Search Results (87)

Characterizing methane (CH₄) adsorption behavior in marine shale reservoirs is of great significance for assessing geological natural gas reserves and elucidating adsorption mechanisms within complex pore systems. In this study, supercritical high-pressure CH₄ adsorption experiments were conducted on Lower Cambrian organic-rich marine Shuijingtuo shales under reservoir-relevant pressure and temperature conditions (30–90 °C and up to 32 MPa). The measured excess isotherms were analyzed using the Polanyi theory-derived Dubinin-Astakhov (D-A) model, which incorporates a pseudo-saturation pressure correction for supercritical conditions and accounts for the adsorbed phase density. The D-A model yielded excellent fits (R² = 0.976–0.990) and temperature-independent characteristic curves for all marine shale samples, confirming its suitability for supercritical CH₄ adsorption in heterogeneous pore systems. A strong positive correlation (R² = 0.87) was observed between total organic carbon (TOC) content and adsorption capacity. This relationship is attributable to the abundant nanoscale organic pores developed within the organic matter, which increase the micropore volume and BET surface area, thereby improving the gas uptake potential. Thermodynamic analysis incorporating real gas behavior and adsorbed phase volume reveals that simplified assumptions (assuming an ideal gas or negligible adsorbed phase volume) systematically overestimate the isosteric heat of adsorption, with this deviation being particularly pronounced at high surface coverages. The model-derived isosteric heat decreases with increasing surface coverage across all shale samples, an observation that is highly consistent with the preferential occupation of high-energy sites within a highly heterogeneous marine nanopore system. The theoretical framework of the isosteric heat of adsorption provided in this study is suitable for other gas–solid adsorption systems and establishes a foundation for future research on other thermodynamic analyses such as the adsorbed phase enthalpy and adsorbed phase specific heat capacity. Full article

Understanding the stress sensitivity of multiscale pore fracture structures (PFS) in coals with different ranks is critical for evaluating coalbed methane (CBM) reservoir behavior. In this study, low-rank and high-rank coals were subjected to effective confining pressure loading–unloading tests under constant pore pressure, and the dynamic evolution of PFS was investigated using low-field nuclear magnetic resonance (LF-NMR), nuclear magnetic resonance imaging (NMRI), and fractal analysis. For the tested specimens, the Fengjiata low-rank coals exhibited higher proportions of seepage pores (SPs) and generally greater stress sensitivity, whereas the Sijiazhuang high-rank coals were dominated by adsorption pores (APs) and showed comparatively stable PFS. SPs are more sensitive to effective stress than APs, and stress-induced pore deformation shows partial irreversibility after unloading. Furthermore, an NMR-based method was proposed to quantify stress-dependent pore compressibility, revealing that pore compressibility decreases logarithmically with increasing effective stress due to the progressive loss of compressible pore space. These findings provide new insights into the multiscale stress response of coal pore fracture systems and improve the evaluation of stress-sensitive permeability evolution in CBM reservoirs. Full article

Understanding how methane moves through nanoporous media is key to predicting performance in unconventional gas reservoirs. At these extremely small scales, pore sizes approach the molecular level, where classical flow assumptions begin to fail and multiple transport mechanisms can occur at the same time. In this work, a unified framework is developed to characterize methane flow regimes using a real-gas corrected Knudsen number. By combining pore size, pressure, and temperature within a single formulation, the approach captures how flow behavior evolves across realistic reservoir conditions. A unified flow regime map is used to characterize the gradual shift in transport behavior—from adsorption-dominated and diffusion-like mechanisms in ultra-tight pores, to transition and slip flow, and eventually to continuum (Darcy) flow in larger pores. The results show that pore size plays the dominant role in determining flow behavior, while pressure introduces a dynamic effect, particularly during reservoir depletion. Sensitivity analysis also highlights that flow regime classification depends not only on thermodynamic conditions but also on molecular-scale parameters such as methane diameter. Comparison with established models and experimental observations shows that the framework captures the expected increase in rarefaction effects at low pressures and small pore sizes. Overall, the results emphasize that gas transport in nanoporous systems is not governed by a single mechanism but evolves over time and across scales. The proposed framework offers a simple, physically grounded tool for identifying dominant transport mechanisms and supporting model selection, while also providing a foundation for more advanced descriptions of gas flow in unconventional reservoirs. Full article

