Variation Of Pore Structure Research Articles

Changes in micropore structure and moisture transport properties in fibrous porous media after ultrasound treatment

The binding force between water molecules and the solid skeleton of fibrous porous media, determined by the microscopic structure and chemical interactions, has a remarkable influence on the dehydration process. Therefore, the micropore structure is a key factor affecting the dewatering performance of fibrous porous media. In this paper, fibrous porous media is treated by ultrasound; then the micropore structure images are obtained. After that, structural characteristics parameters in the field of statistics and fractal theory are obtained by analysing microscopic images to investigate the effect of mechanical action and cavitation on microscopic structure under different sonication conditions. Finally, the effect of structure change on moisture removal is studied experimentally. It turns out that power and treatment times are the main factors that determine the effect of ultrasound on the pore structure of fibrous porous media. With the increase of power and time, the average pore size, the pore size with maximum ratio, the surface porosity and the surface fractal dimension increase non-linearly. Moreover, the growth is nonlinear; the maximum variation of pore structure is achieved at 530 W for 40 min. The moisture content loss rate and the effective moisture content diffusion coefficient are enhanced by the variation of micropore structure.

Experimental and Statistical Investigation of Reservoir Properties with the Effect of Waterflooding Treatment.

An oilfield reservoir over long-term operation may have different petrophysical information, which has a significant impact on oilfield maintenance and finance. Successful oilfield enhanced oil recovery benefits a lot from identifying and analyzing the variations of the critical properties after long-term waterflooding treatments. Since the inspection wells drilled within different development periods contain the core samples that have the petrophysical information at that period, it is necessary to collect and test the samples from different periods to investigate the overall tendency of the petrophysical properties. The samples from four inspection wells, which were drilled in four stages since the very beginning of development, were subjected to in-laboratory core analysis methods to illustrate the variation of some critical parameters in the reservoir. The permeability and porosity variation are revealed clearly by the experimental results. The migration and dissolution of clay minerals play a crucial role in the variation of petrophysical information and pore structure. To quantify the variations above, we applied the multiple linear regression model into our investigation. The dependent variable and all of the predictors in the model come from the experimental results. The quantitative results show the closed correlation between different parameters in the formation. With the development stage moving forward, the weight coefficients for different predictors have multiple trends. The experimental and statistical approach provides a novel understanding of the reservoir properties with the effect of waterflooding treatment

Escape Kinetics of an Underdamped Colloidal Particle from a Cavity through Narrow Pores

It is often desirable to know the controlling mechanism of survival probability of nano - or microscale particles in small cavities such as, e.g., confined submicron particles in fiber beds of high-efficiency filter media or ions/small molecules in confined cellular structures. Here we address this issue based on numerical study of the escape kinetics of inertial Brownian colloidal particles from various types of cavities with single and multiple pores. We consider both the situations of strong and weak viscous damping. Our simulation results show that as long as the thermal length is larger than the cavity size the mean exit time remains insensitive to the medium viscous damping. On further increasing damping strength, a linear relation between escape rate and damping strength emerges gradually. This result is in sharp contrast to the energy barrier crossing dynamics where the escape rate exhibits a turnover behavior as a function of the damping strength. Moreover, in the ballistic regime, the exit rate is directly proportional to the pore width and the thermal velocity. All these attributes are insensitive to the cavity as well as the pore structures. Further, we show that the effects of pore structure variation on the escape kinetics are conspicuously different in the low damping regimes compared to the overdamped situation. Apart from direct applications in biology and nanotechnology, our simulation results can potentially be used to understand diffusion of living or artificial micro/nano objects, such as bacteria, virus, Janus Particle etc. where memory effects play dictating roles.

