Search Results (212)

Efficient coalbed methane (CBM) recovery combined with carbon dioxide (CO₂) sequestration is a promising strategy for sustainable energy production and greenhouse gas mitigation. However, the molecular mechanisms controlling pressure-dependent CH₄ displacement by CO₂ in coal nanopores remain insufficiently understood. In this study, molecular dynamics simulations were conducted to investigate CO₂-driven CH₄ recovery in a slit-pore coal model under driving pressures of 15, 20, and 25 Mpa. The simulations quantitatively captured the competitive adsorption, diffusion, and migration behaviors of CH₄, CO₂, and water, providing insights into how pressure influences enhanced coalbed methane (ECBM) recovery at the nanoscale. The results show that as the pressure increases from 15 to 25 Mpa, the mean residence time of CH₄ on the coal surface decreases from 0.0104 ns to 0.0087 ns (a 16% reduction), reflecting accelerated molecular mobility. The CH₄–CO₂ radial distribution function peak height rises from 2.20 to 3.67, indicating strengthened competitive adsorption and interaction between the two gases. Correspondingly, the number of CO₂ molecules entering the CH₄ region grows from 214 to 268, demonstrating higher invasion efficiency at elevated pressures. These quantitative findings illustrate a clear shift from capillary-controlled desorption at low pressure to pressure-driven convection at higher pressures. The results provide molecular-level evidence for optimizing CO₂ injection pressure to improve CBM recovery efficiency and CO₂ storage capacity. Full article

Coalbed-methane (CBM) extraction involves complex processes such as desorption, diffusion, and seepage, significantly increasing the difficulty of numerical simulation. To enable efficient CBM development, this study establishes an integrated simulation workflow for CBM, encompassing geological modeling, geomechanical modeling, hydraulic fracture simulation, and production dynamic simulation. Specifically, the unconventional fracture model (UFM), integrated within the Petrel commercial software, is applied for fracture simulation, with an unstructured grid constructing the CBM production model. Subsequently, based on the case study of well pad A in the Daning–Jixian block, the effects of well spacing and hydraulic fractures on gas production were analyzed. The results indicate that the significant stress difference between the coal seam and the top/bottom strata constrains fracture height, with simulated hydraulic fractures ranging from 169.79 to 215.84 m in length, 8.91 to 10.45 m in height, and 121.92 to 248.71 mD·m in conductivity. Due to the low matrix permeability, pressure drop and desorption primarily occur in the stimulated reservoir volume (SRV) region. The calibrated model predicts a 10-year cumulative gas production of 616 × 10⁴ m³ for the well group, with a recovery rate of 10.17%, indicating significant potential for enhancing recovery rates. Maximum cumulative gas production occurs when well spacing slightly exceeds fracture length. Beyond 200 mD·m, fracture conductivity has diminishing returns on production. Fracture length increases from 100 to 250 m show near-linear growth in production, but further increases yield smaller gains. These findings provide valuable insights for evaluating development performance and exploiting remaining gas resources for CBM. Full article

Carbon dioxide (CO₂)-enhanced coalbed methane recovery involves a complex process of mixed-gas adsorption, desorption, and diffusion–transport. The literature suggests that an appropriate range of CO₂ injection pressure and an optimal injection time window are critical for coal seams with varying reservoir conditions. That is, higher pressure and longer injection periods do not necessarily lead to better displacement performance. Therefore, in this study, experimental research was conducted on the time-varying characteristics of the displacement–replacement effect of CO₂-enhanced methane (CH₄) extraction from coal seams, and the following results were obtained. (1) The process of displacement–replacement of CH₄ by CO₂ in coal seams can be divided into five stages: a stage of spontaneous CH₄ desorption caused by partial-pressure effects, a replacement-dominated stage, a stage where replacement and displacement act jointly, a displacement-dominated stage, and a stabilization stage. (2) For all three coal samples (anthracite, coking coal, and long-flame coal), cumulative CH₄ desorption increases with increasing CO₂ injection pressure below 5 MPa and finally stabilizes. However, when CO₂ injection pressure exceeds 5 MPa, the effect weakens, possibly due to the dynamic changes in CO₂ partial pressure. (3) The displacement–replacement ratio decreases with increasing CH₄ equilibrium pressure. Additionally, the larger the difference between the CO₂ injection pressure and the CH₄ equilibrium pressure, the better the displacement–replacement effect. Full article

