Search Results (28)

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

As an effective technology to reduce carbon dioxide emissions, carbon capture, utilization, and storage (CCUS) technology has been a major strategic choice and has received widespread attention. Meanwhile, the high cost and strict requirements of carbon dioxide storage and utilization on geographical conditions, industrial equipment, and other aspects limit large-scale applications of CCUS. Taking Shanxi Province as an example, in this paper, we study the economic and environmental characteristics of carbon dioxide capture, storage, and utilization under different combinations of technical routes. Steel, power, cement, and chemical industries are considered. Deep saline aquifers and CO₂-enhanced coalbed methane (CO₂-ECBM) recovery are selected as the two types of sequestration sinks. Urea production, methanol production, microalgae cultivation, and cement curing are selected as the four potential utilization methods. Then, a mixed-integer linear programming (MILP) model is used to optimize the CO₂ utilization pathway based on the principle of least cost, to select the best emission sources, CO₂ pipelines, intermediate transportation nodes, utilization, and storage nodes to achieve reasonable deployment of CCS/CCU projects in Shanxi Province. The results show that CCU with urea production has the lowest cost and is the most economically viable with over 50% reduction in emissions. The second option is CCS which includes CO₂-ECBM and achieves a 50% reduction in emissions. In addition, there is little difference between the cost of cement-cured CCU and that of methanol-produced CCU. CCU for microalgae cultivation has the highest cost. Therefore, the latter three utilization pathways are currently not economical. Full article

CO${}_2$ injection technology into coal seams to enhance CH${}_4$ recovery (CO${}_2$-ECBM), therefore presenting the dual benefit of greenhouse gas emission reduction and clean fossil energy development. In order to gaze into the features of CO${}_2$ injection’s influence on reservoir pressure and permeability, the Thermal-Hydraulic-Mechanical coupling mechanism of CO${}_2$ injection into the coal seam is considered for investigation. The competitive adsorption, diffusion, and seepage flowing of CO${}_2$ and CH${}_4$ as well as the dynamic evolution of fracture porosity of coal seams are considered. Fluid physical parameters are obtained by the fitting equation using MATLAB to call EOS software Refprop. Based on the Canadian CO${}_2$-ECBM project CSEMP, the numerical simulation targeting shallow low-rank coal is carried out, and the finite element method is used in the software COMSOL Multiphysics. Firstly, the direct recovery (CBM) and CO${}_2$-ECBM are compared, and it is confirmed that the injection of CO${}_2$ has a significant improvement effect on methane production. Secondly, the influence of injection pressure and temperature is discussed. Increasing the injection pressure can increase the pressure difference in the reservoir in a short time, so as to improve the CH${}_4$ production and CO${}_2$ storage. However, the increase in gas injection pressure will also lead to the rapid attenuation of near-well reservoir permeability, resulting in the weakening of injection capacity. Also, when the injection temperature increases, the CO${}_2$ concentration is relatively reduced, and the replacement effect on CH${}_4$ is weakened, resulting in a slight decrease in CBM production and CO${}_2$ storage. Full article

Significant stress changes caused by sorption-induced swelling raise the coal wellbore failure potential, which directly impacts the safety and sustainability of CO₂ enhanced coalbed methane (CO₂-ECBM). Additionally, a mixture gas (CO₂/N₂) injection is recommended due to the sharp decline of permeability with pure CO₂ injection. In this study, incorporating the impacts of mixture gas adsorption and poroelastic effects, a semi-analytical model of coal wellbore stability during mixture gas injection is proposed. Model results indicate that the stress field is significantly influenced by the boundary condition and sorption effect. In addition, parametric studies are performed to determine the influence of adsorption parameters, mechanical properties, and gas composition on the stress distribution and then on the wellbore failure index. Furthermore, mixture gas injection with a large proportion of CO₂ or N₂ both cause wellbore instability. Significant compressive hoop stress and shear failure are caused by the mixture gas injection with a large proportion of CO₂. In contrast, the displacement of CH₄ with weakly adsorptive N₂ will result in less compressive and even tensile hoop stress, so shear or tensile failure may occur. Thus, mixture gas (including pure CO₂/N₂) injection must be controlled by coal wellbore failure, providing an accurate estimation of in-situ coal seams’ CO₂ storage capacity from the perspective of wellbore stability. Full article

Coal maceral composition has a great effect on gas adsorption and diffusion. The interaction between maceral composition and supercritical CO₂ (SCCO₂) fluid will affect gas diffusion behavior in coals. Thus, the diffusivity derived from adsorption kinetics of CH₄ and CO₂ in vitrinite- and inertinite-rich coals with low-violate bituminous rank collected from the Hancheng mine of the Weibei coalfield pre- and post-SCCO₂ fluid exposure (SFE) were tested at the conditions of 45 °C and 0.9 MPa. In combination with pore distribution and functional group content, the possible mechanism of the alterations in gas diffusion characteristics in coals with various maceral compositions was addressed. The results show that for vitrinite-rich coals, SFE increases the macropore apparent diffusion coefficient of CH₄, while this treatment decreases the micropore apparent diffusion coefficient of CH₄. However, the reverse trend is found for CO₂ diffusion–adsorption rate. For inertinite-rich coals post-SFE, CH₄ diffusion–adsorption rate increases, while an increase and a decrease in diffusivity CO₂ occur for macropore and micropore, respectively. Generally, SFE shows a stronger impact on CO₂ adsorption rate than CH₄ in coals. The results suggest that the diffusion of CH₄ and CO₂ in coals with different maceral compositions show selectivity to SCCO₂ fluid. The possible reason can be attributed to the changes in pore structure and surface functional group content. SFE causes an increase in macro/mesopore volume of all samples. However, SFE induces a reduction in oxygen-containing species content and micropore volume of inertinite-rich coals, while the opposite trend occurs in vitrinite-rich coals. Thus, the changes in pore volume and surface functional group account for the difference in gas diffusivity of coals with different maceral compositions. With regard to the micropore diffusion–adsorption behavior of CH₄ and CO₂, the impact of oxygen-containing species is superior to pore volume. The oxygen-containing species favor CO₂ diffusion–adsorption but go against CH₄ transport. This effect accounts for the reduction in the micropore diffusion–adsorption rate of CH₄ and the increase in micropore diffusivity of CO₂ in vitrinite-rich coals, respectively. However, the aforementioned effect is the opposite for inertinite-rich coals. Overall, the changes in gas diffusion in coals with different maceral composition during the CO₂-ECBM process requires further attention. Full article

