analyzed the logging cutoff number corresponding to various macrolithotypes, and established natural γ (GR), deep lateral resistivity (LLD), and γ–γ log (GGL) response rules for each coal macrolithotype. The formation mechanisms of different coal macrolithotypes are discussed from the perspective of coal facies and pore structures. The results show that GGL decreased but GR and LLD increased from bright coal to dull coal. Most coal macrolithotypes can be distinguished based on the established thresholds of various logging curves. However, excessively high or low ash yields significantly affect the validity of identification. The vertical coal macrolithotypes attributed to the peat marsh environment in Shanxi Formation mostly comprise three to six sublayers; dull or semi-dull coals are predominant close to the 21 coal seam, and the bright or semi-bright types usually appear in the middle part. The semi-bright and bright coals are usually vitrinite rich, whereas the semi-dull and dull coals are primarily inertinite rich. For pore structure arguments, the highest average specific surface area (SBET) and the total pore volume (VBJH) are found in bright coals, followed by dull and semi-bright coals; those of semi-dull coals are the lowest. However, SBET and VBJH change significantly for different samples, even though the coal macrolithotype is the same. Therefore, the macrolithotype is not the key factor determining the coal parameters of pore structures. Rapid and effective identification of coal macrolithotypes can help determine the CBM enrichment area, the CBM well location, and the exploration horizon.

Characterization of Full Pore and Stress Compression Response of Reservoirs With Different Coal Ranks

The accurate characterization of coal pore structure is significant for coalbed methane (CBM) development. The splicing of practical pore ranges of multiple test methods can reflect pore structure characteristics. The pore\fracture compressibility is the main parameter affecting the porosity and permeability of coal reservoirs. The difference in compressibility of different coal rank reservoirs and pore\fracture structures with changing stress have not been systematically found. The pore structure characteristics of different rank coal samples were characterized using the optimal pore ranges of high-pressure mercury intrusion (HPMI), low-temperature liquid nitrogen adsorption (LT-N2A), low-pressure carbon dioxide adsorption (LP-CDA), and nuclear magnetic resonance (NMR) based on six groups of different rank coal samples. The compressibility of coal matrix and pore\fracture were studied using HPMI data and NMR T2 spectrum under effective stress. The results show that the more accurate full pore characterization results can be obtained by selecting the optimal pore range measured by HPMI, LT-N2A, and LP-CDA and comparing it with the NMR pore results. The matrix compressibility of different rank coal samples shows that low-rank coal > high-rank coal > medium-rank coal. When the effective stress is less than 6 MPa, the microfractures are compressed rapidly, and the compressibility decreases slowly when the effective stress is more than 6 MPa. Thus, the compressibility of the adsorption pore is weak. Nevertheless, the adsorption pore has the most significant compression space because of the largest proportion in different pore structures. The variation trend of matrix compressibility and pore\fracture compressibility is consistent with the increase of coal rank. The compressibility decreases with the rise of reservoir heterogeneity and mechanical strength. The development of pore volume promotes compressibility. The research results have guiding significance for the exploration and development of CBM in different coal rank reservoirs.

Study on the Microscopic Mechanism of Spontaneous Combustion and Oxidation Kinetics of Water-Leached Coal

