Pore Network Connectivity Research Articles

Modeling porosity and permeability evolution of burrow fillings with packstone fabric in the Upper Jurassic Hanifa Formation, central Saudi Arabia: A digenetic backstripping study

Understanding the development and characteristics of burrow fillings (BF) is crucial for predicting porosity, permeability, and fluid flow behavior in bioturbated carbonate reservoirs. Advanced imaging, digital rock modeling, and backstripping techniques offer valuable insights into this development. In this study, we examined the BF in the bioturbated strata of the Upper Jurassic Hanifa Formation (central Saudi Arabia) to understand how sedimentological and diagenetic processes—such as compaction, dolomitization, and dissolution—influence porosity and permeability over time.Backstripping results provide significant insights into the evolution of porosity and permeability within packstone BF of the studied strata. We found that compacted, grain-supported BF exhibit limited improvement in porosity and permeability post-compaction. In contrast, less compacted intervals with open fabric BF can achieve up to 20% porosity and >5000 mD permeability in their grainstone versions. Notably, even with over 10% mud content, open fabric BF maintained substantial permeability (>1000 mD) due to a connected pore network. Moreover, these BF have the potential to recover porosity and permeability after complete pore network occlusion by the mud matrix through diagenetic processes like preferential dolomitization of the mud matrix and subsequent dissolution.The findings underscore the dynamic evolution of porosity in packstones within BF of bioturbated strata. Initially, mud-rich packstones with low porosity and permeability can transform into more permeable packstones through dolomitization and dissolution. Conversely, initially high-quality, mud-poor packstones with high porosity and permeability may degrade due to compaction. The quantification of porosity and permeability using digital rock modeling and backstripping techniques provides an objective and comprehensive understanding of these evolving properties, highlighting key phases and processes affecting reservoir quality. The approach of integrating digital rock modeling and backstripping technique enhances the ability to predict subsurface storage and flow capacity in bioturbated strata.

Introducing a novel Hierarchy-Connectivity factor for characterizing micro-mesoporous materials

To provide a comprehensive description of hierarchical pore networks and optimize micro-mesoporous materials, it is essential to consider pore connectivity as a parameter. This study introduces an innovative hierarchy-connectivity factor (HCF) that includes the volume, size, and connectivity data of pores within the pore network. This study covers micro-mesoporous biosourced carbons, with ordered or disordered mesoporous structures, and three commercial activated carbons derived from petroleum coke and biomass. A dual-shape slit-cylinder model was employed to precisely assess the textural properties of these carbons. The proposed methodology evaluates connectivity exclusively through gas adsorption–desorption isotherm analysis, providing a rapid and practical tool for characterizing the porous network of carbon materials. The efficacy of this HCF is demonstrated in the examination of carbon materials functioning as electrodes for supercapacitors. Overall, this research transcends the field of gas adsorption-based studies, highlighting the importance of evaluating the pore network connectivity within carbon materials in order to optimize them for a particular application. Importantly, this contribution could be extended beyond the scope of carbon materials, encompassing a broader spectrum of porous materials.

Effect of silica fume on the efflorescence, strength, and micro-properties of one-part geopolymer incorporating sewage sludge ash

The application of sewage sludge ash (SSA) in alkali activated cementitious material, i.e., geopolymer, was one of the effective measures to dispose of and recycle the sewage sludge. However, the geopolymer efflorescence could be aggravated due to elevated alkali content, especially for one-part geopolymer, where the dissolution reaction of solid alkali activator was less sufficient. This study focused on the efflorescence of GGBS-SSA based one-part geopolymer, as compared to two-part geopolymer, and the effect of silica fume on the efflorescence, strength and micro-properties of the one-part geopolymer. The results showed that the extent of efflorescence crystallization and the concentration of leached Na+ ions in the one-part geopolymer were more severe than the two-part geopolymer, due to the lower degree of geopolymerization reaction and the heterogeneous dense microstructure of the one-part geopolymer. After adding silica fume to the one-part geopolymer, the extent of efflorescence crystallization and the concentration of leached Na+ ions reduced significantly, and the compressive strength increased from 41.6 MPa to 57 MPa when the silica fume content was 10 %, but then decreased slightly. Adding the silica fume reduced the release rate of reaction heat, which helped to increase the solubility of solid alkali activator and improve the geopolymerization reaction, thus significantly reducing the content of mobile alkali ions and enhancing the compressive strength. Moreover, the silica fume with tiny spherical particle had better ball effect and micro-aggregate filling effect, which refined the pore size and reduced the connectivity of pore network, significantly reducing the diffusion rate and leaching of mobile alkali ions.

