Pore Size Classes Research Articles

Segmental retention models for representing the hydraulic properties of evolving structured soils

AbstractCommon parametrizations of soil hydraulic properties rely on unimodal curves, which cannot accurately represent the properties of many macroporous, aggregated, mixed, or compacted soils. Multimodal hydraulic curves are increasingly used to represent these structured soils in eco‐hydrological models, but the dynamics of the processes that shape soil structure—and the resulting dynamics of soil hydraulic properties—are often neglected. In cases such as compaction recovery, where the structure‐shaping process can be modeled, coupling the evolving pore volumes to soil hydraulic properties in a physically based way remains challenging. Here, we show how modeled or estimated soil structure evolution, when expressed as a time series of porosities in a few pore size classes, can be assimilated into established models of soil hydraulic properties. Our method relies on the division of retention models into smooth segments, whose water contents can be independently adjusted. We apply the approach to examples of modeled soil structure evolution from the published literature: one describing soil structure recovery after compaction and one describing structure formation as a result of organic amendment. In the cases considered, the estimated soil hydraulic conductivity varies more strongly than the modeled porosity which drives it. This shows that transport‐related soil functions can be impacted longer (after compaction) or sooner (after amendment) than suggested by the evolution of structural metrics such as porosity. In general, modeling the evolution of soil hydraulic properties in cases such as these paves the way for holistic, process‐based modeling of land management practices and their impact on soil functioning.

Controls of tectonics and paleoclimate on depositional–diagenetic evolution and pore types of Upper Cretaceous successions (Sarvak Formation) in the Abadan plain, Iran

This study examines the influence of tectonics and paleoclimate on the diagenetic evolution of Upper Cretaceous carbonate reservoirs (Sarvak Formation) in the Abadan Plain, Zagros Basin, Iran. Through petrographic analysis and scanning electron microscopy, the sedimentary facies, diagenetic processes, and pore system within the Sarvak Formation were analyzed. Facies analysis identified eight microfacies, which were deposited in shallow to deep settings of a carbonate ramp. Among these, the grain-dominated facies (packstone and grainstone) of shoal complexes and reef-talus settings were identified as the most important reservoir facies. The formation's complex diagenetic history includes marine, meteoric, and burial diagenetic processes, with meteoric diagenesis playing a significant role beneath paleoexposure surfaces. The Sarvak Formation was categorized into eight flow units, seven pore size classes, eleven electrofacies, eight velocity deviation zones, and thirteen Lorenz zones. The study's findings indicate that the most favorable reservoir units are located beneath the Cenomanian–Turonian exposure surface, characterized by significant dissolution of rudist-dominated facies due to meteoric processes and karstification. Tectonic uplift, along with weathering and erosion within a warm and humid (tropical) paleoclimate, significantly impacts the diagenetic evolution, pore types, and petrophysical properties of the Sarvak Formation. This research underscores the complex interplay between tectonics, paleoclimate, and petrophysical properties in the Abadan Plain, enhancing our understanding of carbonate systems in tectonically active and paleoclimatically dynamic regions.

Petrophysical insights into pore structure in complex carbonate reservoirs using NMR data

The study delves into pore structure attributes within the complex Eocene carbonate of an Indian offshore field, encompassing pore throat, radius and their characteristics. Nuclear Magnetic Resonance (NMR) experimental data reveals crucial insights into pore structures and fluid states. This study compares the NMR T2 distribution curve with capillary pressure data from the Mercury Injection Capillary Pressure (MICP) technique, deriving linear and nonlinear conversion coefficients to transform NMR T2 spectra into equivalent pore radius distribution. Pore radius-dependent porosity partitioning, linked to permeability and the distribution of irreducible water, is conducted utilizing NMR-derived data. Following the T2 cut-off analysis, a two-segment fractal analysis of NMR T2 distribution is also carried out. This analysis unveils associations between fractal dimensions and various petrophysical parameters, including permeability, porosity, T2LM, irreducible water saturation and R50. The NMR-derived pore radius distribution is mostly unimodal, occasionally slightly bimodal. Six different pore size classes (less than 0.05 μm to more than 5 μm) are analysed in relation to permeability, porosity and irreducible water. Small pores (<1 μm) contribute more to irreducible water with low porosity and permeability. The fractal dimension of large pores correlates strongly with porosity, permeability, T2LM, irreducible water and R50 suggesting significant impact on reservoir seepage capacity. In addition to porosity partitioning, the current study demonstrates effectiveness in modelling modified permeability and correlating it with in situ permeability when applied to field NMR log data from the study area. While numerous studies focus on sandstone, our study marks the pioneering attempt at a comprehensive analysis on complex carbonate reservoirs.

