Pore Size Distribution Curves Research Articles

Pore Structure and Heterogeneity Characteristics of Coal-Bearing Marine–Continental Transitional Shales from the Longtan Formation in the South Sichuan Basin, China

Marine–continental transitional shale has become a new field for shale gas exploration and development in recent years. Its reservoir characteristics analysis lags significantly behind that of marine shale, which restricts the theoretical research on the accumulation of marine–continental transitional shale and the progress of exploration and development. The shale pore system is complex and has strong heterogeneity, which restricts the fine evaluation and optimization of the reservoir. Based on nitrogen adsorption–desorption experiments, the morphology and structural characteristics of coal-bearing shale pores were analyzed, and then the micro-pore structure heterogeneity was quantitatively characterized based on fractal theory and nitrogen adsorption–desorption data, and the relationship between pore structure parameters and their influence on fractal characteristics were discussed. The hysteresis loop of nitrogen desorption isotherm mainly belongs to type B, indicating ink bottle, flat plate, and slit are the main pore shapes. The pore size distribution curves are left unimodal or multimodal, with the main peak around 4 nm and 20–60 nm. Smaller pores develop a larger specific surface area, resulting in a high value of fractal dimension (2.564 to 2.677). The rougher the pore surface and the larger the specific surface area provide an adequate adsorption site for shale gas adsorption and aggregation. Thus, fractal characteristics conduced to understand the pore structure, heterogeneity, and gas-bearing property of coal-bearing shale.

Water retention behavior and shear strength of artificially cemented granite residual soil subjected to free drying

Artificially cemented soils have been widely used as filling materials in highway and railway construction. The shear strength evolution of filling materials upon moist variation can determine the stability of subgrade and embankments. This study conducted water retention tests, MIP tests, and multi-stage triaxial shear tests on cement-treated granite residual soil (GRS) to determine its water retention curve (WRC) upon free drying, pore structure, and peak shear strength qf, respectively. The water retention behavior and shear strength evolution upon free drying were modeled based on the dual-porosity structure of cement-treated GRS and the effective stress principle, respectively. Results show that the drying-WRC is bimodal and higher cement dosage yields a more severe decrease in the water retention capacity within a specific suction range. For a given confining pressure, the peak shear strength qf increased with increasing cement dosage or suction value s. The peak shear strength qf also solely depends on the suction value in the peak stress state. In addition, the cement-treated GRS has a bimodal pore size distribution curve, and its macro- and micro-void ratios remain almost unchanged after free drying. The bimodal drying-WRC of the cement-treated GRS can be modeled by differentiating the water retention mechanisms in macro- and micro-pores. Moreover, using the macro-pore degree of saturation as the effective stress parameter χ = SrM, the qf–pf′ relationship (where pf′ is the effective mean pressure at failure) under various suction and stress conditions can be unified, and the qf–s relationships at various net confining pressures σ3,net can be well reproduced. These findings can help design subgrade and embankments constructed by artificially cemented GRS and assess their safe operation upon climate change.

Experimental investigation of CO2 huff-n-puff in tight oil reservoirs: Effects of the fracture on the dynamic transport characteristics based on the nuclear magnetic resonance and fractal theory

CO2 huff-n-puff is a promising method for enhancing oil recovery in tight reservoirs while simultaneously contributing to CO2 geological storage. However, the performance of this method is significantly affected by fractures resulting from hydraulic fracturing and natural fractures. The impact of fracture on oil transport behavior in different pores remains unclear and warrants further investigation. In this study, CO2 huff-n-puff experiments were conducted under induced fracture conditions. A new method for quantifying the relationship between transverse relaxation time T2 and pore radius was proposed by fitting the slopes of the T2 spectrum and the pore size distribution (PSD) curve derived from Micro X-ray computed tomography (Micro-CT). The impact of the fracture on the effective utilization limit was explored, and the fractal dimensions of different cycles were evaluated during CO2 huff-n-puff. The results showed that the oil recoveries of the fracture-free and the fractured samples were 19.97% and 31.16%, corresponding to the effective utilization limits of 1.52 μm and 0.96 μm, respectively. Moreover, based on the fractal characteristics of the whole process of CO2 huff-n-puff, the pore system was divided into micropores (<0.20 μm), small pores (0.20–0.63 μm), intermediate pores (0.63–2.00 μm), and large pores (>2.00 μm). Furthermore, the incremental recovery for each cycle mainly comes from large pores and gradually decreases as the fractal dimension is less than 2.85. The fracture has a significant impact on the migration characteristics of crude oil. The incremental oil in the intermediate pores of the fractured sample, caused by negative convection, was 2.88 times greater than that of the fracture-free sample. Finally, it was found that the gravity effect was more prominent, significantly affecting the remaining oil distributions in fractured reservoirs.

