Pore Space Properties Research Articles

Synergistic action and effect mechanism of coal gangue powder and red mud on the properties of concretes

Red mud and coal gangue powder were employed as substitutes for cement to prepare red mud concrete, coal gangue concrete, and red mud-coal gangue concrete. The effects of red mud and coal gangue powder on the physical properties, workability, and mechanical properties of concretes were evaluated, and the hydration reaction mechanisms were elucidated through microanalysis. Furthermore, the reaction process was investigated through thermogravimetric analysis. The results indicated that the individual addition of red mud and coal gangue powder had negative effect on the strength, workability, and porosity of concretes. However, the combination of red mud and coal gangue powder significantly improved the strength and porosity of the concrete compared to red mud concrete and coal gangue concrete. The red mud-coal gangue concrete with 15 % red mud and 15 % coal gangue powder had lower porosity, and comparable compressive strength with the cement concrete. The microscopic results and thermogravimetric analyses indicated that the synergistic effect of red mud and coal gangue led to the improvement of the strength of red mud-coal gangue concrete. The red mud exhibited the promotion effect on the hydration of coal gangue powder to generate new C-S-H, filling the pores. The denser structure in the red mud-coal gangue concrete was formed, thereby improving the pore space and mechanical properties of the concretes.

Pore-scale investigations of permeability of saturated porous media: Pore structure efficiency

Accurate estimation of permeability and the establishment of correlations with pore space properties have garnered attention due to the increasing demand for oil and gas reservoir development as well as geological carbon sequestration. However, prevailing theoretical and empirical models often rely on porosity and pore sizes as key pore space properties to predict permeability, overlooking the variability of pore structure. To delineate the intrinsic associations between permeability and pore space properties, a self-developed visualized experimental system with controllable pore space properties is used to measure permeability of porous media with varying porosity and particle arrangements. Various parameters, encompassing porosity, pore sizes (maximum pore diameter and pore throat diameter) and fractal parameters (pore fractal dimension, succolarity, and lacunarity) are extracted to quantify pore space properties, and further utilized to develop permeability models. Results demonstrate a positive correlation between permeability and porosity, maximum pore diameter, and average pore throat diameter. Additionally, particle arrangements yield pronounced anisotropy in permeability. Relying solely on porosity and pore sizes to portray pore space properties falls short of comprehensively characterizing permeability. Incorporating fractal characterization through metrics like succolarity and lacunarity emerges as a potent avenue for delineating heterogeneity and pore connectivity. This approach proves useful in characterizing the effect of pore structure variability on permeability. The established permeability model rooted in flow direction succolarity and normalized lacunarity aligns effectively with experimental data, capturing the essential physics of permeability variation.

Deterioration of volcanic tuffs from rock dwellings in Brhlovce (Slovakia) induced by freeze-thaw cycling studied by non-destructive tests and µCT visualization

The aim of this paper is to evaluate the effect of freeze-thaw-induced weathering of volcanic tuffs from the rock dwellings in the village of Brhlovce (Slovakia) based on a non-destructive and experimental study of tuff pore space properties. Traditional techniques for characterization of the porosity and pore size distribution and new experimental techniques like spontaneous imbibition (pore interconnection) or indicative rock pore structure method (pore size distribution) were compared with pore network models acquired by X-ray computed microtomography before and after freeze-thaw cycling in a custom-built thermodilatometer VLAP04. The results offer insight into how frost weathering altered the pore network of volcanic tuffs. Brhlovce tuffs are highly susceptible to frost damage due to their high porosity, pore interconnectivity, and bimodal pore size distribution pattern, where nanometric-sized micropores predominate over larger micrometric-sized capillary pores. Poromechanical principles were applied to confirm that ice crystallization initiates in well-connected micropores with a corresponding crystallization pressure below 10 MPa. After undergoing 100 freeze-thaw cycles with temperature oscillations ranging from − 10 to 10 °C, there was a significant increase in total porosity, pore interconnectivity, and rock permeability. Additionally, the pore size distribution was altered. These parameters influence the transport of fluids and the moisture regime in rock masses, which can serve as a precursor to physical and chemical weathering processes

