Search Results (43)

Water within coal reservoirs exerts dual effects on methane adsorption–desorption by competing for adsorption sites and reducing permeability. The bound water effect, caused by coal wettability, significantly constrains coalbed methane (CBM) production, rendering investigations into coal wettability crucial for efficient CBM development. Compared with other geological formations, coals are characterized by a highly developed microporous structure, making the CO₂ sequestration mechanism in coal seams closely linked to the microscale interactions among gas, water, and coal matrixes. However, the intrinsic mechanisms remain poorly understood. In this study, molecular dynamics simulations are employed to investigate the wettability behaviors of CO₂, CH₄, and water on different coal matrix surfaces under varying temperature and pressure conditions, for coal macromolecules representative of four coal ranks. The study reveals the evolution of water wettability in response to CO₂ and CH₄ injection, identifies wettability differences among coal ranks, and analyzes the microscopic mechanisms governing wettability. The results show the following: (1) The contact angle increases with gas pressure, and the variation in wettability is more pronounced in CO₂ environments than in CH₄. As pressure increases, the number of hydrogen bonds decreases, while the peak gas density of CH₄ and CO₂ increases, leading to larger contact angles. (2) Simulations under different temperatures for the four coal ranks indicate that temperature has minimal influence on low-rank Hegu coal, whereas for higher-rank coals, gas adsorption on the coal surface increases, resulting in reduced wettability. Interfacial tension analysis further suggests that higher temperatures reduce water surface tension, cause dispersion of water molecules, and consequently improve wettability. Understanding the wettability variations among different coal ranks under variable pressure–temperature conditions provides a fundamental model and theoretical basis for investigating deep coal seam gas–water interactions and CO₂ geological sequestration mechanisms. These findings have significant implications for the advancement of CO₂-ECBM technology. Full article

More accurate prediction of CO₂/CH₄ adsorption selectivity coefficients in the CO₂ Enhanced Coal Bed CH₄ Recovery (CO₂-ECBM) project can help to judge the CO₂ adsorption concentration and the desorption purity of CH₄ during the CO₂ injection process, and to achieve the maximization of CO₂ sequestration as well as the optimization of the CH₄ recovery rate. To this end, a coal molecular slit model with 16 sizes including micro-, meso-, and macropores was constructed in this study, and the competitive adsorption characteristics of CO₂ and CH₄ gas mixtures in bituminous coal molecules were investigated using molecular dynamics and giant canonical Monte Carlo simulations. The CO₂/CH₄ adsorption selectivity coefficients (S_c) as a function of gas ratio, gas pressure, pore size, and temperature were analyzed using a large amount of adsorption isotherm data. Based on the simulation results, considering the neglect of pressure and component changes when calculating the adsorption selectivity coefficient using the traditional extended Langmuir (E-L) model, a correction term regarding the pressure of the mixed gas and the mole fraction of CO₂ is set, and a modified equation is proposed. The results show that the adsorption potential energy of CO₂ is significantly higher than that of CH₄, giving it an absolute advantage in the competition. Through multiple regression analysis, the ranking of the influence weights of the four factors on S_c is as follows: pore size > mixed gas pressure > molar fraction of CO₂ > temperature. The negative exponential function can describe the variation of S_c with four factors. The fitting degree between the modified prediction model and the S_c data obtained through simulation reaches 0.84, and the model effect is good. The research results provide theoretical guidance for the optimization of gas injection parameters in the CO₂-ECBM project. Full article

CO₂-enhanced coalbed methane recovery (CO₂-ECBM) represents a promising pathway within carbon capture, utilization, and storage (CCUS) technologies, offering dual benefits of methane production and long-term CO₂ sequestration. This review provides a comprehensive analysis of CO₂-ECBM from a full-chain perspective (Mechanism, Practices, and Outlook), covering fundamental mechanisms and key engineering practices. It highlights the complex multi-physics processes involved, including competitive adsorption–desorption, diffusion and seepage, thermal effects, stress responses, and geochemical interactions. Recent progress in laboratory experiments, capacity assessments, site evaluations, monitoring techniques, and numerical simulations are systematically reviewed. Field studies indicate that CO₂-ECBM performance is strongly influenced by reservoir pressure, temperature, injection rate, and coal seam properties. Structural conditions and multi-field coupling further affect storage efficiency and long-term security. This work also addresses major technical challenges such as real-time monitoring limitations, environmental risks, injection-induced seismicity, and economic constraints. Future research directions emphasize the need to deepen understanding of coupling mechanisms, improve monitoring frameworks, and advance integrated engineering optimization. By synthesizing recent advances and identifying research priorities, this review aims to provide theoretical support and practical guidance for the scalable deployment of CO₂-ECBM, contributing to global energy transition and carbon neutrality goals. Full article