A New Model for Solving Hydrological Connectivity Inside Soils by Fast Field Cycling NMR Relaxometry

In this paper, a new quantitative approach for estimating the structural and functional connectivity inside soil by Fast Field Cycling (FFC) NMR relaxometry is presented, tested by measurements carried out in three samples with different texture characteristics. Measurements by FFC NMR relaxometry have been carried out using water-suspended samples and Proton Larmor frequencies (νL) ranging in the 0.015–35 MHz interval. Two non-degraded soil samples, with different textural characteristics, and a degraded soil collected in a badland area, were analyzed. For a given soil and any applied Proton Larmor frequency, the distribution of the longitudinal relaxation times, T1, (i.e., relaxogram) measured by FFC NMR has been integrated, and the resulting S-shaped curve (i.e., relaxogram integration curve) was represented, for the first time, by Gumbel’s diagram. This new representation of the relaxogram integration curve, transforming the S-shaped curve into a straight line, allowed for distinguishing three linear components, corresponding to three different relaxation time ranges, characterized by three different slopes. Two points, identified by the abrupt slope changes of the relaxogram integration curve plotted in Gumbel’s diagram, are used to identify two characteristic values of relaxation time, T1A and T1B, which define three well-known pore size classes (T1 &lt; T1A micro-pores, T1A &lt; T1 &lt; T1B meso-pores, and T1 &gt; T1B macro-pores). The relaxogram integration curve allowed for calculating the non-exceeding empirical cumulative frequency, F(T1), corresponding to the characteristic T1A and T1B values. The analysis demonstrated that the relaxogram can be used to determine the pore-size ranges of each investigated sample. Finally, using the slope values of the three components of the relaxogram integration curve, a new definition of the Structural Connectivity Index, SCI, and Functional Connectivity Index, FCI, was proposed.

Reservoir characterization of the Oligocene–Miocene siliciclastic sequences (Ghar Member of the Asmari Formation) in the northwestern Persian Gulf

This paper presents an integrated facies, diagenesis, and sequence stratigraphic analysis, and reservoir quality evaluation of the siliciclastic hydrocarbon reservoirs (Ghar Member of the Asmari Formation) in the NW Persian Gulf. Results of core descriptions, thin sections petrography, X-ray computed tomography (CT) scanning, X-ray diffraction, sieve analysis, porosity–permeability measurements, and petrophysical logs are used. Four petrofacies are recognized indicating deposition in shore-line (beach) sand bodies and supratidal to intertidal sub-environments. Calcite and anhydrite cementation, dolomitization, recrystallization, compaction, and fracturing are diagenetic processes which occurred in marine, hypersaline, and shallow burial realms. A large-scale transgressive–regressive sequence and five deepening-upward cycles are defined. Hydraulic flow units and pore-size classes are defined. The best reservoir units (hydraulic units 5 to 8 and pore-size classes 4 and 5) are formed within the well-sorted, rounded, and mature sand/sandstone facies in which intergranular pores are predominant and diagenetic imprints are negligible. These facies are located in lower-half of depositional cycles. Carbonate-dominated facies show variable reservoir properties depending on their diagenetic alterations. In these facies, dolomitization and fracturing improved the permeability. This study shows that reservoir properties of the Ghar Member are strongly facies-controlled and, therefore, are easily predictable within a sequence stratigraphic framework.

Soil water diffusivity as function of the pore size distribution and precompression stress

The idea of the study was to combine the saturation-dependent hydraulic conductivity, K(Se), as intensity parameter with the volume fraction related to the fractional capillary potential for each of the characteristic pore size classes that based on the matric flow potential, φ, and the soil water diffusivity, D(θ). The analysis is focused on the soil hydraulic properties of differently textured soils with differences in the pore size distribution, PSD. The soil-moisture retention, θ(h), and K(Se) as function of the effective saturation were determined with undisturbed samples for each soil profile and horizon and fitted by Mualem–van Genuchten type analytical function. The results indicate a strong linear relationship between φ values and the soil water diffusivity, D(θ), of soils with its volume fractions of macropores (r2: 0.65–0.91), wide coarse pores (r2: 0.55–0.77), and medium pores (r2: 0.83–0.85). The precompression stress, Pc, as mechanical soil strength parameter affects the soil water diffusivity of the chosen datasets especially in the wide coarse pore range (r2: 0.85). The results suggest that the PSD’s-related φ and D(θ) values are affected by both textural and structural effects. The soil water diffusivity itself strongly depends on the PSD and is an effective link between soil’s intensity and capacity parameter.

