Search Results (48)

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

Deep tight sandstone reservoirs are characterized by low porosity and permeability, complex pore structure, and strong heterogeneity. Conducting research on the heterogeneity characteristics of reservoirs could lay a foundation for evaluating their effectiveness and accurately identifying advantageous reservoirs, which is of great significance for searching for “sweet spot” oil and gas reservoirs in tight reservoirs. In this study, the deep tight sandstone reservoir in the Dibei area, northern Kuqa depression, Tarim Basin, China, is taken as the research object. Firstly, statistical methods are used to calculate the coefficient of variation (CV) and coefficient of heterogeneity (TK) of core permeability, and the heterogeneity within the reservoir is evaluated by analyzing the variations in the reservoir permeability. Then, based on fractal theory, the fractal and multifractal parameters of the GR and acoustic logs are calculated using the box dimension, correlation dimension, and the wavelet leader methods. The results show that the heterogeneity revealed by the box dimension, correlation dimension, and multifractal singular spectrum calculated based on well logs is consistent and in good agreement with the parameters calculated based on core permeability. The heterogeneity of gas layers is comparatively weaker, while that of dry layers is stronger. In addition, the fractal parameters of GR and the acoustic logs of three wells with the oil test in the study area were analyzed, and the relationship between reservoir heterogeneity and production was determined: As reservoir heterogeneity decreases, production increases. Therefore, reservoir heterogeneity can be used as an indicator of production; specifically, reservoirs with weak heterogeneity have high production. Full article

Organic framework membranes (OFMs) have emerged as transformative materials for separation technologies due to their tunable porosity, structural diversity, and stability, yet their design and optimization face challenges in navigating vast chemical spaces and complex performance trade-offs. This review highlights the pivotal role of machine learning (ML) in overcoming these limitations by integrating multi-source data, constructing quantitative structure–property relationships, and enabling the cross-scale optimization of OFMs. Methodologically, ML workflows—spanning data construction, feature engineering, and model optimization—accelerate candidate screening, inverse design, and mechanistic interpretation, as demonstrated in gas separations and nascent liquid-phase applications. Key findings reveal that ML identifies critical structural descriptors and environmental parameters, guiding the development of high-performance membranes that surpass traditional selectivity–permeability limits. Challenges persist in liquid separations due to dynamic operational complexities and data scarcity, while emerging frameworks offer untapped potential. The integration of interpretable ML, in situ characterization, and industrial scalability strategies is essential to transition OFMs from laboratory innovations to sustainable, adaptive separation systems. This review underscores ML’s transformative capacity to bridge computational insights with experimental validation, fostering next-generation membranes for carbon neutrality, water security, and energy-efficient industrial processes. Full article

The proton exchange membrane electrolyzer (PEMWE) has been regarded as a promising technology for converting surplus intermittent renewable energy into green hydrogen through electrochemical water splitting. However, the multiphase mass and charge transport processes with countercurrent flow within the PEMWE create complex structure–property relationships that are difficult to optimize. The interdependent effects of multiple structural parameters on the coupled heat transfer, mass transfer, and charge transfer processes further obscure performance optimization mechanisms. To decouple these phenomena and elucidate the underlying mechanisms, a multiphase one-dimensional mathematical model was developed and experimentally validated. Based on the model, the mass transfer, charge conduction, and heat transfer processes inside the PEMWE have been systematically investigated, with a particular focus on the performance-related parameters of the porous transport layer (PTL). The results reveal that PTL thickness and porosity exhibit opposite effects on activation and ohmic overpotential at an elevated current density. Furthermore, a sharp performance decline occurs when PTL gas permeability falls below the critical threshold. These findings provide quantitative guidelines for multiphysics-informed component optimization in high-performance PEMWEs. Full article

