Pore Structure Of Coal Research Articles

Experimental study of supercritical CO2-H2O-coal interactions and the effect on coal spontaneous imbibition characteristics

The interaction among supercritical CO2 (ScCO2)–H2O-coal can impact the pore structure of coal, thereby bringing about changes in the spontaneous imbibition (SI) characteristics of coal. This is of great significance for the effect of hydraulic fracturing and the increase in coalbed methane production. The changes in pore structure and imbibition characteristics of coals with different degrees of metamorphism after ScCO2-H2O treatment were studied using nuclear magnetic resonance technology. The experimental results indicate that ScCO2-H2O treatment can increase porosity, enhance pore connectivity, and significantly increase the imbibition rate. It also alters the proportion of different pores. The proportion of micropores, microfractures and fractures increases, while the proportion of small pores, mesopores and large pores decreases. This change is more significant in low-rank coal. During SI, the main contribution to the imbibition volume of coal samples is consistent with the proportion of pore structure; hence, there are differences in the extent of changes in the SI characteristics between low-rank and high-rank coals. The numerous etched pores produced by ScCO2-H2O treatment exhibit self-similarity, reducing the roughness and complexity of the pores while enhancing connectivity. The emergence of a large number of microfractures and fractures has a minor impact on the change in SI volume and rate but affects the mode of SI.

Gas Content and Geological Control of Deep Jurassic Coalbed Methane in Baijiahai Uplift, Junggar Basin

Deep coalbed methane (CBM) resources are abundant in China, and in the last few years, the country’s search for and extraction of CBM have intensified, progressively moving from shallow to deep strata and from high-rank coal to medium- and low-rank coal. On the other hand, little is known about the gas content features of deep coal reservoirs in the eastern Junggar Basin, especially with regard to the gas content and the factors that affect it. Based on data from CBM drilling, logging, and seismic surveys, this study focuses on the gas content of Baijiahai Uplift’s primary Jurassic coal seams through experiments on the microscopic components of coal, industrial analysis, isothermal adsorption, low-temperature CO2, low-temperature N2, and high-pressure mercury injection. A systematic investigation of the controlling factors, including the depth, thickness, and quality of the coal seam and pore structure; tectonics; and lithology and thickness of the roof, was conducted. The results indicate that the Xishanyao Formation in the Baijiahai Uplift usually has a larger gas content than that in the Badaowan Formation, with the Xishanyao Formation showing that free gas and adsorbed gas coexist, while the Badaowan Formation primarily consists of adsorbed gas. The coal seams in the Baijiahai Uplift are generally deep and thick, and the coal samples from the Xishanyao and Badawan formations have a high vitrinite content, which contributes to their strong gas generation capacity. Additionally, low moisture and ash contents enhance the adsorption capacity of the coal seams, facilitating the storage of CBM. The pore-specific surface area of the coal samples is primarily provided by micropores, which is beneficial for CBM adsorption. Furthermore, a fault connecting the Carboniferous and Permian systems (C-P) developed in the northeastern part of the Baijiahai Uplift allows gas to migrate into the Xishanyao and Badaowan formations, resulting in a higher gas content in the coal seams. The roof lithology is predominantly mudstone with significant thickness, effectively reducing the dissipation of coalbed methane and promoting its accumulation.

Research on Coal Reservoir Pore Structures: Progress, Current Status, and Advancing

Research on Coal Reservoir Pore Structures: Progress, Current Status, and Advancing

Molecular and Pore Structures of Coal on CO2 Hydrate Formation: Insights from the Adsorption-Hydrate Hybrid Process

Molecular and Pore Structures of Coal on CO₂ Hydrate Formation: Insights from the Adsorption-Hydrate Hybrid Process

Modification of pore structure of coal under hot steam injection as revealed by nuclear magnetic resonance

Heat injection provides a feasible approach for the extremely efficient extraction of coalbed gas. Injecting hot steam can effectively improve the pore structure of coal and increase the permeability of coal. To observe the changes in the pore structure of coal during hot steam injection, low magnetic field nuclear magnetic resonance technology is used to study the variations in the pore structure of coal under different heat injection durations. The results show that hot steam can promote the formation, growth, and expansion of coal pore fissures, thereby enhancing the gas permeability of the coal seam. At the same time, the analysis of relevant nuclear magnetic parameters indicates that when the heat injection duration is 15 minutes, hot steam has the best effect on coal modification. In the early stage of hot steam injection, hot steam stimulates the development of the porous structure. In the middle stage, some pore structures collapse and get blocked due to local thermal stress. In the late stage of hot steam injection, hot steam accelerates the conversion of micropore and mesopore structures into macropore or fissure structures, and hot steam has a significant modification effect on coal.