Geographic variations of pore structure of clayey soils along a climatic gradient

Soil pore structure over a wide range of pore sizes regulates soil functions and relies heavily on pedogenic processes. To date, limited knowledge is available on the spatial variations of soil pore structure and underlying mechanisms over a broad geographic scale. Herein, seven typical zonal soils with increased status of soil development in heavy-textures were collected at different genetic horizons under forests along the mid-temperate to tropic climate gradient in eastern China. Pore size distribution (PSD, 0.006 ∼ 800 μm) of aggregates (sized ∼ 1 cm3) and other pore structure characteristics (void ratio, pore area and average diameter) were determined by mercury intrusion porosimetry. Differential PSD displayed tri- or bi-modal patterns and different pore size classes followed diverse geographic trends. Textural pores most influenced by soil type exhibited larger void ratio and pore area in highly than in moderately developed soils; structural porosity most influenced by soil horizon overall decreased with increased soil depth, indicating the decreased levels of hierarchical aggregation whatever soil type. Relative to soil texture and total organic matter (0.70≤|r|≤0.92), total and textural pores were more related to inorganic cementing agents (0.71≤|r|≤0.95, p < 0.001), where kaolinite and free iron/aluminum oxides (mainly crystalline form) exerted the positive effects on void ratio and pore area through clayey pores, in contrast to vermiculite and exchangeable cations; void ratio and pore size of structural pores could be separately promoted by sand-silt size organic matter and vermiculite (r = 0.79 and 0.73, p < 0.001). Additionally, clay-sized organic matter, sand, and free manganese oxides played a secondary effect (R2 = 0.04 ∼ 0.30, p < 0.05) and their specific roles differed with pore size class. The overall results suggest that the dramatic geographic variations of soil pore structure are controlled by both inorganic cementing agents and specific organic components to a large extent at the regional scale.

Soil hydraulic properties and pore dynamics under different tillage and irrigated crop sequences

Soil hydraulic properties and pore continuity are important parameters of soil quality and may differ among tillage systems, crop rotations, and change over time. However, there are some contrasting results, depending on soil type, climate and cultivation history on the spatial and temporal variations. The objective of this study was to determine spatio-temporal dynamics of soil hydraulic properties and pore characteristics (i.e., pore volume, effective porosity, and continuity) on a silt loam long-term tillage field experiment, in Agramunt, NE Spain, during two cropping years. Undisturbed soil samples were used to determine soil water retention, θ(Ψ), and soil hydraulic conductivity, K(Ψ), curves in two different tillage systems (intensive tillage, IT vs no-tillage, NT), two crop sequences (short fallow-maize, FM vs legume-maize, LM) and two positions (within the row of crops, W-row vs between the rows of crops, B-row). The results revealed that LM had greater specific hydraulic conductivity, Kpc in its macroporosity (>1000 µm) than FM due to greater number of effective pores, Npc and effective porosity, εpc. Soil water content, θ was greater under IT than under NT at higher soil water matric potential, Ψ (≥-3 cm H2O), whilst the opposite was observed at lower Ψ (≤ − 50 cm H2O). Long-term NT showed greater hydraulic conductivity, K at higher Ψ (≥ − 10 cm H2O) than IT, and no difference at lower Ψ. Although IT had greater pore volume, ϕpc than NT in the macroporosity and coarse mesoporosity (1000–60 µm) pore size classes, NT had two times greater Kpc than IT due to increased Npc, εpc, and pore continuity, Cwpc. K(Ψ) and pore characteristics showed spatial variations (W-row vs B-row). W-row had significantly greater K at higher Ψ (≥ − 3 cm H2O) than B-row. Temporal dynamics of soil hydraulic properties and pore characteristics were not evident under IT during crop succession. This study shows that LM increases specific hydraulic conductivity of soil macroporosity by increasing the number of effective macropores and the effective porosity. In a Mediterranean climate, this may improve the hydrological functions of agricultural soil and associated crop yield. Further, long-term NT formed a stable number of effective macropores and coarse mesopores, and showed a greater pore continuity in coarse and fine mesopores, resulting in improved soil water flux.