Accurately predicting the permeability of coarse-grained soils is crucial for ensuring geotechnical safety and performance. In this study, 64 coarse-grained soil (CGS) samples were designed using a negative exponential gradation equation (NEGE), and computational fluid dynamics–discrete element method (CFD-DEM) coupled seepage simulations were conducted to generate a permeability coefficient (k) dataset comprising 256 entries under varying porosity and gradation conditions. Three machine learning models—a neural network model (BPNN), a regression model (GPR), and a tree-based model (RF)—were employed to predict k, with hyperparameters optimized via particle swarm optimization (PSO) and four-fold cross-validation applied to improve generalization. Gray relational analysis (GRA) revealed that all input parameters (α, β, d_max, n) significantly influence k (R > 0.6). The interquartile range (IQR) method confirmed data suitability for modeling. Among the models, BPNN achieved the best performance (R² = 0.99, MAE = 1.5, RMSE = 2.9, U₉₅ = 0.4), effectively capturing the complex nonlinear relationship between gradation and permeability. GPR (R² = 0.92) was hindered by kernel selection and noise sensitivity, while RF (R² = 0.97) was limited by its discrete regression nature. Compared to a traditional empirical model (R² = 0.9031), BPNN improved prediction accuracy by 10.13%, demonstrating the advantage of data-driven methods for evaluating CGS permeability. Full article

In the Southeastern Ordos Basin, the Chang 2 low-permeability sandstone reservoir of the Triassic Yanchang Formation is a typical heterogeneous reservoir. Quantitatively characterizing and analyzing its complex pore throat structure has become crucial for enhancing storage and production in the study area. The pore throat structure is a key factor influencing reservoir properties. To achieve this, a comprehensive suite of analytical techniques was employed, including cast thin section (CTS), scanning electron microscopy (SEM), cathodoluminescence (CL), X-ray diffraction (XRD), and mercury intrusion capillary pressure (MICP). This study quantitatively characterizes the pore size distribution of reservoirs in the Southeast Ordos Basin. Based on fractal theory, it clarifies the complexity of the pore throat structure and the degree of microscopic heterogeneity at different scales. Finally, this study reveals the correlation between fractal dimensions and storage and permeability capacities and analyzes the controlling factors. The findings indicate that the predominant lithotype in the study area is fine-grained feldspar sandstone, which develops pore types such as intergranular pores, dissolution pores, and microfractures. Based on the shapes of mercury injection curves and pore throat structural parameters, and in conjunction with SEM images, the samples are categorized into three types. Type I samples exhibit good pore throat connectivity and are characterized by a lattice model. Type II samples are characterized by a tubular pore throat model. Type III samples have poor pore throat connectivity and are characterized by an isolated model. The pore throat network of low-permeability sandstone is primarily composed of micropores (pore throat radius r < 0.1 μm), mesopores (0.1 < r < 1.0 μm), and macropores (r > 1.0 μm). The complexity of the reservoir pore throat structure was quantitatively characterized by fractal theory. The total fractal dimension (D) of all the samples is between 2 and 3, which indicates that the reservoir has capillary fractal characteristics. The average fractal dimension of micropores (D1) is 2.57, while that for mesopores (D2) and macropores (D3) is slightly higher, at an average of 2.68. This suggests that micropores have higher self-similarity and homogeneity. The fractal dimensions D1, D2, and D3 of the three types of reservoirs all exhibit a negative correlation with porosity and permeability. This shows that the more complex the pore throat structure is, the worse the storage and seepage capacity of the reservoir. For type I samples, the correlation of D3 with pore throat structural parameters such as entry pressure, skewness, and maximum mercury saturation is better than that of D2 and D1. For type II and type III samples, D2 shows a significant correlation with pore throat structural parameters. This indicates that the heterogeneity and complexity of mesopores are key factors influencing the pore throat structure of poor-quality reservoirs. Different mineral compositions have varying effects on the fractal characteristics of pore structures. Quartz, feldspar, and clay exert both negative and positive dual impacts on reservoir quality by altering the pore throat structure and the diagenetic processes. The mineral content exhibits a complex quadratic relationship with the fractal dimension. Moreover, micropores are more significantly influenced by the mineral content. The study of the relationship between the fractal dimension and physical properties, pore throat structural parameters, and mineral composition can improve the understanding of the reservoir quality of low-permeability reservoirs. This provides a theoretical basis for exploration and improving the recovery rate in the study area. Full article