Hydraulic fracturing, slotting, and cavity creation are commonly employed techniques to enhance coalbed methane (CBM) development. To investigate the applicability of these three stimulation methods, this study proposes a mathematical method for constructing coal seams with orthogonal cleats, and integrates it with hydraulic fracturing, slotting, and cavity creation to generate three coal seam stimulation models. Considering desorption and diffusion of adsorbed gas, a coupled flow model of CBM in the coal matrix–cleat–hydraulic fracture/slot/cavity system was established. The results indicate that among the three stimulation methods, the stable production period and peak production resulting from slotting are less sensitive to cleat density, enhancing its applicability across diverse coal seams. Hydraulic fracturing yields a higher peak daily production than slotting, reaching 411 m³/d, but its stable production period is shorter, averaging 7.2 years, and its cumulative production is slightly lower than that of slotting when the cleat density is low. In coal seams with low cleat density, the stable production period and cumulative production of the cavity are inferior to those of hydraulic fracturing and slotting. However, when the cleat density is high, the production of cavity creation can reach the levels of hydraulic fracturing and slotting, with cumulative production of 78 × 10⁴ m³. Full article

The accurate prediction of single-well production performance necessitates considering the multiple factors influencing the dynamic changes in coal seam permeability during deep coalbed methane (CBM) extraction. This study focuses on Block D of the Ordos Basin. The Langmuir monolayer adsorption model was selected to describe gas adsorption behavior, and a productivity prediction model for deep CBM was developed by coupling multiple dynamic effects, including stress sensitivity, matrix shrinkage, gas slippage, and coal fines production and blockage. The results indicate that the stress sensitivity coefficients of artificial fracture networks and cleat fractures are key factors affecting the accuracy of CBM productivity predictions. Under accurate stress sensitivity coefficients, the predicted daily gas production rates of the productivity model for single wells showed errors ranging from 1.89% to 14.22%, with a mean error of 8.15%, while the predicted daily water production rates had errors between 0.35% and 17.66%, with a mean error of 8.68%. This demonstrates that the established productivity prediction model for deep CBM aligns with field observations. The findings can provide valuable references for production performance analysis and development planning for deep CBM wells. Full article

Moisture in coal seams significantly impacts methane adsorption/desorption, yet its microscopic mechanism in intact coal remains poorly characterized due to methodological limitations. This study introduces a novel approach that integrates low-field nuclear magnetic resonance (LF-NMR) with volumetric analysis to quantify, in real-time, the effect of moisture on methane dynamics in intact coal samples. The results quantitatively demonstrate that micropores (relative specific surface area > 700 m²/cm³) are the primary adsorption sites, accounting for over 95% of the stored gas. Moisture drastically reduces the adsorption capacity (by ~72% at 0.29 MPa and ~57% at 1.83 MPa) and inhibits the desorption process, evidenced by a strong linear decrease in desorption ratio (DR) (R² = 0.906) and a sharp exponential drop in the initial desorption rate (R² = 0.999) with increasing moisture content. The findings provide a mechanistic understanding that is crucial for optimizing coalbed methane (CBM) recovery and enhancing strategies for outburst prevention and methane emission mitigation. The results reveal distinct adsorption and desorption features of intact coal compared with coal powder, which can be useful in total methane utilization and mining safety enhancement. Full article

Deep coalbed methane (CBM) resources possess significant potential. However, their development is challenged by geological characteristics such as high in situ stress and low permeability. Furthermore, existing production strategies often prove inadequate. In order to achieve long-term stable production of deep coalbed methane reservoirs and increase their final recoverable reserves, it is urgent to construct a scientific and reasonable drainage system. This study focuses on the deep CBM reservoir in the Daning-Jixian Block of the Ordos Basin. First, a thermal–hydraulic–mechanical (THM) multi-physics coupling mathematical model was constructed and validated against historical well production data. Then, the model was used to forecast production. Finally, key control measures for enhancing well productivity were identified through production strategy adjustment. The results indicate that controlling the bottom-hole flowing pressure drop rate at 1.5 times the current pressure drop rate accelerates the early-stage pressure drop, enabling gas wells to reach the peak gas production earlier. The optimized pressure drop rates for each stage are as follows: 0.15 MPa/d during the dewatering stage, 0.057 MPa/d during the gas production rise stage, 0.035 MPa/d during the stable production stage, and 0.01 MPa/d during the production decline stage. This strategy increases peak daily gas production by 15.90% and cumulative production by 3.68%. It also avoids excessive pressure drop, which can cause premature production decline during the stable phase. Consequently, the approach maximizes production over the entire life cycle of the well. Mechanistically, the 1.5× flowing pressure drop offers multiple advantages. Firstly, it significantly shortens the dewatering and production ramp-up periods. This acceleration promotes efficient gas desorption, increasing the desorbed gas volume by 1.9%, and enhances diffusion, yielding a 39.2% higher peak diffusion rate, all while preserving reservoir properties. Additionally, this strategy synergistically optimizes the water saturation and temperature fields, which mitigates the water-blocking effect. Furthermore, by enhancing coal matrix shrinkage, it rebounds permeability to 88.9%, thus avoiding stress-induced damage from aggressive extraction. Full article