Strength and microscopic pore structure characterization of cement-fly ash stabilized organic soil under freeze-thaw cycles

In seasonal frost areas, an organic soil stratum is often encountered during engineering construction due to the widespread existence of organic soils. The soil stratum will experience frost heave in winter and thaw settlement in summer, resulting in a significant variation in its engineering behaviour, especially for organic soil stratum. In this study, with the help of cement, fly ash, and fulvic acid, cement-fly ash stabilized organic soil (CFOS) specimens were prepared and the unconfined compressive (UC) and mercury intrusion porosimetry (MIP) tests were carried out on CFOS specimens. The effects of fly ash content, number of freeze-thaw cycles (FT-N), and curing period on the strength, resilient modulus, and porosity were investigated. Test results revealed that the fly ash content increased from 0% to the optimum content of 5%, the unconfined compressive strength (UCS) and resilient modulus of CFOS with FT-12 increased by 50.30% and 118.92%, respectively. The pore size distribution (PSD) curve and fractal dimension (Dn) of specimens were obtained from the MIP test. The proportion of macropores was the main factor affecting the UCS of CFOS. With the increasing FT-N, the macropore proportion of CFOS with 5% fly ash content increased by 333.33%, and the UCS decreased by 28.25%. Based on a freeze-thaw damage model, the damage parameters of the CFOS specimens were extracted with the Dn as an independent variable. The microscopic pore characteristics and the relationship between the strength, Dn and damage parameter were analyzed. The microscopic pore structure of specimens with different fly ash contents experienced a change subjected to freeze-thaw cycles. The lower the strength, the lower the Dn of CFOS. The damage parameters quantitatively reflected the damage degree of specimens after freeze-thaw cycles on the micro scale. The damage parameter of CFOS with 5% fly ash content increased by 341.57% with the increase of FT-N. The introduction of fly ash was able to reduce freeze-thaw damage to the specimens. The damage parameter and CFOS strength exhibited a significant correlation. The relationships of the strength and microscopic pore structure of CFOS subjected to freeze-thaw cycles were conducive to a better understanding of the freeze-thaw damage mechanism of CFOS on the micro scale. These findings are beneficial for engineering construction design in seasonal frost areas.

Changes in Pore Structure and Fractal Characteristics of Solvents Pretreated High-Rank Coal under Supercritical CO2

CO2 injection in coal seams, which is a significant initiative to mitigate environmental problems caused by greenhouse gases, often leads a sequence of changes in the physical properties of coal reservoirs. To look into how the pore structure changes in the process of CO2 sequestration, we selected fresh coal from Huoerxinhe coal mine in China as the object. Then, acid treatment and SC-CO2 extraction were used to dissolve Organic and inorganic components in coal. Thus, by using SEM, LTGA-N2 apparatus and XRD, the characteristics of pore parameter and fractal dimension variation were discussed. The research results show that, the APS of samples THF, HCL-HF and Y-C increase, while the total PV decreases and the pore connectivity deteriorates. The pore connectivity of Samples THF and HCL-HF is improved (THF-C, HCL-HF-C), but the total pore volume continuously reduces. In addition, solvents treatment and SC-CO2 extraction mainly act on the microporous fraction. After solvents pretreatment, the changes in the pore size distribution curves are mainly manifested in the reduction of number of micropores, especially in the micropores around 3–4 nm. There is a small increase in micropores for samples Y-C and HCL-HF-C, with the pore size mainly concentrated around 4 nm, while the pores of the sample THF-C mainly show an increase within the scope of 3–16 nm. Generally, solvent pretreatment and SC-CO2 extraction help to simplify pore structure. However, the sample HCL-HF-C shows opposite change characteristics. In a short period of time, the larger pore fractal dimension, the less beneficial it is to the flow of CO2, while pore fractal dimension becomes progressively less useful in assessing pore connectivity with increasing time.

