Multiscale Pore Network Research Articles

Incorporation of Sub‐Resolution Porosity Into Two‐Phase Flow Models With a Multiscale Pore Network for Complex Microporous Rocks

AbstractPorous materials, such as carbonate rocks, frequently have pore sizes which span many orders of magnitude. This is a challenge for models that rely on an image of the pore space, since much of the pore space may be unresolved. In this work, sub‐resolution porosity in X‐ray images is characterized using differential imaging which quantifies the difference between a dry scan and 30 wt% potassium iodide brine saturated images. Once characterized, we develop a robust workflow to incorporate the sub‐resolution pore space into a network model using Darcy‐type elements called microlinks. Each grain voxel with sub‐resolution porosity is assigned to the two nearest resolved pores using an automatic dilation algorithm. By including these microlinks with empirical models in flow modeling, we simulate single‐phase and multiphase flow. By fine‐tuning the microlink empirical models, we match permeability, formation factor (the ratio of the resistivity of a rock filled with brine to the resistivity of that brine), and drainage capillary pressure to experimental results. We then show that our model can successfully predict steady‐state relative permeability measurements on a water‐wet Estaillades carbonate sample within the uncertainty of the experiments and modeling. Our approach of incorporating sub‐resolution porosity in two‐phase flow modeling using image‐based multiscale pore network techniques can capture complex pore structures and accurately predict flow behavior in porous materials with a wide range of pore size.

Study on CO2 and CH4 Competitive Adsorption in Shale Organic and Clay Porous Media from Molecular- to Pore-Scale Simulation

Summary The elucidation of the competitive adsorption behaviors between CO2 and CH4 holds great importance in the context of improving natural gas recovery in shale reservoirs. Shale rock, as a complex porous medium, exhibits a highly interconnected multiscale pore network with pore size spanning from several to tens of nanometers. Nevertheless, accurately capturing the adsorption effects and studying the CO2/CH4 competitive adsorption within a large-scale, realistic, 3D nanoporous matrix remains a significant challenge. In this paper, we proposed a novel lattice Boltzmann method (LBM) coupled molecular simulation to investigate CO2/CH4 competitive adsorption in 3D shale nanoporous media. The initial step involves conducting Grand Canonical Monte Carlo (GCMC) simulations to simulate the competitive adsorption behaviors of CO2 and CH4 in kerogen and illite slit pores, with the aim of obtaining the atomic density distribution. Subsequently, a Shan-Chen-based lattice Boltzmann (LB) simulation is used under identical conditions. By coupling the molecular simulation results, the fluid-solid interaction parameters are determined. Finally, LB simulations are performed in designed 3D porous media, utilizing the fluid-solid interaction parameters. The effects of mineral type, CO2 concentration, and pore structure on competitive adsorption behaviors are discussed carefully. Our research offers significant contributions to the improvement of gas recovery and carbon geological sequestration through the examination of CO2/CH4 competitive adsorption in nanoporous media. Additionally, it serves as a link between molecular and pore-scale phenomena by leveraging the benefits of both molecular simulations and pore-scale simulations.

A pore network-based multiscale coupled model for rapid permeability prediction of tight sandstone gas

Tight sandstone has multiscale pore structures. Gas transport in tight sandstones involves several length scales and the complicated physics. Gas transport characteristics in tight sandstone can be represented by apparent gas permeability. Microstructure-based accurate and efficient calculation of gas permeability is challenging. By combining CT and SEM images along with statistical analysis, we present a novel pore network-based multiscale coupled model (MCPNM) to rapidly predict the apparent gas permeability. CT images are adopted to extract the large-scale pore network (LPNM) and the clay component, and SEM image is used to get the properties of small-scale pores. Upscaled model (UM) of small-scale pores is built via statistical analysis and then assigned to the clay domains. The LPNM and UM are coupled as MCPNM by the cross-scale connection structure with variable diameter. The pore spaces at several length scales and the flow characteristics in them are included in the MCPNM. We validate the MCPNM by comparing the calculated apparent gas permeability to the results of available multiscale pore network models and the experimental data. Compared with the available multiscale pore network models, MCPNM solves the accuracy/efficiency trade-off of tight gas permeability prediction. The effects of pressure, temperature, and gas type on gas permeability are studied. The MCPNM simplifies the permeability calculation process and can accurately and rapidly predict tight sandstone gas permeability.

