Pore Surface Fractal Dimension Research Articles

Fractal characteristics of pore structure of hybrid Basalt–Polypropylene fibre-reinforced concrete

The pore characteristics of hybrid basalt–polypropylene fibre-reinforced concrete (HBPRC) are investigated using mercury intrusion porosimetry. The research results indicate that the cumulative pore volume of concrete increases with fibre addition. The pore surface fractal dimension (DS) of HBPRC in gel, capillary, and large pore regions decreases sequentially although it has no physical characteristics in a transition pore region. The incorporation of fibres has an insignificant effect on DS in gel and capillary pore regions; however, it has a reducing effect on DS in the large pore region. Furthermore, the greater the concrete strength, the larger DS becomes and the greater the reducing effect of fibres on DS in the large pore region. Through microscopic and mesoscopic analyses, it has been suggested that bubbles introduced by fibres and the weak dispersion of such fibres are the main reasons for the deterioration of the large pore structure of HBPRC.

Pore morphology and fractal dimension of ash deposited in catalyst diesel particulate filter.

Diesel particle filter (DPF) has been widely acknowledged as the most effective way to mitigate particulate matter emitted from diesel engines. Over time, ash mainly derived from lubricating oil will deposit in DPF, showing negative influence to engine performance, fuel economy, service life of DPF, and so on. Recently, the investigation about DPF backpressure characteristics and DPF regeneration process considering ash has gained attention. As a porous material, ash will play a key role in the DPF permeability. Thus, the pore morphology and fractal dimension of ash derived from three kinds of lube are addressed in this work. The results show that the changing tendency of the micropore specific surface and pore volume is consistent with the ash content in lubricant oil, which is owing to the chemical interaction of Ca and S contained in the oil during the complex DPF regeneration. There is no significant changing tendency of the mesopore and macropore properties because of the heterogeneity and complexity of ash. According to the fractal analyses, the Avnir equation shows excellent predictive accuracy for the pore surface fractal dimension of ash, which reflects that the ash pore surfaces are irregular.

CO2-water-shale interaction induced shale microstructural alteration

To better understand the CO2 enhanced shale gas recovery process, it is of great significance to investigate the CO2-water-shale interaction and its effect on the microstructural alteration in shale. To clarify the microstructural alteration of shale exposure to CO2-water at different CO2 pressures, X-ray diffraction analysis, low-pressure nitrogen gas adsorption (N2GA), nuclear magnetic resonance (NMR) and fractal analysis were carried on shale samples before and after pure water and CO2-water (PCO2 = 6, 10, 14 MPa; T = 45 °C) exposure. After CO2-water exposure, the contents of quartz in shale were increased, while the content of carbonate and clay minerals in shale were decreased with the increase of CO2 pressure. With the increase of CO2 pressure, the total specific surface area (TSSA) in shale were decreased with a maximum reduction of 29.40%, while the porosity was increased from 2.88% to a range of 2.93%-3.16%. The average pore size (Rave) in shale was also increased and the pore connectivity in shale was improved, micropores and mesopores in shale are gradually transferred to macropores. Both the pore surface fractal dimensions obtained from N2GA (DL1) and NMR (DN1), and the pore volume fractal dimensions obtained from N2GA (DL2) and NMR (DN2), were shown a reduction tendency with the increase of CO2 pressure, which reflected that the roughness and complexity of pore structure and morphology in shale decreased. The results indicated that the effect of super-critical CO2 (10 MPa, 14 MPa) on the pore structure alteration was more significant than that of sub-critical CO2 (6 MPa). The fractal dimensions calculated by N2GA and NMR tests had a good consistency, DL1, DN1, and DL2, DN2 shown positive correlations with TSSA and TPV (total pore volume), while shown negative correlations with Rave, which can be used to evaluate the gas adsorption and flow behaviors in shale, respectively.