Influence of Microwave-Assisted Supercritical Carbon Dioxide Treatment on the Pore Structure of Low-Rank Coal.

CO2 injection into coal seams not only enhances coalbed methane (CBM) extraction but also allows for CO2 sequestration. Microwave irradiation is considered to be an effective technology to enhance CBM extraction. In this paper, the effects of microwave irradiation and supercritical CO2 immersion on the pore structure of low-rank coals were investigated by scanning electron microscopy (SEM), mercury-in-pressure (MIP), low-temperature nitrogen adsorption (LTN2GA), and carbon dioxide isothermal adsorption/desorption (CO2IA/D) of coal samples. The results showed that the macropores and micropores of the coal samples were more developed after microwave irradiation. After carbon dioxide immersion, the coal samples showed huge fissures, and the meso- and micropores were reduced. In contrast, microwave-assisted carbon dioxide not only reduced the specific surface area in the meso- and microporous stages and decreased the adsorption sites of methane but also enhanced the pore connectivity in the macroporous stage instead of the appearance of huge fissures. This study illustrates the potential of microwave-assisted supercritical carbon dioxide for enhanced coalbed methane extraction and carbon dioxide sequestration.

Characterization of coal pore structures and flow simulation based on CT and experiments of MIP and SEM

Characterization of pore structures and flow simulation are essential to spatial migration of coalbed methane. In this paper, coal samples from Xinjing and Xinzhuang coal mines are collected and a comprehensive characterization method by using computed tomography (CT) technique and experiments of mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) is proposed. Pore structure parameters are finely characterized and analyzed. Moreover, the flow process for coal samples is visualized and flow characteristics are analyzed. The result shows that pore structures of coal samples are strongly heterogeneous and are dominated by micropores, and filled with kaolinite, calcium monofluorophosphate, pyrite, etc. The flow velocity increases obviously in micropores and narrow throats. The flow is greatly affected by the pore structure. This study is of practical significance for the coalbed methane recoverability evaluation and exploration.

Effects of indigenous microorganisms on the CO2 adsorption capacity of coal

The adsorption capacity of coal is a crucial criterion for CO2 sequestration in deep unminable coal seams. Previous studies have documented how indigenous microorganisms interact with supercritical carbon dioxide (ScCO2) in coal. But how these interactions affect the adsorption capacity were rarely investigated. In this study, we treated bituminous coal samples with ScCO2-H2O and ScCO2-H2O-microorganism interactions in high-pressure reactors for 120 days at 35 °C and 10 MPa, respectively. After the treatments, we characterized both coal pore types and structures. The comparisons between two different post-treatments indicate that no significant change in coal pore types but notable alterations in pore structures are observed. Specifically, the treated coal with microorganisms exhibits a significant increase in micropore specific surface area (SSA) and surface roughness than the one without them. However, changes in meso/macropore structures and overall pore complexity of the coal sample treated with microorganisms were less pronounced than in coal sample treated with ScCO2-H2O alone. FTIR spectra analysis indicated that the absorption peak intensity of oxygen-containing groups and aliphatic functional groups in the sample treated with microorganisms decreased much more significantly than the sample treated without microorganisms. The CO2 adsorption capacity slightly increased in all treated samples at equilibrium pressures below 2.5 MPa. Above this pressure, the coal sample treated with microorganisms maintained a higher CO2 adsorption capacity, while the sample treated without microorganisms exhibited lower CO2 adsorption capacity compared to untreated coal sample. The increased SSA and the reduced oxygen-containing functional groups of coal after the treatment exerted antagonistic effects on CO2 adsorption. These findings suggest that indigenous microorganisms in coal seams affect CO2 adsorption through antagonistic effects, necessitating site-specific microbial assessments.