Coalbed methane (CBM), mainly composed of methane (CH₄) and carbon dioxide (CO₂), has attracted increasing attention due to its dual significance as a clean energy resource and its role in greenhouse gas management. This research systematically examines the adsorption, desorption, diffusion, and bubble evolution dynamics of methane (CH₄) and carbon dioxide (CO₂) in graphene nanopores with diameters of 4 nm, 6 nm, and 8 nm by molecular dynamics simulations. Radial distribution function (RDF) analyses reveal strong solvation of both gases by water, with CO₂ exhibiting slightly stronger interactions. Adsorption and desorption dynamics indicate that CO₂ molecules display shorter residence times on the graphene surface (0.044–0.057 ns) compared with CH₄ (0.055–0.062 ns), reflecting faster surface exchange. Gas-phase molecular number analysis demonstrates that CH₄ accumulates more significantly in the vapor phase, while CO₂ is more prone to adsorption and re-dissolution. Mean square displacement (MSD) results confirm enhanced molecular mobility in larger pores, with CH₄ showing greater overall diffusivity. Structural evolution of the 8 nm system highlights asymmetric bubble dynamics, where large bubbles merge with the upper adsorption layer to form a thicker layer, while smaller bubbles contribute to a thinner layer near the lower surface. CH₄ and CO₂ follow similar pathways, though CO₂ diffuses farther post-desorption due to its weaker surface retention. These results provide fundamental insights into confinement-dependent gas behavior in graphene systems, offering guidance for gas separation and storage applications. Full article

The Erdaoling Mining area, located in Inner Mongolia, NW China, is recognized for its considerable potential in coalbed methane (CBM) exploration and development. However, the complex structures in this region have significant influences on coal reservoir characteristics, particularly pore structure features. This study focuses on the No. 2 coal seam of the Middle Jurassic Yan’an Formation. Three structural patterns were classified based on the existing structural characteristics of the study area. Coal samples of No. 2 coal seam were collected from different structural positions, and were subjected to low-temperature CO₂ adsorption (LTCO₂A), low-temperature N₂ adsorption/desorption (LTN₂A), low-field nuclear magnetic resonance (LF-NMR), and scanning electron microscopy (SEM) experiments, so that the structural controlling effects on pore structure would be revealed. Quantitative analysis results indicate that in terms of asymmetric syncline, from the limb to the core, the total porosity and movable fluid porosity of the coal decreased by 1.47% and 0.31%, respectively, reaching their lowest values at the core. Meanwhile, the dominant pore type shifted from primarily one-end closed pores to “ink-bottle” pores, indicating increased pore complexity. In the fold-thrust structure, the micropore specific surface area, micropore volume, mesopore specific surface area, mesopore volume, and total porosity show clear correlations with variations in coal seam structure. These parameters all reach their maximum values in the fault-cut zone at the center of the syncline, measuring 268.26 m²/g, 0.082 cm³/g, 0.601 m²/g, 1.262 cm³/g, and 4.2%, respectively. Simple pore types, like gas pores and vesicular pores, were identified in the syncline limbs, while open pores, “ink-bottle” pores, and complex multiporous types were mainly developed at fault locations, indicating that faults significantly increase the complexity of coal reservoir pore types. For the broad and gentle syncline and small-scale reverse fault combination, porosity exhibits a decreasing trend from the syncline limbs toward the core. Specifically, the mesopore specific surface area and movable fluid porosity increased by 52.24% and 43.69%, respectively, though no significant effect on micropores was observed. The syncline core in this structural setting developed normal gas pore clusters and tissue pores, with no occurrence of highly complex or heterogeneous pore types, indicating that neither the broad gentle syncline nor the small-scale faulting significantly altered the pore morphology. Comparatively, the broad and gentle syncline and small-scale reverse fault combination was determined to exert the strongest modification on pore structures of coal reservoir, followed by the asymmetric syncline, while the broad syncline alone demonstrated minimal influence. Full article