Parameterization of key rock pore space properties based on tomographic measurements

Celem pracy było wykonanie szerokiej analizy parametrów przestrzeni porowej otrzymanych na podstawie obrazów rentgenowskiej tomografii komputerowej. W pracy określono geometryczne i topologiczne parametry struktury wewnętrznej próbek skał, pełniące kluczową rolę dla prognozowania przepływu mediów złożowych. Materiał badawczy stanowiły próbki dolomitu z kanalikami robaczkowymi powstałymi w wyniku zabiegu kwasowania. Modele 3D przestrzeni porowej zostały wykonane przy zastosowaniu nowoczesnego specjalistycznego oprogramowania Avizo 3D Pro (Thermo Fisher Scientific). Praca polegała na określeniu szeregu parametrów geometrycznych, takich jak: objętość, powierzchnia właściwa, grubość, szerokość i długość oraz sferyczność, maksymalna i minimalna średnica Fereta, liczba Eulera i inne. Do wyliczenia parametrów, oprócz oprogramowania Avizo 3D Pro, wykorzystano również program poROSE (poROus materials examination SoftwarE, Akademia Górniczo-Hutnicza, Kraków). Następnie wybrano największe obiekty w każdej z próbek i przeprowadzono dla nich proces transformacji obrazu w szkielet, co pozwoliło na ich charakterystykę topologiczną. Uzyskano szereg informacji dotyczących kanalików porowych, m.in. o ich średnim promieniu, długości, krętości, liczbie połączeń (węzłów) i liczbie koordynacyjnej. Następnie porównano i przeanalizowano wszystkie parametry dla trzech obiektów pochodzących z trzech badanych próbek. Obiekt o największej objętości zidentyfikowano w próbce C. Wszystkie wybrane do analizy obiekty charakteryzowała podobna wartość średnia średniego promienia kanalików. Podobne są także średnie wartości liczby koordynacyjnej – wynoszą około 3. Najbardziej rozbudowaną strukturą charakteryzują się obiekty w próbkach B i C, na co wskazuje liczba Eulera. Przeprowadzone analizy pokazały, jak wiele informacji na temat szczegółów struktury porowej skał można uzyskać na podstawie wyników tomografii komputerowej. Otrzymane parametry mogą posłużyć do budowy modelu sieci porowej, a także do przeprowadzenia symulacji przepływu mediów przez skałę.

Freeze-thaw effects on pore space and hydraulic properties of compacted soil and potential consequences with climate change

Freezing and thawing affect the pore-space structure in agricultural soils with implications for soil hydraulic properties and water flow. Previous studies have focused on the upper few centimeters of the tilled topsoil, where most freeze-thaw (FT) cycles occur, even though deeper soil layers are also subject to freezing and thawing in cold climates. Thus, little is known about how freezing and thawing affect untilled soil layers, which often show high bulk densities that restrict vertical water movement. Furthermore, it remains unclear how shifts in FT patterns with climate change may change the pore-space structure and water flow through these soil layers. Here we investigated the effects of freezing and thawing on X-ray imaged pore-space characteristics, water retention and near-saturated hydraulic conductivity (K) in untilled soil directly below plough depth. Intact cores were sampled at two sites in central Sweden under the same long-term reduced tillage management. The two soils, a silt loam and a silty clay loam, were subjected to three FT scenarios in a laboratory environment intended to represent FT patterns that are considered likely under current and future winter conditions for this region. The latter scenario was characterised by more FT cycles and a lower freezing temperature. Freezing and thawing increased K in the near-saturated range in both soils, which we attribute to observed small (<0.01 mm3 mm−3) increases in the volume of pores of diameters close to the X-ray resolution limit. Concomitant increases in pore network connectivity and critical pore diameter, especially in the denser silty clay loam soil, probably contributed to this increase in K. The water retention data suggested that changes in pore-space characteristics below X-ray resolution also occurred in both soils. Furthermore, our results indicate that both soils may show higher drainage rates due to shifts in FT patterns in the future, although longer-term changes in pore-space structure with an increasing number of FT cycles would mostly be limited to soils with relatively high clay contents. These soils are often more compacted below plough depth and, thus, benefits from improvements in soil structure such as improved root growth and plant water supply are also expected to be larger.