Deep coal reservoirs commonly exhibit strong multiscale heterogeneity, which directly affects coalbed methane (CBM) storage, diffusion, and flow. In this study, deep No. 8 coal samples from the Daning–Jixian block, Ordos Basin, were comprehensively and quantitatively characterized using low-pressure CO₂ adsorption, low-temperature N₂ adsorption, mercury intrusion porosimetry (MIP), and nuclear magnetic resonance (NMR). A method-constrained calibration framework was developed to assign reliable fractal dimensions to different pore-size intervals and to calculate a volume-weighted comprehensive fractal index. The scale-dependent pore structure was evaluated, and its relationships with coal quality, maceral composition, and maximum vitrinite reflectance (R_o,max) were analyzed. The results show that deep anthracite has a trimodal pore-size distribution, with micropores dominating both specific surface area and pore volume. Fractal behavior is strongly scale-dependent, and calibrated full-pore-size fractal dimensions provide a more reliable measure of reservoir heterogeneity than single-method interpretations. Pore development and heterogeneity are closely associated with coalification degree, coal quality, and maceral composition. Ash tends to inhibit pore development, whereas fixed carbon and vitrinite promote micropore development; inertinite mainly contributes to macropores and fractures. These findings provide a quantitative basis for evaluating pore-structure heterogeneity and optimizing deep CBM reservoir development. Full article

CO₂ methanation with renewable hydrogen is a promising strategy for carbon valorization and synthetic natural gas (SNG) production. However, the molecular mechanisms behind catalyst-dependent adsorption and mass transport in zeolite-confined spaces are still not fully elucidated. Herein, we performed comparative molecular simulations on HZSM-5, Ni/ZSM-5 and Ru/ZSM-5 by combining density functional theory (DFT), grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) methods, aiming to clarify the thermodynamic and mass transport mechanisms of reactant enrichment and product desorption in CO₂ methanation. The electronic structures of the three systems were systematically evaluated via Mulliken charge analysis, differential charge density mapping, and frontier molecular orbital calculations. We further quantified the adsorption thermodynamics and diffusion kinetics of reactants and products, focusing specifically on the effects of temperature and framework Si/Al ratio for Ni/ZSM-5. The results show that Ni doping greatly modulates the local electronic environment of the ZSM-5 framework, enhancing the adsorption of CO₂ (−121.9 kJ·mol⁻¹) and H₂ (−81.6 kJ·mol⁻¹) and weakening the adsorption of CH₄ and H₂O. A higher Si/Al ratio reduces CO₂ adsorption capacity, while elevated temperatures inhibit reactant adsorption and lower the diffusion selectivity of CH₄. This demonstrates that moderately low temperatures and moderate Si/Al ratios can optimize the adsorption and diffusion behaviors of reactants and products. This work provides molecular-level insights into the adsorption and diffusion behaviors of Ni/ZSM-5 and offers theoretical references for the rational development of high-performance CO₂ methanation catalysts. Full article

Dry reforming of methane (DRM) enables the simultaneous conversion of CH₄ and CO₂, yet rapid coking severely restricts the stability of Ni-based catalysts. In this study, Co was incorporated into Ce-, La-, and Zr-promoted Ni catalysts to construct Ni-Co alloy active sites, and their catalytic behavior was systematically evaluated. While single-promoter modification partially suppressed coke deposition at the expense of activity, Ni-Co alloy formation maintained high reforming performance and significantly enhanced stable catalytic performance within the 20 h evaluation period, with the Ce-promoted Ni-Co catalyst exhibiting the most durable anti-coking performance. CO₂-TPD and coke characterization results indicate that promoter species enhance medium-strength basicity and oxygen mobility, thereby facilitating CO₂ adsorption and accelerating the oxidation of surface coke intermediates; in particular, Ce supplies mobile active oxygen species through its oxygen storage-release capacity. DFT calculations further reveal that Co incorporation modulates the electronic structure of Ni sites, optimizing the balance between CH₄ dissociation and CO₂ activation and thus suppressing excessive methane cracking. These findings elucidate the synergistic effect of Ni-Co alloying and promoter modification in DRM and provide mechanistic insight for the rational design of coke-resistant Ni-based catalysts. Full article

The permeability of coal seams plays a critical role in the efficiency of coalbed methane extraction and gas disaster prevention. Traditional permeability models often overlook the anisotropic and dynamic evolution characteristics of coal under varying stress and gas adsorption conditions. This paper proposes a novel permeability evolution model that integrates the effects of effective stress variation and gas sorption-induced deformation on coal permeability. Starting from the concept of face porosity and utilizing a representative voxel approach, the model incorporates the anisotropy of mechanical parameters and adsorption expansion strain to derive the evolution of permeability in three dimensions. The model is validated against experimental permeability data from two distinct coal samples (Sulcis and Sydney), demonstrating its ability to accurately capture permeability changes under different boundary conditions. Furthermore, the concept of “internal expansion strain coefficient” is introduced to quantify the impact of adsorption-induced matrix deformation on permeability. The model provides a theoretical foundation for predicting gas flow behavior in coal seams under complex in-situ conditions and offers significant insights into the optimization of gas extraction strategies. Full article