The fractal characteristics of pore size distribution in cement-based materials and its effect on gas permeability

To study the influence of the pore structure of cement-based materials on macroscopic features (gas permeability), mercury intrusion porosimetry (MIP) and nitrogen adsorption (NA) were applied to 8 groups of paste and mortar samples (including adding mineral admixtures or not and standard or sealed curing conditions). Pore size distribution has a great influence on gas permeability. By calculating pore surface fractal dimensions based on Zhang’s fractal model, the obvious fractal characteristics of micropores (<100 nm) and macropores (> 105 nm) have been found. The pore diameter of the paste is mostly distributed in the micropores, and the average critical pore diameter is 82 nm. For mortar, the pore diameter is mostly distributed in the micropores and transition pores, and the average critical pore diameter is 121 nm, which means that the seepage pore diameters of the paste and mortar are 82 nm and 121 nm, respectively. The pore surface fractal dimensions of the visible pores are larger than those of the micropores, and there is an inverse relationship between the pore surface fractal dimensions and gas permeability. An important guide for engineering production is to use standard curing and add mineral admixtures to mortar materials to improve the impermeability as much as possible, while a contrary condition exists for paste materials.

Impact of coal composition and pore structure on gas adsorption: a study based on a synchrotron radiation facility

AbstractA better knowledge about the impact of coal composition and pore structure on gas adsorption is of great significance for coalbed methane exploration and uneconomic coalbed CO2 storage. Coal is a porous medium with complex organic components, and most pores in coal are nanopores that have a complex geometrical morphology. As a result, it is significantly necessary to investigate the impact of coal composition and pore structure on gas adsorption based on coal 3D nanopore structure. Synchrotron radiation nano‐computed tomography (CT) was applied to acquire 3D coal nanopore structure and synchrotron radiation small‐angle X‐ray scattering (SAXS) was used to obtain the pore surface fractal dimension. The coal with higher pore surface fractal dimension presents higher Langmuir volume. Based on the 3D nanopore structure acquired by synchrotron radiation nano‐CT, gas adsorption capacity was characterized by the adsorbed gas amount on unit pore surface area. It was found that gas adsorption capacity depends on the coal composition. There is a negative correlation between gas adsorption capacity and ash content and oxygen‐containing groups, and a positive correlation between gas adsorption capacity and vitrinite contents. Hysteresis between desorption and adsorption isotherms is dependent upon the ratio of throat number to pore number, and the hysteresis becomes more significant when the ratio is larger. © 2019 Society of Chemical Industry and John Wiley &amp; Sons, Ltd.

Pore Structure Characteristics of RAP-Inclusive Cement Mortar and Cement Concrete Using Mercury Intrusion Porosimetry Technique

Abstract The pore structure characteristics of cement mortar and concrete incorporating reclaimed asphalt pavement (RAP) fine aggregates as part replacements of natural fine aggregates (NAs) were studied using mercury intrusion porosimetry (MIP) technique. NAs were replaced by RAP at 25, 50, 75, and 100 % by volume of total fine aggregates. Mineral admixtures, namely silica fume and activated sugarcane bagasse ash, were incorporated as part replacements of cement as well. MIP technique could identify the mesopores and macropores in the cementitious mixture. Porosity increases with an increase in RAP content in cementitious mixture, owing to larger and porous interfacial transition zone. Total intrusion pore volume increases with an increase in RAP content and is greater than the control mix irrespective of RAP content and mineral admixture. Mesopores and macropores follow a similar trend as total intrusion pore volume, suggesting finer and larger pores in RAP-inclusive cementitious mixtures. Threshold diameters were observed to initially decrease until 50 % RAP content and to increase thereafter for RAP-inclusive cement mortar, suggesting easy penetration of chemical species for higher RAP content mixes. From pore classification studies, entrained air, large capillaries, medium capillaries, and small capillaries were also analyzed. Large capillaries follow a similar trend to threshold diameter, in which the former affects the transport processes in cementitious mixture. RAP-inclusive cementitious mixtures have the ability to resist freeze as well as thaw and salt decay; this is concluded indirectly from pore structure studies. The pore-mass fractal dimension has the ability to describe the pore-solid structure, whereas the pore-surface fractal dimension failed to do so for RAP-inclusive cementitious mixtures.