Multiscale pore network modeling and flow property analysis for tight sandstone: a case study

Abstract Digital rock characterization enables high-fidelity quantification of core samples, facilitating computational studies of physical properties at the microscopic scale. Multiscale tomographic imaging resolves microstructural features from sub-nanometer to millimeter dimensions. However, single-resolution volumes preclude capturing cross-scale morphological attributes due to the inverse relationship between the field of view and resolution. Constructing multiscale, multiresolution, multiphase digital rock model is therefore imperative for reconciling this paradox. We performed multiscale scanning imaging on tight sandstone samples. Based on pore network model integration algorithms, we constructed dual-scale pore network model (PNM) and fracture-pore hybrid network model to analyze their flow characteristics. Results showed that the absolute permeability of the dual-scale PNM exhibited a distinct linear increase with the number of extra cross-scale throats and throat factor, but the rate of increase became smaller when the throat factor exceeded 0.6. For dual-scale pore network with cross-scale throat and throat factor of 1 and 0.7, the predicted porosity matched experimental results well. For the fracture-pore hybrid network model, the relationship between absolute permeability and cross-scale throat properties is similar to the dual-scale PNM. When fluid flow was parallel to the fracture orientation, permeability increased markedly with fracture aperture as a power-law function. However, the dip angle did not induce obvious permeability variation trends across different flow directions.

Multi-scale pore network modelling to evaluate connectivity in ceramic composites

Complex morphologies, such as open or connected feature networks, are present in a wide variety of materials. Characteristics of these networks can impact key performance attributes of the materials themselves, affecting transport properties such as thermal conductivity. Therefore, it is critical to analyze the microstructure of these materials to gain a better understanding of the fundamental characteristics of the morphology. This study utilized pore network modeling as a method to extract morphological information on the solid network formed by boron nitride ceramic flakes in a polymeric resin matrix and uses the characteristics of the model to analyze the connectivity of the flakes. In this work, Micro-CT and FIB/SEM tomography were used in tandem to provide complimentary analyses of the microstructure and nanostructure, respectively, of the flake network to understand how this may contribute to transport properties of the material. Rather than a pore network model (PNM), the flake network model (FNM) was extracted from the tomographic datasets and the coordination number distribution was determined for the flakes detected in each. Micro-CT analysis showed that the flakes had formed a cage-like network around the exterior of the sample with limited connectivity in the interior, likely due to flake agglomeration at the outer surface of the material. A comparison of the full and interior-only Micro-CT FNMs indicated lower connectivity in the interior. This was confirmed by flow rate models generated from the network analysis for the flake contact points. The FNM extracted from the FIB/SEM tomography dataset exhibited similar connectivity compared to the interior-only FNM, indicating that the connectivity of the material was consistent when measured at the micron scale and at the nanometer scale.

Multi-Scale and Multi-Region Pore Structure Analysis on Sandy Conglomerate Whole Core With Digital Rock Model

Abstract Based on industrial computed tomography (CT), the whole core sandy conglomerate is scanned with a resolution of 0.5 mm/voxel, and the representative debris region and filling region subsample is selected to be scanned with a resolution of 15 µm/voxel using micro-CT. Then, four regions of the whole core sandy conglomerate image are segmented with the multi-threshold segmentation algorithm including macro pore, debris, filling, and gravel regions, while binary segmentation is performed on the debris and filling subsamples to segment the debris pores and filling pores respectively. Finally, the multi-scale and multi-region pore network model of the sandy conglomerate was constructed by the integration method to analyze the different types of pore characteristics. It can be found that the integrated sandy conglomerate model can reflect the structural characteristics of macro pore, debris pore, and filling pore at the same time. Meanwhile, the porosity and permeability of the integrated sandy conglomerate model are calculated and they are basically consistent with that of lab test results, which greatly increase the accuracy of the multi-scale multi-region pore network model.