Information-Analytical Software for Developing Digital Models of Porous Structures’ Materials Using a Cellular Automata Approach

An information-analytical software has been developed for creating digital models of structures of porous materials. The information-analytical software allows you to select a model that accurately reproduces structures of porous materials—aerogels—creating a digital model by which you can predict their properties. In addition, the software contains models for calculating various properties of aerogels based on their structure, such as pore size distribution and mechanical properties. Models have been implemented that allow the description of various processes in porous structures—hydrodynamics of multicomponent systems, heat and mass transfer processes, dissolution, sorption and desorption. With the models implemented in this software, various digital models for different types of aerogels can be developed. As a comparison parameter, pore size distribution is chosen. Deviation of the calculated pore size distribution curves from the experimental ones does not exceed 15%, which indicates that the obtained digital model corresponds to the experimental sample. The software contains both the existing models that are used for porous structures modeling and the original models that were developed for different studied aerogels and processes, such as the dissolution of active pharmaceutical ingredients and mass transportation in porous media.

Response of soil–water characteristics to pore structure of granite residual soils

Granite residual soil is a special regional soil with special mineral composition and pore structure characteristics, which is easy to induce serious geological disasters or engineering problems, so it is particularly important to study its mechanical properties of unsaturated soil and its control mechanism. However, the effects of dry density and initial water content on soil–water characteristic curve (SWCC) and their mechanisms are still unclear. Therefore, samples with different dry densities (1.30 g/cm3, 1.50 g/cm3, 1.70 g/cm3) and initial water content (14 %, 20 %, 22 %) were set up in this paper. SWCC test was conducted on the two groups of samples under the humidification path and dehumidification path using filter paper method. Combined with scanning electron microscopy (SEM) test and pore-size distribution (PSD) test, the influence mechanism of different micro-pore structure on SWCC and hysteresis characteristics of granite residual soil was analyzed qualitatively and quantitatively. The results show that the samples with different dry densities basically coincide with each other in the high suction segment. The larger the dry density is, the smaller the range of transition zone in the low suction segment is. As the initial water content of the sample increases from 14 % to 22 %, SWCC changes from a single increase curve to a double increase curve, and the corresponding pore-size distribution curve (PSDC) changes from a trimodal curve to a bimodal curve. The SWCC of granite residual soil has obvious hysteresis effect, and the hysteresis area becomes smaller with the increase of dry density. The inflection point exists in the hysteresis area of SWCCS with higher initial water content. The distribution range of macropore is determined by dry density, and the distribution range of small pore is determined by initial water content. The transformation of SWCC from a single increase curve to a double increase curve is mainly controlled by the distribution range of small pores. The bottleneck effect in the migration process of pore water in the soil and the pore redistribution during water intake and water loss are the main reason for the hysteresis of SWCC. The results of this work provide some guidance for the study of unsaturated soil mechanical properties of granite residual soils.

Thermal property evolution and prediction model of early-age low-heat cement concrete under different curing temperatures

The evolution of low-heat concrete thermal properties is controlled by microstructure growth and affects the early-age mass concrete structure cracking under real curing conditions from the perspective of heat transfer. In this paper, experiments were carried out to obtain the evolution of the hydration heat, thermal conductivity, specific heat capacity, and microstructure of different water-binder low-heat mortars under different curing temperatures. The results show that all the thermal property variation rates are sensitive and positive to the curing temperature. The hydration rate increases with increasing water-binder ratio, and the thermal conductivity and specific heat capacity mainly decrease with increasing hydration degree, but the thermal conductivity has a brief and rapid increase stage when the hydration degree is less than approximately 0.38. At the same curing time, the thermal conductivity is higher with a higher water-binder ratio, and the specific heat capacity is inverse. The microstructure results show that the porosity, pore size distribution curve, and threshold pore size decrease with increasing age and curing temperature and increase with increasing water-binder ratio. Meanwhile, the relations between microstructure and thermal property evolutions also show that thermal properties have an exponential positive correlation with porosity, and their variations are controlled by hydration products and pore water due to the variable hydration process. In addition, based on the results and hydration kinetics, a multiscale thermal parameter prediction model of low-heat concrete considering curing temperature and the water-binder ratio was established by the method of theoretical deduction.