Fractal and pore structure analysis of structural anisotropic coal under different impact loads

Pore structure is the key factor that affects the adsorption/desorption of coal. In order to study the change characteristics of pore structure in structural anisotropic coal under different impact loads, the low- temperature liquid nitrogen adsorption tests were carried out after the split Hopkinson bar (SHPB) impact, then the pore structure variation and fractal dimension were calculated and analyzed. The results show that before and after impact, the adsorption isotherms change from type II (raw coal) to type III (impacted coal). With impact loads increasing, adsorption and desorption isotherms change from hysteresis loops to overlap which are determined by open pore structure and dead end pore structure. Meanwhile, mesoporous and micropores in coal samples are much more obviously damaged by larger impact load, and the adsorption capacity becomes smaller, especially for coal samples in parallel to the bedding. Pore volume mainly distribute in mesoporous and macropores, and SSA mainly distribute in mesoporous and micropores. On the whole, the pore volume and SSA decrease with the increase of impact load. Not only pore volume but also SSA in perpendicular to the bedding are larger than that in parallel to the bedding under same impact load. Fractal dimension D2 decreases with impact load increasing, which causes the adsorption capacity linearly reduces.

Swelling pressure deterioration of compacted GMZ bentonite and its structural damage under heat combined with hyperalkaline conditions

ABSTRACT Compacted bentonite as an artificial barrier/backfill material is susceptible to heat when combined with the hyperalkaline conditions of deep geological repositories. In order to investigate the structural damage of compacted GMZ bentonite and its effect on swelling pressure deterioration, X-ray powder diffraction (XRD) and mercury intrusion porosimetry (MIP) tests were performed on bentonite specimens after immersed in hyperalkaline solution at 20–80°C. The results suggest that the increase in the number and pore sizes of macropores, and the decrease in the crystallinity degree of montmorillonite and silicate minerals are extremely obvious, which well characterises the structural damage of compacted bentonite on a microscopic scale. The structural damage parameter is facilitated by heat combined with hyperalkaline conditions. The structural damage of compacted bentonite is more susceptible to variations in the microscopic pore structure and the dissolution of silicate minerals than that of montmorillonite; and the structural damage of compacted bentonite will cause the long-term deterioration of swelling pressure. Hence, the long-term safety of HLW repository should be evaluated taking into account of the structural damage of compacted bentonite caused by heat combined with chemical conditions.

NMR-Based Study on Soil Pore Structures Affected by Drying–Wetting Cycles

Experimental tests are conducted to investigate pore structure variations during the swelling process of expansive soils that experienced 0–4 drying–wetting cycles. The mathematical connection between the swelling deformations and pore structures is analyzed by nuclear magnetic resonance. The experimental results show that the specimens subjected to 1–4 cycles show higher swelling rates. Swelling curves are divided into rapid swelling, slow swelling and slowly stable stages, and three straight lines are appropriate to fit these curves. Based on a defined T2 cutoff (T2C = 1.84 ms) for the relaxation time, the pores in the test specimens are divided into micropores (r ˂ 1.28 nm) and macropores (r ≥ 1.28 nm). With a lower water content (ω < 19.0%), the proportions of these two pore types are close to each other. When the water content exceeds 19.0%, a significantly large proportion of the macropores that is the main factor which causes swelling deformation in the wetting process of soil is observed. Drying–wetting cycles expand the spaces among the soil particles, and the cycle number shows a linear relationship with the macropores, while it exerts less influence on the micropores. If the soil is subjected to multiple drying–wetting cycles, the ability of the soil to prevent erosion is weakened since the water in the macropores is more readily lost.

Pore structure evolution and fractal characteristics of Zhaotong lignite during drying

While the drying of lignite is critical prior to its usage, the fragmentation and pulverization of lignite during drying inhibit the safe operation of this process. In this study, the changes in the physical structure of Zhaotong lignite during the drying process were examined by study its microstructure. Microstructure aspects like pore size distribution, pore volume distribution, and fractal dimension were used to determine the variation in pore structure, and the relationship between fractal dimension and water content was established. The results showed that open pores and pore connectivity decreased as the degree of drying increased. During the drying process, the number of pores larger than 100 nm decreased first, followed by a decrease in the number of pores smaller than 100 nm. The total pore volume and the pore volume of four types of pores showed a general trend of downward at different drying temperatures. The change in the fractal dimensions of Zhaotong lignite during drying can be divided into three stages. The change in the fractal dimension of the second stage (water content decreasing from 1.19 to 0.62 g/g) was found to be the most significant, since it entailed the destruction of the Zhaotong lignite pore structure.