The CO₂ injection technology for replacing CH₄ to enhance coalbed methane (CBM) recovery (CO₂-ECBM) offers dual benefits, i.e., reducing CO₂ emissions through sequestration and increasing CBM recovery, thereby leading to economic gains. However, there is no clear consensus on how temperature and pressure affect the competitive adsorption characteristics of CO₂ and CH₄ mixed gases in coal. Therefore, the competitive adsorption behavior of CO₂ and CH₄ mixed gases at various pressures and temperatures were investigated using the breakthrough curve method. Anthracite was selected for the adsorption experiment conducted under three gas injection pressure levels (0.1 MPa, 0.5 MPa, and 1 MPa) and at three temperature levels (20 °C, 40 °C, and 60 °C). This study showed that, when the temperature remained constant and the pressure ranged from 0.1 to 1 MPa, the adsorption rates of CO₂ and CH₄ increased as pressure rose. Additionally, the selectivity coefficient for CO₂/CH₄ decreased with an increase in pressure, suggesting that higher pressures within this range are not conducive to the replacement efficiency of CH₄ by CO₂. As the temperature increased from 20 to 60 °C under constant pressure conditions, both the selectivity coefficients for CO₂/CH₄ and the adsorption rates of CO₂ and CH₄ exhibited a downward trend. These findings imply that, within this temperature range, a reduced temperature improves the ability of CO₂ to efficiently displace CH₄. Moreover, CO₂ exhibits a higher isosteric heat of adsorption compared to CH₄. Full article

The interactions among water molecules, coal beds, and gases during the process of coal bed methane mining are highly complex. The water and methane (CH₄)/carbon dioxide (CO₂) molecules compete for adsorption and undergo a series of reactions that affect gas diffusion. In this study, Monte Carlo and molecular dynamics methods were used to investigate the microscopic mechanism of CH₄/CO₂ competitive adsorption and diffusion during CO₂-enhanced coal bed methane mining (ECBM) under different moisture contents, and the geological storage potential of CO₂ was predicted. The results showed that when the CO₂ and water binding sites were independent of each other, the water molecules changed the electrostatic potential around the coal molecules, resulting in enhanced CO₂ adsorption performance, as verified by the surface electrostatic potential. When the water molecules formed a water molecule layer, the adsorption capacity of the secondary adsorption sites provided was larger than that of the surface of the coal molecules, so the CO₂ molecules were preferentially adsorbed on the secondary adsorption sites. However, the number of secondary adsorption sites available was not as large as that on the surface of the coal molecules. The interaction energies revealed that when the displacement effect of CH₄ in the process of CO₂-ECBM and the sequestration effect of CO₂ were considered comprehensively, the best CO₂ sequestration effect and a good CH₄ displacement effect were obtained at a 3% moisture content. The worst CO₂ sequestration effect was found at a 5% moisture content. After CO₂ injection, the main adsorption layer of CH₄ shifted from X = 5 and X = 9 to X = 8.7 and X = 12.5, respectively, and obvious detachment and diffusion occurred. The distribution of the molecular motion and diffusion coefficient revealed the considerable displacement and dispersion of the gas molecules. The distribution of the gas molecular velocity and diffusion coefficient indicated that a 3% moisture content was the ideal condition for CO₂ displacement of CH₄, and the CO₂ sequestration effect was good. Full article

In order to reduce CO₂ emissions from industrial processes, countries have commenced the vigorous development of CCUS (carbon capture, utilization and storage) technology. The high geographical overlap between China’s extensive coal mining regions and CO₂-emitting industrial parks provides an opportunity for the more efficient reduction in CO₂ emissions through the development of Enhanced Coal Bed Methane (ECBM) Recovery for use with CCUS technology. Furthermore, the high geographical overlap and proximity of these regions allows for a shift in the transportation mode from pipelines to tanker trucks, which are more cost-effective and logistically advantageous. The issue of transportation must also be considered in order to more accurately assess the constructed cost function and CCUS source–sink matching model for the implementation of ECBM. The constructed model, when considered in conjunction with the actual situation in Shanxi Province, enables the matching of emission sources and sequestration sinks in the province to be realized through the use of ArcGIS 10.8 software, and the actual transport routes are derived as a result. After analyzing the matching results, it is found that the transportation cost accounts for a relatively small proportion of the total cost. In fact, the CH₄ price has a larger impact on the total cost, and a high replacement ratio is not conducive to profitability. When the proportion of CO₂ replacing CH₄ increases from 1 to 3, the price of CH₄ needs to increase from $214.41/t to $643.23/t for sales to be profitable. In addition, electric vehicle transportation costs are lower compared to those of fuel and LNG vehicles, especially for high-mileage and frequent-use scenarios. In order to reduce the total cost, it is recommended to set aside the limitation of transportation distance when matching sources and sinks. Full article