To address the challenges of low permeability, high gas adsorption, and a fragile structure in coalbed methane reservoirs, this study developed a high-strength composite proppant with an activated carbon skeleton via nitric acid pretreatment, silica–alumina sol coating, and calcination. Orthogonal experiments optimized the preparation conditions: 30–40 mesh activated carbon, Si/Al molar ratio of 4:1, calcination at 650 °C for 2 h. The resulting proppant exhibited an excellent performance: a single-particle compressive strength of 55.5 N, porosity of 33.2%, crushing rate of only 2.3% under 50 MPa closure pressure, and permeability 48.5% higher than quartz sand. In simulated acidic coalbed environments (pH 3–5), its acid corrosion rate was <2.8%, and it enhanced methane desorption by 16.2% compared to pure coal. Additionally, the proppant showed a superior transport performance in fracturing fluids, with better distribution uniformity in fractures than ceramsite, and its hydrophobic surface (contact angle 115.32°) improved fracturing fluid flowback efficiency. This proppant integrates high strength, good conductivity, gas desorption promotion, and corrosion resistance, offering a novel material solution for efficient coalbed methane extraction. Full article

Chinese coalbed methane (CBM) reservoirs exhibit characteristically low recovery rates due to adsorbed gas dominance and “three-low” properties (low permeability, low pressure, and low saturation). CO₂ thermal drive (CTD) technology addresses this challenge by leveraging dual mechanisms—thermal desorption and displacement to enhance production; however, its effectiveness necessitates uniform fracture networks for temperature field homogeneity—a requirement unmet by conventional long-fracture fracturing. To bridge this gap, a coupled seepage–heat–stress–fracture model was developed, and the temperature field evolution during CTD in coal under non-uniform fracture networks was determined. Integrating multi-cluster fracture propagation with stress barrier and intra-stage stress differential characteristics, a stress-barrier-responsive diverting fracturing technology meeting CTD requirements was established. Results demonstrate that high in situ stress and significant stress differentials induce asymmetric fracture propagation, generating detrimental CO₂ channeling pathways and localized temperature cold islands that drastically reduce CTD efficiency. Further examination of multi-cluster fracture dynamics identifies stress shadow effects and intra-stage stress differentials as primary controlling factors. To overcome these constraints, an innovative fracture network uniformity control technique is proposed, leveraging synergistic interactions between diverting parameters and stress barriers through precise particle size gradation (16–18 mm targeting toe obstruction versus 19–21 mm sealing heel), optimized pumping displacements modulation (6 m³/min enhancing heel efficiency contrasted with 10 m³/min improving toe coverage), and calibrated diverting concentrations (34.6–46.2% ensuring uniform cluster intake). This methodology incorporates dynamic intra-stage adjustments where large-particle/low-rate combinations suppress toe flow in heel-dominant high-stress zones, small-particle/high-rate approaches control heel migration in toe-dominant high-stress zones, and elevated concentrations (57.7–69.2%) activate mid-cluster fractures in central high-stress zones—collectively establishing a tailored framework that facilitates precise flow regulation, enhances thermal conformance, and achieves dual thermal conduction and adsorption displacement objectives for CTD applications. Full article

In the race for industrialization and urbanization, the concentration of greenhouse gases like CO₂ and CH₄ is growing rapidly and ultimately resulting in global warming. An Ni-based catalyst over MgO support (Ni/MgO) offers a catalytic method for the conversion of these gases into hydrogen and carbon monoxide through the dry reforming of methane (DRM) reaction. In the current research work, 1–4 wt% strontium is investigated as a cheap promoter over a 5Ni/MgO catalyst to modify the reducibility and basicity for the goal of excelling the H₂ yield and H₂/CO ratio through the DRM reaction. The fine catalytic activities’ correlations with characterization results (like X-ray diffraction, surface area porosity, photoelectron–Raman–infrared spectroscopy, and temperature-programmed reduction/desorption (TPR/TPD)) are established. The 5Ni/MgO catalyst with a 3 wt.% Sr loading attained the highest concentration of stable active sites and the maximum population of very strong basic sites. 5Ni3Sr/MgO surpassed 53% H₂ yield (H₂/CO ~0.8) at 700 °C and 85% H₂ yield (H₂/CO ratio ~0.9) at 800 °C. These outcomes demonstrate the catalyst’s effectiveness and affordability. Higher Sr loading (>3 wt%) resulted in a weaker metal–support contact, the production of free NiO, and a lower level of catalytic activity for the DRM reaction. The practical and cheap 5Ni3Sr/MgO catalyst is scalable in industries to achieve hydrogen energy goals while mitigating greenhouse gas concentrations. Full article