Low-permeability sandstone reservoirs have low permeability, but due to their high porosity and difficulty in development, the development difficulty is relatively high. They can fully tap into the high potential of oil and gas resources in low-permeability sandstone reservoirs and occupy an important position in the global energy supply The study area belongs to low-permeability dense sandstone reservoir, and the destination layer has complex lithology, strong physical inhomogeneity, and complicated pore–permeability relationship, so the conventional core pore–permeability regression method and NMR SDR method do not satisfy the requirements of fine evaluation in terms of the accuracy of permeability calculation. According to the principle of resistivity measurement by electromagnetic waves with Logging While Drilling (LWD), this paper analyzes the reasons for the magnitude of resistivity divergence with Logging While Drilling at different exploration depths. There is a “low invasion phenomenon” during the drilling process of the drill bit. The higher the permeability of the formation, the more severe the “low invasion phenomenon”, and the greater the magnitude of resistivity divergence. In this paper, through the conventional log curve response characteristics and correlation analysis, the P40H/P16H parameter were selected to characterize the magnitude of resistivity divergence, and a fine evaluation model of the reservoir based on the P40H/P16H parameter was established in the study area by relying on the theory of the flow unit, and was applied to the prediction of permeability of new wells. The application results show that the calculated permeability is in good agreement with the results of core analysis, which provides a theoretical basis for the fine evaluation of low-permeability tight reservoirs. Full article

A tight reservoir is characterized by low porosity and permeability as well as a complex pore structure, resulting in low oil recovery efficiency. Understanding the micro-scale distribution of residual oil is of great significance for improving oil production and water flooding recovery rates. In this study, a 3D pore structure model of tight sandstone was established using CT scanning to characterize the residual oil distribution after water flooding. The effects of displacement methods and wettability on residual oil distribution at the micro-scale were then studied and discussed. Moreover, increasing the displacement rate has little effect on the distribution area and dominant seepage channels. Microscopic residual oil is classified into five discontinuous phases according to the oil–water–pore–throat contact relationship. The microscopic residual oil exhibits characteristics of being dispersed overall but locally concentrated. Under water-wet conditions, the injected water tends to strip the oil phase along the pore walls. Under oil-wet conditions, the pore walls have an improved adsorption capacity for the oil phase, resulting in a large amount of porous and membranous residual oil retained in the pores, which leads to a decrease in the overall recovery rate. Full article

The adsorption of coalbed methane alters the pore structure of reservoirs, subsequently affecting the coal seam’s gas adsorption capacity. However, traditional gas adsorption models often neglect this crucial aspect. In this article, we introduce a fractal capillary bundle model that accounts for the expansion of coal seam adsorption. We utilize curvature fractal dimension and capillary fractal dimension to characterize the complexity of the coal seam’s pore structure. By incorporating the concept of fictitious stress, we have described the relationship between gas adsorption, matrix porosity, and permeability changes. We have developed a model that describes the changes in matrix porosity and permeability during the gas adsorption process. After fitting this model to experimental data, it demonstrated high accuracy in predictions. Furthermore, our investigation into how factors such as curvature fractal dimension, capillary fractal dimension, and fictitious stress influence gas adsorption capacity reveals several key findings. Firstly, the specific surface area within the pore structure of coal seams is the primary factor controlling gas adsorption capacity. Secondly, the virtual stress generated during the gas adsorption process alters the coal seam’s maximum gas adsorption capacity, a factor that cannot be overlooked. Lastly, we found that gas adsorption primarily affects the gas migration process, while under high-pressure conditions, gas desorption does not cause significant changes in the matrix porosity and permeability. Full article