Multiscale pore-fracture hybrid pore network modeling for drainage in tight carbonate

Tight carbonate possesses both fractures and multiscale pores. Flow through tight carbonate rocks involves several length scales and the complicated flow physics. Capturing and modeling the entire void space into a single pore network model for drainage is challenging. Here, we present a novel method for modeling of a multiscale pore-fracture hybrid pore network, which enables various void spaces at multiple length scales and the flow characteristics in them to be included. Pores and fractures in a low-resolution CT image are extracted by medial axis-maximal ball algorithm and medial surface algorithm, respectively, to generate a pore-fracture hybrid pore network. Then, a statistics-based small-scale stochastic pore network is established in the CT image solid domains to represent the micropores unresolved by the image. The two pore networks are organically integrated into a multiscale hybrid pore network by cross-scale connections. The statistics of small-scale pore network are determined by experimental permeability. We study the effects of the small-scale pore network density on connectivity, single- and two-phase flow properties of the model to obtain the robust parameter. The influence of fractures on two-phase flow properties is analyzed. Results show the significance of incorporating fractures and micropores in the model. The model is validated via comparison between simulation result and experimental data. This method can accurately, efficiently, conveniently construct multiscale pore-fracture hybrid pore network of tight carbonate rock for evaluating the drainage characteristics.

Multiscale pore network construction for two phase flow simulations in granular soils

Characterizing and modeling the pore structure is the key to simulating single and multiphase flow through granular soils. Modeling the grain-based packing and predicting the multiphase flow properties of granular soils with a wide grain size distribution (GSD) spanning over several orders of magnitude is still a challenging task. This work presents a novel numerical framework for constructing the pore network of granular soils with a wide grain size variation. The present work focuses on using basic properties of granular soils like GSD and porosity to generate pore network properties that are further utilized to simulate the water retention properties of granular soils along drying and wetting paths. A topologically equivalent network of pores and throats is extracted from modeled soil packing at different length scales. By integrating the pore networks extracted from individual length scales in the main simulating domain, a multiscale pore network that represents the pore structure of the soil to be modeled is obtained. The pore-scale phenomena like piston-like advance, corner flow (wetting layers), pore body filling, and snap-off are used to model fluid displacements at the pore scale. The estimated soil water retention curves for various granular soils are in good agreement with experimental data from the literature, and the hysteretic wetting-drying response is also accurately predicted.

Novel Discrete Fracture Network Model for Multiphase Flow in Coal

Multiphase flow intensely affects the movement and accumulation of other fluids in coal and plays an essential role in predicting the permeability of coal during coalbed methane (CBM) production. Fractures have played a decisive role in the transport of CBM after hydraulic fracturing has occurred. In this study, a multiscale pore network model (PNM) was constructed on the basis of focused ion beam scanning electron microscopy (FIB-SEM) image results. Additionally, a novel discrete fracture network model, fracture–pore network model (F–PNM), was proposed to investigate the effect of fracture density, fracture developing direction, and wettability on multiphase flow. The results reveal that the permeability of F–PNM increases with the increase of the fracture density, which could be the result of the predominance of snap off. The permeability decreases as the angle between the fracture and flow direction increases; initially, the permeability decreases steeply and then it tends to remain stable; and for angles between 0° and 15°, the permeability decreased by as much as 61.8%. Moreover, the wettability of coal has limited impact on its water relative permeability; however, it has a measurable effect on gas relative permeability, which could be owing to water accumulation on the coal surface under different wettability conditions. A good wetting performance would have a negative effect on the CBM production and reduce flowback efficiency.

Manufacturing of scaffolds with interconnected internal open porosity and surface roughness.