Characterizing connectivity and tortuosity of pore network based on LF-NMR method to assess the water permeability of white cement mortar

To better understand the macro transport properties and durability of cementitious materials, their microstructure should be characterised more accurately. This study investigated white cement mortars with different water-cement ratios, sand volume fractions, and curing ages. The water permeability was measured using a self-manufactured facility. The low-field nuclear magnetic resonance (LF-NMR) technique combined with the displacement method was used to test the pore size distributions (PSD) of total pores and connected regions. Two-dimensional mortar and pore-based matrix models were established to obtain the tortuosity of the transport path. The results show that the pore connectivity of the mortar ranges from 32 % to 52 %, and the tortuosity ranges from 1.428 to 1.542. The pore connectivity was spatially sensitive, and small pores were densely connected, linking large pores to form a pore network. Although capillary pores dominate the permeability, the role of gel pores cannot be ignored. Considering gel pores can improve the accuracy of the predicted water permeability by 80 %. The relative error of the modified model is [-23.76 %, 37.19 %], which shows a significant improvement.

The impact of porous structure on oil–water two-phase flow under CO2 environment in continental shale reservoirs

CO2 applications for enhanced oil recovery and storage in continental shale reservoirs are promising, and there is a need to evaluate the impact of porous structure on oil–water two-phase flow under CO2 environment. In this study, first, digital cores of quartz-rich, carbonate-rich, and clay-rich shales are established using Focused Ion Beam Scanning Electron Microscopy scanning data processed through generative adversarial networks. Subsequently, the pore networks generated by digital cores are quantitatively analyzed using the generalized extreme value distribution. Finally, pore network modeling is carried out to elucidate the effect of porous structural differences on oil–water flow considering CO2 dissolution and capillary forces. The results show that quartz-rich shale, characterized by nanopore intergranular dominance and the highest pore network connectivity, demonstrates the highest relative permeability of the oil phase. Carbonate-rich shale exhibits intermediate relative permeability of oil phase, while clay-rich shale exhibits the worst. The dissolution of CO2 reduces oil–water interfacial tension and oil viscosity, enhances oil mobilization within nanopores, and notably increases the relative permeability of the oil phase. The permeability of the oil phase is governed by pore structure, displaying positive correlations with core heterogeneity, pore radius, coordination number, and throat length, and negative correlations with throat radius.

A field study of pore-network systems on the tight shale gas formation through adsorption-desorption technique and mercury intrusion capillary porosimeter: Percolation theory and simulations

Properly identifying microscopic pore structure properties of shale reservoirs is essential for developing efficient extraction techniques of shale hydrocarbons. To understand the unique and complex pore network of the Northern Guizhou shale reservoir, shale samples from the Wufeng-longmaxi Formation were investigated using different analytical techniques, X-Ray Diffractometry, low-pressure nitrogen gas adsorption-desorption isotherms, and Mercury intrusion capillary porosimeter. Results indicate that the Wufeng-longmaxi formation is mainly composed of a high quantity of quartz ranging from 0.6 % to 64.1 % and an average extent of 55.825 %, the lowest amount of potassium feldspar ranging from 0.6 % to 2.2 % and an average extent of 1.45 % with Total organic contents ranging from 3.672 to 5.209 %. Similarly, the BJH adsorption techniques for pore volume and area were investigated. The results indicate the dominance of the mesopore with pore size ranging from 2.03 nm to 40nm, average pore volume of 0.073715 m3/g, and average pore area of 13.0324 m2/g. The pore throat distributions from MICP showed drain pressure and median pore throat radius ranging from 0.47 to 13.79 MPa and 0.05–0.08 μm, respectively. Suggested complementary experimental techniques reveal the connectivity of pore networks of the Wufeng-longmaxi Formation shale reservoir, which shows a high production potential of hydrocarbons. The findings of this study can provide a valuable understanding of fluid transport properties and storage space of shale gas that can help decision-makers as they set exploration initiatives for shale gas reservoirs.