Three-dimensional pore network of kaolin using FIB-SEM imaging

The aim of this paper is to propose a new approach permitting the investigation of the 3D pore variation of kaolin K13 in relation with mechanical loading. A mechanical loading consisting of one-dimensional compression and oedometric loading is applied to a sample of centimeter scale. A postmortem protocol using scanning electron microscopy (SEM) coupled with Focused Ion Beam (FIB) was carried out to observe and then obtain a large number of slices of an extracted sub-volume from the small sample. Image analysis using a new processing method enabled the reconstruction of the observed and extracted sub-volume in the form of a rebuilt sub-volume. The segmentation of pores and particles was carried out using an approach based on machine learning and the pore space properties can be quantified. Pore morphology was identified based on two parameters namely the flatness and the elongation. Spatial orientation can also be locally determined. The proposed treatment enabled the distribution of pore size (termed Im_PSD) to be deduced and these were then compared with the results of Mercury intrusion porosimetry technique.

Heterogeneity of pore space properties at the pedon scale of the Phaeozems humus horizon

The variability of soil parameters depends on the chosen method of measurements, the genesis and type of soil land use, and the level of hierarchy of soil structure organization. Computed tomography of soils is an actively growing method of soil structure study, for which many methodological issues remain relevant. The aim of this work was to examine the variability of the main parameters of the pore space (total porosity, number and average pore size) by the example of the humic horizon of a Phaeozem soil. For this purpose, an excessive number (15 microcores of 2 × 3 cm volume) was sampled from soil profile. Based on statistical evaluation of parameter variation, the objective was to determine the optimal number of replicates allowing full characterization of the soil pore space microstructure at the pedon scale. The smallest difference in heterogeneity between pedon and representative elementary volume REV was observed for total porosity (~12 times), while this ratio is larger (~14 times) for number and average pore size. On average, the threshold level, at which the dispersion of properties stopped decreasing, was 7.3 ± 0.6 monoliths for total porosity, 6.5 ± 0.6 monoliths for pore number, and 7.5 ± 0.4 monoliths for LT. Thus, minimal number of replicates necessary for full characteristic of soil structure is 7 monoliths. Sampling and analysis of microcores in 3 repetitions allows to describe the heterogeneity of the structure of the upper pedon horizon only by 25–30%.

Investigating the analytical relationship between pore geometry and other pore space properties in carbonate rocks

Although pore geometry plays an important role in carbonate rock physics modelling, few studies have been carried out on its analytic relationship with other pore space properties such as pore space stiffness. We propose an analytical workflow based on the differential effective medium (DEM) to estimate the elastic properties of carbonate rocks. The validity of our results is then cross-checked with the Xu and Payne model on a real carbonate dataset. This workflow establishes a direct and quantitative link between the pore geometry of carbonate rock and its other pore space properties such as the Biot coefficient and pore space stiffness. This relationship can, furthermore, be utilized in defining rock physics templates (RPTs) to investigate the role of pore geometry on the rock elastic properties. Furthermore, we extended the Biot–Gassmann–Krief (BGK) model through our proposed workflow by establishing a theoretical framework to relate the main components of the BGK model to the pore geometry usually estimated in the laboratory or empirically. This can help to investigate the impact of fluid substitution on each of these main components. Our investigation suggests that the higher the Biot and Gassmann coefficients, the more sensitive the rock is to fluid substitution. Moreover, this analytical workflow has been employed to examine the role of selecting different rotational spheroids (i.e. oblate and prolate) on the modelled velocities. Our results show that the modelled velocities depend on this selection in such a way that prolate pores are less sensitive to the variations in their pore aspect ratio compared with oblate pores.