Manufacturing of three-dimensional scaffolds with multiple levels of porosity are an advantage in tissue regeneration approaches to influence cell behavior. Three-dimensional scaffolds with surface roughness and intra-filament open porosity were successfully fabricated by additive manufacturing combined with chemical foaming and porogen leaching without the need of toxic solvents. The decomposition of sodium citrate, a chemical blowing agent, generated pores within the scaffold filaments, which were interconnected and opened to the external environment by leaching of a water-soluble sacrificial phase, as confirmed by micro-CT and buoyancy measurements. The additional porosity did not result in lower elastic modulus, but in higher strain at maximum load, i.e. scaffold ductility. Human mesenchymal stromal cells cultured for 24 h adhered in greater numbers on these scaffolds when compared to plain additive-manufactured ones, irrespectively of the scaffold pre-treatment method. Additionally, they showed a more spread and random morphology, which is known to influence cell fate. Cells cultured for a longer period exhibited enhanced metabolic activity while secreting higher osteogenic markers after 7 days in culture. STATEMENT OF SIGNIFICANCE: Inspired by the function of hierarchical cellular structures in natural materials, this work elucidates the development of scaffolds with multiscale porosity by combining in-situ foaming and additive manufacturing, and successive porogen leaching. The resulting scaffolds displayed enhanced mechanical toughness and multiscale pore network interconnectivity, combined with early differentiation of adult mesenchymal stromal cells into the osteogenic lineage.

A one dimensional ubiquitiform damage model for quasi-brittle materials

Considering the self-similar characteristic of irregular pore network of materials during the damage evolution process, a pore ubiquitiform model (PUM) is developed to characterize the multiscale pore or microcrack network of quasi-brittle materials, and then based on the PUM, the ubiquitiform damage model (UDM) is proposed to describe the damage evolution process of quasi-brittle materials under quasi-static uniaxial tensile loading. The values of porosity and pore specific surface area estimated from the PUM and the stress-deformation curves including the softening curve calculated from the UDM are in good agreement with previous experimental data, respectively. It can be found that, the PUM can be adopted to characterize the self-similar multiscale pore network and estimate the porosity and pore specific surface area, and the UDM can describe the damage evolution process of quasi-brittle materials under quasi-static uniaxial tensile loading. Meanwhile, the UDM can characterize the self-similar characteristic of pore network during the damage evolution process of quasi-brittle materials through the evolving ubiquitiform complexity.

Novel Shale Permeability Model Coupling Sorption-Induced Differential Deformation and Multiple Flow Regimes

Under certain stress conditions, tight shale is characterized by the features of gas storage rocks, such as low permeability, low porosity, multiscale pore network, and elastic deformation. Flow-regime-based permeability models can reflect the behavior of non-Darcy gas transport but cannot characterize the elastic performance of tight shale. In this study, we introduce a novel permeability model that couples the sorption-induced differential deformation and multiple flow regimes in shale. The model not only determines flow-regime-based permeability for low pore pressures and micro–nano flow paths but also reflects the differential-swelling-index-based poroelasticity for high pore pressures and macro flow paths. The parameter sensitivity of the proposed model is analyzed. The results show that, with the increase in pore pressure at a constant confining pressure, the model is more sensitive to initial permeability than to initial porosity. The compressibility coefficient, which quantifies shale deformability, significantly affects the permeability evolution predicted by the model. The sorption-capacity-controlled differential deformation does not affect the overall permeability evolution but affects the upper and lower limits of permeability distribution in the intermediate process.

Hydrocarbon accumulation model influenced by “three elements (source-storage-preservation)” in lacustrine shale reservoir-A case study of Chang 7 shale in Yan’an area, Ordos Basin

The organic-rich shales of the Chang 7 Member in the Yan’an Formation of the Yan’an area, Ordos Basin is a hot spot for lacustrine shale gas exploration. In this paper, taking the Chang 7 Member shale in the Yan’an area as an example, the main controlling factors of lacustrine shale gas accumulation and the prediction of “sweet spots” are systematically carried out. The results show that the Yanchang Formation shale has the complete gas generating conditions. Shale gas accumulation requires three necessary accumulation elements, namely gas source, reservoir and good preservation conditions. The dynamic hydrocarbon generation process of the Chang 7 shale reservoirs is established according to the thermal simulation experiments of hydrocarbon generation, and the mechanism of catalytic degradation and gas generation in the Chang 7 Member under the background of low thermal evolution degree is revealed. The enriched authigenic pyrite can catalyze the hydrocarbon generation of organic matter with low activation energy, thereby increasing the hydrocarbon generation rates in the low-mature-mature stage. Different types of pores at different scales (2–100 nm) form a multi-scale complex pore network. Free gas and dissolved gas are enriched in laminar micro-scale pores, and adsorbed gas is enriched in nano-scale pores of thick shales, and silty laminates can improve the physical properties of the reservoir. This is because the laminar structure has better hydrocarbon generation conditions and is favorable for the migration of oil and gas molecules. The thickness of the lacustrine shale in the Chang 7 Member is between 40 and 120 m, which has exceeded the effective hydrocarbon expulsion thickness limit (8–12 m). At the end of the Early Cretaceous, the excess pressure of the Chang 7 shale was above 3 MPa. At present, horizontal wells with a daily gas production of more than 50,000 cubic meters are distributed in areas with high excess pressures during the maximum burial depth.