In this article, a series of experiments have been carried out to study the spontaneous combustion and oxidation mechanism of coal after water immersion and investigate its tendency to spontaneous combustion, analyze the difficulty of spontaneous combustion of coal samples under different water immersion conditions, and establish a kinetic model of water immersion coal oxidation (taking the Bulianta 12# coal as a case study). They rely on physical oxidation adsorption, scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), thermogravimetry, and oil bath heating. SEM has been used to analyze the characteristics of coal pore structure under different water immersion conditions (water-saturated coal samples under different water loss conditions until the coal samples are completely dried); FTIR served to investigate the characteristics of the molecular chemical structure of the coal surface before and after the coal is immersed in water. Through programmed temperature oxidation experiments combined with FTIR analyses and gas chromatographic (GC) analysis of gaseous products, it has been possible to study the changes of molecular structure and gas products on the surface of coal samples at different temperatures and water immersion conditions. The oxidation reaction rate of the 12# coal samples of Shendong Mine’s Bulianta Mine under different water content conditions during the spontaneous combustion process has been quantitatively studied. The difficulty of spontaneous combustion of coal samples has been correspondingly addressed. A kinetic model from the perspective of oxygen consumption has been proposed. Thermogravimetry-differential scanning calorimetry (TG-DSC) has been used to analyze and study the exothermal oxidation process before and after coal immersion. From the perspective of the exothermic intensity of the coal-oxygen reaction, an oxidation kinetic model for immersed coal samples has been developed to qualitatively determine its spontaneous combustion tendency. Results have shown that the increase in the specific surface area increases the risk of spontaneous combustion, and coal samples after soaking and drying have a stronger tendency to spontaneous combustion than raw coal. The moisture content of the coal sample leading to the easiest ignition conditions is 16.05%. Regardless of the moisture content, the critical temperature is maintained at 65–75°C, and the temperature of the left coal in the goaf should be prevented from exceeding this critical value.

Influence of active water on gas sorption and pore structure of coal

Active water fracturing is one of the commonly used technologies in area for enhanced coalbed methane (CBM) extraction. However, after the fracturing process, active water residues may block the pore of coal and ultimately reduce the production of CBM. To address this limitation, pore structures of Dongtan (DT) and Houwenjialiang (HW) coal samples were subjected to active water fracturing effect using active water with different concentrations of PAM prepared using polyacrylamide (PAM). Methane adsorption experiment was conducted to probe the effect of active water on the adsorption of methane in coal samples. Additionally, low-temperature nitrogen-adsorption experiments (LT-NAT) and nuclear magnetic resonance (NMR) were probed to characterize the discrepancies in the pore structure and chemical composition surface functional groups of the two coal-sample types. The results showed that considerable discrepancies existed in the pore structures of the two coal-sample types; however, their surface functional groups were similar. The active water treatment had contrasting effects on the adsorption behavior of methane features of the two coal-sample types, that is, as concentration of PAM in the active water increased, the methane adsorption of the treated HW coal sample increased whereas that of the treated DT coal sample gradually decreased. The PAM molecules in the active water residue could adsorb methane, and blockage of pores by the residue will reduce the amount of methane adsorption. The combined effect of the two dominated the influence of active water fracturing on methane sorption. Further optimizing the active water formulation to improve the flowback effect can be a very useful channel of reducing damage to the coal seam.

Effects of Pore Structure of Different Rank Coals on Methane Adsorption Heat

Adsorption thermodynamic characteristics are an important part of the methane adsorption mechanism, and are useful for understanding the energy transmission mechanism of coalbed methane (CBM) migration in coal reservoirs. To study the effect of coal pore characteristics on methane adsorption heat, five different types of rank coals were used for low-pressure nitrogen, low-pressure carbon dioxide, and methane adsorption experiments. Pore structure and adsorption parameters, including maximum adsorption capacity and adsorption heat, were obtained for five coal samples, and their relationships were investigated. The results show that the low-pressure nitrogen adsorption method can measure pores within 1.7–300 nm, while the low-pressure carbon dioxide adsorption method can measure micropores within 0.38–1.14 nm. For the five coal samples, comprehensive pore structure parameters were obtained by combining the results of the low-pressure nitrogen and carbon dioxide adsorption experiments. The comprehensive results show that micropores contribute the most to the specific surface area of anthracite, lean coal, fat coal, and lignite, while mesopores contribute the most to the specific surface area of coking coal. Mesopores contribute the most to the pore volume of the five coal samples. The maximum adsorption capacity has a significant positive correlation with the specific surface area and pore volume of micropores less than 2 nm, indicating that methane is mainly adsorbed on the surface of micropores, and can also fill the micropores. The adsorption heat has a significant positive correlation with the specific surface area and pore volume of micropores within 0.38–0.76 nm, indicating that micropores in this range play a major role in determining the methane adsorption heat.

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.