Multiscale Apparent Permeability Model of Shale Nanopores Based on Fractal Theory

Based on fractal geometry theory, the Hagen–Poiseuille law, and the Langmuir adsorption law, this paper established a mathematical model of gas flow in nano-pores of shale, and deduced a new shale apparent permeability model. This model considers such flow mechanisms as pore size distribution, tortuosity, slippage effect, Knudsen diffusion, and surface extension of shale matrix. This model is closely related to the pore structure and size parameters of shale, and can better reflect the distribution characteristics of nano-pores in shale. The correctness of the model is verified by comparison with the classical experimental data. Finally, the influences of pressure, temperature, integral shape dimension of pore surface and tortuous fractal dimension on apparent permeability, slip flow, Knudsen diffusion and surface diffusion of shale gas transport mechanism on shale gas transport capacity are analyzed, and gas transport behaviors and rules in multi-scale shale pores are revealed. The proposed model is conducive to a more profound and clear understanding of the flow mechanism of shale gas nanopores.

Full‐Sized Pore Structure and Fractal Characteristics of Marine‐Continental Transitional Shale: A Case Study in Qinshui Basin, North China

Based on 10 shale samples collected from 4 wells in Qinshui Basin, we investigate the full‐sized pore structure and fractal characteristics of Marine‐Continental transitional shale by performing organic geochemistry, mineralogical composition, Nitrogen gas adsorption (N2 adsorption) and Nuclear Magnetic Resonance (NMR) measurements and fractal analysis. Results show that the TOC content of the shale samples is relatively high, with an average value of 2.44wt%, and the thermal evolution is during the mature‐over mature stage. The NMR T2 spectrum can be used to characterize the full‐sized pore structure characteristics of shale. By combining N2 adsorption pore structure parameters and NMR T2 spectrums, the surface relaxivity of samples are calculated to be between 1.7877 um/s and 5.2272 um/s. On this basis, the T2 spectrums are converted to full‐sized pore volume and surface area distribution curves. The statistics show that the pore volume is mainly provided by mesopore, followed by micropore, and the average percentages are 65.04% and 30.83% respectively; the surface area is mainly provided by micropore, followed by mesopore, and the average percentages are 60.8004% and 39.137% respectively; macropore contributes little to pore volume and surface area. The pore structure characteristics of shale have no relationship with TOC, but strong relationships with clay minerals content. NMR fractal dimensions Dmicro and Dmeso have strong positive relationships with the N2 adsorption fractal dimensions D1 and D2 respectively, indicating that Dmicro can be used to characterize the fractal characteristics of pore surface, and Dmeso can be used to characterize the fractal characteristics of pore structure. The shale surface relaxivity is controlled by multiple factors. The increasing of clay mineral content, pore surface area, pore surface fractal dimension and the decreasing of average pore size, will all lead to the decreasing of shale surface relaxivity.

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.

Full-size pore structure characterization of deep-buried coals and its impact on methane adsorption capacity: A case study of the Shihezi Formation coals from the Panji Deep Area in Huainan Coalfield, Southern North China

The characteristics and influence on methane adsorption capacity of the pore structure of coals was investigated through an approach that integrates mercury intrusion porosimetry, low pressure N2/CO2 adsorption, and field emission scanning electron microscopy. Shihezi Formation coal samples from the Panji Deep Area in Huainan Coalfield in Southern North China were adopted as the study subjects. The fractal features of heterogeneous coal pore structures were quantified on the basis of N2 adsorption isotherms by using the Frenkel−Halesy−Hill (FHH) model. Pore structure and morphology characterizations confirm that Shihezi Formation coals developed with multiple pore types and heterogeneous pore structures, and the slit-shaped and ink-bottle-shaped pores are mainly distributed at the pore size interval of 3.3–10 nm. Our coal samples present uni-modal and multi-modal pore size distributions, wherein multiple peaks are concentrated in the intervals of 0.45–0.55 nm and 2–20 μm. The major volumes from mesopores (2 nm < pore diameter < 50 nm) and macropores (pore diameter > 50 nm) occur in the full-scale pore size distributions, revealing Shihezi Formation coals are mesopores and macropores rich. Micropores (pore diameter < 2 nm) provide the majority of pore specific surface area, whereas pore volume is mostly contributed by mesopores and macropores. The volumes and specific surface areas of mesopores and macropores are positively correlated with ash yield and inertinite contents but are negatively correlated with maximum vitrinite reflectance (Ro,max). The methane adsorption capacity of Shihezi Formation coals is largely independent of mesopores and macropores but is positively correlated with the pore volume and specific surface area of micropores. This correlation suggests that micropores provide the predominant sites for methane adsorption. The calculated values of pore surface fractal dimension D1 and spatial fractal dimension D2 are 2.440–2.610 (avg. 2.515) and 2.542–2.761 (avg. 2.650), respectively, suggesting that the pore structures of Shihezi Formation coals are highly complex and heterogeneous. The positive correlation between D1 and methane adsorption capacity reveals that high surface roughness of the pore structure is associated with high methane adsorption capacity. However, the irregular pore structure does not influence methane adsorption capacity.

