intraP Pores Research Articles

Reservoir Characteristics of Marine–Continental Transitional Taiyuan Formation Shale and Its Influence on Methane Adsorption Capacity: A Case Study in Southern North China Basin

To further study the reservoir characteristics and adsorption capacity of the Taiyuan Formation shale in the South North China Basin (SNCB), the pore structure and adsorption capacity of shale are discussed using various analysis tests, including elemental geochemistry, organic geochemistry, mineral composition, low-temperature nitrogen adsorption (LTNA), and methane adsorption experiments. The results indicate that the Taiyuan Formation shale formed in a poor oxygen and anaerobic sedimentary environment in still water. The average value of total organic carbon (TOC) content is 2.37%. The organic matter type mainly consists of type III kerogen. The vitinite reflectance (Ro) ranges from 3.11% to 3.50%. The clay mineral content varies greatly, averaging at 40.7%, while the quartz content averages at 37.7%. The Taiyuan Formation shale mainly develops interparticle (InterP) pores, followed by organic pores, intraparticle (IntraP) pores, solution pores, and microfractures. BET specific surface area (SSA) is between 9.47 m2/g and 22.14 m2/g, while pore volume (PV) ranges from 0.0098 cm3/g to 0.022 cm3/g, indicating favorable conditions for shale gas storage. According to the results of the CH4 adsorption experiment, Langmuir volume from Taiyuan Formation shales exhibits 1.35~4.30 cm3/g, indicating excellent adsorption capacity. TOC content shows a positive correlation with both Langmuir volume and BET SSA from Taiyuan Formation shales, suggesting that TOC plays a crucial role in controlling microscopic pores and gas adsorption capacity. Organic matter enhances the shale adsorption capacity by providing abundant pore SSA. Due to formation compaction, the pore size of clay minerals decreases, leading to an increase in pore SSA, while kaolinite exhibits weak hydrophilic ability. Consequently, with the increase in clay minerals and kaolinite content, the shale adsorption capacity is enhanced to a certain extent. However, an increase in the carbonate mineral content may result in a decrease in the proportion of clay minerals, therefore reducing the CH4 adsorption capacity of shale.

Origin and nature of pores in the Toolebuc Formation, a potential unconventional target in Australia

The Toolebuc Formation of Australia, a potential unconventional hydrocarbon resource, has limited studies on its pore structure and sorption characteristics. In this study, shale samples covering the lower mixed argillaceous mudstone (MAM) lithofacies, the middle interbedded calcareous mudstone and shelly thin beds (CM-STB) lithofacies, and the upper interbedded calcareous mudstone and shelly horizons (CM-SH) lithofacies of the Toolebuc Formation were collected. These samples were analysed for pore structure using a combination of scanning electron microscopy, X-ray diffraction, helium pycnometry, mercury intrusion porosimetry, and N2 physisorption techniques. Additionally, methane sorption isotherms were measured under in-situ conditions. The results reveal that most pores are mineral-related intraparticle (intraP) and interparticle pores, with slit, equant and elongated shapes. Organic matter (OM) pores are rare. Porosity, total pore volume and BET specific surface area (SSA) are 3.25–8.26%, 1.32–3.55 cm3/100 g, and 1.26–9.65 m2/g, respectively. Pore volume is dominated by mesopores and macropores while specific surface area is dominated by fine mesopores and micropores. The porosity of the organic matter is significantly low due to the rarity of OM pores in the stage of early oil-window thermal maturity; organic matter consequently occludes pore space and also negatively impacts the average porosity. Clay by contrast is positively correlated to the average porosity. Carbonate provides intraP pores in fecal pellets, but also fills in other pore spaces as occlusion. Methane isotherms exhibit linear shapes, suggesting that a portion of the gas is stored in solution. The lower MAM lithofacies, characterised by rich clay, high porosity and BET SSA, was measured to have an in-situ methane sorption capacity of 4.32 cm3/g; the middle CM-STB lithofacies has intermediate porosity, but exhibits excellent gas generation potential and high in-situ methane sorption capacity (4.12–5.5 cm3/g). Within the CM-STB lithofacies, porosity declines with depth. The upper CM-SH lithofacies is carbonate-rich, exhibiting the lowest porosity and in-situ methane sorption capacity (2.56 cm3/g), but may act as an intraformational seal. The combination and vertical stacking pattern of the three lithofacies provided a favourable setting for gas storage.