The effects of high debris content on pore structure in tight sandstone reservoirs tight sandstone reservoirs are multifaceted. Pore structure is an important factor controlling reservoir quality. Clarifying the effects of different types of rock debris on reservoirs is necessary to study the pore structure and their control factors of tight sandstones. The Western Sichuan Depression with complex rock components, containing multiple types of rock debris, leads to strong heterogeneity of pore throats, so it is necessary to study the factors controlling the development of different types of pore throats in tight reservoirs. In this paper, the Fourth member of Xujiahe Formation (T₃x⁴) is taken as the research object. Based on high-pressure mercury intrusion experiments and the fractal theory, the types of pore throats and their heterogeneity in tight reservoirs were studied, the relationship of fractal dimensions with reservoir physical properties, pore structure, and rock compositions were investigated, and then the controlling factors for the development of different types of pore throats are clarified. The studies show that there are four types of pore throats developed in the T₃x⁴ of the western Sichuan depression, including primary intergranular pore-throats (>350 nm), residual intergranular pore-throats (75–350 nm), dissolution pore-throats (16–75 nm), and intercrystalline pore-throats (<16 nm), among which the homogeneity of dissolution pore-throats are the best, followed by residual intergranular pore-throats and intercrystalline pore-throats, and the primary intergranular pore-throats the most heterogeneous. The permeability has a better relationship with the proportion and fractal dimension of primary intergranular pore-throats and residual intergranular pore-throats of tight reservoir of the Xujiahe Formation. The relation-ship between porosity and the proportion and fractal dimension of primary intergranular pore-throats and dissolution pore-throats is better. Brittle minerals such as quartz and metamorphic debris, as well as early developed films of chlorite and illite mainly control the development of intergranular pore-throats. Potassium feldspar mainly controls the development of dissolution pore-throats, while sedimentary rock debris, volcanic debris, and kaolinite play a destructive role for all types of pore-throats. The high-quality reservoirs in the T₃x⁴ are controlled by the development of primary intergranular pore throats and dissolution pore throats, and they are mainly developed in environments with strong hydrodynamic conditions, large rock grain sizes, high content of brittle minerals such as quartz and metamorphic debris, extensive development of chlorite and illite films, and low content of sedimentary rock debris, matrix, and cemented materials. This study is of guiding significance in clarifying the causes of heterogeneity in different types of pore-throat systems in tight sandstones and the formation mechanism of high-quality reservoirs in tight sandstones with high content of debris. Full article

In order to examine further the characteristics of micropore-throat structures of the tight oil reservoir in the Jiufotang Formation in the Houhe region, this study used whole rock X-ray diffraction, routine physical property analysis, and routine thin section observations to analyze the material composition and physical properties of the tight oil reservoir. CT scanning, high-pressure mercury infiltration, and other test methods were employed to analyze the characteristics of the pore-throat structures in the tight oil reservoir. In addition, the Pearson correlation coefficients quantified the relationships between nine parameters and pore-throat structures. The parameters with high correlations were optimized for analysis, and a comprehensive classification scheme for micropore-throat structures in the tight oil reservoir in the study area was established. The results show that the reservoir in the Jiufotang Formation in the Houhe region is composed of feldspathic and lithic arkosic sandstone, with feldspar and clast pore dissolution pores as the main type of reservoir pore space. The tight oil reservoir has small pore-throat radius, complex structures, poor connectivity, and high heterogeneity. It generally contains micron-sized pores with submicron to nanometer throat widths and small- and medium-sized pores to fine micropore-throat structures. Porosity, permeability, coefficient of variation, skewness coefficient, and average pore-throat radius, were selected for k-means cluster analysis. The micropore-throat structures of the tight oil reservoir were divided into three categories: classes I, II, and III. The study area is dominated by class II pore throats, accounting for 58%. Diagenesis mainly controls the pore-throat structure. These results provide an effective reference for the identification and evaluation of favorable sweet spots in tight oil reservoirs in similar blocks in China. Full article

Pore structure heterogeneity affects the porosity and permeability variation of tight sandstone, thereby restricting sandstone gas production. In total, 11 sandstone samples were taken as a target in the northwest margin of the Junggar Basin. Then, scanning electron microscope and high-pressure mercury injection tests are used to study the distribution of a pore and fracture system in the target sandstone. On this basis, single and multifractal models are used to quantitatively characterize the heterogeneity of pore structure, and the applicability of the classification model in characterizing the heterogeneity of the pore-fracture structure is explored. The results are as follows. (1) The target samples are divided into two types, with the mercury removal efficiency of type A samples ranging from 44.6 to 51.8%, pore size mainly distributed between 100 and 1000 nm, and pore volume percentage ranging from 43 to 69%. The mercury removal efficiency of type B samples ranges from 14 to 28%, and pore diameter distribution is relatively uniform. (2) Different fractal models represent different physical meanings. The calculation results of sponge and thermodynamic fractal models indicate that the heterogeneity of pore structure distribution in the type B sample is significantly stronger than that in type A, which is inconsistent with the conclusions of the Sierpinski model. This is because the aforementioned two models characterize the complexity of pore surface area, while the Sierpinski model characterizes the roughness of pore volume. The comparison shows that there is a significant correlation between the thermal dimensionality value DT and the volume percentage of macropores and mesopores. Therefore, the thermodynamic model can better quantitatively characterize the heterogeneity of macropore and mesoporous pore distribution. (3) The results indicate that higher pore volume range is mainly influenced by mesopores and macropores. From the relationship curve between mercury removal efficiency and single fractal dimension, it can be seen that mercury removal efficiency is greatly affected by distribution heterogeneity of the lower value area of pore volume, and it has no obvious relationship with distribution heterogeneity in the lower value area of the pore volume. Full article