Geometric pore surface area and fractal dimension of catalyzed electrodes in polymer electrolyte membrane fuel cells

Geometric pore surface area and fractal dimension of catalyzed electrodes in polymer electrolyte membrane fuel cells

Predicting Effective Diffusion Coefficients in Mudrocks Using a Fractal Model and Small‐Angle Neutron Scattering Measurements

AbstractThe determination of effective diffusion coefficients of gases or solutes in the water‐saturated pore space of mudrocks is time consuming and technically challenging. Yet reliable values of effective diffusion coefficients are important to predict migration of hydrocarbon gases in unconventional reservoirs, dissipation of (explosive) gases through clay barriers in radioactive waste repositories, mineral alteration of seals to geological CO2 storage reservoirs, and contaminant migration through aquitards. In this study, small‐angle and very small angle neutron scattering techniques have been utilized to determine a range of transport properties in mudrocks, including porosity, pore size distributions, and surface and volume fractal dimensions of pores and grains, from which diffusive transport parameters can be estimated. Using a fractal model derived from Archie's law, we calculate effective diffusion coefficients from these parameters and compare them to laboratory‐derived effective diffusion coefficients for CO2, H2, CH4, and HTO on either the same or related mudrock samples. The samples include Opalinus Shale from the underground laboratory in Mont Terri, Switzerland, Boom Clay from a core drilled in Mol, Belgium, and a marine claystone cored in Utah, USA. The predicted values were compared to laboratory diffusion measurements. The measured and modeled diffusion coefficients show good agreement, differing generally by less than factor 5. Neutron or X‐ray scattering analysis is therefore proposed as a novel method for fast, accurate estimation of effective diffusion coefficients in mudrocks, together with simultaneous measurement of multiple transport parameters including porosity, pore size distributions, and surface areas, important for (reactive) transport modeling.

Comparative analysis of nanopore structure and its effect on methane adsorption capacity of Southern Junggar coalfield coals by gas adsorption and FIB-SEM tomography

Nano-scale porous structure has a significant influence on methane adsorption in coal seam. This work presents a comparative analysis of different experimental techniques for evaluating pore structure and the relationship between pore surface area and roughness with methane adsorption capacity. This sorption capacity subsequently affects the accuracy of total gas-in-place estimates and feasibility of CO2 injection for enhanced natural gas recovery. To evaluate methane adsorption characteristics of nanopores (<100 nm), various laboratory analyses are presented. Surface area, porosity, pore volume and fractal analysis of adsorption pores were determined in 13 coal samples (maximum vitrinite (huminite) reflectance <1.0%) using gas (CO2, N2 and CH4) adsorption and focused ion beam scanning electron microscopy (FIB-SEM) tomography. FIB-SEM images indicate a clustered distribution of multi-scale adsorption pores with similar orientation. The macropores connect some mesopores, which enhance gas flow and have a positive influence on coal permeability. The percent of connected pores compared to the number of total nanopores is ∼2% in selected samples, as calculated by FIB-SEM. Surface area and pore volume distribution determined from FIB-SEM are consistently higher than those obtained from N2 adsorption methods. In general, the FIB-SEM technique can detect both isolated and connected mesopores and macropores but cannot measure micropores. Surface area, pore volume, and porosity that estimates from N2 adsorption tests are prone to underestimate actual conditions. Langmuir volume (9.92–24.42 m3/t) are seemingly independent of maceral composition and coal ranks in this set of samples, having no direct correlations. Methane adsorption capacity increases with increasing the adsorption pore surface area and fractal dimensions and follow a moderate straight-line relationship. Hence, methane adsorption capacity is directly influenced by micropore surface area and micropore roughness. These results are significant for understanding the interaction of coal with gases.

