Search Results (93)

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

This study presents the first application of a deep-towed transmitter–receiver marine controlled-source electromagnetic (TTR-MCSEM) system for gas hydrate exploration in the Shenhu area of the South China Sea. High-resolution electromagnetic data were acquired along a 13 km transect using dynamic source–receiver offsets and a 500 A transmitter. The results reveal the following: (1) unprecedented near-seafloor resolution (20~100 m) for the precise delineation of hydrate-bearing caprock, surpassing conventional ocean-bottom electromagnetic systems; (2) laterally continuous high-resistivity anomalies (~10 Ω·m) extending from the base of the gas hydrate stability zone to the seafloor, which correlate with seismic bottom-simulating reflector (BSR) distributions and suggest heterogeneous hydrate saturation; and (3) fault-controlled fluid migration pathways that supply hydrate reservoirs and lead to seabed methane seepage at structural highs. Through 2D inversion, we show that the inverted resistivity values (~10 Ω·m) are slightly higher than those obtained from resistivity logs (~5 Ω·m). Saturation values derived from inverted resistivity exhibit remarkable consistency with well-log-based measurements. The high efficiency of the system confirms its potential for the transformative quantitative assessment of hydrate systems, seafloor massive sulfides, and marine geohazards. Full article

The dynamic process of water depletion plays a critical role in both surface coalbed methane (CBM) development and underground gas extraction, reshaping water–rock interactions and inducing complex permeability responses. Addressing the limited understanding of the coupling mechanism between heterogeneous pore water evolution and permeability during dynamic processes, this study simulates reservoir transitions across four zones (prospective planning, production preparation, active production, and mining-affected zones) via centrifugal experiments. The results reveal a pronounced scale dependence in pore water distribution. During low-pressure stages (0–0.54 MPa), rapid drainage from fractures and seepage pores leads to a ~12% reduction in total water content. In contrast, high-pressure stages (0.54–3.83 MPa) promote water retention in adsorption pores, with their relative contribution rising to 95.8%, forming a dual-structure of macropore drainage and micropore retention. Multifractal analysis indicates a dual-mode evolution of movable pore space. Under low centrifugal pressure, D₋₁₀ and Δα decrease by approximately 34% and 36%, respectively, reflecting improved connectivity within large-pore networks. At high centrifugal pressure, an ~8% increase in D₀−D₂ suggests that pore-scale heterogeneity in adsorption pores inhibits further seepage. A quantitative coupling model establishes a quadratic relationship between fractal parameters and permeability, illustrating that permeability enhancement results from the combined effects of pore volume expansion and structural homogenization. As water saturation decreases from 1.0 to 0.64, permeability increases by more than 3.5 times. These findings offer theoretical insights into optimizing seepage pathways and improving gas recovery efficiency in dynamically evolving reservoirs. Full article

The development of deep coalbed methane (buried depth > 2000 m) in the Yan’an block of Ordos Basin is limited by low permeability, the pore structure of the coal reservoir, and the gas–water occurrence relationship. It is urgent to clarify the key control mechanism of pore structure on gas migration. In this study, based on high-pressure mercury intrusion (pore size > 50 nm), low-temperature N₂/CO₂ adsorption (0.38–50 nm), low-field nuclear magnetic resonance technology, fractal theory and Pearson correlation coefficient analysis, quantitative characterization of multi-scale pore–fluid system was carried out. The results show that the multi-scale pore network in the study area jointly regulates the occurrence and migration process of deep coalbed methane in Yan’an through the ternary hierarchical gas control mechanism of ‘micropore adsorption dominant, mesopore diffusion connection and macroporous seepage bottleneck’. The fractal dimensions of micropores and seepage are between 2.17–2.29 and 2.46–2.58, respectively. The shape of micropores is relatively regular, the complexity of micropore structure is low, and the confined space is mainly slit-like or ink bottle-like. The pore-throat network structure is relatively homogeneous, the difference in pore throat size is reduced, and the seepage pore shape is simple. The bimodal structure of low-field nuclear magnetic resonance shows that the bound fluid is related to the development of micropores, and the fluid mobility mainly depends on the seepage pores. Pearson’s correlation coefficient showed that the specific surface area of micropores was strongly positively correlated with methane adsorption capacity, and the nanoscale pore-size dominated gas occurrence through van der Waals force physical adsorption. The specific surface area of mesopores is significantly positively correlated with the tortuosity. The roughness and branch structure of the inner surface of the channel lead to the extension of the migration path and the inhibition of methane diffusion efficiency. Seepage porosity is linearly correlated with gas permeability, and the scale of connected seepage pores dominates the seepage capacity of reservoirs. This study reveals the pore structure and ternary grading synergistic gas control mechanism of deep coal reservoirs in the Yan’an Block, which provides a theoretical basis for the development of deep coalbed methane. Full article