The continuous temperature rise has raised global concerns about CO₂ emissions. As the country with the largest CO₂ emissions, China is facing the challenge of achieving large CO₂ emission reductions (or even net-zero CO₂ emissions) in a short period. With the strong support and encouragement of the Chinese government, technological breakthroughs and practical applications of carbon capture, utilization, and storage (CCUS) are being aggressively pursued, and some outstanding accomplishments have been realized. Based on the numerous information from a wide variety of sources including publications and news reports only available in Chinese, this paper highlights the latest CCUS progress in China after 2019 by providing an overview of known technologies and typical projects, aiming to provide theoretical and practical guidance for achieving net-zero CO₂ emissions in the future. Full article

Carbon dioxide (CO₂) is both a primary greenhouse gas and a readily available energy source. In this study, a new underground coal permeability enhancement technique utilizing cryogenic liquid CO₂ (L-CO₂) cyclic injection is proposed. The key parameters that determine the feasibility of this technique are cycle period and cycle number within a fixed working period. The optimal value of these two parameters mainly depends on the pore structure evolution law of coal cores before and after cryogenic L-CO₂ cyclic freeze–thaw. Accordingly, nuclear magnetic resonance (NMR) was employed to study the evolution characteristics of the fracture networks and pore structures in coal cores subjected to different freeze–thaw cyclic modes. The results demonstrated that the amplitude and width of all peaks of the T₂ spectra of the three coal cores (lignite, gas coal, and 1/3 coking coal) increased with an increase in the number of injection cycles. Additionally, as the number of freeze–thaw cycles (N_c) increased, the total porosity and effective porosity of the coal cores increased linearly before stabilizing, while the residual porosity first steadily diminished and afterwards became constant. Furthermore, the variation in the total porosity and residual porosity of the coal cores continuously diminished with an increase in the level of metamorphism. The NMR permeability of the coal cores showed a similar pattern to the porosity. Accordingly, the optimal parameters for cryogenic L-CO₂ cyclic injection during a complete working time were determined to be N_c = 4 and P_c = 30 min. A field test demonstrated that after L-CO₂ cyclic freeze–thaw treatment, the average gas drainage concentration of a single borehole in the test region increased by 1.93 times, while the pure flow of a single gas drainage borehole increased by 2.21 times. Finally, the gas attenuation coefficient decreased from 0.036 to 0.012. We concluded that the proposed permeability enhancement technique transformed coal seams from hard-to-drain to drainable. Full article

In this paper, heat injection and CO₂ injection are combined, and the influence of coal seam parameters on CO₂-ECBM is analyzed to improve the production of CH₄ and CO₂ reserves and the effective control of both greenhouse gases. A multi-physical field coupling model of CO₂-ECBM was established based on Darcy’s law, Fick’s law of diffusion, the extended Langmuir model for adsorption, and the equation of state. Numerical simulation of CO₂-ECBM under different coal seam parameters was carried out by COMSOL Multiphysics. The results show that increasing the injection pressure of the CO₂ injection well and the initial pressure of the coal seam can effectively increase the gas pressure and concentration gradient, which has a positive effect on improving the extraction concentration of CH₄ and the sequestration concentration of CO₂ in the coal seam. The increase of the initial temperature of the coal seam will promote the desorption and diffusion of the binary elemental gas, resulting in a decrease in the concentration of coalbed methane and a decrease in the displacement effect. In the process of displacement, the greater the initial permeability, the greater the fracture opening of the coal seam, which is more conducive to the seepage transport of the gas. The closer to the position of the injection well, the better the displacement effect and the lower the permeability rate ratio. Full article

Practice shows that CO₂/N₂-ECBM is an effective technology to enhance coalbed methane. However, there are few field tests in which the technology is applied to enhance the gas drainage in underground coal mines, and the effect is uncertain. In this study, firstly, the reasons for the decrease of gas drainage efficiency in the exhaustion period were analyzed based on the theory of fluid mechanics. Secondly, the mechanism of N₂ injection to enhance coal seam gas drainage (N₂-ECGD) was discussed: with the gradual decrease of gas pressure in the drainage process, coal seam gas enters a low-pressure state, the driving force of flow is insufficient, and the drainage enters the exhaustion period. The nitrogen injection technology has triple effects of “promoting flow”, “increasing permeability” and “replacing”. Thirdly, the numerical simulations of the nitrogen pressure on drainage effect were carried out based on the fully coupled model. The results show that the higher the nitrogen pressure, the greater the displacement effect between injection and drainage boreholes, the larger the effective range. Finally, a field test of N₂-ECGD was carried out in the Liu Zhuang coal mine in Huainan Coalfield, China. The results show that N₂ injection can significantly enhance the gas flow rate and CH₄ flow rate in the drainage boreholes, and the coal seam gas content decreased 39.73% during N₂ injection, which is about 2.6–3.3 times that of the conventional drainage. The research results provide an important guidance for promoting the application of N₂-ECGD in underground coal mines. Full article