Pore-Type-Dependent Fractal Features of Shales and Implications on Permeability

Pore structure features govern the capacity of gas storage and migration in shales and are highly dependent on the types of pores, i.e., interparticle (InterP) pores, intraparticle (IntraP) pores and organic matter (OM)-hosted pores. However, fractal features in terms of pore types and their respective contributions to permeability have been rarely addressed. On the basis of high-resolution imaging, fractal dimensions (Ds) have been determined from both pore size distributions and digital rock to quantify the heterogeneity in pore morphology and spatial textures. Overall, OM-hosted pores are smaller in size and more abundant in quantity, corresponding to a relatively high D, while IntraP pores are mainly isolated and scarce, translating into lower D values. Additionally, crack-like InterP pores with a moderate level of porosity and the D can play a pivotal role in shale seepage potential. A comparison of the estimated permeability among different pore types highlights that the contribution of interconnected OM pores to the overall permeability remains constrained unless they can link neighboring pore clusters, as commonly observed in organo-clay composites. Furthermore, the pore morphology and fractal features of shale rocks can exhibit noteworthy variations subjected to sedimentology, mineralogy, diagenesis and OM maturation.

Pore Characteristics, Oil Contents and Factors Influencing Laminated Shale in the First Member of the Qingshankou Formation in the Gulong Sag, Northern Songliao Basin

To clarify the reservoir characteristics of laminated shale, the occurrence mechanism of shale oil and its influencing factors in the Gulong Sag, northern Songliao Basin, are studied to better guide the exploration and development of shale oil there. First, X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM) are used to characterize the pore types, pore geneses and factors influencing the pore volume in the study area. Second, the organic matter of the samples is extracted with a mixture of dichloromethane and methanol. Total organic carbon (TOC), nitrogen adsorption and Rock-Eval tests are performed on the samples before and after extraction to reveal the pore size distribution after extraction. The factors influencing free and adsorbed shale oil and the lower limit of pore size are discussed in detail. The results show that interparticle pores (interP pores), intraparticle pores (intraP pores), organic matter pores (OM pores) and microfractures can be found in the laminated shale (Q1) in the Gulong Sag, Songliao Basin, and that the interP pores and intercrystalline pores in clay minerals are the main pores. The FE-SEM results show that the diameters of interP pores vary from several hundred nanometers to several microns, and their morphologies are mainly triangular, strip-shaped or irregular. The morphology of the intercrystalline pores in the clay minerals is generally irregular, depending on the crystal type and arrangement of clay minerals. According to the characteristics of the nitrogen adsorption and desorption curves, the pore morphologies are mainly slit-shaped pores, parallel-plate-shaped pores and ink-bottle-shaped pores. The pore size distribution is mostly bimodal, and the pore volume contribution is the greatest in the pore size range of 10~20 nm. Before and after extraction, the overall characteristics of the pore size distribution change only slightly, but the number of micropores increases significantly. Different minerals have different degrees of influence on the proportions of micropores, mesopores and macropores. Quartz mainly inhibits the formation of micropores, while the overall effect on mesopores and macropores is positive depending on the diagenetic period. Feldspar has a strong positive correlation with the micropore and mesopore proportions but is not highly correlated with the macropore proportions. The influence of the carbonate mineral content on the pore volume is not obvious because of its complex composition. The TOC content and vitrinite reflectance (Ro) are the two most important factors controlling free oil and adsorbed oil, and the contents of mineral components, such as felsic minerals, carbonate minerals and clay minerals, have no obvious correlation with shale oil content. With increasing pore volume, the contents of free oil and adsorbed oil increase, but the proportion of adsorbed oil decreases gradually. The correlation between the specific surface area and adsorbed oil content is poor. At normal temperatures and pressures, the lower limit of the pore diameters that can contain free oil is 4 nm, and the lower limit of the pore diameters that can contain movable oil is 10 nm.

Study on the Pore Structure and Fractal Characteristics of Different Lithofacies of Wufeng-Longmaxi Formation Shale in Southern Sichuan Basin, China.

Gas content and flow characteristics are closely related to shale lithofacies, and significant differences exist in the pore structure and fractal characteristics among lithofacies. In this study, X-ray diffractometer (XRD), field-emission scanning electron microscopy (FE-SEM), gas adsorption (N2 and CO2), and fractal theory were employed to systematically characterize the pore attributes of the marine Wufeng–Longmaxi formation shales. The information of various pores and microfractures among lithofacies was extracted and quantified via high-resolution FE-SEM image stitching technology. Shales were classified into four types based on mineral compositions, and siliceous shales possess the largest SEM-based surface porosity (2.84%) and the largest pore volume (PV) (average 0.0243 cm3/g) as well as specific surface area (SSA) (average 28.06 m2/g). The effect of lithofacies variation on the PV of shale is minor. In contrast, the lithofacies variation has a significant impact on the SSA, and the SSA of siliceous shale is 39.11% higher than that of argillaceous shale. PV and SSA show strong positive correlation with the total organic carbon (TOC) content but negative correlation with clay minerals. Siliceous shales have the greatest fractal dimension D1 (pore surface roughness) (average 2.6821), which is contributed by abundant organic matter pores with more complicated boundaries. The largest fractal dimension D2 (pore structure complexity) (average 2.8263) is found in mixed shales, which is attributed to well-developed intraparticle (intraP) pores associated with carbonate mineral dissolution. This indicates that siliceous shales have the highest methane adsorption capacity and that shale gas desorption, diffusion, and seepage are more difficult in mixed shales.

