Narrow Results

The complex structure and surface property of porous media have significant impact on its accumulation and adsorption capacity. Based on the fractal theory, this paper presents a fractal pore structure model for shales. The effect of different pore structures on fractal dimension is discussed, and the influence of fractal dimension and pore size distribution on porosity is also analyzed. It is shown that the fractal dimension D decreases with the increase of structure parameter q/m for a certain pore diameter ratio, and porosity has positive relationship with fractal dimension. This paper also presents a multilayer fractal adsorption model which takes into account the roughness of adsorption surface by using fractal theory. With the introduction of pseudo-saturated vapor pressure in the supercritical temperature condition, the proposed adsorption model can be applied into a wider range of temperature. Based on the low-pressure nitrogen adsorption and methane isothermal adsorption experiments, the effect of fractal dimension on the adsorption behavior of shales is discussed. Fractal dimension has significant impact on the surface adsorption property and adsorption layer number n. The monolayer saturated adsorption volume V_m increases with the increase of D, while parameter C has the opposite variation trend. Finally, the optimal combination of fractal parameters for describing pore structure of shale samples is selected.

Lacustrine shale gas has received considerable attention and has been playing an important role in unconventional natural gas production in China. In this study, multiple techniques, including total organic carbon (TOC) analysis, X-ray diffraction (XRD) analysis, field emission scanning electron microscopy (FE-SEM), helium pycnometry and low-pressure ${\mathrm{\text{N}}}_2$ adsorption have been applied to characterize the pore structure of lacustrine shale of Upper Triassic Yanchang Formation from the Ordos Basin. The results show that organic matter (OM) pores are the most important type dominating the pore system, while interparticle (interP) pores, intraparticle (intraP) and microfractures are also usually observed between or within different minerals. The shapes of OM pores are less complex compared with the other two pore types based on the Image-Pro Plus software analysis. In addition, the specific surface area ranges from $2.76{\mathrm{\text{m}}}^2/\mathrm{\text{g}}$ to $10.26{\mathrm{\text{m}}}^2/\mathrm{\text{g}}$ and the pore volume varies between $0.52{\mathrm{\text{m}}}^3/100\mathrm{\text{g}}$ and $1.31{\mathrm{\text{m}}}^3/100\mathrm{\text{g}}$. Two fractal dimensions $D_1$ and $D_2$ were calculated using Frenkel–Halsey–Hill (FHH) method, with $D_1$ varying between 2.510 and 2.632, and $D_2$ varying between 2.617 and 2.814. Further investigation indicates that the fractal dimensions exhibit positive correlations with TOC contents, whereas there is no definite relationship observed between fractal dimensions and clay minerals. Meanwhile, the fractal dimensions increase with the increase in specific surface area, and is negatively correlated with the pore size.

Tight oil sandstones have the characteristics of narrow pore throats, complex pore structures and strong heterogeneities. Using nuclear magnetic resonance (NMR) and mercury intrusion porosimetry (MIP), this paper presents an advanced fractal analysis of the pore structures and petrophysical properties of the tight oil sandstones from Yanchang Formation, Ordos Basin of China. Firstly, nine typical tight oil sandstone core samples were selected to conduct NMR and MIP test for pore structure characterization. Next, with the pore size distribution derived from MIP, it was found that the relationships between NMR transverse relaxation time $T_2$ and pore size are more accordant with the power function relations, which were applied to derive pore size distribution from NMR rather than the linear relation. Moreover, fractal dimensions of micropores, mesopores and macropores were calculated from NMR $T_2$ spectrum. Finally, the relationships between the fractal dimensions of different size pores calculated from NMR $T_2$ spectrum and petrophysical properties of tight oil sandstones were analyzed. These studies demonstrate that the combination of NMR and MIP can improve the accuracy of pore structure characterization and fractal dimensions calculated from NMR $T_2$ spectrum are effective for petrophysical properties analysis.