Fractal dimension of wood pores from pore size distribution

Abstract Understanding wood pore geometry and distribution is the basis for studying its macroscopic properties. This research investigated the pore fractal dimension using the pore size distribution of selected softwoods and hardwoods. Mercury intrusion porosimetry explored the detailed structural parameters of wood pore size distributions and assessed their fractal dimension. The results revealed significant variability in pore size distribution, porosity, pore volume, and fractal dimension values. The threshold pressure extracted from porosimetry data can be used as the main parameter to distinguish the pore size distribution regions. Pore sizes ranged from 3 to 35,000 nm, with a corresponding porosity that ranged from 58 to 76%. Three pore size classes were determined and ranged as: macropores (radius 350,000–5000 nm), mesopores (radius 5000–100 nm), and micropores (radius 100–3 nm). The fractal dimension values in the corresponding macropore, mesopore, and micropore size intervals were 2.98–2.998, 2.6–2.92, and 2.53–2.72, respectively, indicating a higher degree of complexity for larger pores.

Anisotropy of soil water diffusivity of hillslope soil under spruce forest derived by X-ray CT and lab experiments

The idea of the study is to indicate direction-dependent differences in hydraulic conductivity, K(Se), and soil water diffusivity, D(θ), as function of the volume fraction related to the fractional capillary potential for each of the characteristic pore size classes by extended anisotropy factors. The study is exemplary focused on a BwC horizon of a Dystric Cambisol under spruce forest formed on the weathered and fractured granite bedrock in the mountainous hillslopes Uhlirska catchment (Czech Republic). Thus, undisturbed soil samples were taken in vertical (0°, y = x-axis) and horizontal (90°, z-axis) direction. The D(θ) values and especially the D(θ)-weighted anisotropy ratios showed that anisotropy increases with the volume fraction of macropores, MaP (d > 0.03 mm), with r2 between 0.89 and 0.92. The X-ray computer tomography (CT) based anisotropy ratio (ACT) is larger for the horizontal sampled soil core with 0.31 than for the vertical with 0.09. This underlines the existence of a predominantly horizontally oriented pore network and the fact that weathered bedrock strata can initiate lateral preferential flow. The study results suggest that combining the hydraulic conductivity as intensity and the capacity parameter by means of diffusivity results in an extended anisotropy ratio which unveils the role of the soil hydraulic characteristics in generation of small-scale lateral preferential flow. In future, the small-scale direction-dependent differences in the soil hydraulic capacity and intensity parameter will be used for model-based upscaling for better understanding of preferential flow at the catchment scale.

Deep learning with multisite data reveals the lasting effects of soil type, tillage and vegetation history on biopore genesis

Soil biopore genesis is a dynamic and context-dependent process. Yet integrative investigations of biopore genesis under varying soil type, tillage and vegetation history are rare. Recent advances in Machine Learning (ML) made faster and more accurate image analysis possible. We validated a model trained on Convolutional Neural Network (CNN) using a multisite dataset from varying soil types (Luvisol, Cambisol and Kandosol), tillage (deep ploughing and without deep ploughing) and vegetation history (taprooted and fibrous-rooted crops) to automatically predict biopore formation. The model trained on the multisite dataset outperformed individually trained single-site models, especially when the dataset contained images with noise and/or fewer biopores. Our model successfully replicated previously established treatment effects but provided new insights at more detailed scales and for different pore-size classes. These insights demonstrated that effects of deep ploughing on soil biopores can persist for more than 50 years and are more pronounced on the Luvisol rather than the Cambisol soil type. The effects of perennial fodder crops with high biopore generating capacity were also shown to persist for at least a decade. Re-growing the same fodder crops or a mixture with grass had no further impact on biopore density but generated a shift in pore-size classes from large to smaller biopores. We suspect this is likely to have resulted from three possible scenarios; (1) newly created fine pores (1–4 mm); (2) blockage of large-sized pores by earthworm faeces; (3) decrease in pore diameter. In summary, by using a single robust model trained on the multisite dataset, we were able to generate new insights on pore-size distribution as affected by site, vegetation, and deep ploughing. We have demonstrated that Deep Learning-based image analysis can provide easier biopore quantification and can generate models that provide novel insights across different research settings consistently and accurately.

