Search Results (29)

In situ physical coal fracturing is one of the key technologies for deep coal resource extraction, among which the liquid nitrogen cyclic freeze–thaw (LNCFT) technique demonstrates remarkable fracturing effects and promising application potential in physical coal breaking. To determine economically viable mining and coalbed methane (CBM) extraction cycles, this study builds on previous research and conducts a series of experiments to investigate the effects of different cold condition temperatures and freeze–thaw cycles on the mesoscopic surface structure and macroscopic mechanical properties of deep, water-rich coal-rock samples. A statistical damage constitutive model for saturated coal-rock under coupled freeze–thaw and loading, incorporating a damage threshold, was established to more accurately describe the damage patterns and mechanisms. The results indicate that lower cold condition temperatures lead to greater mesoscopic crack propagation, lower uniaxial compressive strength, and significantly reduced freeze–thaw failure cycles. Under −45 °C, saturated coal-rock samples experienced macroscopic failure after only 23 freeze–thaw cycles, which is 9 and 15 cycles fewer than those under −30 °C and −15 °C, respectively. Furthermore, measurements of wave velocities in three directions before and after testing revealed that freeze–thaw cycles caused particularly pronounced damage in the direction perpendicular to the bedding planes. Additionally, the established coupled statistical damage constitutive model provides a more accurate and intuitive analysis of the entire process from damage to failure under different cold conditions, showing that as the temperature decreases and freeze–thaw cycles increase, the coal-rock’s brittleness diminishes while plastic deformation and ductile failure characteristics are enhanced. In summary, for coal and CBM extraction using the LNCFT technique, it is recommended to extract gas once after approximately 35 cycles of liquid nitrogen injection. This study provides a theoretical basis for the application of liquid nitrogen cyclic freeze–thaw technology in deep coal fracturing. Full article

Coal mine gas with methane concentrations below 8% cannot sustain stable self-combustion, posing significant challenges for safe utilization and greenhouse gas mitigation. To address this limitation, we developed a large-scale industrial square rotary regenerative thermal oxidizer (RTO) capable of high-efficiency oxidation under ultra-low methane conditions. This work integrates multi-scale computational fluid dynamics (CFD) modeling, laboratory and pilot-scale physical experiments, and multi-physics coupled simulations to capture the complex interactions of fluid flow, species transport, and thermal response in regenerative ceramics. Compared with conventional circular or three-bed RTOs, the proposed square rotating design achieves 13% higher heat storage utilization, 15% smaller floor area, and enhanced spatial uniformity of the temperature field. Multi-scale simulations reveal that increasing methane molar fraction (CH₄) from 0.012 to 0.017 raises the peak temperature from 1280 K to 1350 K, reduces the burnout height from 1.18 m to 1.15 m, and, under constant oxygen supply, extends the high-temperature zone to 1450 K with a stabilized burnout position at 1.06 ± 0.01 m. Incorporating a 15° conical expansion combustion chamber increases local turbulent kinetic energy by 17.4%, accelerating oxidation while maintaining methane removal rates > 98% within an optimized bottom blowing time of 30–90 s. This study not only provides validated design thresholds for ultra-low concentration methane oxidation—such as temperature windows, buffer zones, and switching cycles—but also offers an engineering framework for scaling RTO systems to industrial coal mine applications. This advances both energy recovery efficiency and methane emission control, demonstrating clear advantages over existing RTO configurations. Full article

A polyurethane slurry was developed to simultaneously enhance the strength and permeability of geological formations, differing from the conventional fracture grouting used for soft-soil reinforcement. Injected via splitting grouting, the slurry cures to form high-strength, highly permeable channels that increase reservoir permeability while improving mechanical stability (dual-enhanced stimulation). To quantify its diffusion behavior and guide field application, we built a splitting-grouting model using the finite–discrete element method (FDEM), parameterized with the reservoir properties of coalbed methane (CBM) formations in the Ordos Basin and the slurry’s measured rheology and filtration characteristics. Considering the stratified structures within coal rock formed by geological deposition, this study utilizes Python code interacting with Abaqus to divide the coal seam into coal rock and natural bedding. We analyzed the effects of engineering parameters, geological factors, and bedding characteristics on slurry–vein propagation patterns, the stimulation extent, and fracturing pressure. The findings reveal that increasing the grouting rate from 1.2 to 3.6 m³/min enlarges the stimulated volume and the maximum fracture width and raises the fracturing pressure from 26.28 to 31.44 MPa. A lower slurry viscosity of 100 mPa·s promotes the propagation of slurry veins, making it easier to develop multiple veins. The bedding-to-coal rock strength ratio controls crossing versus layer-parallel growth: at 0.3, veins more readily penetrate bedding planes, whereas at 0.1 they preferentially spread along them. Raising the lateral pressure coefficient from 0.6 to 0.8 increases the likelihood of the slurry expanding along the beddings. Natural bedding structures guide directional flow; a higher bedding density (225 lines per 10,000 m³) yields greater directional deflection and a more intricate fracture network. As the angle of bedding increases from 10° to 60°, the slurry veins are more susceptible to directional changes. Throughout the grouting process, the slurry veins can undergo varying degrees of directional alteration. Under the studied conditions, both fracturing and compaction grouting modes are present, with fracturing grouting dominating in the initial stages, while compaction grouting becomes more prominent later on. These results provide quantitative guidance for designing dual-enhanced stimulation to jointly improve permeability and mechanical stability. Full article