Fractal dimension is an important parameter in the evaluation of tight reservoirs. For an outcrop section of the Nenjiang formation in the Songliao Basin, China, the pore structure and pore fractal characteristics of shale parasequences were investigated using fractal theory. In addition, factors causing pore structure changes were analyzed using the results of low-temperature nitrogen adsorption and scanning electron microscope (SEM) experiments. Conducive to gas migration and secondary pores development such as dissolution, results showed that nanoscale pores dominated by fracture-like morphology and consequent good internal connectivity were observed in each pore size section within the target layer. Each parasequence is characterized by a sequential upward decrease of average pore size and an upward increase of total pore volume, with an increasing number of pores from 2nm to 50nm. Pores are isolated from each other, with poor connectivity and relatively complex composition of brittle minerals and clay minerals. Main components of the brittle minerals, quartz and feldspar, occur in 20–50% and higher clay mineral content ranging from 50% to 70%. In the parasequence cycle, clay mineral gradually decreases while the brittle mineral content increases. Fractal dimension is negatively correlated with clay mineral content and positively correlated with brittle mineral (quartz and feldspar) content. The fractal dimension calculated by the imaging method and the FHH method shows an upward increasing tendency in each of the parasequence cycles. This is as a result of different phenomena, varied sediment hydrodynamic forces leading to particle size differences and increased brittle minerals resulting in microcracks, therefore, the fractal dimension of the large pores (imaging method) increases upward in the parasequence. Simultaneously, with increased content and accompanied dissolution of brittle minerals causing an increase of small pores from base to top of the parasequence, the fractal dimension of the small pores (FHH method) grows.

Existing methods of well-logging interpretation often contain errors in the exploration and evaluation of carbonate reservoirs due to the complex pore structures. The differences in frequency ranges and measurement methods deviated between the acoustic well logs and indoor ultrasonic tests cause inconsistent results. Based on the elastic wave equation and the principle of the control variable method, a 2D axisymmetric borehole model with complex pore structures was developed, and the numerical simulation method for acoustic log was constructed. The modeling results show that the power function can well describe the effects of pore structure on the acoustic waves, while the velocity of the Stoneley wave is not sensitive to the pore structure. Crack-like pores with pore aspect ratio (AR) less than 0.1 significantly affect the velocities of P- and S-waves, whereas “spherical” pores have fewer effects. The models with larger pore sizes have high velocities of P- and S-waves. The velocities calculated by the equivalent medium theory are always higher than the numerical simulation results. The velocity deviation caused by the difference in frequency is much smaller than the pore structure. A fractal approach to quantify the effects of pore structures is applied in the acoustic logging data. The fractal dimension increases with the pore AR or size when the porosity is constant, which can be described by a simple power function. This gives us new ideas and methods for pore structure evaluation in the lower frequency range than the conventional petrophysical model.

In recent years, the application of fractal theory in construction materials has drawn tremendous attention worldwide. This special issue section containing seven papers publishes the recent advances in the investigation and application of fractal-based approaches implemented in construction materials. The topics covered in this introduction mainly include: (1) the fractal characterization of construction materials from nano- to micro-scales; (2) combining fractals methods with other theoretical, numerical and/or experimental methods to evaluate or predict the macroscopic behavior of construction materials; (3) the relationship of fractal dimension with the macro-properties (i.e. mechanical property, shrinkage behavior, permeability, frost resistance, abrasion resistance, etc.) of construction materials.