Unconventional natural gas development requires a deeper insight into how CH₄ and CO₂ adsorb and diffuse in the pores of coal seams. Graphene (GRA) is frequently employed in microscopic mechanism simulations on coal surfaces because its structure closely resembles that of the coal seam matrix. In this study, molecular dynamics simulations were conducted to systematically investigate the diffusion, adsorption, and desorption behaviors of CH₄ and CO₂ within the pore system of hydrated graphene under three representative temperature and pressure conditions: 190 K-6 MPa, 298 K-0.1 MPa, and 320 K-8 MPa. The results show that heatinfg and depressurization significantly enhance the diffusion ability of gas molecules and promote their desorption from the graphene surface. Low temperature and high pressure are conducive to the formation of a stable adsorption layer, and more hydrogen bond structures are formed between CO₂ and water. However, under high-temperature conditions, this ordered structure is significantly weakened. The density distribution further reveals the spatial distribution characteristics of water molecules and gases and their evolution trends with changes in temperature and pressure. This research is conducive to a deeper understanding of the multiphase behavior of coalbed methane and its regulatory mechanism, providing theoretical support for the gas storage and displacement processes. Full article

CH₄ is a powerful greenhouse gas that is thought to be one of the main causes of global warming. The catalytic conversion of methane in the presence of oxygen into hydrogen-rich syngas, known as the partial oxidation of methane (POM), is highly appealing for environmental and synthetic concerns. In search of a cheap catalytic system, the Ni-supported MgO-based (5Ni/MgO) catalyst and the promotional supplement of 1–3 wt.% Sr over 5Ni/MgO are investigated for the POM reaction. Catalysts are characterized by N₂ sorption isotherm analysis, X-ray diffraction spectroscopy, Raman spectroscopy, temperature-programmed desorption techniques, and thermogravimetry. Increasing the loading of strontium over Ni/MgO induced a strong interaction of NiO with the support, pronouncedly. In the presence of oxygen during the POM, the moderate-level interaction of NiO with the support grows markedly. Overall, at a 600 °C reaction temperature, the 5Ni2Sr/MgO catalyst shows 72% CH₄ conversion (~67% H₂ yield) at 14,400 mL/h/g_cat GHSV and ~86% CH₄ conversion (84% H₂ yield) at 3600 mL/h/g_cat GHSV. Achieving a higher activity towards the POM over cheap Ni, Sr, and MgO-based catalysts might draw the attention of environmentalists and industrialists as a low-cost and high-yield system. Full article

Variable-flow displacement has been effectively used to enhance oil recovery; however, it has rarely been investigated for coalbed methane production, and the CH₄ diffusion laws in this process are not clear. In this paper, we carried out a study on the CH₄ diffusion law in the bidirectional diffusion process displaced by variable-flow gas injection. The emission and desorption quantity of CH₄ under variable-flow gas injection, the displacement effect under the principle of equal time and quantity, and the applicability of the CH₄ diffusion model for the bidirectional diffusion process were analyzed. The results indicate that the variable-flow injection modes emit more CH₄ compared to constant flow injection. The CH₄ emission and desorption quantities for each injection mode are as follows: step-changed > sinusoidal-changed > constant flow. Secondly, the order of CH₄ emission and desorption quantity in each gas injection mode is as follows: step-changed > sinusoidal-changed > constant flow. When CO₂ is the injection gas source, the outlet CH₄ emission and desorption quantity are larger than N₂ injection. Thirdly, through the analysis of the principle of equal time and equal quantity, the variable-flow injection modes consume less gas for each volume of emitted CH₄, resulting in a more effective displacement. Finally, the diffusion fitting effect of the bidisperse model for CH₄ in the bidirectional diffusion process is better than that of the unipore model, and the bidisperse diffusion model can better fit the m_t/m_∞ curve of CH₄ diffusion during the variable-flow gas injection replacement process. Full article