Moisture dependence of responses in physical–chemical property and CH4 ad-/desorption capability of coals to microwave radiation

For purpose of understanding feasibility of coalbed methane (CBM) production in practical water (H2O)-containing reservoirs by using microwave radiation, the dielectric property highly associated with microwave-absorbing and resultant temperature-rising capabilities of coals were addressed primarily; afterward, the temperature-rising rule of coals and its dependence on moisture were examined; finally, the impacts of moisture on changes about main physical–chemical property and resultant CH4 ad-/desorption performance of coals were investigated under microwave field. Results indicated that the coals display microwave-absorbing potential to raise temperature. The moisture initially increases the temperature-rising rate owing to its strong microwave-absorbing capability while decreases the final bulk temperature because of its endothermal evaporation. The resultant rising temperature decomposes and transforms minerals. Moreover, the temperature rise upgrades crystallite structure of the dried coals. As for the wetted coals, the rapid temperature rise disorders crystallite structure of the Sihe and Yima coals due to potential steam explosion. Furthermore, the radiation decreases the aromaticity and the total surface oxygenic groups of the dried coals; the moisture whittles down these decreasing trends. In general, the micro- and mesopore surface area and volume of the coals drops after radiation. The microwave radiation smooths pore surface of the wetted coals while roughens that of the dried coals. The complexity of the coal pore structure almost reduces after radiation. Besides, the microwave radiation significantly reduces the CH4 adsorption capability of the dried coals by 6.74–36.69 %; the moisture further reduces the CH4 adsorption capability by 20.48–43.03 %. Ultimately, the microwave radiation almost enhances the ad-/desorption hysteresis of CH4 on the dried coals while weakens that of the wetted coals. Such alterations depend on the responses of pore abundance and pore surface roughness to microwave. These findings confirm that microwave radiation is a promising option to produce CBM, particularly for H2O-bearing CBM reservoirs.

Study on I/III mixed fracture characteristics of coal in different regions of China

The fracture mechanical property of coal rock constitutes an important mechanical constraint for hydraulic fracturing of coal reservoir, and its three-dimensional fracture characteristics hold great significance for coal-bed methane mining. In this study, non-invasive methods were employed to investigate the pore structure and permeability of coal, aiming to understand the gas storage, distribution, and mobility of coal from various regions. To assess the influence of geological stress on coal during coal-bed methane mining, a three-point bending test was conducted to determine the characteristics of coal I/III mixed fractures. The experimental results indicate that with the decrease of the modal mixing parameter Me, the fracture area expends, the fracture load Pf and fracture energy Ef increase, while the fracture toughness declines. The 3D fracture criterion is utilized to predict the toughness ratio of coal rock I/III mixed fracture. The 3D-MTS and 3D-MMTSN criteria offer the upper and lower limits of prediction respectively. The 3D-MTSED criterion proves to be a more appropriate fracture criterion for analyzing the 3D fracture.

Effects of surface tension and contact angle on pore structure development of coal samples under chemical solution erosion

To explore the influence of surfactant concentration on the pore structure and permeability of coal samples during the chemical enhancement of coalbed methane production, different kinds and different concentrations of surfactants were added to the chemical solution, and the coal samples were soaked. Methods such as low-field nuclear magnetic resonance testing (NMR), fractal theory, permeability testing, surface tension testing, and contact angle testing were employed to analyze the variation patterns of coal sample pore structure, fractal characteristics, and permeability, and to explore the correlation between surface tension, contact angle, and the degree of pore structure development. The results show that the increase in total porosity of coal samples, the increase in the seepage pore porosity, the decrease in Dt, and the growth rate of permeability increase with the increase in surfactant concentration, and are negatively correlated with the surface tension of the solution and the contact angle of the coal-solution interface, while the decrease in Ds is not significantly correlated with surfactant concentration, surface tension, or contact angle. In terms of the erosion effect of a chemical solution on coal samples, the influence of contact angle is greater than that of surface tension, while surface tension has the greatest impact on the development of adsorption pores. By adding different surfactants, the surface tension of the chemical solution and the contact angle of the coal-solution interface can be controlled, further promoting the erosion of coal samples, which is of positive significance for the chemical enhancement of coalbed methane production.