The effects of wettability and permeability on hydrocarbon foam performance in unconsolidated porous media: An experimental investigation at elevated pressure and temperature conditions

Propped fractures in hydraulically fractured reservoirs function as unconsolidated porous media with permeability orders of magnitude greater than the matrix. This disparity in permeabilities leads to poor conformance control and sweep efficiency. The foam generated inside the fractures can be effective in addressing such challenges. It can reduce the gas mobility and enhancing the fracture-matrix interactions. However, the foam performance is influenced by the wetting conditions and pore space properties of the porous medium. In this work, for the first time, the effect of a complex interplay between permeability and wettability on hydrocarbon foam performance was investigated in unconsolidated porous media. To this end, methane foam experiments were conducted on sandpacks of varying wettability and permeability at high-pressure and high-temperature conditions. Additionally, interdependence between permeability and key foam parameters, including surfactant concentration and foam quality, were probed. Foam performance was evaluated based on the steady-state pressure drops across the medium and the foam’s apparent viscosity. The results showed a non-monotonic dependency of foam strength on the permeability of porous media, irrespective of their wetting conditions. We observed that foamability and foam stability were significantly enhanced at ascending permeability due to enlarged pores and pore-to-throat ratios, leading to reduced gas mobility. However, excessively large pore bodies adversely affected the bubble generation and downgraded the foam performance. Furthermore, foam quality significantly influenced the foam performance at all permeabilities. Distinct foam behavior was observed vis-à-vis variations in foam quality and separated by the low- and high-quality regimes. The results showed a modulated correlation between steady-state foam strength and permeability throughout the low-quality regime; however, no common trends were observed in the high-quality counterpart. Additionally, the transition foam quality was higher in oil-wet media than in water-wet systems. This shift broadened with ascending permeability and can be attributed to the wettability of porous media. Interestingly, variations in surfactant concentration produced similar trends in all wettability cases and displayed insensitivity to permeability. The results indicate that an increase in concentration improved the viscoelastic properties of the aqueous films, improving the foam strength in water- and oil-wet media of all permeabilities.

Study on the Effect of Fly Ash Admixture on the Performance of Recycled Pervious Concrete

In order to solve the problems of recycling of a large amount of coarse aggregate produced after the crushing of waste concrete and urban flooding, this project prepares the recycled coarse aggregate of 4.75-9.5 mm into 100 mm × 100 mm × 100 mm cubic permeable concrete specimens, and researches the influence law of different fly ash admixture on the pore space, water permeability and mechanical properties of the recycled aggregate permeable concrete, and determines the best design The optimal design ratio was determined. The experimental results show that the permeable concrete specimens prepared with recycled coarse aggregate of 4.75-9.5mm particle size and mixed with fly ash were cured in water, and the longer the curing age, the higher the compressive strength. With the increase of fly ash dosage, the compressive strength first rises and then decreases, and the porosity and permeability coefficient decreases continuously. When the fly ash dosage is 15%, the 28d compressive strength of recycled aggregate pervious concrete is the highest, which is 12.8 MPa. At this time, the porosity is 23.9%, and the permeability coefficient reaches 3 mm/s. Considering the overall situation, the optimum dosage of fly ash for recycled aggregate pervious concrete is recommended to be about 15%.

Joint Distributions of Local Pore Space Properties Quantitatively Explain Simulated Air Flow Variations in Paper

The gas flow through sheet-like porous materials such as paper can show marked lateral variations due to a heterogeneous, locally varying microstructure. Hence, reliable predictions of such lateral flux variations require an appropriate consideration of local variations in the microstructure. The flow through such sheet-like materials is commonly described with Darcy’s law in which permeances are formulated in terms of microstructure properties, such as porosities, tortuosities, or hydraulic radii. This work proposes an extension of existing permeance models that directly considers the variation and the cross-dependence between local microstructure properties. The extended model is applied to local air fluxes through a paper sheet to exemplarily reveal the joint impact of local porosities and local tortuosities on the air flux. The key extension is to consider a joint distribution of porosity and tortuosity. The latter is constructed from the univariate property distributions using a copula approach and yields local tortuosities including their variation for any encountered local porosity. These values jointly enter any permeance model that qualitatively captures the dependence of the air flux on the porosity. To assess the merit of the model, variations in the air flux and in the pore space properties are independently determined from the same measured microstructure of paper. Air flux variations are provided by computational fluid dynamics simulations on multiple, nonoverlapping segments taken from the microstructure. A statistical analysis of the entire microstructure provides the distribution of local porosity, tortuosity, and thicknesses. Our model quantitatively explains that porosity-dependent variations in the tortuosity, in particular the ones associated with high-volume pathways, decisively determine air flux variations.