Nano-Scale Pore Structure Characterization and Its Controlling Factors in Wufeng and Longmaxi Shale in the Zigong Area, Southwest Sichuan Basin

The nano-scale pore systems in shale reservoirs control shale gas transportation and aggregation, which is of great significance for the resource evaluation of shale oil and gas and the selection of a “sweet spot”. Taking twelve marine shale samples from the Wufeng–Longmaxi Formation in the Zigong area, southwest Sichuan Basin, as the research target, we carried out a series of experiments, including total organic carbon (TOC) analysis, X-ray diffraction (XRD), gas adsorption (CO2 + N2), and mercury intrusion porosimetry (MIP), to study the full-scale pore structure characterization and controlling factors of pore volume and specific surface area. The results presented the following findings. (1) Marine shale samples from the target area are rich in organic matter, with an average TOC value of 3.86%; additionally, the mineral composition was dominated by quartz and clay minerals, with average contents of 44.1% and 31.4%, respectively. (2) The full-scale pore size distribution curves of pore volume developed multimodally, with the main peaks at 0.5 nm–2 nm, 3 nm–6 nm, and 700 nm–2.2 um; moreover, the full-scale pore size distribution curves of a specific surface area developed unimodally, with the main peak ranging from 0.5 nm to 1.2 nm. (3) Pore volume was mainly contributed by mesopores and macropores, with an average contribution of 46.66% and 42.42%, respectively, while the contribution of micropores was only 10.91%. The specific surface area was mainly contributed by micropores and mesopores, with an average contribution of 64.63% and 29.22%, respectively, whereas the contribution of micropores was only 6.15%. (4) The TOC content mainly controlled the pore volume and specific surface area of micropores and mesopores, while the clay and feldspar content generally controlled the pore volume and specific surface area of macropores. Additionally, the quartz content had an inhibitory effect on the development of all pore types. These results will help researchers understand the laws of gas accumulation and migration.

Mechanical performances and micro-level properties of basalt and PVA fiber reinforced engineered cementitious composite after high temperatures exposure

After high-temperature exposure, synthetic fibers with low melt temperature tend to exert negative influence on mechanical performance of engineered cementitious composite (ECC) and its elements. Basalt and PVA fiber reinforced hybrid-fiber ECC (basalt/PVA HF-ECC) is a promising way to address such drawback, due to the low cost, sustainability and high-temperature stability of basalt fiber. However, relevant researches only focus on fiber replacement and are still on primary stage, meanwhile their results show further performance improvements are required. In present study, basalt/PVA HF-ECC with superimposed basalt fiber content is proposed. Mechanical and micro-level related tests were conducted to investigate the effect of superimposed basalt fiber content after high-temperature exposure. Compressive test results showed basalt fiber could increase compressive strength at elevated temperatures. The optimal basalt fiber volume fraction was 0.8%, whereby compressive strength was higher than that of control PVA-ECC by 62.05%, 55.45% and 37.24% at 400 °C, 600 °C, and 800 °C. Tensile test results exhibited that stress-strain curves of basalt/PVA HF-ECC showed brittle behavior when PVA fiber melted at elevated temperatures, and first cracking strength also showed continuous increase as basalt fiber content increased. The optimal basalt fiber volume fraction was 1.2%, whereby first cracking strength was higher than that of control PVA-ECC by 48.43%, 42.38%, 36.30% and 50.41% at 23 °C, 400 °C, 600 °C, and 800 °C. Mercury intrusion porosimetry (MIP) tests showed basalt fiber reduced average pore diameter at elevated temperatures, while hardly exerted influence on porosity. At 600 °C and 800 °C, 0.8% and 1.2% content of basalt fiber decreased average pore diameter of control PVA-ECC by 13%–37%, and deduced peak heights of pore size distribution curves. SEM observation discussed and proved the basalt fiber contribution to strength improvement after high-temperature exposure. Rupture space related analysis was also used to discuss the tensile strain capacity deterioration at elevated temperatures. The present study investigates high-temperature mechanical performances and micro-level properties of basalt/PVA HF-ECC with superimposed basalt fiber content, thus promotes development of heat-resistant ECC with much reliable performances.