Deep and ultra-deep reservoirs are characterized by low porosity and permeability, pronounced heterogeneity, and complex pore structures, complicating permeability evaluations. Permeability, directly influencing the fluid production capacity of reservoirs, is a key parameter in comprehensive reservoir assessments. In the X Depression, low-porosity and low-permeability formations present highly discrete and variable core data points for porosity and permeability, rendering single-variable regression models ineffective. Consequently, accurately representing permeability in heterogeneous reservoirs proves challenging. In the following study, lithological and physical property data are integrated with mercury injection data to analyze pore structure types. The formation flow zone index (FZI) is utilized to differentiate reservoir types, and permeability is calculated based on core porosity–permeability relationships from logging data for each flow unit. Subsequently, the average permeability for each flow unit is computed according to reservoir classification, followed by a weighted average according to effective thickness. This approach transforms logging permeability into drill stem test permeability. Unlike traditional point-by-point averaging methods, this approach incorporates reservoir thickness and heterogeneity, making it more suitable for complex reservoir environments and resulting in more reasonable conversion outcomes. Full article

Carbonate rocks are important for the petroleum industry, as they contribute significantly to hydrocarbon reserves, although their analysis is complex due to the high cost of core sampling and their high heterogeneity; for this, synthetic rocks aim to provide relatively homogeneous samples with analogous characteristics to natural rocks. In this research, synthetic carbonate rocks were produced by mixing a fixed ratio between limestone powder, obtained from limestone mining waste, and epoxy resin as a cementing material, using compaction energy for consolidation. The work aimed to produce homogeneous samples with high strength, reproducing the intergranular pore system for future applications in rock–fluid interaction analysis. The characteristics and structure of the samples were investigated through X-ray computed microtomography, petrographic images, petrophysical, chemical and geomechanical tests. Results showed a direct increasing relationship between porosity and permeability and a tendency for mechanical strength (UCS) to decrease with increasing porosity. When compared with the natural carbonate rocks, these presented similarities in their mechanical properties and petrophysical measurements, showing that the methodology can be considered as an alternative for the obtention of a realistic material that can be used for future experiments regarding rock mechanics and rock–fluid interaction for prediction of carbonate rocks’ behavior. Full article

The complex pore structure of unconventional oil and gas reservoirs is one of the reasons for the difficulties in resource evaluation and development. Therefore, it is crucial to comprehensively characterize the pore structure, understand reservoir heterogeneity from multiple perspectives, and gain an in-depth understanding of fluid migration and accumulation mechanisms. This review outlines the methods and basic principles for characterizing microporous systems in unconventional reservoirs, summarizes the fractal analysis corresponding to the different methods, sorts out the relationship between the fractals and reservoir macroscopic physical properties (porosity, permeability, etc.) with the reservoir microscopic pore structures (pore structure parameters, pore connectivity, etc.). The research focuses on cutting-edge applications of characterization techniques, such as improved characterization accuracy, calibration of PSD ranges, and identification of different hydrogen compositions in pore systems for dynamic assessment of unconventional reservoirs. Fractal dimension analysis can effectively identify the quality level of the reservoir; complex pore-throat structures reduce permeability and destroy free fluid storage space, and the saturation of removable fluids is negatively correlated with D_f. As for the mineral composition, the fractal dimension is positively correlated with quartz, negatively correlated with feldspar, and weakly correlated with clay mineral content. In future qualitative characterization studies, the application and combination of contrast agents, molecular dynamics simulations, artificial intelligence techniques, and 4D imaging techniques can effectively improve the spatial resolution of the images and explore the adsorption/desorption of gases within the pores, and also help to reduce the computational cost of these processes; these could also attempt to link reservoir characterization to research on supercritical carbon dioxide-enhanced integrated shale gas recovery, carbon geological sequestration, and advanced underground hydrogen storage. Full article