The low efficiency of the microbial gasification of coal limits the application of bio-coal bed methane technology. The co-fermentation of coal and biomass provides a new approach for improving the degradation rate of coal. In this study, a co-fermentation system comprising five different coal orders with five microalgae was constructed in the laboratory, and the methanogenic characteristics of coal–algae co-fermentation and its microbiological mechanism were systematically investigated in terms of gas production, soluble organic matter, and microbial community characteristics. The results showed that the combination of lignite and Nannochloropsis exhibited optimal methane production, with a methane yield of 26.43 mL/g coal. Biogenic methane yields for lignite–Porphyra and anthracite–Porphyra were 23.43 mL and 21.28 mL, respectively, demonstrating the potential for algae to enhance gas production even in high-rank coals. pH monitoring revealed that algal species played a critical role in the acidification process. Dunaliella caused a continuous pH decrease, reaching 3.76 by day 30, while Nannochloropsis maintained a neutral pH of 6.95, optimizing the fermentation environment. Significant differences in soluble organic matter were observed between the lignite and anthracite fermentation systems, with lignite systems producing more volatile fatty acids, including acetic and butyric acids. Microbial community analysis revealed that Methanosarcina, an acetic acid-utilizing methanogen, was dominant in lignite and anthracite systems, while Syntrophomonas played a key role in lignite–Nannochloropsis co-fermentation. These findings provide valuable insights into optimizing coal microbial gasification and selecting appropriate algal species to enhance methane production efficiency. Full article

Deep coalbed methane (CBM) reservoirs are characterized by high hydrocarbon content and are considered an important strategic resource. Due to their inherently low permeability and porosity, horizontal well drilling is commonly employed to enhance production, with the length of the horizontal section playing a critical role in determining CBM output. However, during extended horizontal drilling, wellbore instability frequently occurs as a result of drilling fluid invasion into the coal formation, posing significant safety challenges. This instability is primarily caused by the physical intrusion of drilling fluids and their interactions with the coal seam, which alter the mechanical integrity of the formation. To address these challenges, interpenetrating and semi-interpenetrating network (IPN/s-IPN) hydrogels have gained attention due to their superior physicochemical properties. This material offers enhanced sealing and support performance across fracture widths ranging from micrometers to millimeters, making it especially suited for plugging applications in deep CBM reservoirs. A self-degradable interpenetrating double-network hydrogel particle plugging agent (SSG) was developed in this study, using polyacrylamide (PAM) as the primary network and an ionic polymer as the secondary network. The SSG demonstrated excellent thermal stability, remaining intact for at least 40 h in simulated formation water at 120 °C with a degradation rate as high as 90.8%, thereby minimizing potential damage to the reservoir. After thermal aging at 120 °C, the SSG maintained strong plugging performance and favorable viscoelastic properties. A drilling fluid containing 2% SSG achieved an invasion depth of only 2.85 cm in an 80–100 mesh sand bed. The linear viscoelastic region (LVR) ranged from 0.1% to 0.98%, and the elastic modulus reached 2100 Pa, indicating robust mechanical support and deformation resistance. Full article