Supercritical Methane Adsorption Characteristics and Pore Structure Characteristics of Deep Middle Rank Coal

Abstract In order to accurately analyze the adsorption characteristics, pore structure, and fractal law of deep middle rank coal, high-pressure isothermal adsorption tests based on magnetic levitation high-pressure thermobalance and pore structure tests based on mercury intrusion method were carried out on coal samples collected from typical kilometer level deep wells. The fractal law of sponge model was studied. The results showed that: a. The weight method adsorption test showed “excess adsorption” phenomenon with the increase of equilibrium pressure, and the modified Gibbs surface excess adsorption theory was in line with the Langmuir adsorption model; b. The distribution of macropore volume in deep middle rank coal accounts for a dominant proportion of 57.44% to 62.47%, and the proportion of microporous specific surface area reaches 72.96% to 74.27%, indicating that microporous adsorption capacity is the strongest; c. The pore structure parameters and adsorption constants exhibit strong nonlinear characteristics.

Scale-span feature of surface analysis, macromolecular and pore structure in coal: Implications for inherent evolutionary mechanism of morphological microstructure

Morphological microstructural evolution of tectonic coal determines CBM storage and transportation mechanism as well as the occurrence of coal and gas outburst. Scale-span morphological feature of original and tectonic coals was characterized by AFM, Raman spectrum and physisorption method. The results demonstrate that metamorphism could remold the surface property of coal from rugged band to flat porous structure with pore form factor from 0.572 to 0.831, promoting the formation of regular and round pores. Tectonism facilitates ability of gas sorption in coal, breaking the basic structural integrity. Metamorphism could facilitate the ordered macromolecular structural evolution on recombination and aromatization as well as condensation; besides, tectonism may promote the evolution and development of microcrystalline structure in advance. The aforementioned results have revealed essential differences in morphological microstructure between original and tectonic coals, which are of great guiding significance to safe mining of CBM and prediction of coal and gas outburst.

Influence of Supercritical Conditions on Isothermal Adsorption Capacity Calculation of Methane and Model Optimization.

The study of isothermal adsorption models for coal and methane under supercritical conditions helps us to understand the gas content distribution in coal seams and improve gas management in coal mines. Coal samples from the Hebi mine and Longshan coal mine were selected for this research. Using the weight method, isothermal adsorption curves of supercritical methane at temperatures of 308, 313, and 318 K were measured. The adsorption phase density of supercritical methane was obtained by using the intercept method and model fitting, and the absolute adsorption capacity of methane was calculated. Simultaneously, the pore volume and specific surface area of the coal samples were tested, and the isothermal adsorption model for supercritical methane was studied and optimized. The results indicate that using mercury intrusion, low-temperature N2, and low-temperature CO2 adsorption methods for testing coal sample pore structures allows for multiscale characterization of the coal pore structure. Based on the Gibbs excess adsorption theory, the phase density of methane obtained from the intercept method and model fitting effectively represent the isothermal adsorption curve of supercritical methane. By combination of the specific surface area and pore volume distribution of the coal with the absolute adsorption capacity of methane, the number of methane adsorption layers on the coal surface was calculated to be between 1.11 and 1.3 layers. This suggests that the isothermal adsorption model for supercritical methane on coal is not solely micropore filling or single molecular layer adsorption but primarily single molecular layer adsorption with concurrent micropore filling adsorption. Based on this adsorption mechanism, an L-DA isothermal adsorption model for supercritical methane on coal was established. The L-DA isothermal model shows good fitting results and effectively explains the adsorption characteristics of supercritical methane on coal.

Thermal index parameter investigation of coal–oxygen adsorption during low-temperature oxidation of coal

Oxidative spontaneous combustion is an inherent property of coal. In which, coal oxygen adsorption heat is the decisive speed step of coal spontaneous combustion self-acceleration process, and coal oxygen adsorption heat is closely related to coal pore structure. By utilising an automatic physicochemical adsorption instrument, it was found that the form of pore existence in coal was dominated by mesopores, that the pore structure, oxygen uptake capacity and temperature varied non-linearly, and that pore size was positively correlated with pore volume; Using a self–built air adsorption calorimetric experimental system, it was found that when oxygen was involved in the adsorption reaction, the presence of oxygen would lead to a significant time lag in the adsorption equilibrium. Additionally, the transition temperature of coal adsorption mode has been determined to be 40 °C, shedding light on the mechanism behind the thermal self-accelerating reaction of coal oxygen adsorption; Using the correlation method, it was obtained that the index affecting the physical adsorption heat release is the percentage of mesopore volume in the coal, while the index affecting the chemical adsorption heat release is the pore size. The research findings are anticipated to offer theoretical backing for advancing flame-resistant materials.