Achieving fast Zn-ion storage kinetics by confining nitrogen-enriched carbon nanofragments in a honeycomb-like matrix

Contriving ingenious and stable carbon structures is critical but of great challenge to improve the electrochemical kinetics in Zn-ion storage devices. In this study, nitrogen-enriched carbon nanofragments confined in a three-dimensional honeycomb-like hierarchical porous matrix are designed and constructed with a simple in situ confined thermal polymerization and carbonization strategy. By employing the as-prepared hierarchical carbon nanostructures for an advanced Zn-ion hybrid supercapacitor, a record high-rate capability has been obtained, manifested as a high specific capacity of 101.3 mAh g−1 at an extraordinary fast discharge rate of 100 A g−1. Additionally, a maximum energy density of 130.2 Wh kg−1 at a power density of 185 W kg−1, and a peerless power density of 92500 W kg−1 at the corresponding energy density of 93.7 Wh kg−1 are achieved, demonstrating the feasibility for high-power Zn-ion storage devices. Further kinetics and mechanism studies reveal that the fast Zn2+ storage kinetics originates from the N-enriched chemistry and the multiscale pore network which provide a hydrophilic surface, boost charge transfer, and expedite the ion diffusion.

Understanding gas transport mechanisms in shale gas reservoir: Pore network modelling approach

This report summarizes the recent findings on gas transport mechanisms in shale gas reservoir by pore network modelling. Multi-scale pore network model was developed to accurately characterize the shale pore structure. The pore network single component gas transport model was established considering the gas slippage and real gas property. The gas transport mechanisms in shale pore systems were elaborated on this basis. A multicomponent hydrocarbon pore network transport model was further proposed considering the influences of capillary pressure and fluid occurrence on fugacity balance. The hydrocarbon composition and pore structure influences on condensate gas transport were analyzed. These results provide valuable insights on gas transport mechanisms in shale gas reservoir. Cited as: Song, W., Yao, J., Zhang, K., Yang, Y., Sun, H. Understanding gas transport mechanisms in shale gas reservoir: Pore network modelling approach. Advances in Geo-Energy Research, 2022, 6(4): 359-360. https://doi.org/10.46690/ager.2022.04.11

Engineering a superhydrophilic TiC/C absorber with multiscale pore network for stable and efficient solar evaporation of high-salinity brine

Achieving stable and efficient solar-driven evaporation of high-salinity brine is highly desirable while challenging because of salt inevitably blocking the evaporator surface during vapor generation. One of the effective resolvents is to construct macropores to ensure sufficient diffusion and convection of concentrated salt back into the bulk water. Herein, inspired by the interconnected pores of the balsa wood, superhydrophilic porous TiC/C absorber (SPTCA) with macro-/micro-/nano-pores network is successfully built by carbothermic reduction. The macropores channel plays the role of water transport, steam overflow, and salt ion diffusion backflow. The micro-/nano-pores channel enables broadband light absorption and keeps the moisture in a small range to achieve high-efficiency evaporation. With this rational design, the evaporator alloys for a stabilized evaporation of 1.78 kg/m2 h from simulated seawater for over 100 days and 1.75 kg/m2 h from high-salinity brine (15 wt.% NaCl) for over 50 days under 1 sun irradiation. The SPTCA provides a promising avenue for architecting the multiscale-pores network for solar desalination of high-salinity brine.