Characterization of the Full-Sized Pore Structure and Controlling Factors of the Coal-Bearing Shale in the Wuxiang Block, South-Central Qinshui Basin, China

The characterization of the full-sized pore structure is important for the evaluation and prediction of the reservoir of shale gas with strong heterogeneity. It is of great scientific significance to explore the pore structure characteristics of overmature coal-bearing shale. Core descriptions, X-ray diffraction (XRD), vitrinite reflectance (Ro), field emission scanning electron microscopy (FE-SEM), high-pressure mercury intrusion porosimetry (MIP), and low-pressure N2/CO2 gas adsorption (N2-/CO2-GA) experiments were performed on overmature coal-bearing shale samples from the Wuxiang block, south-central Qinshui Basin, China. The results show that the total organic carbon (TOC) ranged from 0.29 to 8.36%, with an average of 3.84%, and the organic matter (OM) is dominated by type III kerogen. The minerals in the shale primarily consist of clay (43–85.5%, averaging 52.1%) and quartz (12.6–61.2%, averaging 43.5%). The major clay minerals are illite-smectite (I/S) and illite, ranging from 22.5 to 55.6% (mean 41.4%) and 8.7–52.7% (mean 32%), respectively. FE-SEM images reveal that intraparticle pores (IntraP pores) and interparticle pores (InterP pores) are widely developed in clay minerals, and organic pores are occasionally present. Mesopores make the greatest contribution to the total pore volume (PV), and micropores are the major contributors to the specific surface area (SSA). Clays are the main controllers of micropore development. Mesopores developed in the clay mineral layers are promoted by I/S but inhibited by illite. Macropores and microfractures are mainly developed in clays and quartz and do not correlate significantly with the TOC, or mineral composition, due to the influence of compaction and cementation. The TOC and minerals affect pore structure characteristics mainly by influencing micropores.

Structural deformation and its pore-fracture system response of the Wufeng-Longmaxi shale in the Northeast Chongqing area, using FE-SEM, gas adsorption, and SAXS

Tectonism caused strong deformation of the Wufeng-Longmaxi Formation in the periphery of the Sichuan Basin, which affected the pore-fracture of the organic-rich shale and migration of shale gas. Samples of structurally deformed shale from the anticline and detachment structure and undeformed shale were collected in the Northeast Chongqing area, China. The investigations of shale structural deformation and its pore-fracture system were performed using optical microscopy, field emission scanning electron microscopy (FE-SEM), low-pressure gas adsorption (LP-GA), and small-angle X-ray scattering (SAXS). The research shows that deformed samples with curved beddings and friction mirrors on the macro level, as well as the microscopically curved organic matter (OM) bands and OM folds, indicate the significant structural transformation and predominantly ductile deformation. The micro-fractures in deformed samples are more developed with different combinations in the two geological structures, showing brittle deformation. Compared with undeformed samples, the number of OM pores in deformed samples is reduced significantly, while the interparticle (interP) pores, intraparticle (intraP) pores, and micro-channels are more developed. The connected pore size distribution (PSD) of all samples is mainly from 0.3 nm to 10 nm. The pore volume (PV) of micropore in the deformed samples is between 0.100 and 0.500 cm3/100 g, and the specific surface area (SSA) is in the range of 2.175–13.554 m2/g. Undeformed samples have a larger PV and SSA than the deformed samples. TOC and the structural deformation control the structure of connected pore, and their PV and SSA are positively correlated with TOC content. Fractal dimension reveals that the surface of connected and closed pore (pore size 3.81–95.34 nm) becomes more complex, and the distribution of small pore (pore size 1.25–3.81 nm) becomes more uniform under the structural deformation of the shale. As the degree of shale deformation increases, the mesopore and macropore volume of connected pores decrease significantly, while the strongly structural deformation of samples results in a substantial increase in micropore volume and SSA. Besides, the PV of both closed and connected pores tends to decrease and the average pore size increases linearly. Furthermore, the shale deformation result in a weakening of the methane adsorption capacity. Shale gas can migrate through the fracture network to the central zone of the detachment structure or the core of the anticline, which has a significant effect on shale gas enrichment and preservation.