Injecting CO₂ into coal seams to enhance coal bed methane (ECBM) recovery has been identified as a viable method for increasing methane extraction. This process also has significant potential for sequestering large volumes of CO₂, thereby reducing the concentration of greenhouse gases in the atmosphere. However, for deep coal seams where formation pressure is relatively high, there is limited research on CO₂ injection into systems with higher methane adsorption equilibrium pressure. Existing studies, mostly confined to the low-pressure stage, fail to effectively reveal the impact of factors such as temperature, high-pressure CO₂ injection, and coal types on enhancing the recovery and sequestration of CO₂-displaced methane. Thus, this study aims to investigate the influence of temperature, pressure, and coal types on ECBM recovery and CO₂ sequestration in deep coal seams. A series of CO₂ core flooding tests were conducted on various coal cores, with CO₂ injection pressures ranging from 8 to 18 MPa. The CO₂ and methane adsorption rates, as well as methane displacement efficiency, were calculated and recorded to facilitate result interpretation. Based on the results of these physical experiments, numerical simulation was conducted to study multi-component competitive adsorption, desorption, and seepage flow under high temperature and high pressure in a deep coal seam’s horizontal well. Finally, the optimization of the total injection amount (0.7 PV) and injection pressure (approximately 15.0 MPa) was carried out for the plan of CO₂ displacement of methane in a single well in the later stage. Full article

Coalbed methane (CBM), recognized as a sustainable and environmentally friendly energy source, plays a crucial role in mitigating global climate change and advancing low-carbon energy solutions. However, the prevalence of low-permeability coal seams poses a significant challenge to effective CBM extraction. Improving coal permeability has emerged as a viable strategy to address the issue of low-permeability coal. Conventional CBM stimulation methods fall short in overcoming this obstacle. In contrast, the enhanced technique of CBM extraction by water-based ultrasonic cavitation holds great promise due to its use of high energy intensity, safety, and efficiency. Nevertheless, the inadequate theoretical framework for managing this technology impedes its widespread adoption for large-scale applications. This study investigated the impact of water-based ultrasonic cavitation treatment on coal’s properties and permeability through mechanical testing and permeability measurements conducted before and after treatment. This study also explored the process by which this technology, known as WUC-ECBM, improves coal’s mechanical properties and permeability. The findings suggest a potential stimulation technique (WUC-ECBM) for use in CBM extraction, and its physical mechanism. Full article

Carbon Dioxide-Enhanced Coalbed Methane (CO₂-ECBM), a progressive technique for extracting coalbed methane, substantially boosts gas recovery and simultaneously reduces greenhouse gas emissions. In this process, the dynamics of coalbed fractures, crucial for CO₂ and methane migration, significantly affect carbon storage and methane retrieval. However, the extent to which fracture roughness, under the coupled thermal-hydro-mechanic effects, impacts engineering efficiency remains ambiguous. Addressing this, our study introduces a pioneering, cross-disciplinary mathematical model. This model innovatively quantifies fracture roughness, incorporating it with gas flow dynamics under multifaceted field conditions in coalbeds. This comprehensive approach examines the synergistic impact of CO₂ and methane adsorption/desorption, their pressure changes, adsorption-induced coalbed stress, ambient stress, temperature variations, deformation, and fracture roughness. Finite element analysis of the model demonstrates its alignment with real-world data, precisely depicting fracture roughness in coalbed networks. The application of finite element analysis to the proposed mathematical model reveals that (1) fracture roughness ξ markedly influences residual coalbed methane and injected CO₂ pressures; (2) coalbed permeability and porosity are inversely proportional to ξ; and (3) adsorption/desorption reactions are highly sensitive to ξ. This research offers novel insights into fracture behavior quantification in coalbed methane extraction engineering. Full article