Multi-scale pore structure, pore network and pore connectivity of tight shale oil reservoir from Triassic Yanchang Formation, Ordos Basin

The accurate evaluation of microscopic pore structure characteristics of shales is of great significance for shale oil development. In order to gain a better understanding of complicated pore structure of lacustrine shales, silty shale samples from the Upper Triassic Yanchang formation in Ordos Basin were investigated using FE-SEM, N2 adsorption, MIP, helium pycnometry and Nano-CT. FE-SEM observations show that five typical pores (interparticle pores, intraparticle pores, dissolution-related pores, organic matter pores and microfractures) are developed in shale samples. The full-scale pore size distributions, determined by combining N2 adsorption, MIP and Nano-CT, show that the pore sizes of the samples range from 2 nm to more than 20 μm, while the dominant pores are in the range of 20–100 nm. A certain difference exits in porosity obtained by Nano-CT, N2 adsorption, MIP, helium pycnometry and the combination methods. The N2 adsorption tends to underestimate the total pore volume and porosity. The MIP may overestimate or underestimate the porosity, due to the matrix compression or limited penetration of mercury, respectively. The combined porosity using multiple methods is reasonably consistent with helium pycnometry. Pore network models of these samples are extracted using the centerline algorithm. From the pore network model, the proportion of connected pores of the studied samples ranges from 15.51% to 37.45%. The coordination numbers obtained from pore network range from 1.34 to 1.84, indicating the relatively poor pore connectivity of the studied samples. Samples rich in organic matter have high proportion of connected pores in the region of interest (ROI) of 3D network. Pore throats show preferential orientation in XY plane (parallel to bedding) and then their two diagonal directions, while the Z axis of the pore structure (perpendicular to bedding) is less connected. These parameters improve the understanding on storage space and transport property of lacustrine shale oil.

Permeability of cementitious materials using a multiscale pore network model

This paper presents a new multiscale pore network modelling framework for predicting saturated and unsaturated permeability of OPC-based cementitious materials using a novel algorithmic implementation. The framework fundamentally relies on the data on cement composition and current understanding of cement hydration kinetics and microstructural features. Central to the modelling framework is the ability to numerically estimate pore size distribution (PSD) from existing models and the ability to obtain snapshots of unsaturated microstructure for various degrees of saturation. The framework is an amalgamation of three important existing models: (i) particle packing model for predicting nanoscale PSD, (ii) cement hydration kinetics to estimate microscale PSD, and (iii) a pore network model to estimate the permeability. The proposed pore network modelling is validated against an extensive set of experimental data that includes a very wide range of materials. The predicted intrinsic permeability falls well within the accepted experimental range. Though fewer experimental data are available to compare, the predicted unsaturated permeability shows highly promising results.

The Impacts of Pore Structure and Relative Humidity on Gas Transport in Shale: A Numerical Study by the Image-Based Multi-scale Pore Network Model

The Impacts of Pore Structure and Relative Humidity on Gas Transport in Shale: A Numerical Study by the Image-Based Multi-scale Pore Network Model

Rapid multiscale pore network modeling for drainage in tight sandstone

Tight sandstone possesses multiscale pores. Capturing and modeling the important pore properties into a pore network for drainage is challenging and computationally expensive. We present a method for modeling of a multiscale pore network, which enables us to handle huge rock micro computed tomography (μCT) images with high computational efficiency and minimal experimental input data. This method uses micro-links to represent the upscaled transport properties of micropores unresolved by the image and connects pore clusters that have topological relationships in the image to extract a connected pore network from it. The size distribution of micro-links is optimized and determined by experimental permeability. The effects of the number distribution of micro-links on flow properties are investigated to obtain the robust parameter. The presented method is validated on a digital core and the published experimental data. This method is successfully applied to rapidly evaluate the influence of mineral components on drainage behavior in two tight sandstone rocks, which involves a large number of computations. The two μCT images both contain more than 109 voxels and are handled by a personal computer, proving the potential in rapidly, accurately, and conveniently handling huge tight sandstone μCT image for evaluating the drainage characteristics.