Numerical investigation of the tensile behaviors of needle-punched nonwoven geotextiles

A numerical modeling method to reproduce the structures of needle-punched nonwoven geotextile based on the actual manufacturing process (web formation and web bonding) was developed. The geotextile model was built based on the statistical characteristics of the computed tomography (CT) observations, physical and mechanical properties of fibers, and physical parameters of the geotextile. A combination of the finite element model and pore network model method was proposed to obtain the pore size characteristics. The appearance and pore size characteristics before and under uniaxial tensile strains were investigated. It is shown that the method can simulate the geotextile manufacturing process satisfactorily. The pore size distribution (PSD) curves obtained from the model show good agreement with the CT-based results. Moreover, the method can reasonably simulate the tensile behaviors of geotextile under uniaxial tensile strain. The specimen necking phenomenon during the uniaxial tensile test was well reproduced. The PSD curves moved towards the direction of larger pore size with increasing uniaxial strain, and characteristic pore sizes (O98, O95, O50, O30, O10) all show a relatively steady increasing trend.

Hydro-mechanical behavior and microstructure evolution of red clay-bentonite backfills

Soil-bentonite (SB) cutoff walls have been extensively used as vertical engineering barriers to control groundwater flow and subsurface contaminant migration. As the pollution containment capability of the SB backfills is intimately related to the hydro-mechanical behavior, consolidation tests and hydraulic conductivity tests were carried out to investigate the effects of bentonite content on the hydro-mechanical behavior of red clay-bentonite backfills, and the microstructure evolution of backfills under vertical loading was evaluated based on mercury intrusion porosimetry (MIP) tests and field emission scanning electron microscope (FESEM) observations. The results of microstructure analysis also evaluated the influence of bentonite on the clay fabric which changed the relationship between hydraulic conductivity and void ratio. The results indicated that the void ratio and hydraulic conductivity decreased linearly with the increase of the effective consolidation stresses, and the compression index decreased with the increase of bentonite content. Meanwhile, the pore size distribution (PSD) curve of the backfill specimen almost turned into unimodal characteristics with only micropores distributed after consolidation. Additionally, the flow paths became more tortuous under compression which decreased the hydraulic conductivity. The montmorillonite aggregates in red clay-bentonite backfill formed into tortuous network structure which bound the clay aggregates together and effectively increased the tortuosity of flow paths.

Impact of Polymer Chain Rearrangements in the PA Structure of RO Membranes on Water Permeability and N-Nitrosamine Rejection.

The use of solvents is overall recognized as an efficient method to improve the water permeability of polyamide thin film composite membranes (PA-TFC). The objective of this work was to test the performance of the membranes after exposing them to n-propanol (n-PrOH) to improve the permeability of the membranes while maintaining the rejection factor for small uncharged organic molecules, namely N-nitrosamines (NTRs). After the membranes were exposed to n-PrOH, the water permeability of the UTC73AC membrane increased by 98%, with minimal change in rejection. N-nitrosodiethylamine (NDEA) rejection decreased (3.4%), while N-nitrosodi-n-propylamine (NDPA) and N-nitrosodi-n-butylamine (NDBA) rejection increased by 0.9% and 2.8%, respectively. In contrast, for the BW30LE membrane, water permeability decreased (by 38.7%), while rejection factors increased by 14.5% for NDEA, 6.2% for NDPA, and 15.0% for NDBA. In addition, the morphology of the membrane surface before and after exposure to n-PrOH was analyzed. This result and the pore size distribution (PSD) curves obtained indicate that the rearrangement of polymer chains affects the network or aggregate pores in the PA layer, implying that a change in pore size or a change in pore size distribution could improve the permeability of water molecules, while the rejection factor for NTRs is not significantly affected.

Effect of clinoptilolite on properties and fractal characteristic of pore structure of polypropylene fiber-reinforced cement-based composite

To further analyze the effect mechanism of clinoptilolite on the macro-mechanics and microstructure of polypropylene fiber-reinforced cement-based composite, the samples with 0, 5%, 10%, 15%, 20% and 25% clinoptilolite contents were investigated by compression test, flexural test, X-ray diffraction (XRD), scanning electron microscope (SEM) and mercury intrusion porosimetry (MIP), and the pore structure characteristics were quantitatively analyzed by fractal theory. Results showed that: With the increase of clinoptilolite content, the compressive strength, flexural strength and porosity of samples first increased and then decreased. The addition of clinoptilolite reduced the 7-day strength but increased the 28-day compressive strength, 60-day compressive strength, 28-day flexural strength and 60-day flexural strength of the samples by about 6%, 14.3%, 13.9% and 19.1%. It was recommended that the clinoptilolite content should not exceed 15%. Due to the filling effect and pozzolanic effect of clinoptilolite, adding clinoptilolite into the matrix could promote a hydration reaction to produce more C-S-H gels and reduce the proportion of more harmful pores and harmful pores, effectively improving the internal pore structure of the matrix. Meantime, it was found that the evolution law of pore structure based on fractal dimension calculated by the thermodynamic fractal model was consistent with the variation law of the pore size distribution curves and pore volume measured by MIP. Furthermore, a polynomial relationship model between clinoptilolite content, the quantitative fractal characteristic of pore structure and the macroscopic mechanical behaviors was established, which indicated that the pore fractal dimension could quantitatively describe the effect of clinoptilolite content on the pore microstructure.