Accurate measurement of coal seam gas content forms the core foundation for coal mine disaster prevention and coalbed methane resource development. However, as coal mining in China extends into deeper strata, multi-component gases (CH₄, CO₂, N₂, etc.) exhibit competitive adsorption effects and dynamic differentiation characteristics. These behaviors pose significant challenges to traditional measurement standards based on the “single methane-dominated system” assumption. This study systematically analyzes the competitive adsorption mechanisms of multi-component gases. By integrating gas data from major coal-producing provinces in China (such as Guizhou, Anhui, and Xinjiang), we reveal the heterogeneous distribution characteristics of gas components and their controlling factors, including coalification degree, burial depth, and maceral components. Case studies from Henan and Shanxi provinces demonstrate that neglecting the strong adsorption hysteresis of CO₂ and the rapid desorption characteristics of N₂ induces a systematic error exceeding 25% in total gas content measurements within areas high in non-methane components. To address these issues, we propose a “hierarchical measurement and real-time correction” framework for multi-component gas content. This framework categorizes gas measurement into four levels—ranging from single-component to all components—based on the complexity of geological conditions. It combines differentiated measurement with dynamic correction to improve the accuracy of gas content evaluation under complex geological conditions. This research provides a more reliable theoretical basis for gas pressure prediction and mine gas grade identification, offering substantial theoretical significance and engineering value for enhancing safety guarantees in deep coal mining. Full article

Direct emission of low-concentration methane not only aggravates global warming but also causes serious energy waste. Catalytic combustion is considered an effective strategy for methane abatement because it enables methane oxidation at relatively low temperatures. In this work, a series of Mn–Ce–Cu/γ-Al₂O₃ catalysts with different nominal Mn/Ce ratios were prepared by the incipient wetness impregnation method and applied to low-concentration methane catalytic combustion. The results showed that Mn–Ce co-modification significantly improved the activity of Cu/γ-Al₂O₃ catalysts, and the catalytic performance strongly depended on the Mn/Ce ratio. Among all samples, 7Mn-3Ce-10Cu exhibited the best activity, with the temperatures required for 10%, 50% and 90% methane conversion (T₁₀, T₅₀ and T₉₀) of 380.8, 427.3 and 478.7 °C, respectively. Apparent activation energy (E_a) analysis further showed that 7Mn-3Ce-10Cu possessed the lowest E_a value of 83.81 kJ mol⁻¹, indicating that the optimized Mn/Ce ratio effectively lowered the apparent kinetic barrier for methane oxidation. X-ray diffraction (XRD), transmission electron microscopy (TEM) and nitrogen (N₂) adsorption–desorption results suggested that Mn–Ce co-modification changed the phase composition, improved the dispersion state of active oxide species and generated a more favorable pore structure for reactant diffusion. Oxygen temperature-programmed desorption (O₂-TPD) and X-ray photoelectron spectroscopy (XPS) results further indicated that the enhanced activity of 7Mn-3Ce-10Cu was closely associated with improved oxygen desorption behavior, a higher proportion of surface oxygen species and favorable surface redox characteristics of Cu, Mn and Ce species. Moreover, 7Mn-3Ce-10Cu maintained methane conversion above 90% during a 50 h stability test at 500 °C, and the inhibition caused by 5% H₂O was partially reversible. These results demonstrate that Mn–Ce co-modification is an effective strategy for improving low-cost Cu-based catalysts for low-concentration methane combustion. Full article