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.

Automatic 3D cell segmentation of fruit parenchyma tissue from X-ray micro CT images using deep learning

BackgroundHigh quality 3D information of the microscopic plant tissue morphology—the spatial organization of cells and intercellular spaces in tissues—helps in understanding physiological processes in a wide variety of plants and tissues. X-ray micro-CT is a valuable tool that is becoming increasingly available in plant research to obtain 3D microstructural information of the intercellular pore space and individual pore sizes and shapes of tissues. However, individual cell morphology is difficult to retrieve from micro-CT as cells cannot be segmented properly due to negligible density differences at cell-to-cell interfaces. To address this, deep learning-based models were trained and tested to segment individual cells using X-ray micro-CT images of parenchyma tissue samples from apple and pear fruit with different cell and porosity characteristics.ResultsThe best segmentation model achieved an Aggregated Jaccard Index (AJI) of 0.86 and 0.73 for apple and pear tissue, respectively, which is an improvement over the current benchmark method that achieved AJIs of 0.73 and 0.67. Furthermore, the neural network was able to detect other plant tissue structures such as vascular bundles and stone cell clusters (brachysclereids), of which the latter were shown to strongly influence the spatial organization of pear cells. Based on the AJIs, apple tissue was found to be easier to segment, as the porosity and specific surface area of the pore space are higher and lower, respectively, compared to pear tissue. Moreover, samples with lower pore network connectivity, proved very difficult to segment.ConclusionsThe proposed method can be used to automatically quantify 3D cell morphology of plant tissue from micro-CT instead of opting for laborious manual annotations or less accurate segmentation approaches. In case fruit tissue porosity or pore network connectivity is too low or the specific surface area of the pore space too high, native X-ray micro-CT is unable to provide proper marker points of cell outlines, and one should rely on more elaborate contrast-enhancing scan protocols.

Engineering of a decellularized bovine skin coated with antibiotics-loaded electrospun fibers with synergistic antibacterial activity for the treatment of infectious wounds.

An ideal antibacterial wound dressing with strong antibacterial behavior versus highly drug-resistant bacteria and great wound-healing capacity is still being developed. There is a clinical requirement to progress the current clinical cares that fail to fully restore the skin structure due to post-wound infections. Here, we aim to introduce a novel two-layer wound dressing using decellularized bovine skin (DBS) tissue and antibacterial nanofibers to design a bioactive scaffold with bio-mimicking the native extracellular matrix of both dermis and epidermis. For this purpose, polyvinyl alcohol (PVA)/chitosan (CS) solution was loaded with antibiotics (colistin and meropenem) and electrospun on the surface of the DBS scaffold to fabricate a two-layer antibacterial wound dressing (DBS-PVA/CS/Abs). In detail, the characterization of the fabricated scaffold was conducted using biomechanical, biological, and antibacterial assays. Based on the results, the fabricated scaffold revealed a homogenous three-dimensional microstructure with a connected pore network, a high porosity and swelling ratio, and favorable mechanical properties. In addition, according to the cell culture result, our fabricated two-layer scaffold surface had a good interaction with fibroblast cells and provided an excellent substrate for cell proliferation and attachment. The antibacterial assay revealed a strong antibacterial activity of DBS-PVA/CS/Abs against both standard strain and multidrug-resistant clinical isolates of Acinetobacter baumannii, Pseudomonas aeruginosa, and Escherichia coli. Our bilayer antibacterial wound dressing is strongly suggested as an admirable wound dressing for the management of infectious skin injuries and now promises to advance with preclinical and clinical research.

Investigating Soil Pore Network Connectivity in Varied Vegetation Types Using X-ray Tomography