A study on the impact of ultrasonic-stimulated clean fracturing fluid on the pore structure of medium to high rank coal

The pore structure of coal plays a key role in the effectiveness of gas extraction. Conventional hydraulic fracturing techniques have limited success in modifying the pore structure using clean fracturing fluid (CFF), and the stimulating effects of ultrasonic can enhance the effectiveness of CFF in modifying coal pore structures. To research the effects of ultrasonic stimulation on the pore structure of medium to high-rank coal when using CFF, this study employed mercury intrusion porosimetry (MIP) and low-temperature nitrogen adsorption (LT-N2A) methods to analyze the changes in pore structures after cooperative modification. The results indicate that the pore volume and surface area of medium to high rank coal exhibit an increase and followed by a decrease with increasing Ro,max values, while the average pore diameter and permeability demonstrate a decrease and followed by an increase with Ro,max. Although there are some variations in the results of MIP and LT-N2A analysis for different pore size ranges, the overall findings suggest that ultrasonic stimulation in conjunction with CFF effectively alters the coal pore structure. The most significant improvement was observed in coking coal, where pore volume increased by 22%, pore area decreased by 11% and tortuosity decreased by 47%. The improvement of lean coal is the smallest, the pore volume increases by about 7%, and the surface area decreases by about 14%. It is found that the modification of coal pore volume is mainly concentrated in transition pores and macropores. These research outcomes provide valuable insights into the application of ultrasonic technology in coalbed gas extraction.

The Pore Structure Multifractal Evolution of Vibration-Affected Tectonic Coal and the Gas Diffusion Response Characteristics

Vibrations caused by downhole operations often induce coal and gas outburst accidents in tectonic zone coal seams. To clarify how vibration affects the pore structure, gas desorption, and diffusion capacity of tectonic coal, isothermal adsorption-desorption experiments under different vibration frequencies were carried out. In this study, high-pressure mercury intrusion experiments and low-pressure liquid nitrogen adsorption experiments were conducted to determine the pore structures of tectonic coal before and after vibration. The pore distribution of vibration-affected tectonic coal, including local concentration, heterogeneity, and connectivity, was analyzed using multifractal theory. Further, a correlation analysis was performed between the desorption diffusion characteristic parameters and the pore fractal characteristic parameters to derive the intrinsic relationship between the pore fractal evolution characteristics and the desorption diffusion characteristics. The results showed that the vibration increased the pore volume of the tectonic coal, and the pore volume increased as the vibration frequency increased in the 50 Hz range. The pore structure of the vibration-affected tectonic coal showed multifractal characteristics, and the multifractal parameters affected the gas desorption and diffusion capacity by reflecting the density, uniformity, and connectivity of the pore distribution in the coal. The increases in the desorption amount (Q), initial desorption velocity (V0), initial diffusion coefficient (D0), and initial effective diffusion coefficient (De) of the tectonic coal due to vibration indicated that the gas desorption and diffusion capacity of the tectonic coal were improved at the initial desorption stage. Q, V0, D0, and De had significant positive correlations with pore volume and the Hurst index, and V0, D0, and De had negative correlations with the Hausdorff dimension. To a certain extent, vibration reduced the local density regarding the pore distribution in the coal. As a result, the pore size distribution was more uniform, and the pore connectivity was improved, thereby enhancing the gas desorption and diffusion capacity of the coal.

Effect of [C12mim][Cl]-NaCl compound solutions on pore structure and wetting characteristics of bituminous coal based on Pearson correlation coefficient