How can pore characteristics of soil aggregates from contrasting tillage systems affect their intrinsic permeability and hydraulic conductivity?

Studies on how the alteration in the soil pore architecture can influence the permeability of small soil aggregates are still scarce. These aggregates affect the water dynamics mainly at the intra-aggregate pore scale. Thus, investigating the intra-aggregate pore space properties becomes crucial for comprehending the matter and energy transported into the soil. This paper focuses on analyzing the permeability and hydraulic conductivity of small soil aggregates from contrasting tillage systems (no-tillage – NT, minimum tillage – MT, conventional tillage – CT, and secondary forest – F) through synchrotron-based X-ray Computed Tomography. The 3D image segmentation was performed using machine and deep learning strategies. Permeability was measured based on the image-based parametrization of the Kozeny-Carman equation, which is dependent on porosity, hydraulic radius, and tortuosity. The porosity, hydraulic radius, and pore connectivity were not affected by the soil tillage systems at the scale of analysis (aggregates of a few millimeters). Conventional tillage had higher pore tortuosity relative to NT, demonstrating less favorable fluid transport at the aggregate scale. However, only the aggregates under F had significantly higher permeability and hydraulic conductivity relative to NT, MT, and CT. Our results brought new insights into the effects of different tillage systems on the soil structure and their possible impact on water dynamics at the microscale.

Exploration of the X-ray Dark-Field Signal in Mineral Building Materials.

Mineral building materials suffer from weathering processes such as salt efflorescence, freeze–thaw cycling, and microbial colonization. All of these processes are linked to water (liquid and vapor) in the pore space. The degree of damage following these processes is heavily influenced by pore space properties such as porosity, pore size distribution, and pore connectivity. X-ray computed micro-tomography (µCT) has proven to be a valuable tool to non-destructively investigate the pore space of stone samples in 3D. However, a trade-off between the resolution and field-of-view often impedes reliable conclusions on the material’s properties. X-ray dark-field imaging (DFI) is based on the scattering of X-rays by sub-voxel-sized features, and as such, provides information on the sample complementary to that obtained using conventional µCT. In this manuscript, we apply X-ray dark-field tomography for the first time on four mineral building materials (quartzite, fired clay brick, fired clay roof tile, and carbonated mineral building material), and investigate which information the dark-field signal entails on the sub-resolution space of the sample. Dark-field tomography at multiple length scale sensitivities was performed at the TOMCAT beamline of the Swiss Light Source (Villigen, Switzerland) using a Talbot grating interferometer. The complementary information of the dark-field modality is most clear in the fired clay brick and roof tile; quartz grains that are almost indistinguishable in the conventional µCT scan are clearly visible in the dark-field owing to their low dark-field signal (homogenous sub-voxel structure), whereas the microporous bulk mass has a high dark-field signal. Large (resolved) pores on the other hand, which are clearly visible in the absorption dataset, are almost invisible in the dark-field modality because they are overprinted with dark-field signal originating from the bulk mass. The experiments also showed how the dark-field signal from a feature depends on the length scale sensitivity, which is set by moving the sample with respect to the grating interferometer.