The ecological environment in southwestern China is fragile. Due to the significant preferential flow in vertical and horizontal directions and poor water conservation ability, vegetation degradation still exists under conditions of abundant rainfall. Therefore, the pore connectivity and infiltration characteristics in shallow soil under typical local vegetation need to be studied. A calculation model for the vertical connectivity of soil macropores was independently constructed, and differences in soil macropore structures and the degree of vertical connectivity in typical vegetation types (natural secondary forest, natural grassland, Yunnan pine plantation, eucalyptus plantation, cypress plantation, mulberry bushes) were investigated by CT scanning technology of undisturbed soil columns. The results showed that the vertical connectivity of large pores in the shallow soil of the region can be quantitatively described by X-ray tomography, and the total surface area and cumulative curvature of macropores in natural grassland soil were two or three times that in artificial vegetation. The concentration area of macropores in the soil of artificial forestland was closer to the surface, and the tendency of macropore preferred path decreased by 76.18% around 30 cm depth in the soil. The vertical connection of soil macropores in artificial forests was significantly lower than that of natural secondary forestlands (33.03%) and natural grasslands (36.75%). The restoration of the plantation improved surface soil pore structure, and the vertical connectivity of soil is nearly 20% less than that of natural vegetation types (natural secondary forestland, natural grassland), which reduced water outflow rate by nearly 44% and electrolyte content by nearly 14% at a depth of 30 cm. This study provided data and research directions for the study of hydrological processes in local forest vegetation and technical support for solving the problems of soil water loss and forestland water conservation in southwestern China.

Nanoscale Three-Dimensional Network Structure of a Mesoporous Particle Unveiled via Adaptive Multidistance Coherent X-ray Tomography.

Mesoporous nanoparticles provide rich platforms to devise functional materials by customizing the three-dimensional (3D) structures of nanopores. With the pore network as a key tuning parameter, the noninvasive and quantitative characterization of these 3D structures is crucial for the rational design of functional materials. This has prompted researchers to develop versatile nanoprobes with a high penetration power to inspect various specimens sized a few micrometers at nanoscale 3D resolutions. Here, with adaptive phase retrievals on independent data sets with different sampling frequencies, we introduce multidistance coherent X-ray tomography as a noninvasive and quantitative nanoprobe to realize high-resolution 3D imaging of micrometer-sized specimens. The 3D density distribution of an entire mesoporous silica nanoparticle was obtained at 13 nm 3D resolution for quantitative physical and morphological analyses of its 3D pore structure. The morphological features of the whole 3D pore network and pore connectivity were examined to gain insight into the potential functions of the particles. The proposed multidistance tomographic imaging scheme with quantitative structural analyses is expected to advance studies of functional materials by facilitating their structure-based rational design.

Pore-fracture and permeability heterogeneity of different marcolithotypes of medium-rank coals in Jixi Basin, China

The pore-fracture system of different marcolithotypes (bright, semi-bright, semi-dull and dull coals) is quite different, which has great influences on coalbed methane (CBM) exploitation. Here the optical microscope (OM), scanning electron microscope (SEM), mercury intrusion porosimetry (MIP), low-temperature nitrogen adsorption (LTND) and X-ray computed tomography (X-CT) are performed on different marcolithotype coals from Jixi Basin. From bright coals to dull coals, “ink bottle” shaped pores account for a gradually increasing proportion of the transition pores, but contribute fewer mesopores. The results of LTND and MIP indicate that bright coals have the poorest mercury removal efficiency and lowest BET specific surface area, while the opposite appears to be the case for dull coals. According to the pore size distribution based on LTND and MIP, from bright coal, semi-bright coal, semi-dull coal to dull coal, the total pore volume gradually decreases, with the proportion of macropore volume occupying 76.2%, 5.1%, 3.7% and 1.7%, respectively. On the basis of the 3D reconstruction of the pore-fracture system obtained from X-CT, the bright coal is characterized by high connected porosity and total porosity, presenting the best pore-fracture system. The pore network of bright coal is three-way interconnected with the largest number of pores and the shortest and densest throats, which facilitates the communication of fluids between the pores. In contrast, dull coals with a unidirectionally connected pore network model, fewer pores and sparse throats, have a poor seepage capacity. In addition, the X-CT seepage simulation results and the gas permeability results confirm the superiority of the pore-fracture system in bright coals.