Factors affecting shale microscopic pore structure variation during interaction with supercritical CO2

Substantial physiochemical changes occur during the interaction between supercritical carbon dioxide (ScCO2) and shale. It is important to clarify the variation and controlling factors of the microscopic pore structure (MPS). Six samples collected from Longmaxi Formation with various material compositions were treated with ScCO2. X-ray diffraction (XRD), scanning electron microscope (SEM), gas adsorption, and the maximum adsorption quantity (MAQ) model were employed to determine parameters before and after ScCO2 treatment. XRD data document feldspar, carbonate, and clay decrease after ScCO2 treatment. Specifically, the dissolution of clay and carbonate increases the micropore volume and mesopore specific surface area (meso-SSA), respectively. From gas adsorption results, two types based on the difference in total organic content (TOC) are identified, i.e. an accelerating improvement (samples with a TOC less than 3.00 %) and a moderating improvement (samples with a TOC greater than 3.00 %). Data from gas adsorption also support increased MPS parameters with pressure, indicating the variation can be classified into three discernible phases, i.e. a negligible variation at low-pressure, a swift increase at medium-pressure, and a sharp increase at high-pressure. Temperature, however, has a limited improvement on the MPS. These findings are likely to impact the MPS variation in the process of CO2-enhanced shale gas recovery (CO2-EGR).

Study on the variation of rock pore structure after polymer gel flooding

Abstract Polymer gels have received an increasing attention as profile control agents in the heterogeneous reservoir water-flooded layers. The article focus on the profile control mechanism of polymer gels by plugging macropore. The micro-structures of polymer gel and pore were characterized by scanning electron microscopy (SEM). And the core displacement equipment was used to evaluate the properties of plugging properties. The variation of pore structure is measured by SEM, mercury intrusion meter and nuclear magnetic resonance instrument. Investigation results show that the molecules form a network structure with many dense layers after 15 days of cross-linking reaction, which results in the increase of the viscosity of the system from 15.3 to 6325 mPa‧s. After the polymer gel forms a micelle at the middle end of the core and blocks the pores, the plugging rate of the core reaches 99.2%. The results obtained in this paper reveal the mechanism of plugging characteristics of polymer gels, and further prove the great potential of polymer gels to enhance oil recovery in oilfields.

Effects of pulse wave on the variation of coal pore structure in pulsating hydraulic fracturing process of coal seam

Coal pore structure is not only closely related to gas adsorption, but also affect the dynamic characteristics of gas desorption and diffusion. In order to understand the influence of pulsating hydraulic fracturing on the variation of coal pore structure, in this paper, the mercury intrusion method and liquid nitrogen adsorption method were used to quantitatively analyze the changes of coal porosity and pore size distribution under the action of pulse wave. And combined with scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) test to study the variation of minerals on the surface of coal samples. The results show that the influence of pulsating load on coal porosity is greater than that of static pressure load. The cumulative volume of micropores and their proportion decrease with the increase of pulsating frequency and pressure; the cumulative volume and proportion of macropores show an increasing trend; the cumulative volume of mesopores increases with increasing frequency, but the proportion increases in the low frequency phase, and decreases in the high frequency phase. There are two types of pores in the coal sample affected by the pulse waves: one is the expansion of the original pores, and the other is the new pores because the mineral crystals embedded in the coal body are washed out. The results have an important theoretical basis for the analysis of gas microscopic dynamics during pulsating hydraulic fracturing.

The Influence of Atmospheric Pressure on Air Content and Pore Structure of Air-entrained Concrete

To study the effect of atmospheric pressure on the properties of fresh and hardened air-entrained concrete, three kinds of air entraining agents were used for preparing air-entrained concrete in the plateaus (Lhasa, 61 kPa) and the plains (Beijing, 101 kPa). Air content, slump, compressive strength and pore structure of the three air-entrained concretes were tested in these two places. It is found that the air content of concrete under low atmospheric pressure (LAP) is 4%-36% lower than that of concrete under normal atmospheric pressure (NAP), which explaines the decrease of slump for air-entrained concrete under LAP. Pore number of hardened concrete under LAP is reduced by 48%-69%. While, the proportion of big pores (pore diameter >1 200 μm) and air void spacing factor are increased by 1.5%-7.3% and 51%-92%, respectively. The deterioration of pore structure results in a 3%-9% reduction in the compressive strength of concrete. From the results we have obtained, it can be concluded that the increase of critical nucleation energy of air bubbles and the decrease of volumetric compressibility coefficient of air in the concrete are responsible for the variation of air content and pore structure of concrete under LAP.