This study presents a comprehensive multifractal characterization of full-scale pore structures in middle- to high-rank coal reservoirs from the Western Guizhou–Eastern Yunnan Basin and establishes a permeability prediction model integrating fractal heterogeneity and pore throat parameters. Eight coal samples were analyzed using mercury intrusion porosimetry (MIP), low-pressure gas adsorption (N₂/CO₂), and multifractal theory to quantify multiscale pore heterogeneity and its implications for fluid transport. Results reveal weak correlations (R² < 0.39) between conventional petrophysical parameters (ash yield, volatile matter, porosity) and permeability, underscoring the inadequacy of bulk properties in predicting flow behavior. Full-scale pore characterization identified distinct pore architecture regimes: Laochang block coals exhibit microporous dominance (0.45–0.55 nm) with CO₂ adsorption capacities 78% higher than Tucheng samples, while Tucheng coals display enhanced seepage pore development (100–5000 nm), yielding 2.5× greater stage pore volumes. Multifractal analysis demonstrated significant heterogeneity (Δα = 0.98–1.82), with Laochang samples showing superior pore uniformity (D₁ = 0.86 vs. 0.82) but inferior connectivity (D₂ = 0.69 vs. 0.71). A novel permeability model was developed through multivariate regression, integrating the heterogeneity index (Δα) and effective pore throat diameter (D₁₀), achieving exceptional predictive accuracy. The strong negative correlation between Δα and permeability (R = −0.93) highlights how pore complexity governs flow resistance, while D₁₀’s positive influence (R = 0.72) emphasizes throat size control on fluid migration. This work provides a paradigm shift in coal reservoir evaluation, demonstrating that multiscale fractal heterogeneity, rather than conventional bulk properties, dictates permeability in anisotropic coal systems. The model offers critical insights for optimizing hydraulic fracturing and enhanced coalbed methane recovery in structurally heterogeneous basins. Full article

Gas extraction is an important means to reduce coalbed gas and ensure safe coal production. Injecting N₂/CO₂ into a coalbed can enhance coal seam gas extraction, but problems with N₂/CO₂ sources underground have prevented the wide application of this technology in coal mines. The air contains a large amount of N₂, but only a few studies have investigated the injection of air into coalbeds to facilitate gas extraction. In this study, a thermal–hydraulic–solid coupling model for air-enhanced coalbed gas extraction (Air-ECGE) was established. Additionally, the impact of air injection on coalbed methane extraction was simulated, and field experiments were conducted on air injection to enhance gas extraction. The results showed that injecting high-pressure air into a coalbed can effectively facilitate gas desorption and gas migration within the coalbed, greatly improving the efficiency of gas extraction in the coalbed. In addition, owing to the large pressure gradient that can lead to fast coalbed gas seepage, the gas production rate of the extraction borehole is directly proportional to the gas injection pressure. Further, the spacing of the boreholes limits the influence range of the gas injection: the larger the spacing, the larger the influence range, and the higher the gas extraction rate of the extraction borehole. After injecting air into the coalbed of the Liuzhuang coal mine, the extraction flow rate and concentration of gas from the extraction boreholes both increased significantly. A certain delay effect was also observed in the gas injection effect, and the gas extraction flow rate only decreased after a period of time after the gas injection had stopped. Full article

To elucidate the gas adsorption characteristics of medium-rank coal, this study collected samples from fresh mining faces in the Qinshui Basin. A series of experiments were conducted, including low-temperature carbon dioxide adsorption, low-temperature liquid nitrogen adsorption, mercury intrusion, and methane isothermal adsorption experiments, which clarify the pore structure characteristics of medium-rank coals, reveal the gas adsorption behavior in medium-rank coal, and identify the control mechanism. The results demonstrate that the modified Dubinin–Radushkevich (D-R) isothermal adsorption model accurately describes the gas adsorption in medium-rank coal, with fitting errors remaining below 1%. Comprehensive pore structure analysis reveals that the coal pore volume consists primarily of absorption pores (<2 nm), transitional pores (10–100 nm), and seepage pores (>100 nm), while the specific surface area is predominantly contributed by absorption pores (<2 nm). At low pressures, gas molecules form monolayer adsorption on absorption pore (<2 nm) and adsorption pore (2–10 nm) surfaces. With increasing pressure, multilayer adsorption dominates. As pore filling approaches the maximum capacity, the adsorption rate decreases progressively until reaching an equilibrium, at which point the adsorption capacity attains its saturation limit. The adsorption data of the gas in medium-rank coal can be explained by the improved D-R isothermal adsorption model. The priority of gas filling in pores is different, and the absorption pore is normally better than the adsorption pore. The results provide a new idea and understanding for the further study of the coalbed gas adsorption mechanism. Full article