Quantitative Comparison of 3D Pore Space Properties With Magnetic Pore Fabrics—Testing the Ability of Magnetic Methods to Predict Pore Fabrics in Rocks

AbstractPore fabrics characterize the anisotropy of pore space in rocks and influence the direction of fluid flow. This is important in reservoir characterization, and petroleum and geothermal energy exploitation. X‐ray computed micro‐tomography (XRCT) is commonly used to analyze pore fabrics, but limited by the micron‐scale resolution for representative 1‐inch rock cores. The magnetic pore fabric (MPF) method has been proposed to capture pores down to 10 nm. Although empirical relationships between MPF and pore space properties or permeability anisotropy are available, their application is compromised by large variability. This study integrates He pycnometry and XRCT‐derived pore space models with MPFs, and provides a quantitative comparison for calcarenite (∼50 vol% porosity and complex pore structure), and molasse sandstone (10%–30% porosity and relatively homogeneous pore fabrics). The preferred orientation of pores obtained from XRCT is described by a total shape ellipsoid, calculated by summing the second‐order tensors reflecting the best‐fit ellipsoids of individual pores. This ellipsoid is then compared to the MPF magnitude ellipsoid in terms of fabric orientation, degree and shape of anisotropy. The MPF and total shape ellipsoids are generally coaxial. The MPF has a smaller anisotropy degree than the total shape ellipsoid, and their relationship depends on the ferrofluid properties. The anisotropy shapes show large variability. Nevertheless, the good agreement of principal directions in most samples makes MPFs a valuable and efficient complementary tool to analyze a large number of samples, in combination with XRCT on selected samples, for a field‐scale pore space characterization.

Development of restoration mortars and artificial stones for use in restoring cultural heritage sites made from volcanic tuffs

Many historically significant buildings and cultural heritage sites are made of volcanic tuffs. These sites are subject to a progressive weathering and are partly in ruinous conditions. Preparing a suitable stone replacement mortar for repair work or even whole replacement stones for these rocks is of great interest. During the quarrying of tuff rocks, more than 40% of the material is rejected and cannot be further used. Utilizing this material, the production of replacement materials with optimized properties for the restoration and repair of severely damaged natural stones represents a resource-saving measure for the preservation of these buildings. In addition to the effective use of the natural stone waste, these artificial materials, in contrast to the very heterogeneous tuffs, offer a constant quality and consistent properties. A more effective exploitation can lead to a more sustainable usage of this natural resource and minimize the interference to the environment. This study describes the development of stone replacement mortars and the creation of artificial replacement stones utilizing source material from Mexico and Armenia. The resulting replacement material shows similar properties, aesthetics, and weathering behavior like the original rock, while different properties could be optimized. Petrophysical properties as well as the weathering behavior of the volcanic tuff rock were analyzed along with the developed replacement mortar and artificial stone. As hydric and thermohydric expansion processes are important factors for the durability of the investigated rocks, replacement materials were developed and optimized which, except for the pore structure of the cement and concrete matrix, show a high correspondence to the natural stones in terms of macroscopic appearance and technical properties. The compatibility of the replacement material with the original rock was investigated. Moreover, based on the evaluation of the mechanical as well as the pore space, water transport and weathering properties, the replacement stones were evaluated as equally suitable construction material, while the replacement mortars vary in regard to their results. Accordingly, the replacement stones developed in this study can be considered as full-fledged rock substitutes for the Armenian tuffs, while the replacement mortars for the Mexican tuffs partly show a good suitability and could be partially optimized.

Permeability prediction in the South Georgia rift basin–applications to CO2 storage and regional tectonics

Absence of a permeability log necessary to assess reservoir quality and injectivity for potential CO2 storage in the heterogeneous and complex South Georgia Rift (SGR) basin provides the motivation for this study. The focus of this study was on the Triassic-Jurassic red beds buried, entrenched beneath the Cretaceous-Cenozoic Coastal Plain sediments. Moreover, the significant cost typically between $10 M and $100 M associated with drilling and logging for in situ permeability coupled with the limited resolution of existing core data further makes this work necessary. The purpose is to relate, use the interpretation of the predicted permeability distribution to assess feasibility for safe and long-term CO2 sequestration. This study also intends to establish the impacts of active and passive tectonism that has shaped and/or re-shaped the evolution of the basin on the present-day permeability. A methodology was applied that utilizes the pore space and geohydraulic properties of the reservoir from existing laboratory and well data to produce a newly derived permeability log. It shows a non-uniform distribution with depths possibly due to geologic changes in the confined and heterogeneous red beds. The derived log displays characteristics consistent with observations from the porosity and resistivity logs. The interpretation of these logs provides evidence for the presence of low permeable, tightly cemented, and compacted red beds. We conclude that the low permeability aided by the low resistivity depicted in the red beds suggests increased confining stress and reduced injectivity, and that the uncharacteristically low permeability reflects a deformed basin shaped with episodes of uplift and erosion.