The Influence of Overburden Stress and Molding Water Content on the Microstructure of Remolded Loess

This study aims to reveal the mechanisms of the microstructural evolution of remolded loess under different molding water contents and overburden stresses. Utilizing L6 loess from Yan’an, remolded soil specimens were fabricated under various pressures and moisture contents, followed by conducting one-dimensional consolidation tests. The macroscopic porosity, pore size distribution curves (PSD), and microstructure of these remolded loess samples were examined. Experimental findings indicate that an increase in molding water content leads to an augmentation in macroscopic pore volume and elongated pore shapes, concurrently exerting substantial influence on the primary pore size and pore volume of both macropores (&gt;0.4 μm) and minipores (0.4–4 μm), with minimal impact on micropores (&lt;0.4 μm). The escalation of overburden stress solely contributes to the reduction in pore size and pore volume of macropores. Variations in the Menger fractal dimension occur only beyond the optimal water content, while overburden stress exhibits a minimal effect on the Menger fractal dimension. Furthermore, remolded loess exhibited a certain yield stress, and when the overburden stress was lower than the yield stress, there was almost no change in various types of pores. Finally, a microstructural evolution model of remolded loess under different molding water contents and overburden stresses was proposed. These findings are expected to provide new insights into the microstructural evolution and deformation mechanisms of loess in high embankment construction sites.

The effect of soil porosity size distribution on integral energy content of water in different moisture slopes

Abstract The water required by the plant is provided through the porous environment of the soil around the root. Therefore, indicators such as soil texture and effective porosity have been defined to measure the amount of water available to the plant. The pore size determines water energy and the plant's ability to absorb moisture, which is evaluated in this article. Multivariate regression analysis between the integral energy of water in different ranges of humidity as the dependent variable and the indices of soil pore size distribution as the independent variable has been investigated. Due to the fact that none of the indices of the pore size distribution curve have a significant relationship with the integral energy value of water in the permanent wilting point, therefore, it was not possible to estimate this index using the indices of the pore size distribution curve. The results showed that the elongation index of soil pore size distribution curve is the only factor determining the amount of integral energy of water in different soil water contents.

Effect of Static and Dynamic Methods of Compaction on Mechanical Properties of Silt

Engineered fills, such as roads, earthen dams, embankments, and earthen sites, are enhanced by compaction to increase strength and decrease compressibility. Static and dynamic compaction are the most popular and important methods in geotechnical engineering practice with different compaction mechanisms. However, the significant differences in the physical and mechanical properties between both compaction methods that have been reported in the literature were limited to clay or sand. Limited attempts have been made to quantify the variations in silt’s mechanical attributes that were caused by the sample preparation method. A series of consolidated drained triaxial shear tests and some mercury intrusion porosimetry (MIP) tests were constructed to interpret this. The results showed that static and dynamic compaction had a significant influence on the mechanical properties of the silt coupled with the molding water content. The shear strength of the dynamically compacted samples was higher than those of the statically compacted samples, as was cohesion (c), and the sample preparation method had little effect on the internal friction angle (φ). Furthermore, samples that were compacted at optimum water content (wot) had greater strength than those compacted on the dry or wet side of the optimum. Compared with the static compaction sample, the pore size distribution curve of the dynamic compaction sample shifted to the left with the peak pore size, distribution density, and the interaggregate pores proportion decreased. The strength discrepancy could be traced to differences in the silt structure features, for example, pore size distribution and particle orientation between statically or dynamically compacted specimens.