3D CFD-DEM study on fine particle migration in packed proppant layers

Fine particle migration is a crucial issue in hydraulic fracturing-based production from unconventional reservoirs and can significantly affect the effective conductivity of the packed proppant layers. In this study, to investigate the conveying behavior of fine particles, a Eulerian-Lagrangian method, namely the computational fluid dynamics–discrete element method (CFD-DEM), is adopted for 3D simulations. Two types of particles are involved in this process: larger proppant particles and fine particles. Proppant particles form a background skeleton, and a two-phase flow of fine particles and fluid is constrained in the connected pore networks. We adopt a unified CFD-DEM framework to resolve the non-slip boundary condition on the proppant particle surface and track fine particles using a four-way coupling strategy. In particular, a forcing term based on the fictitious domain method is embedded in a regular volume-averaged Navier-Stokes equation to solve the fluid motion. A perfectly packed skeleton of a face-centered cube is designed for subsequent numerical studies. The results show that the fluid and particle flow velocities are higher near the fracture surfaces than inside the interlayers. In addition, it is found that as the fine particle concentration increases, the average horizontal velocity increases, whereas the settling velocity tends to decrease.

Mechanical properties and pore network connectivity of sodium montmorillonite as predicted by a coarse-grained molecular model

The study of ionic transport through hydrated sodium montmorillonite (Na-MMT)—the main swelling component of bentonite—is of significant interest to better understand its ability to contain contaminants. From a macroscopic viewpoint, porosity and tortuosity can be viewed as scaling factors connecting diffusion coefficients in the clay material to free diffusion coefficients of ions in water. In this work, the mechanical properties, porosity and tortuosity of Na-MMT were calculated using a bottom-up approach. First, the constructed coarse-grained mesoscopic models of Na-MMT were fitted to all-atom molecular dynamics simulations. Thirty different models—each containing one thousand 120 Å-wide hexagonal Na-MMT platelets—were generated. The dry densities considered herein range from 0.8 to 1.3 g/cm3. Second, calculated the models' elastic constants were in excellent agreement with experimental values reported in the literature. Third, each system's porosity, pore size distribution and pore network were analysed. Fourth, the pore network information was used to create a 3D image of hydrated Na-MMT, and random walk simulations were employed to evaluate its tortuosity. Finally, the porosity and tortuosity values estimated using the models were compared to macroscopic experimental values describing tritium and iodide diffusion in Na-MMT dominant bentonite. The values obtained using the models were fairly consistent with experimental values reported in the literature.

Assessing the impact of bioturbation on reservoir quality: Integrated spontaneous imbibition, pressure decay profile permeability and pore-throat characterization study of the Cretaceous Bhuj formation, Kachchh basin, India

The Lower Cretaceous Bhuj Formation in Kachchh basin is well known for its hydrocarbon potentials including proven gas reserves in Kachchh offshore. In the onshore these reservoir equivalent rocks outcrops and includes sandstones (quartz arenites), with higher degree of bioturbations. The present work, deals with determining of impact of variable bioturbation degree sandstones reservoir quality. The petrographic properties were integrated with spot permeability and spontaneous imbibition study to understand petrophysical contrast between burrow and non-burrowed part.In moderately bioturbated samples (BI-3), spot permeability for burrows was 18 times higher than non-burrowed matrix. While in higher bioturbated, samples (BI-4) the permeability contrast between burrows and non-burrowed area was moderately lower (10.26 times). Similarly in highly bioturbated sandstones of BI-5 the permeability contrast between burrow and non-burrow was less up to 5.45 times. Thus, highly bioturbated sandstones appears to be less heterogeneous as compared with moderately bioturbation sandstones. To support this study, qualitative and quantitative investigation of the pore network, pore structure and pore connectivity through Scanning Electron Microcopy (SEM) indicated a larger proportion of intergranular pores in burrow fill than in the surrounding matrix for all types of bioturbation classes. High pressure Mercury Injection (HPMI) data for quantitative validation of these inferences also estimated a higher presence of connected macropore volume in burrowed regions of Bioturbation Index 4 and 5. In contrast, the Bioturbation Index-3 indicated highest connected mesopore volume for the non-burrowed area. The imbibition characteristic corroborated with the better pore connectivity and larger pore throat size in the burrow volume than the non-burrowed matrix estimated from other studies. The Bioturbation Index 4 and 5 had higher initial imbibition rates and imbibition capacities than Bioturbation Index-3. The well-developed and connected pores in the mesopore region tend to form a large transition zone, while a higher proportion of connected pore volume in Bioturbation Index 5 is exhibited in a higher late imbibition rate. In essence, a conceptual model is presented to describe spontaneous imbibition in bioturbated media, in order to understand the enhanced petrophysical quality in bioturbated sandstones.