The Impact of Mineral Dissolution on Drainage Relative Permeability and Residual Trapping in Two Carbonate Rocks

Carbon dioxide injection into deep saline aquifers is governed by a number of physico-chemical processes including mineral dissolution and precipitation, multiphase fluid flow, and capillary trapping. These processes can be coupled; however, the impact of fluid–rock reaction on the multiphase flow properties is difficult to study and is not simply correlated with variations in porosity. We observed the impact of rock mineral dissolution on multiphase flow properties in two carbonate rocks with distinct pore structures. Observations of steady-state hbox {N}_2–water relative permeability and residual trapping were obtained, along with mercury injection capillary pressure characteristics. These tests alternated with eight stages in which 0.5% of the mineral volume was uniformly dissolved into solution from the rock cores using an aqueous solution with a temperature-controlled acid. Variations in the multiphase flow properties did not relate simply to changes in porosity, but corresponded to the changes in the underlying pore structure. In the Ketton carbonate, dissolution resulted in an increase in the fraction of pore volume made up by the smallest pores and a decrease in the fraction made up by the largest pores. This resulted in an increase in the relative permeability to the nonwetting phase, a decrease in the relative permeability to the wetting phase, and a modest, but systematic decrease in residual trapping. In the Estaillades carbonate, dissolution resulted in an increase in the fraction of pore volume made up by pores in the central range of the initial pore size distribution, and a corresponding decrease in the fraction made up by both the smallest and largest pores. This resulted in a decrease in the relative permeability to both the wetting and nonwetting fluid phases and no discernible impact on the residual trapping. In summary, the impact of rock matrix dissolution will be strongly dependent on the impact of that dissolution on the underlying pore structure of the rock. However, if the variation in pore structure can be observed or estimated with modelling, then it should be possible to estimate the impacts on multiphase flow properties.

Changes of pore structure and chloride content in cement pastes after pore solution expression

Pore solution expression is a widely accepted approach to extract pore solution of cement-based materials by appllying high pressure. In this study, the variations of pore solution distribution and chloride content in cement pastes before and after pore solution expression were examined. The results showed that the value of chloride concentration index Nc were mostly higher than 1.0 for cement pastes immersed in NaCl solution, and decreased with the chloride concentration of soaking solution and water-to-binder (w/b) ratio. During the pore solution expression, the pores larger than 40 nm were totally removed and the porosity of smaller pore was decreased. Based on a proposed physical model on structure of cement paste, the value of Nc was calculated according to the variations of pore structure and chloride content during pore solution expression. The calculated results showed similar trend as the experimental results obtained by pore solution expression method.

In-situ SAXS study of pore structure during carbonization of non-caking coal briquettes

Carbonization can enhance the utilization of coal. Pore structure is the main feature of the physical structure of coal and its carbonization products as char and coke. In this work, we performed an in-situ small angle X-ray scattering (SAXS) study on high temperature carbonization (1200 °C) of a kind of lignite and a kind of sub-bituminous coal. The variation of pore structure including pore size distribution, porosity and specific surface area of coal illustrates the different stages of the carbonization process.

The Impacts of Nano-Micrometer Pore Structure on the Gas Migration and Accumulation in Tight Sandstone Gas Reservoirs

The uncertainties between reservoir quality and gas migration and accumulation in tight sandstone gas reservoirs are intrinsically attributed to complex microscopic pore structures. Integrated analysis including the physical simulation experiment of gas migration and accumulation, X-ray computed tomography (X-CT), and casting thin section (CTS) were conducted on core plug samples collected from the Upper Paleozoic Permian Lower Shihezi and Shanxi tight sandstone of the Daniudi area in the Ordos Basin to investigate the impacts of pore structure on the gas migration and accumulation. Physical simulation suggested that the gas flows in migration in tight sandstone reservoirs were characterized by deviated-Darcy linear flow and non-linear flow regimes. Minimum and stable migration pressure square gradients determined by application of apparent permeability were employed as key parameters to describe gas flow. Pore structure characterization revealed that the tight sandstone reservoir was characterized by wide pore and throat size distributions and poor pore-throat connectivity. The pore–throat combinations could be divided into three types, including the macropore and coarse throat dominant reservoir, full-pore and full-throat form, and meso-small pore and fine throat dominant form. Comparative analyses indicated that pore and throat radii determined the gas flow regimes by controlling the minimum and stable migration pressure gradients. Gas accumulation capacity was dominated by the connected effective porosity, and the gas accumulation process was controlled by the cumulative effective porosity contribution from macropores to micropores. Variations in pore structures resulted in differences in gas migration and accumulation of tight sandstone reservoirs. The macropore and coarse throat-dominant and the full-pore and full-throat reservoirs exhibited greater gas migration and accumulation potentials than the small pore and fine throat dominate form.