Micro‐ and nanoscale pore‐throat structure, fractal characteristics, and permeability estimation models in deep sandstones: Palaeogene Shahejie Formation, Nanpu Sag, NE China

Long‐term burial and strong diagenesis modification result in highly heterogeneous pore structures in deep clastic reservoirs; these heterogeneities exert a controlling influence over the development of these deep petroleum systems. A clear understanding of the pore structure simplifies the tasks of identifying and modelling reservoirs for the purposes of hydrocarbon exploration. This study characterizes the mineral compositions, physical properties, and pore systems of the first member of the Palaeogene Shahejie Formation (Es1) in the Nanpu Sag using thin section and scanning electron microscope observations, X‐ray diffraction measurements, mercury intrusion capillary pressure, and nuclear magnetic resonance data. Our results show that the Es1 sandstones typically fall into one of three different pore‐throat sizes: mainly 10 μm (type‐I), 1 μm (type‐II), and less than 1 μm (type‐III). Between type‐I and type‐III reservoirs, the pore system changes from residual intergranular pore (InterP) and secondary InterP to secondary intragranular pores (IntraP) and intercrystalline pores. The fractal theory is selected to quantify the heterogeneity of the pore‐throat structure, due to the existence of an irregular and complex pore system (IntraP and intercrystalline pores), the type‐III reservoir has the highest fractal dimension, which indicates that the complex pore‐throat structure is not conducive to the formation of high‐quality deep sandstone reservoirs. Using the fractal dimension and displacement pressure, a novel permeability prediction is proposed, compared with the previous method, the new model is simpler and more effective. Our research can deepen the geological understanding of deep‐burial sandstone reservoirs and further guide petroleum exploration.

Reservoir Characteristics of the Lower Permian Marine-Continental Transitional Shales: Example from the Shanxi Formation and Taiyuan Formation in the Ordos Basin

The pore types and pore structure parameters of the heterogenetic shale will affect the percolation and reservoir properties of shale; therefore, the research on these parameters is very important for shale reservoir evaluation. We used X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), low-pressure CO2 adsorption analysis, mercury injection capillary pressure (MICP), and high-pressure methane adsorption analysis to analyze the characteristics of different pore types and their parameters of the Lower Permian Shanxi Formation and Taiyuan Formation in the Ordos Basin. The influence of different mineral contents on the porosity and pore size is also investigated. The Shanxi Formation (SF) is composed of quartz (average of 38.4%), plagioclase, siderite, Fe-dolomite, calcite, pyrite, and clay minerals (average of 50.1%), while the Taiyuan Formation (TF) is composed of calcite (average of 37%), siderite, Fe-dolomite, quartz, pyrite, and clay minerals (average of 32.3%). The most common types of pores observed in this formation are interparticle pores (InterP pores), intraparticle pores (IntraP pores), interclay pores, intercrystalline pores (InterC pores), organic matter pores (OM pores), and microfractures. CO2 adsorption analysis demonstrates the type I physisorption isotherms, showing microporous solids having comparatively small external surfaces. The similar types of isothermal shapes of the Shanxi Formation (SF) and Taiyuan Formation (TF) suggest that both types have similar pore size distribution (PSD) within the measured pore range by the low-pressure CO2 adsorption experiment. The micropore pore size of the TF is larger than that of the SF. MICP shows the larger pores (>50 nm), and most of the volume was adsorbed by macropores. Methane gas sorption capacity increases with increasing pressure. Clay minerals and quartz played an important role in providing adsorption sites for methane gas. The overall analysis of both formations shows that TF has good reservoir properties than SF.

Pore Type, Pore Structure, and Controlling Factors in the Late Triassic Lacustrine Yanchang Shale, Ordos Basin, China

Organic-rich lacustrine shales in the Upper Triassic Yanchang Formation with thermal maturity mainly in the oil window are the main shale oil and shale gas system in the lacustrine strata of the Ordos Basin, China. Pore systems are important for the storage and transfer of shale oil and gas. The main objectives of this study are to identify the pore types and pore structures and investigate the controlling factors for pore types, pore structures, and total porosities of the lacustrine Yanchang Shale. In this study, organic-rich mudstones, mudstones with siltstone interlayers, siltstone, and sandstones were selected from 15 wells in the southern Ordos Basin. X-ray diffraction, pyrolysis, scanning electron microscopy (SEM), low-pressure nitrogen adsorption analysis, and helium porosimetry were conducted to investigate the mineral compositions, pore types, pore structures, porosities, and controlling factors. Siltstone and sandstone interlayers heterogeneously developed in the Yanchang Shale. The petrology, mineral composition, geochemistry, pore type, pore structure, and porosity of siltstone interlayers are different from those of mudstones. The siltstone and sandstone interlayers usually have more quartz and feldspars, greater detrital grain sizes, and relatively better grain sorting but are lower in clay minerals, total organic carbon (TOC), amount of free liquid hydrocarbons values (S1), and total residual hydrocarbons values (S2), compared to mudstones. Interparticle (interP), intraparticle (intraP) pores, and organic pores (OPs) were developed in both siltstones and mudstones. OPs were observed in samples with lower thermal maturity (e.g., 0.5–0.85%). The inorganic pore size is greater than that of OPs. Additionally, the inorganic pore diameters in siltstone interlayers are also greater than those in mudstones. Organic-rich mudstones generally have higher pore volumes (PVs) of pores with sizes less than 10 nm, pore volumes of pores with sizes between 10 and 50 nm (PV, 10–50 nm), and specific surface area (SSA), but they have lower PVs of pores with sizes greater than 50 nm, total PV, and porosity when compared to siltstone and sandstone interlayers. The dominant pore type in mudstones is OPs and TOC (first order), sources and OM types (second order), and thermal maturity (third order), while the abundances of rigid grains with greater sizes and grain sorting are the main controlling factors of pore structures, SSA and PV. Both inorganic pores and organic pores are abundant in the siltstone interlayers. The pore size distribution (PSD), PV, and porosity of siltstone interlayers are related to the abundance of rigid grains (first order), grain sorting (second order), grain size (third order), and carbonate cement content. The total PV and porosity of Yanchang Shale reservoirs may have increased with the increased abundance of siltstone and sandstone interlayers.