Direct evidence and mechanism for biocrusts-induced improvements in pore structure of dryland soil and the hydrological implications

As a crucial living skin inhabiting the soil-atmosphere interface, biocrusts play a vital role in various soil properties and processes, especially surface soil pore structure and soil pore-related hydrological processes, such as infiltration and evaporation. However, it remains unclear how biocrusts affect pore structure in the micrometric point of view. In this study, X-ray computed tomography and image analysis were employed to quantify the differences in soil pore structure between bare soil and three types of biocrusts (cyanobacterial, cyanobacterial-moss mixed, and moss crusts) at 0–50 mm depth. Three-dimensional images were segmented and analyzed to assess morphological and geometrical properties, such as porosity, pore volume, degree of anisotropy, Euler number, pore shape, pore connectivity, and pore tortuosity. Our results showed that the porosity of three types of biocrusts was, on average, 100% higher than bare soil, and the porosity ranked in order of moss crusts > mixed crusts > cyanobacterial crusts > bare soil. In comparison to bare soil, pore surface area density, mean pore volume, and node density of biocrusts were increased by 45%, 422%, and 52% on average, respectively. Biocrusts had larger fractal dimensions but lower degrees of anisotropy, mean tortuosity, and Euler number, which indicates the pore structure of biocrust became more complex and stable with higher connectivity compared to bare soil. Additionally, elongated pores had the largest contribution to porosity, and these pore proportions were significantly higher in biocrusts than bare soil. More importantly, the pore network model and network analysis revealed that biocrusts have a higher connected porosity (17% vs. 4%), connected porosity/isolated porosity (11.7 vs. 0.6), average coordination number (8 vs. 5), and number of channels (14,480 vs. 807), but a lower average channel length (2.7 vs. 3.8 mm), which indicates that the physical and topological structures of pore network in biocrusts were reconfigured and exhibited a better pore network connectivity. Moreover, the biocrusts increased soil water holding capacity by 87% on average, but decreased saturated hydraulic conductivity by 55% as compared to bare soil. From the reshaped pore structure, improved soil water holding capacity, and decreased infiltrability of the biocrusts in comparison to bare soil, we conclude that biocrusts play a vital role in surface soil water balance, which subsequently affect surface hydrological processes (e.g., runoff generation and evaporation) in drylands.

Carbonation of supersulfated cement concrete after 8 years of natural exposure

Supersulfated cement (SSC) has garnered attention as a potential lower-carbon alternative to traditional Portland cement, but its carbonation resistance has raised concerns about its wider implementation. This study aims to evaluate the carbonation behavior of SSC concrete after 8 years of natural exposure, examining the carbonation depths, changes in hydration products, and microstructures of SSC concrete with different water-to-cement ratios. The results showed that the carbonation rate of SSC concrete with a C30 grade was 1.02 mm/year, while the carbonation rate of SSC concrete with a C50 grade was 0.54 mm/year. During the exposure process, ettringite in the SSC concrete was found to be the first hydration product to carbonate, leading to a dense pore structure. However, once C–S–H gels began to carbonate, the pore structure rapidly coarsened, resulting in a significant increase in pore network connectivity. Carbonation in SSC concrete generated calcium carbonate with aragonite as the dominant polymorph due to the extensive carbonation of ettringite. The results of this study on the carbonation behavior of SSC concrete under natural conditions can serve as a reference for validating the results of accelerated carbonation tests and provide a foundation for developing a carbonation model for SSC concrete under complex environmental conditions.

Enhancing the thermo-mechanical properties of calcium aluminate concrete at elevated temperatures using synergistic flame-retardant polymer fibres

This paper presents a systematic study on the feasibility of using synergistic flame-retardant polymer (SFRP) fibres for enhancing thermo-mechanical properties of calcium aluminate concrete at elevated temperatures. A series of tests were conducted to investigate the effect of SFRP fibres and cement type on thermal conductivity, compressive, splitting and flexural behaviours, and microstructural and micromechanical evolutions of concrete at different temperature levels, based on which the underlying mechanisms for enhancing the thermo-mechanical properties of calcium aluminate concrete were explored. Experimental results indicate that the combined usage of calcium aluminate blinders and SFRP fibres can significantly promote the strength-sustaining capacity (e.g., less than 45% compressive strength loss at 800 °C) and effectively prevent explosive spalling of concrete. The underlying mechanisms for enhanced thermo-mechanical properties of concrete can be explained by (i) the further reaction between hydration products and pozzolanic materials, which reduces the strength loss of concrete, and (ii) the melting, foaming and overflowing of SFRP fibres, which improves the connectivity of pore network and lowers the conductivity of concrete, and thereby improves the permeability and slows down the mechanical degradation of concrete.