A critical challenge in coalbed methane (CBM) extraction is the severe formation damage induced by conventional foam fracturing fluids, primarily through polymer retention and hydrogen bond disruption within the microporous matrix. This study presents a molecularly engineered, low-damage foam fracturing fluid that leverages synergistic nanoparticle–surfactant interactions to construct a robust interfacial pseudo-gel network, thereby decoupling effective fracture stimulation from adverse geochemical damage. The primary novelties of this work are threefold: (i) establishing a direct, quantitative cause-and-effect relationship between molecular interfacial architecture and reservoir protection, (ii) proposing a comprehensive “interfacial control” design paradigm that engineers viscoelasticity at the gas–liquid interface rather than through bulk polymer gelation, and (iii) demonstrating the complete decoupling of foam stability from formation damage in a polymer-free system. A systematic optimization methodology was employed: initial foaming agents were screened via the Waring Blender method, evaluating foam volume, half-life, and a derived comprehensive index; subsequently, synergistic binary surfactant mixtures and foam stabilizers were assessed to formulate the final systems. An optimized formulation, designated Foam System I (0.5 wt.% fluorosurfactant FK + 0.5 wt.% nano-silica RX + 2.0 wt.% KCl), demonstrated exceptional foam quality (Γ = 77.1 ± 1.5%) and kinetic stability (T₁/₂ > 350 s). Rheological characterization confirmed shear-thinning behavior conforming to the Herschel–Bulkley model (n = 0.38–0.42, R² > 0.98) and a structural recovery of 92.5 ± 2.1%—comparable to crosslinked polymer gels but achieved without any bulk viscosifier. Core flood analyses revealed that Foam System I induced a permeability damage of only 12.75 ± 1.8%, representing a 55–75% reduction compared to polyethylene glycol (PEG)-stabilized reference fluids (28.36–51.91%). X-ray photoelectron spectroscopy (XPS) correlated this enhanced reservoir compatibility with an 18.0 ± 2.0% suppression of oxygen-containing functional group adsorption, attributed to the steric hindrance conferred by the fluorinated hydrophobic moieties. This work establishes an “interfacial control” paradigm wherein gel-like stabilization for proppant transport is achieved via interfacial viscoelasticity rather than bulk polymer gelation, thereby directly addressing the critical imperative to harmonize fracture conductivity with reservoir protection in unconventional energy development. The findings are validated for shallow CBM reservoir conditions (25–35 °C), with extension to higher-temperature formations identified as a priority for future investigation. Full article

To elucidate coalbed methane (CBM) adsorption mechanisms in deep coal reservoirs, the macromolecular structures of coal samples with different coal ranks were characterized using FTIR, XPS, and C NMR, followed by the construction of corresponding molecular models. Grand Canonical Monte Carlo (GCMC) simulations were employed to investigate methane adsorption behavior within the coal matrix at 313.15 K and pressures up to 20 MPa. The results showed that as coal rank increased (R_o,max = 1.63% to 3.18%), the coal macromolecular structure transformed from a side-chain-rich configuration to a highly aromatized and directionally stacked structure. This structural maturation leads to a more compact coal matrix, evidenced by a reduction in free volume from 5108.39 ų to 3999.87 ų and a decline in accessible free volume from 8.23% to 6.26%, thereby restricting the effective space for methane storage. At 20 MPa, although the pore walls of high-rank coal exhibit stronger localized adsorption capacity, the bulk adsorption capacity follows the order: DZ > ZC > SH. This suggests that under deep, high-pressure conditions, the pore-volume compression effect associated with increasing coal rank governs the upper limit of adsorption per unit mass of coal. As pressure increases into the deep reservoir regime, the state of methane in coal micropores gradually shifts from surface adsorption to a high-density, quasi-liquid filling behavior. Consequently, the influence of specific surface area diminishes, while effective free volume emerges as the primary determinant of high-pressure adsorption capacity. The impact of coal rank on deep methane adsorption reflects a competition between enhanced adsorption potential and restricted storage space. The densification-induced compression of effective free volume is identified as the dominant factor limiting the adsorption capacity of deep CBM. This study provides a molecular-scale understanding of deep CBM occurrence mechanisms and establishes a theoretical framework for resource evaluation. Full article

This work investigates the behavior of carbon dioxide (CO₂) near the surface of different single-atom alloys to evaluate their potential as catalysts for decarbonization processes. Specifically, 26 transition metals from the first three transition series, alloyed with three low Miller index copper supports, were considered. Adsorption energies and distances of linear CO₂, trigonal CO₂, and CO* + O* on the surfaces were calculated using the semiempirical computational method MOPAC-PM7. Additionally, activation energies were determined from previously published research. The proposed methodology is less computationally demanding than DFT studies, and results show good agreement with both experimental and simulated data. This approach provides a computationally efficient methodology for screening promising materials that convert CO₂ into valuable products, such as methane and methanol. Full article