CO₂-enhanced coalbed methane recovery (CO₂-ECBM) has been demonstrated as an effective enhanced oil recovery (EOR) technique that enhances the production of coalbed methane (CBM) while achieving the goal of CO₂ sequestration. In this paper, the grand canonical Monte Carlo simulation is used to investigate the dynamic mechanism of CO₂-ECBM in anthracite pores. First, an anthracite pore containing both organic and inorganic matter was constructed, and the adsorption and diffusion characteristics of CO₂ and CH₄ in the coal pores under different temperature and pressure conditions were studied by molecular dynamics (MD) simulations. The results indicate that the interaction energy of coal molecules with CO₂ and CH₄ is positively associated with pressure but negatively associated with temperature. At 307.15 K and 101.35 kPa, the interaction energies of coal adsorption of single-component CO₂ and CH₄ are −1273.92 kJ·mol⁻¹ and −761.53 kJ·mol⁻¹, respectively. The interaction energy between anthracite molecules and CO₂ is significantly higher compared to CH₄, indicating that coal has a greater adsorption capacity for CO₂ than for CH₄. Furthermore, the distribution characteristics of gas in the pores before and after injection indicate that CO₂ mainly adsorbs and displaces CH₄ by occupying adsorption sites. Under identical conditions, the diffusion coefficient of CH₄ surpasses that of CO₂. Additionally, the growth rate of the CH₄ diffusion coefficient as the temperature increases is higher than that of CO₂, which indicates that CO₂-ECBM is applicable to high-temperature coal seams. The presence of oxygen functional groups in anthracite molecules greatly influences the distribution of gas molecules within the pores of coal. The hydroxyl group significantly influences the adsorption of both CH₄ and CO₂, while the ether group has a propensity to impact CH₄ adsorption, and the carbonyl group is inclined to influence CO₂ adsorption. The research findings are expected to provide technical support for the effective promotion of CO₂-ECBM technology. Full article

The technology of CO₂ geological storage and CH₄ intensive mining (CO₂-ECBM) in coal seams integrates greenhouse gas emission reduction and new fossil energy development and has great development prospects. The CO₂ injection, CO₂ sequestration mechanism and storage capacity, and CH₄ stimulation effect constitute the core content of the effectiveness of CO₂-ECBM, among which CO₂ injection is the most critical. Traditional seepage analysis methods often struggle to tackle flow-related issues influenced by microscale effects and intricate channels. This paper highlights the advantages of employing lattice Boltzmann (LBM) numerical simulations to study CO₂ seepage behaviors when teaching a Rock Mass Seepage Mechanics Course. This course primarily covers topics such as the pore structure of rock, unstable liquid seepage, gas seepage theory and related subjects. Its goal is to provide students with a solid theoretical foundation to address the complexities of fluid seepage in pours media encountered in practical scenarios. A novel LBM-based methodology was employed to estimate the CO₂ seepage capacity by incorporating the effects of different concentrations of [Bmin]Cl solution (0 wt%, 1 wt%, 3 wt%, and 5 wt%). The CO₂ velocity distribution cloud map of each coal sample was simulated; the average velocity distribution curve of each coal sample was obtained; and the velocity profile of the seepage channel of each coal sample was described. This study can provide theoretical guidance for the technology of CO₂ geological storage and CH₄ intensive mining in coal seams. Full article

The present work aims to analyse the influence of present-day burial depths of coal seams on the sorption properties towards CH₄ and CO₂, respectively. For medium-rank coals located in the southwestern area of the Upper Silesian Coal Basin (USCB), the gravimetric sorption measurements were carried out with pure gases at a temperature of 30 °C. The variability of CO₂/CH₄ exchange sorption and diffusivity ratios was determined. It was revealed that in coal seams located at a depth above 700 m, for which the sorption exchange ratio was the greatest, the process of CO₂ injection for permanent storage was more beneficial. In the coal seams lying deeper than 700 m with a lower CO₂/CH₄ sorption ratio, the CH₄ displacement induced by the injection of CO₂ (CO₂-ECBM recovery) became more favourable. Full article