Calcium carbonate whisker (CW) can work as a cost-effective and environment friendly micro-fiber in reinforcing cementitious composites. Influence of high temperature on micro-structure of CW reinforced cement paste by nanoindentation and mercury intrusion porosimetry test is studied in this research. Up to 500${}^{\circ }$C, the indentation depth, elasticity modulus, indentation hardness and interfacial transition zone (ITZ) width of CW reinforced cement paste are near or even better than that at room temperature, due to the coupling effect of CW transformation from aragonite to calcite and internal autoclaving. However, when the temperature is higher than 700${}^{\circ }$C, nano-mechanical properties of CW reinforced cement paste degenerated significantly, due to the decomposition of CW and hydration products. Similarly, with the increase of temperature up to 400${}^{\circ }$C, the porosity and pore size increase little or even decrease, while the fractal dimension of pore volume increases. With the introduction of CW, the pore parameters and fractal dimension are decreased up to 400${}^{\circ }$C, due to the filler effect of CW. When the temperature is higher than 700${}^{\circ }$C, the pore diameter and fractal dimension of CW reinforced cement paste are significant higher than that of pure cement paste, due to the decomposition of CW and hydration products. In CW reinforced cement paste, the fractal dimension was increased with the increased temperature and porosity in this research. There are negative correlations between the pore volume fractal dimensions and the strengths of CW reinforced cement paste. Fractal dimension is a useful tool to evaluate the change of pore structure at high temperature.

Face slab concrete suffers from serious frost damage in the cold regions in China. How to improve the frost resistance of face slab concrete in cold regions is one of the important issues in concrete-faced rockfill dam (CFRD) design and construction. The results in this paper indicate that the frost resistance of concrete can be improved by adding fly ash, fiber, MgO and shrinkage-reducing admixtures (SRAs), and their efficiencies are in the following sequence: (fly ash $+$ fiber) $>$ fiber $>$ fly ash $>$ MgO $>$ SRA. The incorporation of 0.8kg/m³ polyvinyl alcohol (PVA) fiber and 20wt.% fly ash together enhances the compressive strength and tensile capacity of concrete by 6–7% at the late age, whereas the addition of 6wt.% MgO or 1wt.% SRA reduces the compressive strength and tensile capacity by about 4–10% at various ages. The $D_s$ of concrete added with fly ash, fiber, MgO and SRA is within the range of 2.619–2.796. The frost resistance of concrete correlates linearly with the air void parameters, pore structures and $D_s$. The utilization of fly ash and/or PVA fiber refines and optimizes the pore structure, thus increasing $D_s$ and improving the frost resistance. On the contrary, MgO and SRA in this study are less effective in refining the pores than PVA fiber and fly ash, thereby producing smaller $D_s$ and relatively weaker frost resistance.

Dam concrete suffers from serious abrasion damages in southwestern China, the abrasion resistance of concrete is therefore one of the most important factors determining the reliability even the safety of the dams. In the present work, the effects of fly and/or silica fume on the mechanical properties, drying shrinkage, as well the cracking and abrasion resistance of concrete were investigated, then the pore structures of concrete added with fly ash and silica fume and the pore surface fractal dimensions ($D_s$) were determined by the mercury intrusion porosimetry (MIP) method and a fractal model, respectively. Finally, the relationships between the abrasion resistance of concrete and the porosity as well as the $D_s$ were revealed and discussed. The results indicate that silica fume significantly increases the drying shrinkage especially at early age, while the incorporation of fly ash and silica fume together can decrease the early plastic shrinkage-induced cracking risk and the final shrinkage to some degrees. Besides, the utilization of 5wt.% silica fume and 20wt.% fly ash together increases the abrasion resistance and mechanical property by about 4–9% at various ages. In addition, the compressive strength and the abrasion resistance of concrete are linearly correlated with the concrete porosity and the $D_s$. Both the fly ash and silica fume could decrease the porosity of concrete and increase the $D_s$, therefore the concrete containing fly ash and silica fume have desired mechanical property and abrasion resistance. Moreover, $D_s$ has a more profound effect on the abrasion resistance of concrete than the concrete porosity. The addition of 5wt.% silica fume and 20wt.% fly ash together is suitable for the concretes used for wearing surfaces in terms of mechanical property, cracking and abrasion resistance.