Water injection technology is a compelling technical measure to improve the water content of coal seams and reduce dust generation during mining. At present, the methods to enhance the wetting effect in the research progress are limited to increasing the dosage of a single wetting agent and using a variety of anionic and cationic wetting agents in large doses. More research is needed on the synergistic enhancement of the wetting effect by inorganic salts and ionic liquids. Aiming at this problem, this paper studies the influence of the combination of NaCl and surface active ionic liquid [C12mim][Cl] on the wetting effect and pore structure of bituminous coal. The influence of composite solution and ratio change on coal pore structure and wetting characteristics is studied based on Pearson correlation coefficient, combined with the experimental results of nitrogen adsorption at low temperature, scanning electron microscope and wetting characterization. The results show that bituminous coal exhibits different sizes of hysteresis rings after [C12mim][Cl] and NaCl treatments. With the change in composite ratio, the pore connectivity of coal samples is improved, and the surface roughness of modified bituminous coal is affected by the shift in composite ratio. With the enhancement of the wetting effect, the pore depth increases. On the other hand, adding 0.6 mol/L NaCl makes the composite solution reach a critical micelle concentration, which could synergistically enhance the surface activity and wetting effect of [C12mim][Cl] solution. There is a negative correlation between the volume of bituminous coal macropores and the contact angle, which indicates that the optimal solution ratio can be determined by analyzing the bituminous coal macropore volume change.

The effects of supercritical CO2 transient high-pressure impact on coal pore structure characteristics

The impact of supercritical CO2 transient high-pressure fracturing on coal pore structure is studied here. This examination uses a CO2 fracturing test platform to obtained coal samples at fracturing pressures of 22.6, 26.7, and 30.6 MPa, and we investigated the effects of CO2 transient high-pressure impacts on the pore structure of the coal by means of low-temperature N2 adsorption experiments and CO2 adsorption experiments. The results demonstrate that the specific surface area of the coal samples increased by 60.4%, 200.7%, and 92.6%, and the cumulative total pore volume increased by 56%, 267%, and 77.8% under the pressure impacts of 22.6, 26.7, and 30.6 MPa, with a significant increase in the number of pores. The original pore morphology of coal can be changed by the supercritical CO2 transient high-pressure impact, and the creation of new pores across the whole pore diameter section can be catalyzed. The impact fracturing on the pore structure is mainly attributed to the impact of supercritical CO2 and extraction. The meso-pores and macro-pores of the coal are further expanded by the impact of supercritical CO2, while the micro-pores with chemical properties are primarily modified by the extraction. An impact pressure of 26.7 MPa has a more pronounced effect on the expansion of meso-pores and macro-pores, and its effect on micro-pores is less significant compared to that of the other two fracturing samples. Therefore, it is possible that a specific fracturing pressure can more effectively expand meso-pores and macro-pores while reducing the impact on micro-pores.

Effect of Coal Rank and Coal Facies on Nanopore-Fracture Structure Heterogeneity in Middle-Rank Coal Reservoirs.

Considerable variations in microscopic and industrial components (ash, moisture, volatile matter, etc.) have been reported within identical coal seams. These disparities in coal quality and pore structure within the same coal seam profoundly affect the drainage of deep coalbed methane (DCBM). This study focuses on 22 coal samples collected from two wells in the Benxi Formation of the central and eastern parts of the Ordos Basin. First, the coal facies were determined for all samples using submicroscopic components, and then, the adsorption pore and seepage pore structures were studied through CO2/N2 adsorption and mercury intrusive tests. Subsequently, the study delves into the correlation between coal rank, coal facies, and the distribution of the pore structures across various pore sizes, elucidating the primary controlling factors influenced by coal rank and coal phase. The results are as follows: (1) For a given coal seam, R o,max exhibits minimal variation among the samples, which suggests R o,max is not the primary factor affecting pore structure. Conversely, the ash content occupies the pore space, thereby revealing a negative correlation between the ash content and adsorption pore volume (PV). (2) On the basis of the texture preservation index (TPI) and gelatification index (GI), coal facies were classified into moist forest swamp facies (type A), moist herbaceous swamp facies (type B), and water-covered herbaceous swamp facies (type C). Type A is characterized by higher TPI, lower GI, and ash content, whereas type C exhibits lower TPI, higher GI, and ash content. (3) Type A samples, with the lowest ash content, display larger PV and specific surface area (SSA) compared with type B, while type C has the lowest values. Type C, with the highest vitrinite content, predominantly consists of semibright and bright coal, prone to microcracks, which results in a higher seepage PV compared with types A and B. (4) The coal facies represent variations in ash content and microscopic components, which significantly impacts both adsorption and seepage pores. Moist forest swamp facies samples are characterized by micropore development and the highest content of adsorbed gas. Herbaceous swamp facies samples display macropore development and the highest content of free gas.