Pore characteristics and pore structure deformation evolution of ductile deformed shales in the Wufeng-Longmaxi Formation, southern China

Marine shales in southern China experiences multiple complex tectonic movements, which highly affects shale pore characteristics and pore evolution. The effects of tectonic deformation on nanopore structure and petrophysical properties of organic shales remain unclear. Three ductile deformed shale samples were collected and analyzed through an integrated experiment procedure for a comparative research against undeformed shale samples. Field emission-Scanning electron microscopy (FE-SEM), low pressure N2 and CO2 adsorption and mercury injection porosimetry (MIP) analyses were performed to characterize pore structure of the shales. The results reveal that the mineral compositions of these shale samples are controlled by sedimentary environment and source and exhibits no obvious relationship with tectonic deformation. With increasing deformation degree, the micropore proportion gradually decreases (from 17% to 6%), and the macropore proportion increases from 16% to more than 20%. Micropore volumes and proportion in the fold core samples are the smallest (4.2 × 10−3 cm3/g and 6.3%) and account for only one-third of the undeformed shale values, while the meso- and macropore values are larger-1.2 times those of the undeformed shale values. The evolution characteristics and deformation modes of organic matter (OM), interparticle (interP) and intraparticle (intraP) pores and microfractures under ductile deformation were examined. All pore types become interconnected during deformation and form microfractures ranging from a few nanometers to tens of microns, which is the main reason for the enhanced shale connectivity caused by deformation. Under the same porosity conditions, the deformed shale permeability is much higher than the undeformed shale permeability, and stronger deformation corresponds to higher permeability. The fold core permeability is generally higher than the limb permeability and exceeds 30 times. The different ductile deformation positions were divided into five levels, and the corresponding reservoir quality was evaluated. This study is of great significance to better understand the pore structure evolution process for shale exploration and development under tectonic deformation.

Multi-Angle Investigation of the Fractal Characteristics of Nanoscale Pores in the Lower Cambrian Niutitang Shale and Their Implications for CH₄ Adsorption.

Shale gas has received widespread interest due to its successful commercial development in China. Pore structures in shale can directly control its gas storage and migration properties. In this study, field emission scanning electron microscopy (FE-SEM), low-pressure N₂/CO₂ adsorption and highpressure methane adsorption were used to investigate the nanoscale pore structures of the Lower Cambrian Niutitang Formation in the southeastern Upper Yangtze platform. The fractal parameters of the pore structures were also calculated using the Frenkel-Halsey-Hill (FHH) model. The relationships between the fractal dimensions and TOC content, mineral composition and pore structure parameters were also discussed. The results show that organic matter and clay minerals are primary factors affecting the nanoscale pore development. Slit-shaped pores and ink-bottle-shaped pores are the predominant pore types in the Niutitang shale. The Brunauer-Emmett-Teller (BET) surface areas vary from 4.91 m²/g to 34.33 m²/g, and the pore volumes range from 0.689 m³/100 g to 2.964 m³/100 g. Two fractal dimensions (D1 and D₂) of the Niutitang shale were obtained using the FHH model, with D1 ranging from 2.605 to 2.684, and D₂ ranging from 2.681 to 2.865. D1 adequately characterizes the surface roughness of the pore structures, while D₂ represents the complexity of the pore types. Inter-particle (InterP) pores commonly have greater shape complexities than OM pores and intra-particle (IntraP) pores, based on analyses using Image-Pro Plus software. In addition, the TOC content and clay minerals have great effects on the fractal dimension D1. Meanwhile, the fractal dimension D1 increases with increasing BET surface area, but there is no definite relationship between the fractal dimensions and pore volumes. Both the fractal dimensions D1 and D₂ are negatively correlated with pore sizes. Further investigation indicates that the fractal dimension D1 exhibits a strong positive relationship with the methane adsorption capacity indicating that Niutitang shales with greater values of the fractal dimension D1 have higher methane adsorption capacities.

Micro- and Mesopores Occurring in Transitional Shales: An Examination Based on Pore Radius and Origin.