Enrichment of free oil in alkaline lacustrine Fengcheng Formation in Mahu Sag

Estimation of free hydrocarbon quantities in tight formations where pore sizes are at the nano and micron size is very important to understand formation producibility. In the Mahu sag, the Fengcheng Formation is being considered as an important source rock. Although the importance, characteristics of the Fengcheng Formation in terms of controlling factors on the enrichment of free vs. adsorbed hydrocarbons in pore spaces is still not clear. In this study, 13 samples were collected from the Fengcheng Formation and a combination of analytical techniques: X-ray diffraction analysis (XRD), nitrogen gas adsorption, total organic content (TOC), and Rock Eval pyrolysis were employed to analyze the microstructures and geochemical properties of the samples. These analyses were performed on two groups: 1) intact samples and 2) when the soluble hydrocarbon were extracted from the samples. The pore surface area and pore volume were measured slightly higher in the latter group, which were around 2.12 and 1.23 times of the intact samples, respectively. Moreover, deconvolution of pore size distribution (PSD) curves demonstrated that soluble oil mainly exists in the pore phases: 1, 2, 3, and 4 with the mean pore diameter of 15, 35, 51, and 67 nm, respectively. Overall, the oil in the samples of the Fengcheng Formation can become enriched with mobile (vs. adsorbed) fluids if the quantities of the free generated oil and pore size, each exceeds 0.7 wt% and 10.94 nm, respectively. This study can help researchers deepen their understandings of oil mobility and the evaluation of formation producibility in reservoirs deposited in an alkaline lake sedimentary environment.

Effect of Fines Content on Pore Distribution of Sand/Clay Composite Soil

Plant sand fixation is the most durable and environmentally friendly sand mitigation measure for windblown sand hazards. For aeolian–sand composite soil, the fines content is the main factor affecting the pore size distribution characteristics and determines the soil water retention characteristics. Sandy soil with uniform particle size was used as the material, low-plasticity clay as a fine particle, and composite soils with different fines content were prepared. The evolution of the pore size distribution of composite soils was quantitatively determined using low-field nuclear magnetic resonance technology, and the influence of the fines content on the pore size distribution was obtained. The results show that as the fines content increases, the pore size first increases and then decreases. The pore size distribution curve gradually changes from a single-peak structure to a double-peak structure and becomes a unimodal structure again. The corresponding dominant pore size of composite soils increases first and then decreases with the increase in the fines content. A critical fine particle content is identified to control the strong local honeycomb pore structure of composite soils, which provides a theoretical basis for the material selection and design of plant sand fixation measures.

Microstructural investigation into the damage behaviour of MX80 bentonite pellet under wetting/drying path using Mercury Intrusion Porosimetry (MIP) and Micro-Computed Tomography (μCT)

MX80 bentonite pellet has been considered as a promising sealing/backfilling material in geological disposal facilities for high-level and intermediate-level radioactive wastes. As the sealing/backfilling material which is subjected to hydration by pore water or vapour from host rock can be damaged by wetting-induced cracking, it is essential to clarify the suction effect on the damage behaviour of the material. In this study, a combined Mercury Intrusion Porosimetry/Micro-Computed Tomography (MIP/μCT) method was applied to investigate the microstructural damage of the MX80 bentonite cylinder-shaped 32-mm pellets equilibrated under suctions s from 113 to 1.5 MPa for wetting and from 113 to 262 MPa for drying. The enlarged pore size distribution (PSD) curves by MIP/μCT revealed an insignificant drying-induced closure of cracks, as opposed to massively developed wetting-induced cracks, with the size of the largest cracks reaching several millimetres. This is confirmed by the μCT observation. Indeed, the μCT images showed a negligible effect of drying on cracking, contrary to the effect of wetting characterised by significant cracking from the marginal area to the central part of pellet, indicating significant damage of pellet by wetting. Additionally, only horizontal cracks were identified at s ≥ 82 MPa. Vertical cracks emerged and propagated only at s ≤ 57 MPa, together with further expanding of horizontal cracks, due to the preferentially horizontal orientation of clay particles resulting from the one-dimensional compaction in the pellet fabrication. A damage coefficient D was defined as the ratio of the volume of macro-pores to the total volume of pellet. D increased slightly upon drying from suction 113 to 262 MPa and significantly upon wetting from suction 113 to 3.2 MPa, but kept almost unchanged when wetting furthered from suction 3.2 to 1.5 MPa because the clogging of macro-pores by the exfoliation effect slowed down the cracking rate.