As a clean energy source, coalbed methane (CBM) produces almost no exhaust gas after combustion, and its extraction and efficient utilization play a key role in supporting sustainable development. Therefore, molecular dynamics simulations were used to research the diffusion of CH₄ in coal after injecting CO₂/N₂ in different sequences and to clarify the efficiency of CBM extraction under different injection sequences, so as to contribute to sustainable development. The results show that the adsorption amounts of CO₂ and N₂ in different injection sequences are obviously different. To narrow the gap between the two injection amounts, the injection pressure of N₂ can be appropriately increased and that of CO₂ can be reduced, or N₂ can be injected preferentially instead of CO₂. When CO₂ is injected first, the interaction energy between CH₄ and coal is stronger and increases slightly with displacement time as a whole. The interaction energy curve of the N₂ injection decreases, and the displacement effect becomes worse and worse. From the diffusion and relative concentration distribution of CH₄, it can be seen that the diffusion of CH₄ molecules outside the grain cell is more obvious when N₂ is injected first. In terms of the number of CH₄ molecules diffusing outside the crystal cell, it is less when CO₂ is injected first than when N₂ is injected first. The average value of the velocity distribution of CH₄ increases slightly when CO₂ is injected first and decreases significantly when N₂ is injected first, but the average value is overall higher for N₂ injection first. From the difference in diffusion coefficients before and after the gas injection, it can be seen that the decrease in permeability due to the expansion of the coal matrix by CO₂ is more obvious than the increase in permeability due to the contraction of the coal matrix by N₂. Full article

The tectonically deformed coal (TDC) reservoirs with abundant gas resources and low permeability are expected to become one of the target coal seams for carbon dioxide geological storage-enhanced coalbed methane recovery (CO₂-ECBM). The pore–fracture structure plays a crucial role in determining the effectiveness of CO₂ storage. Fractal analysis provides a valuable approach to quantitatively describe the complex and heterogeneous pore–fracture structures across various scales in coal matrixes. Accordingly, the TDC samples in the Huainan–Huaibei coalfield and primary-undeformed coal (PUC) samples in the Qinshui Basin were selected for pore–fracture structure parameter tests using the mercury intrusion porosimetry (MIP) and low–temperature nitrogen adsorption (LNA) methods. Their multiscale pore–fracture parameters were analyzed using different fractal methods based on pore diameter. According to the fractal results, a multiscale classification standard for pore–fracture structures was devised in this study that is suitable for the controlling gas migration process. A parameter of 8 nm is set as the separating pore diameter for gas migration and storage. It was observed that the connectivity of migration pores (>8 nm) in TDC samples was stronger compared to PUC samples, reflected in larger pore volumes and smaller fractal dimensions. However, its complex development of seepage pores (150–300 nm) may hinder the flow of CO₂ injection. As for the storage pores (<8 nm), the fractal dimension of the 2–8 nm pores in TDC was found to be similar to that of PUC but with larger pore volumes. The fractal dimension of the filling pores (<2 nm) in TDC samples was relatively lower, which facilitates efficient gas volume filling. Therefore, the pore–fracture structure of the TDC samples is found to be more advantages for CO₂ injection and storage compared to the PUC. This suggests that TDC reservoirs holds promising geological potential for CO₂-ECBM implementation. Full article

The dissolution of supercritical carbon dioxide (ScCO₂) in water forms a ScCO₂–H₂O system, which exerts a transformative influence on the physicochemical characteristics of coal and significantly impacts the CO₂-driven enhanced coalbed methane (CO₂-ECBM) recovery process. Herein, the effect of ScCO₂–H₂O treatment on the physicochemical properties of coal was simulated in a high-pressure reactor. The migration of major elements, change in the pore structure, and change in the CH₄ adsorption capacity of coal after the ScCO₂–H₂O treatment were detected using plasma emission spectroscopy, the low-temperature liquid nitrogen adsorption method, and the CH₄ adsorption method, respectively. The results show that (1) the ScCO₂–H₂O treatment led to mineral reactions causing a significant migration of constant elements in the coal. The migration of Ca ions was the most significant, with an increase in their concentration in treated water from 0 to 16–970 mg·L⁻¹, followed by Na, Mg, and K. Al migrated the least, from 0 to 0.004–2.555 mg·L⁻¹. (2) The ScCO₂–H₂O treatment increased the pore volume and pore-specific surface area (SSA) of the coal via the dissolution and precipitation of minerals in the coal pores. The total pore volume increased from 0.000795–0.011543 to 0.001274–0.014644 cm³·g⁻¹, and the total pore SSA increased from 0.084–3.332 to 0.400–6.061 m²·g⁻¹. (3) Changes in the CH₄ adsorption capacity were affected by the combined effects of a mineral reaction and pore structure change. The dissolved precipitates of the minerals in the coal pores after the ScCO₂–H₂O treatment caused elemental migration, which not only decreased the mineral content in the coal pores but also increased the total pore volume and total pore SSA, thus improving the CH₄ adsorption capacity of the coal. This study provides theoretical support for CO₂ sequestration and ECBM recovery. Full article