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

China’s natural gas demand is growing under the “dual carbon” goal. However, the peaking capacity of gas storage remains insufficient. Oil reservoir-based underground gas storage (UGS) has, thus, emerged as a critical research focus due to its potential for efficient capacity expansion. The complexity of seepage and phase change characteristics during the multi-cycle injection–production process has not been systematically elucidated. This study combines experimental and numerical simulations to examine the seepage and phase change characteristics. This study innovatively reveals the synergistic mechanism of permeability, pressure, and cycle. The control law of multi-factor coupling on the dynamic peaking capacity of UGS is first expounded. Oil–water mutual drive reduced oil displacement efficiency by 2.5–4.7%. Conversely, oil–gas mutual drive improved oil displacement efficiency by 3.0–4.5% and storage capacity by 4.7–6.5%. The fifth-cycle oil–gas mutual displacement in high-permeability cores (74 mD) under high pressure (22 MPa) exhibited reductions in irreducible water saturation (7.06 percentage points) and residual oil saturation (6.38 percentage points) compared with the first-cycle displacement in low-permeability cores (8.36 mD) under low pressure (16 MPa). Meanwhile, the gas storage capacity increased by 13.44 percentage points, and the displacement efficiency improved by 10.62 percentage points. Multi-cycle huff-and-puff experiments and numerical simulations revealed that post-depletion multi-cycle huff-and-puff operations can enhance the oil recovery factor by 2.74–4.22 percentage points compared to depletion. After five-cycle huff-and-puff, methane content in the produced gas increased from 80.2% to 87.3%, heavy components (C₈₊) in the remaining oil rose by 2.7%, and the viscosity of the remaining oil increased from 2.0 to 4.6 mPa·s. The deterioration of the physical properties of the remaining oil leads to a reduction in the recovery factor in the cycle stage. This study elucidates seepage mechanisms and phase evolution during multi-cycle injection–production, demonstrating the synergistic optimization of high-permeability reservoirs and high-pressure injection techniques for enhanced gas storage design and efficiency. Full article

Low permeability has always limited the efficient extraction of coalbed methane (CBM) in China. To investigate the permeability enhancement effect of supercritical CO₂ on coal seams, experiments were conducted using a self-developed supercritical CO₂ immersion system and a single-component gas (CO₂ and CH₄) seepage experimental apparatus, considering different immersion times and injection pressures. The gas seepage characteristics of CO₂ and CH₄ in coal seams were studied under various conditions. Additionally, nuclear magnetic resonance (NMR) was used to obtain the porosity components of the coal samples at different immersion times. The changes in permeability before and after the experiment were compared to analyze the permeability enhancement effect of supercritical CO₂ on the coal samples. The results show that the original porosity of the coal sample was 2.06%. After 5, 10, 15, and 20 days of immersion, the porosity of the coal samples increased by 2.78%, 3.26%, 3.22%, and 2.86%, respectively. After immersion in supercritical CO₂, the porosity exhibited a trend of initially increasing and then decreasing. During the single-component gas seepage experiment following supercritical CO₂ immersion, the outlet flow rates of both CO₂ and CH₄ reached their maximum on the 10th day of immersion. Compared with the 0-day immersion, the outlet flow rates of CO₂ and CH₄ increased by 4.49 times and 3.23 times, respectively. After immersion, the CH₄ permeability within the coal sample was stronger than that of CO₂. Full article