Petrophysical properties of representative geological rocks encountered in carbon storage and utilization

This study evaluates petrophysical properties (especially porosity, permeability, tortuosity, and diffusivity) of representative geological rocks in the context of injectivity, storage space, and caprock integrity for effective utilization and long-term storage of carbon dioxide. A total of 10 geological rocks were selected as representative storage media for consideration as saline aquifers & depleted oil and gas reservoirs (sandstones and carbonates), basalts, and cap rocks, as well as utilization in organic-rich shale and coal seams. An integrated suite of laboratory tests, including liquid immersion porosimetry, gas expansion porosimetry, grain size distribution, mercury intrusion porosimetry, and gas diffusion, were performed on these various rock samples. The results exhibit a disparity of petrophysical properties among two broad groups of rocks: rocks selected for possible storage of CO2 have porosities of ∼10-25%, permeabilities of ∼10−16-10−13 m2, μm-sized pore-throat size distribution, and mostly good pore connectivity; in contrast, the potential caprocks have porosities of ∼0.5-5%, permeabilities of ∼10−20-10−18 m2, pore throat sizes of <50 nm, and probably poorly connected pore networks. An understanding of the measured facets of pore structure and contribution of fractures is also critical in the context of different testing principles and data interpretation of petrophysical analyses, as well as observational scales in the laboratory and field, and therefore reliable confidence of CO2 storage and utilization performance. Our work further illustrates the controlling influence of grain size distribution and geological processes on pore size distribution and pore connectivity for a wide range of rock types and lithologies, and particularly presents the extent and behavior of CO2 gas diffusion with a custom-designed apparatus for a holistic understanding of various petrophysical attributes of widely different geological rocks.

Differential development characteristics of secondary pores and effects on pore structure and movable fluid distribution in tight gas sandstones in the lower Permian, northeastern Ordos Basin, China

The lower Permian tight gas sandstones (TGSs) in the northeastern Ordos Basin have developed a complex pore network composed mainly of secondary pores with strong reservoir heterogeneity, which affects evaluation and exploitation of gas resources. In this study, lower Permian TGSs were analyzed through the thin sections, scanning electron microscopy, X-ray diffraction, He porosimetry, high-pressure mercury injection capillary pressure, X-ray computed tomography, and nuclear magnetic resonance experiments to elucidate the differential developmental characteristics of secondary pores and their effects on the pore structures and movable fluid distribution. Secondary pores in the lower Permian TGSs include dissolution pores associated with feldspar dissolution and micropores related to authigenic clay minerals. Based on fractal results, we designated the first-order dissolution pores as those with pore throat diameters greater than 1.98 μm, the second-order dissolution pores as those with pore throat diameters between 0.05 and 1.98 μm, and the pore throats diameters of micropores are mainly smaller than 0.05 μm. Both volumetric feldspar leaching calculations and petrographic statistics suggest that the secondary porosity observed in the lower Permian TGSs may not represent a net increase in porosity and more likely represents a redistribution of the original porosity. Dissolution requires sandstones with nonnegligible initial porosity and permeability: coarse-grained, quartz-rich, low proportion of ductile grains sandstones are more likely to maintain high porosity during burial, and dissolution of coarse-grained feldspar facilitates the development of first-order dissolution pores, thus generating the highest redistributed secondary porosity. The first-order dissolution pores have wide pore throat diameters, which enable seepage of movable fluids; the second-order dissolution pores have intermediate pore throat diameters, which allow moderate movable fluid seepage; and the micropores have small pore throat diameters, which are not conducive to seepage of movable fluids. Combinations of different types of secondary pores lead to differences in pore structures and distributions of movable fluids in the lower Permian TGSs. Specifically, TGSs with high first-order dissolution porosity have better pore structures, corresponding to the highest movable fluid saturation (MFS) and movable fluid porosity (MFP). When micropores dominate the pore system, the pore structure and pore network connectivity are poor, and the MFS and MFP decrease.