Structures and fractal characteristics of pores in low volatile bituminous deformed coals by low-temperature N2 adsorption after different solvents treatments

Coal is a porous medium with fractal characteristics. With the removal of soluble fractions under solvent treatment, the structures and fractal characteristics of pores are bound to produce changes. Exampled by low volatile bituminous coals with different degrees of deformation from Huoerxinhe and Changcun coal mines in Qinshui basin, North China, the paper investigated on the changes of pore structure parameters and minerals compositions of coals before and after different solvents treatments, including tetrahydrofuran (THF), hydrochloric acid (HCl) and chlorine dioxide (ClO2). And Insight into the relationships between fractal dimensions and pore structure parameters and coal compositions were provided. Results show that: The shapes of N2 adsorption/desorption isotherms rarely change but their adsorption quantities differ sharply after different solvents treatments for coal with same deformation, indicating no change in pore type but a change in the number of pore in the same pore size. Compared to raw coals, with the removal of soluble fractions in coals pore volume (PV) and specific surface area (SSA) tend to ascend after THF extraction; changes in PV and SSA depend jointly on the dissolution of carbonate minerals and the swelling of clay minerals after HCl treatment; and the varieties of PV and SSA, to a great extent, are closely related to the swelling of clay minerals after ClO2 treatment. Pore structure fractal dimension (DA) and surface fractal dimension (DB) are obtained based on Frenkel-Halsey-Hill model. DA is mainly affected by the proportion of transitional pores and mesopores on PV; therefore, DA can be used to describe pore volumetric roughness of these transitional pores and mesopores. Meanwhile, DB is mainly influenced by the proportion of micropores on SSA; therefore, DB can be utilized to represent pore surface roughness of these micropores. As coal deformation increases, DA grows for raw coals and coals after extraction with THF; therefore, PV has a significant migration to the pores of small apertures causing the growing contribution of micropores to PV. However, DA declines for coals treated by HCl and ClO2, potentially because the welling of clay minerals blocks the pores of small apertures and increase the proportion of the pores of large apertures on PV. DB shows no clear trends with increasing coal deformation. A “reversed U-shaped” curve relationship is observed between moisture in coal and two fractal dimensions, which is due to the effect of water molecule interfacial tension. Ash correlates negatively with DA and positively with DB because of the various influences of minerals on pore homogeneity in pores of different apertures.

A Study of Sandstone Permeability Anisotropy Through Fractal Concept

Most correlation equations of rock permeability are usually based on the Euclidean geometry concept. Pore geometry and structure of most porous rocks are very complex, therefore non-Euclidean geometry concept, e.g. fractal theory, is needed to handle such a complexity. This paper presents a new equation for sandstone permeability involving other properties and fractal dimensions of pore space and surface. The equation is derived by combining Newton’s Law of viscosity, Darcy equation, and fractal geometry concept. It is shown that parameters such as tortuosity, internal surface area, and shape factor can be replaced by fractal dimensions. As natural porous media are mostly anisotropic, this study enables us to identify factors that affect the anisotropy. Eighteen sandstone samples with porosity and permeability range from 21 to 37% and 2.76 to 3,644 millidarcies, were employed in this study. The pore space and surface fractal dimensions for each orthogonal direction for each sample was determined by box counting method. The results of this study demonstrate that calculated directional permeability of the high permeability samples is very close to the measured one after corrections were made for pore sizes of less than one micron. This finding suggests that micropores of the samples may be a major factor not contributing to fluid flow. For the low and medium permeability samples, however, an additional pore geometrical correction is needed. The additional correction factor is considerably different for different directions of fluid flow, indicating that the anisotropy is due to the difference in directional pore structural characteristics.