Coalbed methane (CBM) development is strongly controlled by pore structure evolution in deformed coals and its influence on hydraulic fracturing behavior. To clarify the multifractal characteristics of cross-scale pores and their control on fracturing effectiveness, this study investigated eight different deformation coals from the Ordos Basin using low-temperature CO₂/N₂ adsorption (LT-CO₂A/LT-N₂A) and high-pressure mercury intrusion porosimetry (HMIP). Micropores (<2 nm), mesopores (2–50 nm), and macropores (>50 nm) were systematically characterized, and their pore size distributions (PSDs) were quantitatively analyzed using the Coal Structure Index (CSI) and multifractal theory. The results indicate that the multifractal parameters of macropores are significantly distinct from those of mesopores and micropores, exhibiting lower H (0.824–0.893) and D₁ (0.766–0.853), and higher α₀ (1.422–1.541), ΔD (1.230–1.408), and Δα (1.459–1.642). Macropores controlled by tectonic deformation exhibit stronger heterogeneity compared to mesopores and micropores in local parts of the coal mass; PSD varies significantly with deformation rising, derived from the differential pore structure evolution during brittle–ductile transition and the multi-scale synergistic effects including maturity and composition. Combined with field fracturing curves, the results further indicate that the α₀, ΔD, and Δα of macropores are negatively correlated with breakdown pressure, with correlation coefficients of 0.51, 0.61, and 0.59, respectively, and that strong local heterogeneity of macropores favors fracture initiation and propagation and reduces breakdown pressure. Cataclastic coal is the most favorable for hydraulic fracturing, followed by undeformed coal, whereas granulated coal shows the poorest fracturing performance. Full article

Permeability is a critical parameter governing the gas flow behavior of the coalbed methane (CBM) reservoir during the exploration and exploitation of CBM, as well as the geological storage of CO₂ in the coalbeds. It is strongly associated with the multi-scale fractures developed in coal. Based on the distribution characteristics of micro-fractures, a multi-scale permeability model for coal reservoirs was established by introducing the permeability tensor, which comprehensively considers adsorption-induced coal swelling, pore pressure, effective stress, and micro-fractures. Further, the dynamic evolution law and mechanism of multi-scale permeability of coal reservoirs under different adsorption pressures were discussed. The results indicate that the increase in effective stress on the coal caused by adsorption-induced swelling essentially leads to a decrease in the equivalent multi-scale permeability of coal. Two key indicators, namely equilibrium pressure and rebound pressure, were defined to quantitatively characterize the evolution law of the equivalent multi-scale permeability during gas adsorption or desorption processes. The effective stress generated by the CO₂ adsorption-induced swelling effect in the low-rank coal is 1.47 times that in the middle-rank coal and 2.51 times that in the high-rank coal. Additionally, the effective stress generated by the CO₂ adsorption-induced swelling effect in the low-rank coal is 5.15 times that generated by N₂, while this level is 4.32 times higher than that in the middle-rank coal. Therefore, compared with the low- and middle-rank coal, the high-rank coal exhibits a smaller decrease in multi-scale permeability due to its weaker adsorption-induced swelling effect. During N₂ adsorption, the pore pressure effect dominates over the adsorption-induced swelling effect, resulting in a decrease in the effective stress on the coal with increasing gas pressure. Consequently, the equivalent multi-scale permeability of coal will increase much more significantly with an increase in injected N₂ pressure than with an increase in CO₂ pressure. By accounting for the differences between the effects of adsorption-induced swelling and pore compression on the equivalent multi-scale permeability of coal reservoir, the injectivity of CO₂ can be improved by mixing it with N₂. Full article

Understanding the evolution of coal pore–fracture structures under coupled stress paths and creep deformation is critical for enhancing coalbed methane extraction and preventing coal and gas outbursts. In this study, coal samples from the Ningtiaota Mine were investigated using online Nuclear Magnetic Resonance (NMR) technology combined with triaxial loading–creep coupled experiments. The dynamic evolution of pore–fracture structures (PFSs) under different deviatoric stress levels was characterized and visualized in real time and across multiple scales. The results reveal a pronounced stress-dependent pore evolution during creep. Under low-stress conditions, seepage pores were compressed and gradually transformed into adsorption pores, whereas under high-stress conditions, seepage pores expanded and interconnected, dominating deformation and failure. Fractal theory was employed to quantify pore structure complexity, and repeated experiments demonstrated a significant positive correlation between the fractal dimension and the fractional order. Based on these findings, a fractal-dimension-based fractional creep model was developed by introducing a Riemann–Liouville fractional dashpot. The proposed model accurately captures the nonlinear creep behavior of coal and provides a microstructural interpretation of the fractional order. This study provides theoretical and experimental support for long-term stability assessment of deep coal–rock masses and prediction of coalbed methane migration. Full article