Soil development and management effects on hydraulic properties along a granitic soil toposequence in southern China

A profound understanding of the effects of natural soil development and anthropogenic land management on soil hydraulic properties is essential for soil productivity and water conservation. The objective of our research was to explore the effects of soil development on soil bulk density (BD), pore size distribution, saturated hydraulic conductivity (Ksat), and soil water retention characteristics under horseweed grass (Erigeron canadensis) and corn (Zea mays)-rapeseed (Brassica napus) rotation systems in weathered granitic soils in southern China. Pore size classes determined were macropores (effective diameter > 1000 μm), coarse mesopores (effective diameter 60–1000 μm), fine mesopores (effective diameter 10–60 μm), and micropores (effective diameter < 10 μm). Soil samples from each treatment were collected to a depth of 40 cm in 10 cm increments. The results showed that Ksat was highest in Entisols (6.08 cm h−1) and lowest in Alfisols (2.58 cm h−1), and these values were averaged across management systems and depths. The saturated water content was 12.8% higher in Alfisols than that in Entisols. With soil development, soil water retention increased for all measured soil water pressures. The highest macroporosity as well as coarse and fine mesoporosity were recorded in Entisols, while the highest microporosity was found in Alfisols. Improvement in soil physical and hydraulic properties was seen in the horseweed grass system (HG) compared with that in the corn-rapeseed rotation system (CR). The bulk density was 6% lower for the HG than that for the CR. The HG had 30% higher Ksat and 10.3% greater saturated water content than the CR. The HG also had 15.2% increased macroporosity compared with the CR, which may enhance water infiltration and reduce runoff loss.

MINERALOGICAL, BIB‐SEM AND PETROPHYSICAL DATA IN SEAL ROCK ANALYSIS: A CASE STUDY FROM THE VIENNA BASIN, AUSTRIA

The Vienna Basin is a major hydrocarbon province with a long exploration history. Within the basin, secondary migration from Upper Jurassic source rocks into stacked Middle Miocene (Badenian) sandstone reservoirs was formerly considered to have occurred almost entirely along major fault zones. However recent exploration data has suggested that in areas where no major faults are present, oil may have migrated vertically through the sandy mudstone intervals separating individual reservoir units, which are therefore imperfectly sealed. In order to investigate possible secondary migration through the semi‐permeable mudstones, this study links variations in gross depositional environment (GDE) to variations in mudstone properties (e.g. mineralogy and pore size distribution). The study focussed on the mudstones which seal reservoir sandstones referred to locally as the “8.TH” and “16.TH” units. The bulk mineralogical composition of 56 mudstone and sandy mudstone (and minor intercalated muddy sandstone) samples from seal layers in 22 wells was studied by X‐ray diffraction analysis, broad ion beam – scanning electron microscopy (BIB‐SEM), mercury intrusion porosimetry (MICP) and N2 adsorption. These data are interpreted in the context of GDE maps of the Vienna Basin which were previously established using seismic and well log data.Results indicate that the gross depositional environment strongly controlled the pore space characteristics of the mudstones. The sandy mudstones in the NW part of the study area were influenced by a complex eastward‐prograding deltaic system which deposited coarse detritus into a major palaeo depression (“Zistersdorf Depression”) located in the centre of the basin. Higher overall porosity and a dominance of larger pore size classes, probably resulting in reduced seal quality, were observed for sandy mudstones from well locations within feeder channels and also from within the Zistersdorf Depression. Similarly, sandy mudstones from locations associated with the long‐term input of coarser sediments in shoreline, coastal and proximal offshore settings in the NW and central parts of the study area are considered to be of lower sealing quality compared to fine‐grained mudstones deposited in distal, open‐marine settings which prevailed in the SE part of the study area throughout the Middle Miocene.In general, pore geometries were influenced by mineralogical composition; quartz‐ and detrital carbonate‐rich samples show equidimensional pores, while more elongated pores (with a higher average aspect ratio) characterize clay‐rich samples. Furthermore, matrix mesopores (2‐50 nm) determined by N2 sorption are more abundant in clay‐rich versus quartz‐rich samples, and show a pronounced positive trend with increasing percentage of illite‐smectite mixed‐layer clay minerals.This study shows that regional‐scale mudstone seals in the Vienna Basin have been influenced by variations in sedimentation associated with lateral variations in gross depositional environment during the Middle Miocene. The observed pore characteristics will serve as input data for future models of secondary migration.