A quantitative characterization of the change in coal molecular structures with different cyclical microwave modification parameters and a better understanding of the reaction mechanism of the modification are of great significance for the commercial extraction of coal bed methane (CBM). Therefore, long-flame coal samples obtained from the Ordos Basin, China, were modified by microwave radiation with different times, and the long-flame coal molecular structure parameters were determined by solid-state ¹³C nuclear magnetic resonance (ss¹³C NMR), Fourier transform infrared (FTIR) spectrometry, and X-ray photoelectron spectrometry (XPS). Atomistic representations of the raw long-flame coal molecular model and modified long-flame coal molecular models were established. The temperature rise, pore volume increase, mineral removal, and functional group changes after the modification have a negative effect on methane adsorption. After the modification, the decrease in surface area of the micropores reduced the adsorption site of methane in coal. As a result, the methane adsorption amount decreased linearly with the decreasing surface area. The CH₄ adsorption isotherms of the long-flame models were dynamically simulated and analyzed. The results of this study can prove that after multiple cycles of microwave modifications, the functional groups in long-flame coal were fractured, and the number of micropores was reduced, which effectively decreased the methane adsorption performance in long-flame coal seams, thereby promoting methane extraction. Microwave modification is a promising method for enhancing CBM recovery. Full article

The study block is located on the eastern edge of the Ordos Basin and is one of the typical medium coalbed methane blocks in China that have previously been subjected to exploration and development work. The rich CBM resource base and good exploration and development situation in this block mean there is an urgent need to accelerate development efforts, but compared with the current situation for tight sandstone gas where development is in full swing in the area, the production capacity construction of CBM wells in the area shows a phenomenon of lagging to a certain degree. In this study, taking the 4 + 5 coal seam of the YJP block in the Ordos Basin as the research object, we carried out technical research on an integrated program concerning CBM geology and engineering and put forward a comprehensive seismic geology analysis method for the prediction of the CBM content. The study quantitatively assessed the tectonic conditions, depositional environment, and coal seam thickness as potential controlling factors using gray relationship analysis, trend surface analysis, and seismic geological data integration. The results show that tectonic conditions, especially the burial depth, residual deformation, and fault development, are the main controlling factors affecting the coalbed methane content, showing a strong correlation (gray relational value greater than 0.75). The effects of the depositional environment (sand–shale ratio) and coal bed thickness were negligible. A weighted fusion model incorporating seismic attributes and geological parameters was developed to predict the gas content distribution, achieving relative prediction errors of below 15% in validation wells, significantly outperforming traditional interpolation methods. The integrated approach demonstrated enhanced spatial resolution and accuracy in delineating the lateral CBM distribution, particularly in structurally complex zones. However, limitations persist due to the seismic data resolution and logging data reliability. This method provides a robust framework for CBM exploration in heterogeneous coal reservoirs, emphasizing the critical role of tectonic characterization in gas content prediction. Full article

Deep coalbed methane (CBM) reservoirs hold substantial resource potential and play a crucial role in China’s unconventional natural gas development. However, the vertical propagation behavior of hydraulic fractures in deep CBM formations remains inadequately understood, posing challenges for optimizing fracturing parameters to control fracture height growth and enhance fracture development within the coal seam. To address this, this study establishes numerical simulation models to investigate hydraulic fracture propagation in directional wells, incorporating three typical lithological combinations representative of deep CBM reservoirs. Through these models, the influence mechanisms of bedding density, stress ratio, rock friction coefficient, and fracturing parameters on vertical fracture propagation and post-fracture productivity were systematically analyzed. The results reveal that the fracture propagation characteristics vary significantly with lithological combinations. Initially, hydraulic fractures penetrated adjacent formations near the wellbore while simultaneously generating branched fractures, leading to the formation of a complex fracture network. As propagation continues, branch fractures exhibited reduced width compared to the primary fracture. Well-developed bedding planes in the roof or floor, combined with lower stress ratios and friction coefficients, effectively constrained vertical fracture growth. Furthermore, optimizing fracturing fluid volume, reducing injection rate, and lowering proppant concentration promoted fracture development within the coal seam, thereby enhancing post-fracture well productivity. These findings provide a theoretical foundation for the optimization of hydraulic fracturing strategies in deep CBM reservoirs, contributing to more effective reservoir stimulation and resource recovery. Full article