The microscopic pores in shales can be characterized based on different features, including pore morphology, pore size distribution and pore origin. In this work, shale samples from the Lower Permian Shanxi Formation and Upper Carboniferous Taiyuan Formation in the Linxing Block of the Ordos Basin were examined via experiments including X-ray diffraction (XRD), total organic carbon (TOC) analysis, field emission scanning electron microscopy (FE-SEM), automatic acquisition of large-image technology (MAPS), and low-temperature gas (CO₂ and N₂) adsorption/desorption experiments. The results show that clay minerals (average value of 53.9 wt.%) and quartz (average value of 39.0 wt.%) dominate the sample composition, with some K-feldspar, plagioclase, siderite, etc. The TOC values are between 1.15% and 8.46%, and all the shales are of middle-high maturity (the vitrinite reflectance (Ro) ranges from 1.23% to 1.75%). Based on FE-SEM images, the observed pore types include interparticle (interP) pores, intraparticle (intraP) pores, and organicmatter (OM) pores, and the pores and fractures at different scales are examined by MAPS. The distributions of micro- and mesopores and the proportions of the different pore sizes were determined from the N₂ and CO₂ adsorption results. The pore size distribution results suggest that N₂ mainly penetratesmesopores (2-50 nm), while CO₂ mainly penetrates large micropores (0.7-2 nm). Micropores are the largest contributors to the total pores. The mesopores were further classified as small, medium and large mesopores, and the proportion of mesopores decreased with increasing pore size.

Pore-scale simulations of electrical and elastic properties of shale samples based on multicomponent and multiscale digital rocks

The special intrinsic characteristics of shale formations, such as multiscale pore systems and complex mineral components, make the simulation of shale properties more difficult than that of sandstone properties. As such, one prerequisite of numerically predicting the physical properties of shale samples is to construct accurate shale models that include those special features. To this end, the study presents a novel hybrid stochastic modeling algorithm, called DEM-QSGSA, which integrates the discrete element method (DEM) and the quartet structure generation set algorithm (QSGSA), to build the necessary multicomponent and multiscale shale models. To investigate the effects of the components on the shale properties, the DEM-QSGSA was used to generate a dozen digital shale models with different mineral components containing quartz, feldspar, calcite, clay minerals, pyrite, and organic matter. The accuracy of the models was verified by comparing the characteristics of pore systems and the percentages of mineral components of the generated models with the ones of the SEM images of real shale samples. The electrical resistivity, bulk modulus, and shear modulus of these shale models were obtained using FEM. The results of electrical and elastic properties of these digital rocks indicate that the increase in abundance of the organic matter (OM) pores, intraparticle (intraP) pores, clay minerals, or OM results in a decrease in the electrical resistivity and elastic moduli when the pore systems of shale models are saturated with water. However, when the volume fraction of pyrite becomes larger, the elastic moduli increases and electrical resistivity decreases. Moreover, comparison of the sensitivity indices of the variables shows that pyrite has the largest effect on the electrical and elastic properties of shale samples, whilst clay minerals exert a moderate impact on them.

Porosity characteristics of different lithofacies in marine shale: A case study of Neoproterozoic Sinian Doushantuo formation in Yichang area, China

This study evaluates the pore network systems in the Neoproterozoic Sinian Doushantuo (Z1d) shale from the Yichang area in the Middle Yangtze region. All samples were classified based on their lithologies and examined by geochemical and petrographic analysis, assisted with carbon dioxide/nitrogen (CO2/N2) adsorption, mercury injection capillary pressure (MICP) and focused ion beam-scanning electron microscopy (FIB-SEM) to fully characterize the nanoscale porosity of the different shale lithofacies. The calcareous shale lithofacies group (I-C), mixed shale lithofacies group (II-M) and siliceous shale lithofacies group (III-S) were identified via ternary mineralogy diagrams. Clearly, different lithofacies had different porosities, pore volume, surface area and pore-size distribution (ranging from 0.3 nm to 36 μm). The mixed shale lithofacies group (II-M) had the highest total pore volumes, followed by the calcareous shale lithofacies group (I-C) and the siliceous shale lithofacies group (III-S). Peak values (2–20 nm, 20–50 nm, 50–60 nm and 90–200 nm) obtained from N2 adsorption were identified as organic matter (OM) pores, dissolution intraparticle (intraP) pores and intercrystalline pores (using SEM images analysis). Porosity shows a non-monotonic trend with TOC content and a maximum at 4.5–5.5 wt% TOC, similar relationships between TOC and mesopore/macropore volumes, due to pore collapse and compaction. The FIB-SEM images revealed solid bitumen and two types of kerogen in the examined shale samples; these different organic matter types had clearly distinct pore characteristics. While the smaller OM particles mobilized and converted into solid bitumen in which OM pores were larger and more abundant, the large-scale OM particles were most likely non-porous inert maceral and sponge-like porous amorphous kerogen. Overall TOC content and organic types are the key controlling factors to the nanoscale porosity development in the Doushantuo Shale, and the Sinian Doushantuo shale (in the Yichang area) has great potential for shale gas exploration and exploitation in South China.