Anisotropy of unsaturated hydraulic properties of compacted mineral capping systems seven years after construction

The mechanical compaction of soil material of mineral landfill systems affects the continuity and connectivity of the complex soil pore network. A horizontally-oriented layering is intended to generate a slope-induced lateral water flow out of mineral capping systems that is sufficient to minimise the statutory required vertical percolation through the underlying waste body and the potential leachate.In this case, soil compaction affects both the porosity and water retention as capacity values and the hydraulic conductivity as intensity parameter. The idea of this study was to combine information on both soil properties in an extended anisotropy factor based on the soil water diffusivity. The analysis is focused on the direction-dependent soil hydraulic properties of a mechanically compacted landfill capping system.In particular, the volume fractions were related to the fractional capillary potential for each of the characteristic pore size classes. Three different soil profiles of top, middle, and bottom slopes of the mineral capping system of the Rastorf landfill in Northern Germany were sampled seven years after construction. Undisturbed soil cores of 100 cm3 and 438 cm3 were extracted in vertical (ver) and horizontal directions (hor) in depths of 20, 50, and 80 cm representing the main layers. The soil water retention and unsaturated hydraulic conductivity, K, functions were determined by suctions plates, permeameter, and the evaporation method. In the coarse pores range (pressure head values of h ≥ −300 hPa), the standard anisotropy ratio, AR, (K(Se)hor/K(Se)ver) as a function of effective saturation, Se, in the sealing liner in 80 cm depth was larger than 1, indicating higher horizontal than vertical K(Se) values. Thus, AR-values above 1 in the range close to water saturation especially in 80 cm depth suggest the tendency of lateral water flow out of mineral capping system due to a sufficient hydraulic potential and thus its reasonable functionality, even seven years after construction.The anisotropy factor was extended in two steps; for AR* and AR**, the pore size class-related matric flux potential, ϕ, and the soil water diffusivity, D(θ), were proposed to combine intensity parameters with capacity-based volume fractions of pore size classes and the fractional capillary potential. The ϕ- and D(θ)-weighted anisotropy ratios, AR* and AR**, indicate that anisotropy increases with the volume fraction of macropores (r2AR* of 0.69−0.77; r2AR** of 0.71−0.80) and wide coarse pores (r2AR* of 0.57−0.78; r2AR** of 0.79−0.89) in both directions. The results suggest that by combining both the intensity and the capacity parameters of the soil hydraulic properties in an extended anisotropy ratio improves the information on compacted mineral capping systems.

Nano-Scale Pore Structure and Its Multi-Fractal Characteristics of Tight Sandstone by N2 Adsorption/Desorption Analyses: A Case Study of Shihezi Formation from the Sulige Gas Filed, Ordos Basin, China

Fractal dimension is a critical parameter to evaluate the heterogeneity of complex pore structure in tight sandstone gas and other low permeability reservoirs. To quantify the fractal dimension of tight sandstone at various pore size classes and evaluate their implications on mineral composition and nano pore structure parameters, we conducted an integrated approach of N2 adsorption/desorption experiment (N2-GA), X-ray diffraction (X-RD), and field emission scanning electron microscopy (FE-SEM) on Sulige tight sandstone reservoirs. By comparing the nine types of fractal dimensions calculated from N2 adsorption data, we put forward the concept of “concentrated” fractal dimensions and “scattered” fractal dimensions (DN2, DN3, DN5, DN7 and DN8) for the first time according to its concentration extent of distribute in different samples. Result shows that mineral composition has a significant influence of a different level on specific surface area (SSA), pore volume (PV), and fractal dimensions (DN), respectively, where the “scattered” fractal dimension is more sensitive to certain specific property of the reservoir, including mineral content and the specific surface area contribution rate (Sr) of type II mesopores (Mesopore-II: 10~50nm). In addition, three type of hysteresis loops were distinguished corresponding to different pore shape combination of N2-GA isotherm curve, which reveals that pore structure heterogeneity is mainly controlled by inkbottle-shaped pores and the volume contribution rate (Vr) of mesopores in this study area. These findings could contribute to a better understanding of the controlling effect of pore heterogeneity on natural gas storage and adsorption.