The utilization of phosphorus slag (PHS) to replace the fly ash in the construction of hydraulic projects has attracted a growing attention in China. In this study, the influence of PHS fineness and content on cement hydration, mechanical strength, permeability as well as the pore structure and fractal dimension ($D_s)$ of concrete have been discussed. The results indicate that the PHS addition retards the cement hydration and hence decreases the hydration heat within three days. The incorporation of PHS with a Blaine specific surface area of 505m²/kg could participate in the early pozzolanic reaction and consequently offsets the retarding effect to some extent. The incorporation of 20–40wt.% PHS declines the early strength of concrete, but this reduction effect on strength can be eliminated to some degrees by mechanically grinding the PHS. The compressive strengths of concrete added with PHS with a high fineness of 505m²/kg (abbreviated as PHS-H) are about 16.0–20.6% higher at three days and 8.9–11.0% higher at 180 days compared that of the control concrete. The contribution of PHS-H to the pore structure refinement is more significant than that of PHS with a low fineness of 302m²/kg (abbreviated as PHS-L) at various ages because PHS-H is of much higher reactivity and can consume more Ca(OH)₂ than PHS-L which leads to a denser microstructure and a lower chloride diffusion coefficient ($D_{\mathrm{\text{RCM}}})$. The incorporation of PHS decreases the $D_s$ at three days, whereas the concrete incorporated with PHS has much higher $D_s$ than that of control one at late age. The $D_{\mathrm{\text{RCM}}}$ value increases with increasing the porosity and the most probable aperture, while $D_s$ has a more significant effect on $D_{\mathrm{\text{RCM}}}$ than the porosity and the most probable aperture. The concrete added with 20wt.% PHS-H exhibits the highest $D_s$ and the lowest $D_{\mathrm{\text{RCM}}}$ value at long-term age among the five concrete mixtures in this work.

Permeability is a critical parameter for characterizing the migration behavior of a fluid in porous media, thus being of great importance in guiding the geotechnical engineering practice. In this study, an improved permeability model was developed on the basis of the fractal features of the porous geomaterials. It further takes into account the periodic variation of the cross-section of the pore path in the flow direction and the roughness of the pore surfaces. This model indicates that the permeability is a function of the porosity, maximum pore radius, proportion of minimum pore radius to the maximum pore radius, roughness of the capillary tube, and variation in the cross-sectional characteristics of the tube. It provides a better insight into the transport mechanism of fluids in porous geomaterials. The permeability of the model is related to the variable cross-sectional characteristics of the pore channel, i.e. the belly-to-throat ratio and the roughness, as a quadratic function. These two parameters are the key to affecting the level of permeability. The model was verified over a wide range of permeability variations by changing from high-permeability media (10² mD) to low-permeability media (10⁻³ mD). Accurate permeability prediction of high–low-permeability porous media can be achieved by considering the variation of parameters.

Acoustic wave propagation is sensitive to many rock properties including fluid content, porosity, and pore structure, among others. Pore structure is one of the important parameters in controlling both seismic wave velocity and permeability in sandstones and carbonate rocks. For a given porosity of two similar rocks with different pore structure, their acoustic wave speeds can differ 2 km/s and permeability can span nearly six orders of magnitude from 0.01 mD to 20 D in both sandstones and limestones. In this paper, we introduce a two-parameter velocity model defined by porosity and a new pore structure parameter called as frame flexibility factor. Using this model, we define three pore structure types (PST) to quantify the pore structure effects on elastic properties of sedimentary rocks. These three PSTs have their distinct characteristics on synthetic shot gathers and common midpoint gathers. This study indicates that it may be feasible to use this new concept and method to detect pore structure variations in reservoir rocks from field seismic data. This study also helps explain why analysis of amplitude variation with offset (AVO) in some cases fails for fluid detection: pore structure effect on seismic waves can mask all the fluid effects, especially in carbonate rocks.