In situ change of fractal structure in coal with coking capability during high-temperature carbonisation

ABSTRACT Coal is an important energy and chemical raw material. Carbonization is an effective way for comprehensive utilization of coal, especially for coal with coking capability. The complex pore structure of raw coal and its carbonization solid products can be well characterized by a fractal description. Owing to a lack of in situ studies, the change in the fractal structure of coal with coking capability during carbonization remains unclear. Here we report briefly on an in situ study on high-temperature (1000°C) carbonization of four kinds of Chinese typical bituminous coal with coking capability, namely gas coal, fat coal, coking coal and lean coal, by synchrotron radiation small-angle X-ray scattering (SAXS). The change of fractal structure of the coal is analyzed in terms of corresponding mechanisms.

Research on coalbed methane microscopic seepage characteristics of medium and high-rank coal

ABSTRACT In the present study, the microscopic seepage characteristics of coalbed methane (CBM) in medium and high-rank coal are investigated. Moreover, the influence of the coal pore structure on the CBM migration is studied. To this end, X-ray μCT scanning technology is utilized to construct the microscopic pore structure model of medium and high-rank coal. Then, the established model is transformed into a vectorized CAD solid model. Numerical simulation under different working conditions is carried out. The obtained results show that the pore system of coal is in a connected state. However, not all pores can penetrate the entire seepage direction. It is found that pore connectivity is the most important factor determining the CBM seepage performance. As coal metamorphism increases, the corresponding seepage pore connectivity decreases, pore throat radius decreases, seepage channels in coal seams decrease and the coal permeability decreases. Meanwhile, permeability anisotropy and the pore pressure of the coal have different changing laws along different seepage paths. It should be indicated that there is a dominant seepage direction. As the degree of coal metamorphism increases, the permeability in all directions gradually decreases.

Experimental study of effect of slickwater fracturing on coal pore structure and methane adsorption

Driven by high pressure, slickwater may intrude into the pores of coal seams, causing changes in the pore structure, and ultimately affecting the flow of Coalbed Methane (CBM). In this study, slickwater prepared with different concentrations of polyacrylamide (PAM) is used to soak coal samples from Inner Mongolia under high pressure to explore the effect of slickwater fracturing on coal seam pores. To study the evolution characteristics of the pore structure of coal samples treated with slickwater, low temperature nitrogen adsorption experiments and methane adsorption experiments are combined. The experimental results show that under the action of external pressure, slickwater invades the pore structure of coal, resulting in a significant decrease in pore volume and specific surface area. Furthermore, with the increase of the pressure and viscosity of the slickwater, the slickwater residue blocks the micropores in the coal pore structure more severely. The damage to methane adsorption by residue is more serious than that to nitrogen adsorption, reflecting that more residue remains in the micropores of coal samples. The development of a gel breaker suitable for slickwater can promote the degradation of PAM polymer molecules and the reduction of residual liquid viscosity after fracturing, and improve the flowback effect. This may be an effective way to reduce reservoir damage.

Differences in Petrophysical and Mechanical Properties Between Low- and Middle-Rank Coal Subjected to Liquid Nitrogen Cooling in Coalbed Methane Mining

Abstract Exploring the damage differences between different coal rank coal reservoirs subjected to liquid nitrogen cooling is of great significance to the rational development and efficient utilization of coalbed methane (CBM). For this purpose, the mechanical properties, acoustic emission (AE) characteristics, and energy evolution law of lignite and bituminous coal subjected to cooling treatment were investigated based on Brazilian splitting tests. Then, pore structure changes were analyzed to reveal the differences in microscopic damage between lignite and bituminous coal after a cooling treatment. The results showed that compared with bituminous coal, the pore structure of lignite coal changed more obviously, which manifested as follows: significant increases in porosity, pore diameters, and pore area and a larger transformation from micropores and transition pores to mesopores and macropores. After the cooling treatment, the thermal damage inside lignite and bituminous coal was 0.412 and 0.069, respectively. Thermal damage reduced the cohesive force between mineral particles, leading to the deterioration of the macroscopic physical and mechanical properties. Simultaneously, denser acoustic emission ringing counts and larger accumulated ringing counts were observed after cooling. Moreover, the random distribution of thermal damage enhanced the randomness of the macrocrack propagation direction, resulting in an increase in the crack path tortuosity. With more initial defects inside coal, a more obvious thermal damage degree and wider damage distribution will be induced by cooling treatment, leading to more complicated crack formation paths and a higher fragmentation degree, such as that of lignite coal.