Anisotropy of unsaturated soil hydraulic properties of eroded Luvisol after conversion to hayfield comparing alfalfa and grass plots

The ploughing-induced compaction of the interface between topsoil and subsoil negatively affects the connectivity and continuity of the complex pore system through plough pans as artificial boundary resulting in water-logged conditions. The conversion of arable land into hayfield is an opportunity for breaking up plough pans and recovering pore networks in the long-term. The basic idea of the current study was to investigate the potential pore structure recovery effect by growing either deep-rooting alfalfa or shallow-rooting grass on former conventionally-tilled cropland. The alfalfa and grass plots located in moraine region in Northeast Germany on erosion-affected truncated Albic Luvisol, were sampled nine years after the conversion to hayfield. Undisturbed soil cores (300 cm3) were extracted in vertical (0°) and horizontal directions (90°) from the boundary between Ap and Al-Bt- horizons (about 25−35 cm soil depth). The soil water retention and the hydraulic conductivity under variable-saturated conditions were determined by using a combination of methods including suction plate, pressure chamber, double membrane through-flow, and evaporation experiments. The anisotropy ratio, AR, (horizontal versus vertical) of the unsaturated hydraulic conductivity function, K(Se), was less pronounced for the soil at the alfalfa as compared to that of the grass plot, especially for wide coarse pores (wCP: >−60 hPa). The AR-ratio was differently depending on the pore size for soil from grass and alfalfa plot, and ranged between AR-values of 1 and 7. The pore size class-related matric flow potential, ϕ, derived from K(Se) function was used for comparing the volume fraction of pore size classes with a pore size-related fraction of the capillary potential. The volume fraction of the wide coarse pores was negatively related with ϕ (r2: 0.91). The smaller AR-values for the soil of the alfalfa plot suggest a tendency for better changing the platy structure into a prismatic-platy structure through the more intense rooting in the vertical direction than in the soil of the grass plot.

Effects of land-use/cover change on soil hydraulic properties and pore characteristics in a semi-arid region of central Iran

The semi-arid regions of central Iran have vastly undergone the manipulation and conversion of rangelands to dry farmlands. Soil hydraulic properties and pore characteristics may differ in various land-use/cover types. This study evaluated the effects of good and poor rangeland, dry farmland and abandoned farmland on the soil hydraulic properties and pore characteristics in a semi-arid region of central Iran. A completely randomized design was used to analyze the effects of land-use/cover on soil hydraulic properties and pore characteristics. Water infiltration into the soil at inlet matric suction (h) values of 2, 5, 10 and 15 cm was measured using a tension infiltrometer in different land-use/cover types with 18 replications. Wooding's analytical method was used to model the infiltration data and the best-fit values for Gardner’s parameters of macroscopic capillary length (λc) and saturated hydraulic conductivity (Ks) were estimated. Pore characteristics were also estimated using the Watson and Luxmoore method. The results indicated that saturated and near-saturated hydraulic conductivity values [Kh] and λc were significantly influenced by the land-use/cover type. For h < 5 cm, good rangeland and dry farmland had the highest and lowest means of hydraulic conductivity, steady-state flux and sorptivity, respectively. The K values at h lower than 5 cm were found to be as follows: good rangeland > poor rangeland > abandoned farmland > dry farmland. Good rangeland had a greater number of large pore-size class (i.e., > 0.06 cm) and total porosity. Dry farmland and good rangeland had the lowest and highest proportions of large pore-size class (> 0.06 cm), respectively. Inappropriate management practices such as over-grazing of poor rangeland, cultivation and harvesting machinery stress and soil organic carbon decomposition in the dry farmland decreased the frequency of very large pore-size classes (i.e., > 0.15 cm). Although very large and large pores contributed to less than 1% of the soil volume, more than 50% of the total water flow would happen through these pore-size classes. Preserving rangelands in good condition can maintain soil structure and stability and would enhance water infiltration into the soil. These findings can be used by decision makers and land managers for holistic management in ecosystems of semi-arid areas.