A series of spherical activated carbons (SACs) with different pore structures were prepared from chloromethylated polydivinylbenzene by ZnCl₂ activation. The effects of activation temperature and retention time on the yield and textural properties of the resulting SACs were studied. All the SACs are generated with high yield of above 65% and exhibit relatively high mesopore fraction (me%) of 35.7%–43.6% compared with conventional activated carbons. The sample zlc28 prepared at 800°C for 2 h has the largest BET surface area of 891 m² g^-1 and pore volume of 0.489 cm³ g^-1. SEM and XRD analyses of zlc28 verify the presence of developed porous structure composed of disordered micrographite stacking with large amounts of interspaces in the order of nanometers.

In this study, we report a new aluminosilicate fibrous porous ceramics-supported catalyst for the removal of ${\text{NO}}_x$. The influence of raw materials and sintering process on the structure and properties of aluminosilicate fibrous porous ceramics was studied. The composition and the structure of the catalysts were characterized by XRF, XRD, SEM-EDS, and XPS analysis, and the experiments of denitrification were performed to determine the influence of the porous structure and the calcination process on the denitrification efficiency. The results indicated that the glass fibers facilitate the formation of a uniform porous network structure within aluminosilicate fibrous porous ceramics, and that the optimal sintering temperature is 1000^∘C. Under the condition, the loaded catalysts were highly dispersed on the surface of aluminosilicate fibrous porous ceramics, which possessed high porosity and large average pore sizes. The aluminosilicate fibrous porous ceramics-supported catalyst exhibited excellent denitrification efficiency and catalytic stability, which is expected to be used for the treatment of ${\text{NO}}_x$ in flue gas.

Pores in cementitious materials are multi-sized ranging from several nanometers to several micrometers. Many properties of cementitious materials are directly or indirectly related to pore structure. Mercury intrusion porosimetry (MIP) is a common method for pore analysis. However, the accuracy of this method is now being questioned by more and more researchers. In this paper, pore size distribution and porosity of hard cement paste were measured by both MIP and backscattered electron image analysis (BSE-IA). Then, the merits and demerits of the two methods were analyzed. The results showed that BSE-IA was more accurate than MIP in quantitative study of large size pores However, small size pores couldn't be measured by BSE-IA due to the limitation of resolution of backscattered electron images.

Based on the statistic theory and image processing technology, a three-dimensional reconstructed model of pore structure for sandstone is established. On the basis of the model, the numerical simulation experiment is carried out to analyze the influence of pore structure parameters (pore size, shape and porosity) on the deformation, stress distribution and failure distribution of sandstone under the splitting load. Specific modeling process: firstly generating the spatial position of the pore with the statistical principle. Then using the finite element analysis software to establish the corresponding model and using the image processing technology in the image processing software to divide the model net. Importing the restructure 3D pore structure into the finite element analysis software and getting reconstruction of the pore model. The research results show that: the effect of void morphology on the damage of sandstone is obvious. With the change of the pore shape from the ellipsoid to the sphere, the fracture zone of the splitting disc is increased and more dispersed. With the pore shape is changed from sphere to ellipsoid, the maximum stress along the Y direction is firstly decreased and then increased. When the pore size increases, the failure mode becomes a single tensile failure from an induced-tension-shear failure, and the maximum stress along the Y direction is decreased. But the influence degree is weakened with the increase of the pore size of the model. Along with the increase of the porosity, the maximum stress along the Y direction is increased; therefore, the sandstone is easy to damage. But the degree of influence decreases with the increase of pore size. The results reveal the influence of the parameters of the pore structure on the mechanical properties of rock, which provide some reference for the study of the effect of pore structure on the rock physical mechanical properties.

The chloride permeability of concrete with the addition of air-entraining agent was researched. Samples with different agent to binder ratios and vibration time were prepared. The gas content, pore structure, electric flux and permeability coefficient were analyzed. Experimental results show with the agent ratio of 0.0125 ‰ and vibration time of 40 s, the concrete can resist chloride permeation effectively.