Process-based and dynamic 2D modeling of shale samples: Considering the geology and pore-system evolution

Accurate characterization of shale samples is of great importance for predicting and evaluating their intrinsic properties as they undergo various geological processes. As such, constructing shale models not only entails accounting for the numerous components and pores, but also requires considering the effects of thermal maturation on the evolutions of the pore systems and components. In this study, a new dynamic and process-based modeling for considering the geological evolutions occurred in pore systems is developed, which makes a significant improvement in reproducing the complex geological features in shale samples. Three types of pores, including organic matter (OM), interparticle (interP), and intraparticle (intraP) pores, and several other components including kerogen, pyrite, clay minerals, globigerinids, calcite, feldspar, quartz, and petroleum products, are all considered in our process-based modeling. In order to demonstrate the performance of our process-based method, two-dimensional (2D) dynamic shale models of multiple cases with different geological processes are constructed. Then, to characterize the constructed samples, the occurrences and distributions of all components are studied using a two-point correlation function. Aside from statistical comparisons, the gas flow in these shale models is also simulated to exhibit the permeability variations produced in the process-based models. Moreover, the degree of cementation, the fractions of interP, intraP, and OM pores are varied to mimic new geological scenarios, which aims to further evaluate the performance of the proposed technique. The results indicate that our process-based model presents an excellent performance. Furthermore, the presented modeling technique can be extended to investigating the effects of various components and pores on the intrinsic properties of shales in the future.

Structure characteristics and evolution mechanism of nanopore in transitional coal-bearing shale

Nanopore plays a crucial role in shale gas adsorption, and its characteristics and evolution are significant for the evaluation of shale reservoirs with strong heterogeneity. In this study, transitional coal-bearing shale samples from the south margin of North China were selected for investigation of pore structure characteristics and evolution mechanism. Results indicated that nanopores of the studied shales are fractal with strong heterogeneity. The higher fractal dimension leads to larger surface area and micropore volume of shale samples. The abundant terrigenous material and limited hydrocarbon generation and expulsion are favorable for the development of intraP pores in clay, so that the Organic matter (OM) pores are poorly developed and are partially filled with clay minerals and pyrite framboids. Shale samples having higher levels of clay content and lower levels of quartz content tend to develop more heterogeneous micropores, producing a more complex pore structure in shale. The evolution of nanopore in studied shales is simultaneously determined by the coupling relationship between clay content and the thermal maturation of OM. In the early oil and low clay content stage, the low degree of the thermal evolution of OM leads to generation of less acidic fluid during the decarboxylation of kerogen, resulting in a weak physical combination and chemical corrosion between OM and clay minerals, with the macropores taking the main place in the pore structure. In the main to late oil and medium clay content stage, hydrocarbon generation of OM leads to the development of pores in kerogen and the gradual generation of more acidic fluids that dissolve clay minerals, so that the mesopores are dominant. In the wet gas and high clay content stage, increases in hydrocarbon generation intensity and clay mineral content cause development of more complexes of OM and clay, with a large number of micropores produced in shale reservoirs.

Study of subcritical and supercritical gas adsorption behavior in different nanopore systems in shale using lattice Boltzmann method

Adsorption has been studied for decades in either petroleum or chemical industry. However, the detailed adsorption behavior is yet not well understood in nanoporous media with complex pore structures and surface properties, especially for subcritical gases when capillary condensation might occur. Shale (mudrock) has complex pore systems as a result of heterogeneous mineralogy and diagenesis, and interparticle (interP) and intraparticle (intraP) pores are two main components. Understanding gas adsorption and phase transition behavior in different pore systems is essential for petrophysical characterization, reserve estimation and production forecast. In this study, we use lattice Boltzmann method (LBM) to study nitrogen (subcritical) and methane (supercritical) adsorption behavior in reconstructed shale nanopore systems. The model accounts for the van der Walls (vdW) forces between fluid molecules by incorporating a modified pseudopotential model based on Peng-Robinson equation of state (PR-EOS). Rock-fluid interaction is also considered by introducing phenomenological surface forces, which take two different forms for subcritical and supercritical gases respectively. The correlation between parameters defined in LBM and physical properties (i.e. density, pressure, isosteric heat of adsorption) is established systematically. Using the developed model, we observe quantitative agreement between LBM and lattice density functional theory (LDFT)/experimental data in terms of adsorption isotherms and density distributions. We then compare adsorption behavior in two ‘model’ interP and intraP pore systems and a synthesized polyethylene porous medium with more complex pore shapes. We observe three adsorption stages for subcritical gas adsorption: a) early capillary condensation near grain contacts or sharp corners, forming isolated pendular rings or fluid clusters; b) growing, merging, and spreading of the isolated condensed phase, forming circular gas/liquid meniscus; c) the condensed phase becomes fully-connected across the geometry, leaving isolated gas bubbles. We observe a signature behavior for the adsorption isotherm of intraP pore system, where a stepped feature occurs around relative pressure of 0.8, and the adsorption uptake for the polyethylene porous medium is bounded by the upper and lower limits given by the interP and intraP pore systems. On the other hand, we find that supercritical methane adsorption in mesoporous media is less sensitive to pore shape, but rather controlled by the specific surface area. The existence of micropores also affects the adsorption uptake.