A permeability prediction method based on pore structure and lithofacies

Permeability prediction using linear regression of porosity always has poor performance when the reservoir with complex pore structure and large variation of lithofacies. A new method is proposed to predict permeability by comprehensively considering pore structure, porosity and lithofacies. In this method, firstly, the lithofacies classification is carried out using the elastic parameters, porosity and shear frame flexibility factor. Then, for each lithofacies, the elastic parameters, porosity and shear frame flexibility factor are used to obtain permeability from regression. The permeability prediction test by logging data of the study area shows that the shear frame flexibility factor that characterizes the pore structure is more sensitive to permeability than the conventional elastic parameters, so it can predict permeability more accurately. In addition, the permeability prediction is depending on the precision of lithofacies classification, reliable lithofacies classification is the precondition of permeability prediction. The field data application verifies that the proposed permeability prediction method based on pore structure parameters and lithofacies is accurate and effective. This approach provides an effective tool for permeability prediction.

Carbon Storage and Enhanced Oil Recovery in Pennsylvanian Morrow Formation Clastic Reservoirs: Controls on Oil–Brine and Oil–CO2 Relative Permeability from Diagenetic Heterogeneity and Evolving Wettability

The efficiency of carbon utilization and storage within the Pennsylvanian Morrow B sandstone, Farnsworth Unit, Texas, is dependent on three-phase oil, brine, and CO2 flow behavior, as well as spatial distributions of reservoir properties and wettability. We show that end member two-phase flow properties, with binary pairs of oil–brine and oil–CO2, are directly dependent on heterogeneity derived from diagenetic processes, and evolve progressively with exposure to CO2 and changing wettability. Morrow B sandstone lithofacies exhibit a range of diagenetic processes, which produce variations in pore types and structures, quantified at the core plug scale using X-ray micro computed tomography imaging and optical petrography. Permeability and porosity relationships in the reservoir permit the classification of sedimentologic and diagenetic heterogeneity into five distinct hydraulic flow units, with characteristic pore types including: macroporosity with little to no clay filling intergranular pores; microporous authigenic clay-dominated regions in which intergranular porosity is filled with clay; and carbonate–cement dominated regions with little intergranular porosity. Steady-state oil–brine and oil–CO2 co-injection experiments using reservoir-extracted oil and brine show that differences in relative permeability persist between flow unit core plugs with near-constant porosity, attributable to contrasts in and the spatial arrangement of diagenetic pore types. Core plugs “aged” by exposure to reservoir oil over time exhibit wettability closer to suspected in situ reservoir conditions, compared to “cleaned” core plugs. Together with contact angle measurements, these results suggest that reservoir wettability is transient and modified quickly by oil recovery and carbon storage operations. Reservoir simulation results for enhanced oil recovery, using a five-spot pattern and water-alternating-with-gas injection history at Farnsworth, compare models for cumulative oil and water production using both a single relative permeability determined from history matching, and flow unit-dependent relative permeability determined from experiments herein. Both match cumulative oil production of the field to a satisfactory degree but underestimate historical cumulative water production. Differences in modeled versus observed water production are interpreted in terms of evolving wettability, which we argue is due to the increasing presence of fast paths (flow pathways with connected higher permeability) as the reservoir becomes increasingly water-wet. The control of such fast-paths is thus critical for efficient carbon storage and sweep efficiency for CO2-enhanced oil recovery in heterogeneous reservoirs.