Nanoscale pore structure and fractal characteristics of a marine-continental transitional shale: A case study from the lower Permian Shanxi Shale in the southeastern Ordos Basin, China

Shale samples collected from seven wells in the southeastern Ordos Basin were tested to investigate quantitatively the pore structure and fractal characteristics of the Lower Permian Shanxi Shale, which was deposited in a marine-continental transitional (hereinafter referred to as the transitional) environment. Low-pressure nitrogen adsorption data show that the Shanxi Shale exhibits considerably much lower surface area (SA) and pore volume (PV) in the range of 0.6–1.3 m2/g and 0.25–0.9 ml/100 g, respectively. Type III kerogen abundant in the transitional Shanxi Shale were observed to be poorly developed in the organic pores in spite of being highly mature, which resulted in a small contribution of organic matter (OM) to the SA and PV. Instead, I/S (illite-smectite mixed clay) together with illite jointly contributed mostly to the SA and PV as a result of the large amount of inter-layer pores associated with them, which were evident in broad-ion-beam (BIB) imaging and statistical analysis. Additionally, the Shanxi Shale has fractal geometries of both pore surface and pore structure, with the pore surface fractal dimension (D1) ranging from 2.16 to 2.42 and the pore structure fractal dimension (D2) ranging from 2.49 to 2.68, respectively. The D1 values denote a pore surface irregularity increase with an increase in I/S and illite content attributed to their more irregular pore surface compared with other mineralogical compositions and OM. The fractal dimension D2 characterizing the pore structure complexity is closely related to the pore arrangement and connectivity, and I/S and illite decrease the D2 when their contents increase due to the incremental ordering degree and connectivity of I/S- or illite-hosted pores. Meanwhile, other shale constituents (including kaolinite, chlorite, and OM) that possess few pores can significantly increase the pore structure complexity by way of pore-blocking.

Fractal characteristics of nano-pores in the Lower Silurian Longmaxi shales from the Upper Yangtze Platform, south China

To better understand the fractal characteristics of nano-pore structure and their effects on methane adsorption in marine shales, we have conducted ultra-low pressure nitrogen adsorption, methane adsorption, X-ray diffraction (XRD), total organic carbon (TOC), and vitrinite reflectance for eleven shale samples of the Lower Silurian Longmaxi Formation from the Upper Yangtze Platform, south China. Two fractal dimensions D1 and D2 (at relative pressures of 0–0.5 and 0.5–1, respectively) were obtained using the fractal Frenkel–Halsey–Hill (FHH) method based on low-pressure nitrogen adsorption isotherms. The relationships among composition of the Longmaxi shale (TOC, clay minerals, brittle minerals), the pore structure parameters (i.e. average pore diameter, surface area, pore volume), and the fractal dimensions were investigated. The results show that the Longmaxi shale possesses fractal geometries with fractal dimension D1 values ranging from 2.3542 to 2.4715 and fractal dimension D2 values ranging from 2.5818 to 2.7497. The good positive correlation between fractal dimensions D1 and the BET surface area and the strongly negative relationship between fractal dimensions D2 and the average pore diameter indicate that D1 can adequately characterize pore surface fractal dimension, while D2 represents the pore structure fractal dimension. Organic matter has a positive influence on the BET surface area and the BJH pore volume and a negative effect on the average pore diameter. The BJH pore volume and the average pore diameter have a positive relationship trend with the clay minerals content, whereas a negative linear trend with the brittle minerals content. The mineralogical compositions and TOC of the Longmaxi shale have different impacts on the pore structures, which in turn exert different effects on the fractal dimensions D1 and D2. Both the fractal dimensions D1 and D2 have positive effects on the Langmuir volume of methane adsorption. However, the fractal dimensions D1 and D2 have opposite influence on the Langmuir pressures of methane adsorption. Thus the fractal dimensions can well be used to evaluate gas shale reservoir and shale gas accumulation in south China.