Study on the infiltration mechanism of injecting chelating agent into deep low permeable coal seam

Coal seam water injection technology which provides a guarantee for coal mine safety production is significant for the prevention and control of coal mine disasters. The selection of additives is very important for the implementation of coal seam water injection technology. Although the research on additives has been extensive in recent years, the mechanism of additives which can increase the permeability of coal and change mechanical properties of coal is still unclear. After selecting the chelating agent as the water injection additive, we characterized the pore structure of coal by low temperature N2 adsorption, mercury injection, scanning electron microscopy, etc., and tested the strength of coal by uniaxial hydraulic testing machine, as a result, several interesting results were found. Firstly, the minerals in the coal seam will damage the permeability of the coal seam and impede the flow of water in the coal seam. Secondly, the chelating ligand can interact with minerals like calcite and dolomite. When the concentration of the chelating agent is 500 mg/L, Ca2+ can be detected in the immersion solution and the leaching concentration of Ca2+ is 43.76 times greater than that in the ultra-pure water immersion solution. The change of pH has a greater impact on the leaching concentration of Fe and Mg, while the change of solid–liquid ratio has a smaller impact on their leaching concentration. Thirdly, as the dissolution of minerals, the size and volume of pore in coal increase, specific surface area decrease, connectivity of pore become better, the permeability of coal increase by an average of 129%, which has a promoting effect on water injection. Fourthly, the existence of minerals will affect the mechanical strength of coal. The uniaxial compressive strength of coal decreases after the chelating ligands interacted with the coal and the pH of the solution and soaking time also obviously affect the strength of coal. These results are helpful to open up a new direction for the selection of water injection additives that can realize the green and safe mining of coal.

Study on the Quantitative Characterization and Seepage Evolution Characteristics of Pores of Loaded Coal Based on NMR.

Quantitative characterization of the pore structure and gas seepage characteristics of loaded coal is of great significance to the study of high-efficiency gas drainage in coal seams. Aiming at the problem of imperfect characterizations of coal seepage characteristics based on nuclear magnetic resonance (NMR), a calculation method for the pore permeability of coal with different pore diameters is proposed. The pore structure and seepage characteristics of coal have been quantitatively studied using a nuclear magnetic resonance (NMR) system. The results show that with increasing external load, the proportion of the pore volume of the coal sample in the range of 0.01–0.52 μm gradually decreases, while that in the range of 5.11–352.97 μm increases. In this process, the porosity increases from 0.9967 to 1.0103%, the connectivity increases from 0.1718 to 0.2391, and the permeability increases from 2.64 × 10–6 to 8.20 × 10–6 μm2. The calculation of the coal sample connectivity and permeability using the improved NMR permeability component proves that 94.37–352.97 μm pores are the main channel of fluid flow. When the axial pressure increases, the coal body permeability in the aperture range of 94.37–352.97 μm rapidly increases. The improved permeability component calculation model can better reflect the variation law of pore permeability of the loaded coal body.

Study on the mechanism of the influence of HNO3 and HF acid treatment on the CO2 adsorption and desorption characteristics of coal

In order to cooperate with the application of acid fracturing enhanced permeability technology, this paper proposes using CO2 adsorption and desorption system and low-temperature nitrogen adsorption method to conduct experimental research on the changes in the CO2 adsorption and desorption characteristics of coal after treatment with HF solution and HNO3 solution. In addition, Combined with X-ray diffraction experiments, the indirect relationship between mineral dissolution and adsorption and desorption characteristics is analyzed. The research results show that: The acid solution transforms the pore structure of coal through the dissolution of mineral impurities. In the microporous stage, hydrofluoric acid is mainly used for pore formation and nitric acid for pore expansion. Acid treatment changed the adsorption characteristics of the coal sample, reduced the saturated adsorption capacity a, and increased the adsorption constant b. And a and b were linearly related to the specific surface area and pore volume, respectively. This paper has determined that adsorption equilibrium pressure 1.91 MPa and 2.57 MPa are the enhanced adsorption thresholds of hydrofluoric acid and nitric acid, respectively. After exceeding the threshold, acid treatment will no longer play a role in increasing CO2 adsorption. In addition, the acid treatment also changed the desorption characteristics of the coal sample, increased the initial desorption amount and the final desorption amount, shortened the desorption time. The effect of hydrofluoric acid was more evident than that of nitric acid.