Peat macropore networks – new insights into episodic and hotspot methane emission

Abstract. Peatlands are important natural sources of atmospheric methane (CH4) emissions. The production and emission of CH4 are strongly influenced by the diffusion of oxygen into the soil and of CH4 from the soil to the atmosphere, respectively. This diffusion, in turn, is controlled by the structure of macropore networks. The characterization of peat pore structure and connectivity through complex network theory approaches can give conceptual insight into how the relationship between the microscale pore space properties and CH4 emissions on a macroscopic scale is shaped. The evolution of the pore space that is connected to the atmosphere can also be conceptualized through a pore network modeling approach. Pore regions isolated from the atmosphere may further develop into anaerobic pockets, which are local hotspots of CH4 production in unsaturated peat. In this study, we extracted interconnecting macropore networks from three-dimensional X-ray micro-computed tomography (µCT) images of peat samples and evaluated local and global connectivity metrics for the networks. We also simulated the water retention characteristics of the peat samples using a pore network modeling approach and compared the simulation results with measured water retention characteristics. The results showed large differences in peat macropore structure and pore network connectivity between vertical soil layers. The macropore space was more connected and the flow paths through the peat matrix were less tortuous near the soil surface than at deeper depths. In addition, macroporosity, structural anisotropy, and average pore throat diameter decreased with depth. Narrower and more winding air-filled diffusion channels may reduce the rate of gas transport as the distance from the peat layer to the soil–air interface increases. The network analysis also suggests that both local and global network connectivity metrics, such as the network average clustering coefficient and closeness centrality, might serve as proxies for assessing the efficiency of gas diffusion in air-filled pore networks. However, the applicability of the network metrics was restricted to the high-porosity near-surface layer. The spatial extent and continuity of the pore network and the spatial distribution of the pores may be reflected in different network metrics in contrasting ways. The hysteresis of peat water content between wetting and drying was found to affect the evolution of the volume of connected air-filled pore space in unsaturated peat. Thus, the formation of anaerobic pockets may occur in a smaller soil volume and methanogenesis may be slower when the peat is wetting compared to in drying conditions. This hysteretic behavior might explain the hotspots and episodic spikes of CH4 emissions, and therefore, it should be taken into account in biogeochemical models.

Digital rock physics: Defining the reservoir properties on drill cuttings

Well drilling with core extraction is one of the most expensive procedures in the oil industry. The potential alternative, which may open opportunities for cost-efficient acquisition of petrophysical data, can be an approach based on digital analysis of drill cuttings. The main purpose of this research was to evaluate the possibility of using digital cuttings analysis and rock physics for reservoir properties evaluation. The carbonate samples of core and cuttings were simultaneously collected from the same intervals during the drilling of deviated wells in the Carboniferous reservoir of the oil deposit in the Republic of Tatarstan, Russia. Based on the core analysis using the optical study of thin sections and XRD, we investigated sedimentological and mineralogical features and classified all lithotypes of well intervals. The standard samples were produced from all defined facies and their reservoir properties were studied in routine tests. The largest cuttings from each well interval were separated and their structures were compared using μCT with core subsamples of the same intervals. We assessed the changes of cuttings structure occurring in the drilling process, which include the colmatation of the pore space with drilling fluid and disintegrated rock particles and the formation of secondary fractures. Detailed analysis of cuttings from one well interval consisting of the border between the facies of high porous grainstones with packstones and dense micrite limestones demonstrated that high porous varieties were eroded in the drilling process. The analysis of chemical composition using XRF-mapping, XRD and SEM with EDS showed that most of the cuttings were constituted by disintegrated rock particles or form elements (e.g. separate fusulinids) cemented by drilling mud. However, more durable cuttings from dense micrite limestones facies were generally retained their dominant calcite composition and original rock structure, with only secondary fractures. Finally, we analyzed pore space properties and performed digital simulations of wave propagation velocities to compare the characteristics of these cuttings and core subsamples from this lithotype, scanned in the same μCT settings. The results demonstrate close characteristics of porosity 0.08–0.6%, dominant pore sizes 1–60 μm and velocities Vp = 6130–6240 m/s, Vs = 3180–3230 m/s. However, the comparison of obtained values with measured on standard samples shows a significant difference, which is caused by deviation from REV. The potential use of cuttings for assessing reservoir and petrophysical properties can be possible in case of provision with the factors of cuttings structure alteration, representativity of samples, as well as optimization for these purposes of the drilling process with cuttings selection.