Effects of tillage intensity on pore system and physical quality of silt-textured soils detected by multiple methods

Understanding the effects of agricultural management practices on soil functionality is an ongoing challenge in environmental science and agricultural practice. In the present study we quantified the effects of changes in tillage intensity on soil physical quality and pore size distribution after 6, 10 and 23 years. At three long-term tillage experimental sites in central Europe we analysed soils under four different soil management systems: conventional mouldboard tillage; chiselling + rotary harrow; rotary harrow; and no till. These treatments differed in mechanical intensity and depth. Pore size distributions were calculated from soil water retention curves based on high-resolution measurements. Subsequently, fractions of functional pore size classes and indicators of soil physical quality were determined and compared between the treatments. In addition, we evaluated the performance of two calculation approaches for pore size distribution: (1) fitting of a smoothing cubic spline; and (2) a bimodal van Genuchten function. The parametric function yielded a higher proportion of storage pores by approximately 3–5%. The combination of multiple measurement and evaluation methods enabled detailed comparison of soil physical characteristics between different tillage treatments. No-till soils showed a distinct lack of transmissive pores and higher bulk density, but similar plant-available water capacity, compared with the other treatments. Under all soil management systems, aeration deficits were observed, emphasising the high vulnerability for compaction of silt-dominated arable soils with a low organic matter content. Hence, the design of agricultural soil management strategies on such soils needs to consider the risks of compaction as thoroughly as erosion or chemical degradation.

Stratigraphic and lithofacies control on pore characteristics of Mississippian limestone and chert reservoirs of north-central Oklahoma

We have determined how stratigraphy and lithofacies control pore structures in the Mississippian limestone and chert reservoir of north-central Oklahoma. There are 17 lithofacies and 29 high-frequency cycles documented in the Mississippian interval of this study. The high-frequency cycles have thicknesses ranging from 0.3 to 30.5 m (1–100 ft) and are mainly asymmetric regressive phases. The pore characteristics, measured through digital-image analysis (DIA) of thin-sections photomicrographs ([Formula: see text]100), exhibit unique correlations with core porosity, permeability, and lithofacies within a sequence-stratigraphic framework. There are five fundamental correlations observed. First, porosity from DIA and laboratory core measurements has a strong positive relationship ([Formula: see text]). However, some values from DIA porosity yield relatively higher values, specifically in spiculitic mudstone wackestones and argillaceous spiculitic mudstone wackestones. The difference is hypothesized due to the presence of isolated nanopores that are not accessible by helium during measurement of core porosity. Second, the relationship between pore circularity and permeability is indeterminate. The indeterminate relationship is related to a complex internal pore network, intensive diagenetic alteration, an unconnected microfracture network, and isolated pores. Third, positive moderate to strong correlations ([Formula: see text]) between porosity and permeability are observed only in four lithofacies. Fourth, coarse-grained lithofacies within the uppermost depositional sequence of the Mississippian interval have a heterogeneous pore-size distribution, whereas fine-grained lithofacies tend to exhibit a homogeneous pore-size distribution. Fifth, higher reservoir quality is associated with the upper intervals of high-frequency shallowing-upward cycles. This confirms that the sequence-stratigraphic variability of lithofacies is important to predict reservoir quality and its distribution. An alternative graphical method of pore-size distribution is also developed. To be a useful “technique,” examples of the plot are demonstrated using samples in this study. The plot successfully provides simple identification of pore-size classes, quantitative percentage of pore-size class, dominant pore class, and approximate minimum and maximum pore size.

Cell wall microstructure, pore size distribution and absolute density of hemp shiv.

This paper, for the first time, fully characterizes the intrinsic physical parameters of hemp shiv including cell wall microstructure, pore size distribution and absolute density. Scanning electron microscopy revealed microstructural features similar to hardwoods. Confocal microscopy revealed three major layers in the cell wall: middle lamella, primary cell wall and secondary cell wall. Computed tomography improved the visualization of pore shape and pore connectivity in three dimensions. Mercury intrusion porosimetry (MIP) showed that the average accessible porosity was 76.67 ± 2.03% and pore size classes could be distinguished into micropores (3–10 nm) and macropores (0.1–1 µm and 20–80 µm). The absolute density was evaluated by helium pycnometry, MIP and Archimedes' methods. The results show that these methods can lead to misinterpretation of absolute density. The MIP method showed a realistic absolute density (1.45 g cm−3) consistent with the density of the known constituents, including lignin, cellulose and hemi-cellulose. However, helium pycnometry and Archimedes’ methods gave falsely low values owing to 10% of the volume being inaccessible pores, which require sample pretreatment in order to be filled by liquid or gas. This indicates that the determination of the cell wall density is strongly dependent on sample geometry and preparation.