Comparative analysis of pore structure parameters of coal by using low pressure argon and nitrogen adsorption

The accurate characterization of coal pore structure is of great significance for in-depth understanding of interior properties and gas adsorption, desorption and diffusion characteristics. In this study, the low pressure argon adsorption (LP-ArGA) and low pressure nitrogen adsorption (LP-N2GA) were all used to analyze the pore parameters of coal with different metamorphism. The results indicated that the adsorption isotherm type and desorption hysteresis type of coal samples obtained by the LP-ArGA and LP-N2GA belong to the same type, but the adsorption capacity of the former is higher than that of the latter. The pore size distribution (PSD) of coal samples shows a significant multi-peak distribution feature. Compared with the LP-N2GA, the LP-ArGA can accurately analyze the pore parameters between 2 and 4 nm or part of 2–7 nm. The mesopore volumes obtained by the LP-ArGA is 1.66–2.84 times that of the LP-N2GA, and there is no obvious law of macropore volume. The macropore volumes of QN and PM samples decreased by 17.2% and 50.9%, respectively, while GHS sample increased by 166.0%. The corresponding specific surface area (SSA) showed the same properties. Fractal curve fitting results obtained by the two methods are highly correlated, but compared with LP-ArGA, D1 and D2 obtained by the LP-N2GA can well reflect pore characteristics. The fractal dimension obtained by the two methods and the variation trend with the metamorphic degree are all different, which is mainly due to the difference of adsorption volume. The purpose of this paper is to introduce a new method for the analysis of coal pore structure in order to better understand the pore characteristics.

Study on the Effect of Polysulfide Content on the Micromorphology and Spontaneous Combustion Characteristics of Coal

This paper aimed to study the effect of the polysulfide content on the micromorphology and spontaneous combustion characteristics of coal, in order to develop more targeted prevention and treatment strategies. To this end, this study selected the method of mixing different sulfides with very low sulfur content raw coal to prepare the coal samples to be tested. Various parameters, such as true density, porosity, micromorphology, and oxygen uptake of the different sulfur samples, were tested. The results reveal that sulfide had a certain expansion effect on the coal body and improved the pore structure of coal, and the porosity increased with the increase of the sulfur content. After adding iron (II) disulfide (FeS2) and iron (II) sulfide (FeS) powder to the original coal sample, the number of fine particles on the surface increased significantly. After increasing the oxidation temperature, the lamellar structure disintegrated, and the massive coal body was broken into several fine particles, which promoted the spontaneous combustion of coal. Polysulfide promotes the low-temperature oxygen absorption of coal and shortens the natural firing period of coal. FeS has a slightly greater effect on increasing the tendency of coal to spontaneously combust and shortening the shortest natural firing period of coal. Before the addition of FeS2 and FeS to the coal samples, the coal production amount was not much different below 80–90°C, and then, the gap gradually widened. Under the same temperature condition of coal, carbon monoxide (CO) production basically occurred first as the sulfur content increased. When FeS2 and FeS were added, the sulfur content of the coal samples was 3 and 4%, respectively, and the production of CO and ethene (C2H4) was the largest. Although the peak areas of aliphatic hydrocarbon, aromatic hydrocarbon, hydroxyl group, and carbonyl group in the coal samples with FeS were different, they all reached their maximum value when the sulfur content was 4%.

Experimental study on the adverse effect of gel fracturing fluid on gas sorption behavior for Illinois coal

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.