To address the critical challenges of wellbore instability in deep coal seam drilling operations, this investigation developed an innovative organic–inorganic composite nanosealing agent (NS) through chemical modification of nano-silica. Advanced characterization techniques including Fourier Transform Infrared Spectroscopy, laser particle size analysis, and Scanning Electron Microscopy revealed that the optimized NS possessed a uniform particle distribution (mean diameter 86 nm) and enhanced surface hydrophobicity, effectively mitigating particle agglomeration. Systematic experimental evaluation demonstrated the material’s multifunctional performance: the NS-enriched drilling fluid achieved an 88.7% reduction in sand bed invasion depth and 76.4% decrease in filtrate loss at optimal concentration. Notably, comparative inhibition tests showed the NS outperformed conventional KCl and KPAM inhibitors, achieving 91.2% shale rolling recovery rate and 65.3% lower swelling rate than deionized water baseline. Core flooding experiments further confirmed superior sealing capability, with 2% NS addition attaining 88% sealing efficiency for low-permeability cores (0.5 mD) and establishing a 10 MPa breakthrough pressure threshold. Field implementation in the SSM1 well at Shenmu Huineng Liangshui Coal Mine validated the technical efficacy, the NS-enhanced drilling fluid system achieved 86.7% coal seam encounter rate with zero wellbore collapse incidents, while core recovery rate improved by 32.6% to 90.4% compared to conventional systems. This research breakthrough provides a scientific foundation for developing next-generation intelligent drilling fluids, demonstrating significant potential for ensuring drilling safety and enhancing gas recovery efficiency in deep coalbed methane reservoirs. 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

Coal is a typical dual-porosity structural material. The injection of CO₂ into coal seams has been shown to be an effective method for storing greenhouse gasses and extracting coal bed methane. In light of the theory of dual-porosity media, we investigate the impact of non-homogeneity on seepage anisotropy and examine the influence of CO₂ gas injection on the anisotropy of coal and the permeability of fractures. The results demonstrate that under constant pressure conditions, coal rock has the greatest permeability variation in the direction of face cleats and the smallest changes in the direction of vertical bedding. The more pronounced the heterogeneity, the more evident the change in permeability and the less pronounced the decreasing stage of permeability. Additionally, the larger the diffusion coefficient is, the less pronounced the permeability change. The change in permeability is inversely proportional to the size of the adsorption constant and directly proportional to the size of the fracture. As the matrix block size increases, the permeability also increases, whereas the decrease in permeability becomes less pronounced. The findings of this study offer a theoretical basis for further research into methods for enhancing the CO₂ sequestration rate. 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

The application of the hydrodynamic method has enhanced the extraction of coal bed methane (CBM). In this method, fracturing fluid rapidly penetrates the coal reservoir, altering its intrinsic pore structure and microscopic mechanical properties. These changes impact the properties of the coal reservoir and CBM depletion. It is, therefore, crucial to explore how these micro-characteristics evolve following water invasion. In this context, using nanoindentation tests, the microscopic characteristics of three coal samples were measured under dry conditions and at water saturations corresponding to 44% and 75% relative humidity. The influence of water immersion on the pore structure was also assessed using mercury injection experiments. Moreover, cluster analysis was used to categorize the extensive measured data into three sub-components: fractures (large pores), inertinite, and vitrinite, to investigate the impact of water saturation on microscopic properties. The findings indicate that cluster analysis is well-suited to these data, showing excellent agreement with porosity and maceral tests. The relationship between the elastic modulus and hardness of dry and wet coal samples varies across the sub-components. There is a notable dependency in the case of vitrinite, whereas water content tends to reduce this dependency. It is also found that water content negatively affects elastic modulus and hardness and reduces the anisotropy ratio. The mechanical properties of inertinite are highly responsive to water immersion, whereas vitrinite exhibits lesser sensitivity. The softening mechanisms of coal when immersed in water, such as calcite phase dissolution, swelling stress fracturing, and weakening of macerals, are identified. This study offers new perspectives on the impact of moisture on the alteration of micromechanical properties in coal. Full article

Fishbone drilling (FbD) involves drilling multiple micro-holes branching out in various directions from the primary vertical or deviated wellbore. FbD is similar to multilateral micro-hole drilling and can be employed to boost hydrocarbon production in naturally fractured formations or during refracturing operations by connecting existing natural fractures. Key design elements in fishbones include determining the number, length, and spacing between the branches, and the angle at which the branches deviate from the main borehole. Fishbone wells have emerged as a promising technology for improving well performance and reducing environmental impact. In this paper, we present a comprehensive review of the different applications of fishbone wells in conventional and renewable energy systems. We discuss the potential of fishbone wells for enhanced oil and gas recovery, as well as their application in unconventional resources such as coal bed methane. Moreover, we examine the feasibility of fishbone wells in renewable energy systems, such as geothermal energy and carbon capture, utilization, and storage (CCUS). We highlight the various benefits of fishbone wells, including reduced carbon footprint, enhanced efficiency, and increased sustainability. Finally, we discuss the challenges and limitations associated with fishbone wells in different energy systems. This review provides a comprehensive overview of the potential and challenges of fishbone wells in reducing carbon footprint and improving well performance in a wide range of energy systems. Full article