Pore characterization and its impact on methane adsorption capacity for organic-rich marine shales

Shale matrix pore structure controls the gas storage mechanism and gas transport behaviors. We employed various techniques to characterize the complex pore structures of 12 shale samples collected from two marine shale formations in upper Yangtze area (UYA) in China. The characterization techniques include field emission scanning electron microscopy (FE-SEM), high-pressure mercury intrusion porosimetry (MIP), and low-pressure N2/CO2 adsorption. The excess methane adsorption capacity was measured for each sample and the results were modeled using Langmuir model. Based on the FE-SEM image analyses, micro- and meso-pores within organic matter and inter-particle pores between or within clay minerals are the most commonly developed in these shale samples. Both uni- and multi-modal pore size distributions (PSDs) were observed, and a significant portion of pores are in the pore size range between 3 and 100nm. It was also found that the micropore (<2nm) is the major contributor to the overall specific surface area (SSA), whereas most of the pore volume is occupied by mesopores (2–50nm). Two different fractal dimensions, pore surface fractal dimension (D1) and spatial geometry fractal dimension (D2), were estimated from low pressure N2 isotherms, with D1 ranging between 2.469 and 2.682, and D2 ranging between 2.576 and 2.863, indicating that the surface and volume of pore structure are heterogeneous. Samples with higher D1 can provide more adsorption sites for methane and tends to have relatively high adsorption capacity, whereas shales with higher D2 do not influence the gas adsorption and storage capacities. Methane adsorption capacity increases with the increase of both micropore volume and micropore surface area, and this confirms that microporosity is the governing factor on methane adsorption capacity and storage.

Effect of humic acids, sesquioxides and silica on the pore system of silt aggregates measured by water vapour desorption, mercury intrusion and microtomography

SummaryMost of the information on soil aggregation and porosity comes from studies of natural soil in which the effects of the different constituents that form the structure overlap. The aim of this research was to study the effects of these constituents separately on well‐characterized artificial aggregates in order to understand them better. To do this, the pore system of model silt aggregates, amended with different amounts of humic acids, iron and aluminium hydroxides or colloidal silica, was investigated at three levels of magnification with water vapour desorption (nanometre sizes), mercury intrusion (micrometre sizes) and microtomography (tens of micrometres). Humic acid and aluminium hydroxide increased aggregate porosities measured by all methods. An increase in porosity with increasing additions of each constituent was indicated only by water desorption. We did not observe any well‐defined trends in the dynamics of average pore radii. The pore surface fractal dimension determined by mercury intrusion was negatively correlated with that measured by water desorption. The pore system in granular media comprises larger voids joined by narrower necks; therefore, we attempted to relate their sizes with a novel approach that combined microtomography with mercury intrusion and extrusion data. We observed a decrease in the size of pore necks that give access to voids of the same sizes with increasing additions of all constituents. With additions of humic acid this effect was the smallest. The mercury intrusion data showed the formation of separate concretions of iron hydroxides and silica in silt aggregates.

Fractal characteristics of pores in non-marine shales from the Huainan coalfield, eastern China

The joint development of coal-bed methane (CBM) and shale gas at the Huainan coalfield in East China has drawn the attention of industrial and academic communities. In CBM and shale gas systems, a large amount of gas might be stored as adsorbed gas, which is closely related to the pore structure. In this paper, we report on the fractal characteristics of pores of non-marine organic shales from the Huainan coalfield. Measurements of X-ray diffraction, total organic carbon, vitrinite reflectance, and nitrogen adsorption were conducted on 13 shale samples to characterize the fractal dimensions and pore properties. Results indicated that pore morphology is dominated by cylindrical and slit shaped types. A small number of wedge shaped pores can be identified, but bottle neck pores are rare. The average pore diameter is between 4.44 and 22.80 nm, and pores with a diameters less than 50 nm are dominant. The pore surface fractal dimension (D1) and pore structure fractal dimension (D2) can be used to indicate overall fractal characteristics. The D1 values are primarily affected by shale constituents. In terms of immature and low total organic carbon samples, the thermal maturity of theses shales is a cardinal factor governing pore structures, especially for surface area. D2 values are obviously controlled by the type and connectivity of pores, and they are significant in analyzing pore structures, especially in assessing average pore diameter. Similarly to coals, shales with high D1 values are favorable for methane adsorption, while high D2 values go against adsorption capacity.