Full-scale pores and micro-fractures characterization using FE-SEM, gas adsorption, nano-CT and micro-CT: A case study of the Silurian Longmaxi Formation shale in the Fuling area, Sichuan Basin, China

Pore-fracture structure of shales is important to shale gas resource potential evaluation, exploration and development. However, investigations about the pore-fracture structure of shales were mainly focused on nano-submicron pores, while the contribution of micrometer-scaled pores and micro-fractures to pore system of shale reservoirs lack sufficient attention. Here, using a combination of field-emission scanning electronic microscopy (FE-SEM), gas adsorption (N2 and CO2) and CT scanning (Nano-CT and Micro-CT), the Longmaxi shale cored from JYA well, in the Fuling area, was selected to comprehensively evaluate the characteristics of the pore-fracture structure at different scales. The results show that there are diverse types of pores in the organic-rich shale, mainly consisted of organic pores, inorganic pores (interP pores and intraP pores) and micro-fractures. Many pores are ink bottle-shaped characterized by narrow necks and wide bodies, while some others are slit-shaped. Pore size range from 0.305 nm to 98.5 μm. Pores with diameter of 0.305–2 nm (micro-pores), 2–50 nm (meso-pores), 50 nm-2 μm (macro-pores) and 2–98.5 μm (micro-fractures) account for 33.59%, 36.28%, 14.04% and 16.09% of the total pore volume, respectively. Moreover, shale permeability and specific surface area were calculated to be 0.005 mD and 39.44 m2/g, respectively, based on the full-scaled pore splicing. About 92% of specific surface area is contributed by pores with diameter of 0.305–5 nm. Micro-fractures connect with each other and form net-shaped structure with great connectivity from observational results. The micro-fractures with size of 20–98.5 μm provide the main permeability (about 90%). Micro-pores and small meso-pores in shale reservoirs provide large adsorption space for adsorbed gas, while micro-fractures enhance the seepage capability of shale gas, which are favorable for shale gas accumulation.

Nanopore Structure and Fractal Characteristics of Lacustrine Shale: Implications for Shale Gas Storage and Production Potential

In order to better understand nanopore structure and fractal characteristics of lacustrine shale, nine shale samples from the Da’anzhai Member of Lower Jurassic Ziliujing Formation in the Sichuan Basin, southwestern (SW) China were investigated by total organic carbon (TOC) analysis, X-ray diffraction (XRD) analysis, field emission scanning electron microscopy (FE-SEM), and low-pressure N2 adsorption. Two fractal dimensions D1 and D2 (at the relative pressure of 0–0.5 and 0.5–1, respectively) were calculated from N2 adsorption isotherms using the Frenkel–Halsey–Hill (FHH) equation. The pore structure of the Lower Jurassic lacustrine shale was characterized, and the fractal characteristics and their controlling factors were investigated. Then the effect of fractal dimensions on shale gas storage and production potential was discussed. The results indicate that: (1) Pore types in shale are mainly organic-matter (OM) and interparticle (interP) pores, along with a small amount of intraparticle (intraP) pores, and that not all grains of OM have the same porosity. The Brunauer–Emmett–Teller (BET) surface areas of shale samples range from 4.10 to 8.38 m2/g, the density-functional-theory (DFT) pore volumes range from 0.0076 to 0.0128 cm3/g, and average pore diameters range from 5.56 to 10.48 nm. (2) The BET surface area shows a positive correlation with clay minerals content and quartz content, but no obvious relationship with TOC content. The DFT pore volume shows a positive correlation with TOC content and clay minerals content, but a negative relationship with quartz content. In addition, the average pore diameter shows a positive correlation with TOC content and a negative relationship with quartz content, but no obvious relationship with clay minerals content. (3) Fractal dimension D1 is mainly closely associated with the specific surface area of shale, suggesting that D1 may represent the pore surface fractal dimension. Whereas fractal dimension D2 is sensitive to multiple parameters including the specific surface area, pore volume, and average pore diameter, suggesting that D2 may represent the pore structure fractal dimension. (4) Shale with a large fractal dimension D1 and a moderate fractal dimension D2 has a strong capacity to store both adsorbed gas and free gas, and it also facilitates the exploitation and production of shale gas.