Effect of different types of fracturing fluid on the microstructure of anthracite: an experimental study

ABSTRACT A systematic understanding of the effects of different fracturing fluids on the chemical and pore structure of coal is of great significance for the exploitation of coalbed methane (CBM). In this study, Fourier-transform infrared spectroscopy and mercury intrusion porosimetry were used to analyze the chemical and pore structure of coal samples treated by different fracturing fluids. The results show that fracturing fluid treatment has little effect on the coal aromatic structure. The oxygen-containing and hydroxyl structures of the coal samples treated by acidic and viscoelastic surfactant (VES) fracturing fluids increased by >17%, which is beneficial for gas desorption and thereby increases CBM yield, whereas those of the coal sample treated with deionized water decreased by 3.8%. The acidic and VES fracturing fluid treatments are also beneficial for coal pore connectivity, which promotes gas migration. The three types of fracturing fluids increased the macropore volume by >350%. The deionized water treatment reduced mesopores and micropores by 20% and 5%, respectively, while the transition pores increased by 2.3%, resulting in a 2.6% increase of the specific surface area, which is not conducive to gas desorption and migration. The acidic fracturing fluid treatment increased the mesopore volume by 53.3%, reduced the transition pore and micropore volumes by 5.3% and 32.5%, respectively, and reduced the surface area by 13.2%, which reduces the amount of adsorption sites and facilitates gas migration. However, the adsorption pores and mesopores after VES fracturing fluid treatment decreased by 17.6% and 26.7%, respectively, which indicates that this treatment does not transform most adsorption pores into migration pores.

Effect of Mechanical Vibration with Different Frequencies on Pore Structure and Fractal Characteristics in Lean Coal

The mechanical vibrations caused by underground operations can easily lead to coal and gas outbursts in coal mines. Using the MVGAD-I experimental platform that we designed, the raw coal (0 Hz) was treated with vibration frequencies of 25, 50, 75, and 100 Hz, and the coal samples with different frequency vibrations were obtained. The total pore volume (TPV), specific surface area (SSA), pore size distribution, and the pore fractal dimension (PFD) of five coal samples were analyzed by mercury intrusion porosimetry and low-pressure nitrogen adsorption data. We found that the TPV, SSA, and PFD of the coal samples fluctuate with the increase of vibration frequency. The changes of the TPV and SSA of coal samples treated with 25 and 75 Hz vibrations were significantly greater than those subjected to vibrations of 50 and 100 Hz. Compared with the raw coal (0 Hz), the TPV and SSA of macropores, mesopores, and micropores increased the most in 75 Hz vibration coal sample. Therefore, the 75 Hz vibration excitation can improve the permeability of a body of coal mass and is conducive to the diffusion and seepage of coalbed methane and its production.. The influence of 25 Hz vibration on the TPV and SSA of macropores and mesopores is not obvious, but the TPV and SSA of minipores and micropores decrease significantly, which is not conducive to gas diffusion and adsorption. In addition, 25 and 75 Hz vibrations obviously damaged the fractal characteristics of both mesopores and micropores, resulting in the change of gas adsorption and diffusion ability. The rational use of a 75 Hz vibration is beneficial to both the production of gas and the prevention of outbursts, while a 25 Hz vibration should be avoided. The results are expected to reveal the microscopic mechanism of a vibration-induced outburst and provide theoretical guidance for employing the appropriate frequency of vibration to improve the rate of gas drainage and reduce the risk of outbursts.

Research on the Influence of Gas on Coal Pores based on NMR Technology

In order to study the change characteristics of the pore structure of coal under different CH4 pressure conditions, nuclear magnetic resonance (NMR) technology was used to test the degree of influence on porosity and pore size distribution of coal after the adsorption of CH4 under different pressures. The test results show that under the action of gas pressure, the NMR signal amplitude of the NMR T2 spectrum changes significantly. From the distribution area of the T2 spectrum of the coal sample, it can be known that the peak peak area of small holes&gt;the peak peak area of medium and large holes&gt;the peak peak area of cracks indicates that the coal sample The micropores in the coal body are the most developed, and the connectivity is not strong, and it is difficult to circulate; in the range of gas pressure P=0.5~2.0MPa, as the gas pressure increases, new pores will be generated in the coal body, and the micropores are interconnected to form micropores or medium Pores and mesopores are interconnected to form cracks, thereby increasing the porosity and permeability of the coal sample.