Application of machine learning in colloids transport in porous media studies: Lattice Boltzmann simulation results as training data

Colloid transport through a porous medium changes geometrical and hydraulic properties of the pore space. The impact of this effect depends on the colloid types and pore space surface properties which determine the likelihood of pore clogging. Colloid attachment and subsequent detachment are key factors in pore clogging. In this study, the impact of four major fluid and colloids properties on the pore surface and hydraulic conductivity alteration during colloids transport were evaluated using machine learning. These four parameters include solution ionic strength, zeta potential, colloid size and fluid flow velocity.A combined lattice Boltzmann-smoothed profile method was used to simulate accurately coupled mechanisms governing colloid transport to evaluate the impact of the four parameters on the resulting pore space properties after colloid transport. The result of several simulations revealed significant changes of pore surface coverage by the attached colloids, and conductivity, void fraction and coordination number of colloid agglomerates created during transport of individual colloids. Since the simulation of the impact of combination of all possible sets of four parameters is very time consuming, an Artificial Neural Network (ANN) was used as a prognostic method to use the results of several simulations to predict the behavior for a wide range of pore, colloidal and fluid properties. Reported results from a set of 162 simulation case studies for different possible combination of solution ionic strength, zeta potentials, colloid size and flow velocity were selected as input parameters for the machine learning. Four output parameters, namely, pore surface coverage, conductivity, void fraction and coordination number of the colloidal particles were selected.To lower the prediction error value, which is targeted to be lower than 10%, networks were trained 50 times using a MATLAB code, and in each training, after at most 10 epochs, networks were trained. A maximum relative error value of 8.95% was obtained, which is very well within the range of training quality criteria. The results show that the ANN can profoundly predict the simulation outcomes for a wide range of ionic strength (IS) and can be directly used to obtain the value of dependent variables through simple calculations using network weights and transfer functions.

Microstructure evolution of young sea ice from a Svalbard fjord using micro-CT analysis

AbstractWe analysed the three-dimensional microstructure of sea ice by means of X-ray-micro computed tomography. Microscopic (brine- and air- pore sizes, numbers and connectivity) and macroscopic (salinity, density, porosity) properties of young Arctic sea ice were analysed. The analysis is based on ice cores obtained during spring 2016. Centrifuging of brine prior to CT imaging has allowed us to derive confident relationships between the open, vertically connected and total porosity of young sea ice at relatively high temperatures. We analysed the dependence of the microscopic properties on vertical position and total brine porosity. Most bulk properties (salinity, density) and pore space properties (pore sizes and their distribution) show a strong dependence on total brine porosity, but did not change significantly over the course of the field work. However, significant changes were observed for pore numbers (decreasing over time) and pore connectivity (increasing over time). CT-based salinity determinations are subject to larger than standard uncertainties (from conductivity), while the CT method yields important information about the salinity contributions from closed and open pores. We also performed a comparison of CT-based air porosity with calculations based on density from hydrostatic weighing. The consistency is encouraging and gives confidence to our CT-based results.