Experimental study on the pore structure variation of self-compacting rubberised concrete under fatigue load

In this paper, the correlation analysis between mechanical fatigue properties and pore structure was performed on the self-compacting rubberised concrete (SCRC). Four-point bending destructive and non-destructive fatigue tests were conducted for obtaining mechanical properties of the SCRC, while the mercury intrusion porosimetry (MIP) test was applied to the pore structure analysis. Several parameters were utilised for the description of pore structure, including porosity, mean pore diameter, threshold, fractal dimension, actual minimum diameter and the most probable pore diameter. The Pearson formula was introduced for the study on the correlation among the fatigue life, the fatigue damage and different pore structure parameters. The experimental results show that the fatigue load has a strong effect on the pore structure change. As the damage developing, the pore structure of the SCRC deteriorated: the porosity, the mean pore diameter, the actual minimum diameter and the most probable pore diameter increase. Meanwhile, the capillary pore fractural dimension decreases slightly. On the basis of the factors, the parameters of pore structure can be served as indices for the fatigue life prediction. The correlation parameters between the fatigue life and the optimal parameters of pore structure (the porosity, the most probable pore diameter, the mean pore diameter and the mesopore fractural dimension) have been determined in this paper.

Self-Regeneration Performance and Characterization of Silver-Containing Activated Carbon Fibers Coated by Titanium Dioxide.

In this study, wood-based activated carbon fibers (WACF) were modified by Ag nanoparticles (AgNPs) and TiO2 films. The coating of TiO2 films decreased the AgNPs agglomeration and exfoliation on WACF. As the soaking concentration of AgNO3 solution (Sconc) increased, AgNPs size and content increased, while the pore volume (especially micropore volume) of fibers reduced. However, at higher Sconc in the range of 0.2 to0.4 mol/L, only slight variations in AgNPs content and pore structure were observable for WACF/TiO2/Ag (Ag-containing WACF coated by TiO2 film). WACF/TiO2/Ag-0.1 (0.1 was the soaking concentration of AgNO3 solution, mol/L) represented the best self-regeneration performance under the visible light irradiation. The self-regeneration performance of WACF/TiO2/Ag was determined by the synergistic effects of two factors: adsorption and photodegradation. The abundant pores of WACF/TiO2/Ag-0.1 increased the methylene blue (MB) concentration of TiO2 surrounding and facilitated the MB photodegradation. Meanwhile, their suitable Ag content enhanced MB photodegradation. Furthermore, the principal pathway of a chemical reaction between Ag+ and WACF was interpreted based on the data of surface elemental constituents and surface functional groups.

Heterogeneity of reservoir quality and gas accumulation in tight sandstone reservoirs revealed by pore structure characterization and physical simulation

To better understand the quality of the heterogeneous reservoir and gas accumulation in tight sandstone reservoirs, it is necessary to reveal the internal relationship between the reservoir qualities, pore structure and gas migration. In this study, we employ integrated methods including geological data analysis, casting thin section, SEM, XRD and Micro-CT (μ-CT) combined with physical simulation experiments of gas migration for the Upper-Paleozoic Shanxi and Lower Shihezi formations in the Ordos Basin. The results reveal three main pore-throat structures in tight sandstone reservoir in Ordos basin, including the macro-meso pores (>10 μm) with medium throats (1–3 μm) (MMPMT), small pores (2–10 μm) with fine throats (0.5–1 μm) (SPFT) and the micro-nano pores (<2 μm) with nano (<0.5 μm) or no throats (MPNT), and evident variations in pore structures between “sweet spot” and poor reservoir in tight layers. Critical charge pressure square gradient (CCPG), critical migration pressure square gradient (CMPG) and ultimate gas saturation (UGS) are identified to describe gas migration process. “Sweet spot” is characterized by a lower CCPG and CMPG and higher UGS compared with poor reservoir. Comparative analyses indicate the capacity of pore-throat to participate in gas migration is positively correlated with average radii, suggesting meso-small pores (20–2 μm) with medium-fine throats (1–3 μm) are the main pathway for gas migration in tight sandstone reservoirs. The CCPG is determined by the volume fraction and average radii of MMPMT, and CMPG is controlled by the proportion and average radii of SPFT. The configurations of pore structures control the gas accumulation process and affect the degree of accumulation (UGS). Pore structure variation is the primary factor controlling the heterogeneities of reservoir quality and gas accumulation in tight sandstone reservoirs.