In China, most medium- and shallow-depth coalbed methane (CBM) reservoirs are in the middle to late stages of development. Exploiting CBM in unsaturated low-permeability reservoirs remains particularly challenging. This study investigates the evolution of reservoir pressure in rock strata during CBM extraction from a low-permeability coal seam in the Ordos Basin. By integrating the seepage equation, material balance equation, and fluid pressure theory, we establish a theoretical and numerical model of reservoir pressure dynamics under varying bottom-hole flowing pressures. The three-dimensional surface of reservoir pressure is characterized by the formation of a stable pressure drop funnel. The results show that gas–liquid flow capacity is significantly constrained in low-permeability reservoirs. A slower drainage control rate facilitates the formation of stable seepage channels and promotes the expansion of the seepage radius. Under ultra-low permeability (0.5 mD) to low permeability (2.5 mD) conditions, controlling the bottom-hole flowing pressure below the average value aids the effective expansion of the pressure drop funnel. Numerical simulations indicate that the seepage and desorption radii expand more effectively under low decline rates in low-permeability zones. Calculations based on production data reveal that, under ultra-low permeability conditions, Well V₁ exhibits a narrower and more elongated pressure drop funnel than Well V₂, which operates in a low permeability zone. Furthermore, well interference has a lesser effect on the expansion of the pressure drop funnel under ultra-low permeability conditions. These differences in the steady-state morphology of the pressure drop funnel ultimately lead to variations in production capacity. These findings provide a theoretical foundation and practical guidance for the rational development of low-permeability CBM reservoirs. Full article

The study of pore characteristics in tectonic coal is essential for a deeper understanding of gas diffusion, seepage, and other transport processes within coal seams, and plays a crucial role in the development of coalbed methane resources. Based on low-temperature N₂ and CO₂ adsorption experiments, this study investigated the pore structure characteristics of four tectonic coal samples collected from the Hegang and Jixi basins in China. The results show that the mylonitic coal sample exhibits a clear capillary condensation and evaporation phenomenon around a relative pressure (P/P₀) of 0.5. The degree of tectonic deformation in coal has a significant impact on its pore characteristics. As the degree of deformation increases, both the pore volume and specific surface area of the coal gradually increase. The pore volume and specific surface area of micropores are primarily concentrated in pores with diameters of 0.5–0.7 nm and 0.8–0.9 nm, while those of mesopores are mainly distributed in pores with diameters of 2.3–6.2 nm. The proportion of pore volume and specific surface area contributed by micropores is much greater than that of mesopores. The fractal dimension is positively correlated with the degree of tectonic deformation in coal. As the fractal dimension increases, the average pore diameter decreases, closely tied to the destruction and reconstruction of the coal’s pore structure under tectonic stress. These findings will contribute to a deeper understanding of the pore structure characteristics of tectonic coal and effectively advance coalbed methane development. Full article

Coal resource extraction and utilization are essential for sustainable development and economic growth. This study integrates a pseudo-triaxial mechanical loading system with low-field nuclear magnetic resonance (NMR) to enable the preliminary visualization of coal’s pore-fracture structure (PFS) under mechanical stress. Pseudo-triaxial and cyclic loading–unloading tests were combined with real-time NMR monitoring to model porosity recovery, pore size evolution, and energy dissipation, while also calculating the fractal dimensions of pores in relation to stress. The results show that during the compaction phase, primary pores are compressed with limited recovery after unloading. In the elastic phase, both adsorption and seepage pores transform significantly, with most recovering post-unloading. After yield stress, new fractures and pores form, and unloading enhances fracture connectivity. Seepage pore porosity shows a negative exponential relationship with axial strain before yielding, and a logarithmic relationship afterward. The fractal dimension of adsorption pores decreases during compaction and increases afterward, while the fractal dimension of seepage pores decreases before yielding and increases post-yielding. These findings provide new insights into the flow patterns of methane in coal seams. Full article

Influencing factors and sensitivity analysis of coal permeability are significant for reasonably setting coalbed methane (CBM) extraction parameters and increasing CBM output. Seepage tests were conducted on gassy coal using a seepage test system for damaged coal and rock mass under various conditions of axial pressure, confining pressure, and gas pressure. Moreover, the influences of different factors on the permeability of gassy coal and the sensitivity of permeability to these factors were analyzed. Research results show that under the same confining pressure, the relationship between permeability and axial pressure of gassy coal meets the quadratic polynomial function; under the same axial pressure, the permeability changes with the confining pressure as a power function. The permeability of gassy coal is far more sensitive to confining pressure than to axial pressure during axial seepage. Under the same axial pressure and confining pressure (same stress), the permeability of gassy coal reduces at first and then increases in a V-shaped trend with growing gas pressure. There is a turning point in the seepage tests, that is, the critical gas pressure. When the gas pressure is lower than the critical value, the slippage effect plays the leading role in the variation of permeability of the coal; on the contrary, effective stress plays the dominant role. In the non-isobaric deviatoric stress state, the permeability of gassy coal is most sensitive to the confining pressure, followed successively by gas pressure and axial pressure. The research results provide a theoretical basis for precise gas extraction and control in coal seams. 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