Small-pore Region Research Articles

Three-dimensional numerical simulation of Ostwald ripening characteristics of bubbles in porous medium

Ostwald ripening behaviors of bubbles in porous medium are observed commonly in various fields, including CO<sub>2</sub> geological storage, preparation of porous materials, and fuel cells. A three-dimensional pore network model based on concentration coupling calculation has been developed to investigate the ripening characteristics of bubbles in porous medium on a pore scale. This model takes into account the shape of bubble, the structure of porous medium, and mass transfer between gas and liquid. By solving the gas phase concentration of each pore body in the three-dimensional pore network, the model can track the evolution process of each bubble. A microfluidic chip with a four-pore structure is used to validate the reliability of the model through visual experiments. To analyze the effect of porous medium heterogeneity on the bubble ripening process, two different three-dimensional pore network structures with varying pore sizes are constructed and the ripening processes of bubbles in two regions are simulated numerically. The results show that the initial distribution of bubbles can affect the ripening process of porous medium. When bubbles are uniformly distributed, in the ripening process, they exhibit regular and systematic changes in their spacing. However, in the case of uneven bubble distribution, as the bubbles transfer from smaller pore region to larger pore region, they also undergo individual mass transfer towards the larger bubble region in their respective areas. Consequently, the remaining bubbles no longer maintain a spaced distribution pattern. Additionally, the differences in initial size among bubbles can accelerate the ripening process, resulting in a significantly shorter ripening time than that in a uniform distribution. The choice of pore number has a significant influence on continuous-scale equivalent parameters, such as average capillary pressure and saturation. As the number of pores increases, the capillary pressure and saturation exhibit a more regular, nonlinear variation. A relationship between capillary pressure and saturation in the small pore region and in the large pore region are established, which deviate from the assumptions made in the existing literature. This result provides important guidance for constructing the continuous-scale ripening model that can be used to predict the evolution process of CO<sub>2</sub> during geological storage and provide guidance for studying the influence mechanism of heterogeneity during long-term CO<sub>2</sub> storage.

A Dual-Permeability Approach to Study Anomalous Moisture Transport Properties of Cement-Based Materials

Anomalous moisture transport in cement-based materials is often reported in the literature, but the conventional single-porosity moisture transport models generally fail to provide accurate simulation results. Previous studies suggested that the anomalous moisture transport could be caused by different moisture transport velocity in large and small pores. Based on this concept, the present study proposes a continuous dual-permeability model for cement-based material. The proposed model includes the transport contribution of both liquid water and water vapor, which are governed by liquid advection and vapor diffusion, respectively. We explicitly consider that moisture transport in the large pore region is faster than the small pore region. The volumetric fraction of each region is determined when fitting the measured sorption isotherms by using a bimodal equation. The validation with experimental data shows that the dual-permeability model can well simulate both the “normal” and the anomalous moisture transport. The applicability of the proposed model implies that the “dual-porosity property” could be one of reasons that cause anomalous moisture transport in cementitious materials. In addition, results show that vapor diffusion can be neglected for moisture transport in both porosities at high relative humidity (RH), while at low RH, vapor diffusion must be considered.

Thermal-hydro coupling model of immiscible two-phase flow in heterogeneous porous structures at high temperatures

The multi-phase flow characteristics and preferential paths in the heterogeneous pore structures of reservoir rocks at high pressures and temperatures are much significant to evaluate the exploitation efficiency of subsurface hydrocarbon resources and carbon sequestration effects. Therefore, these characteristics and paths should be accurately described and understood. However, only a few existing evaluation models consider the effect of thermo-hydro (T-H) coupling on these characteristics and paths. In this study, based on the commercial programs COMSOL with MATLAB, we developed a T-H coupling flow model to characterize the immiscible two-phase interface dynamics and flow paths in heterogeneous pore structures at various two-phase temperature differences. Ten numerical cases were carried to evaluate the effects of the reservoir and injection temperatures on the preferential paths of two-phase flow in the same two-phase temperature difference range. The results indicate that during CO2-water non-isothermal displacement process, the reservoir temperature affects the preferential paths of two-phase flow and injection temperature has a larger effect on branch flow in small pore regions. The injection rate considerably affects the two-phase flow paths as a result of temperature effects. This study provides a reference for quantitatively analyzing multi-phase flow in reservoir rocks at formation temperatures.

A hybrid model for estimation of pore size from ortho-positronium lifetimes in porous materials

The present paper proposes a novel model for estimating the free-volume size of porous materials based on the analysis of various experimental ortho-positronium (o-Ps) lifetime data. The model is derived by combining the semi-classical (SE) physics model, which works in the region of large pores (pore size R> 1 nm), with the conventional Tao-Eldrup (TE) model, which is applicable only for the small-pore region (R< 1 nm). Thus, the proposed model, called the hybrid (HYB) model, is able to smoothly connect the o-Ps lifetimes in the two regions of the pore. Moreover, by introducing the o-Ps diffusion probability parameter (D), the HYB model has reproduced quite well the experimental o-Ps lifetimes in the whole region of pore sizes. It is even in a better agreement with the experimental data than the most up-to-date rectangular TE (RTE) and Tokyo models. In particular, by adjusting the value of D, the HYB model can also describe very well the two defined sets of experimental o-Ps lifetimes in the pores with spherical and channel geometries. The merit of the present model, in comparison with the previously proposed ones, is that it is applicable for the pore size in the universal range of 0.2−400 nm for most of porous materials with different geometries.

Enhancing Immiscible Fluid Displacement in Porous Media by Capillary Pressure Discontinuities

Fluid displacement in porous media plays an important role in many industrial applications, including biological filtration, carbon capture and storage, enhanced oil recovery, and fluid transport in fuel cells. The displacement front is unstable, which evolves from smooth into ramified patterns, when the mobility (ratio of permeability to viscosity) of the displacing fluid is larger than that of the displaced one; this phenomenon is called viscous fingering. Viscous fingering increases the residual saturation of the displaced fluid, considerably impairing the efficacy of fluid displacement. It is of practical importance to develop suitable methods to improve fluid displacement. This paper presents an experimental study on applying the discontinuity of capillary pressure to improve immiscible fluid displacement in drainage for which the displacing fluid (air) wets the porous media less preferentially than does the displaced fluid (silicone oil). The concept involves using a heterogeneous packing system, where the upstream region features large pores and small capillary pressure, and the downstream region features small pores and large capillary pressure. The increase in capillary pressure prevents fingering from directly crossing the media interface, thus enhancing the displacement. The experimental apparatus was a linear cell comprising porous media between two parallel plates, and glass beads of 0.6 and 0.125 mm diameter were packed to compose the heterogeneous porous media. The time history of the finger flow was recorded using a video camera. Pressure drops over the model from the inlet to the outlet were measured to compare viscous pressure drops with capillary pressures. The results show that the fluid displacement was increased by the capillary discontinuities. The optimal displacement was determined through linear regression by adjusting the relative length of the large- and small-pore region. The results may assist in the understanding of fingering flow across the boundaries of different grain-sized bands for the gas and oil reservoir management, such as setting the relative location of the injection and production wells. The findings may also serve as a reference for industrial applications such as placing the grain bands in an adequate series to improve the displacement efficacy in biological filtration.

Experimental investigation on spontaneous counter-current imbibition in water-wet natural reservoir sandstone core using MRI.

Counter-current imbibition is a process whereby a wetting phase spontaneously imbibes into a porous media, displacing the non-wetting phase. This process is considered an important oil recovery mechanism during water flooding in fractured oil reservoirs. In this study, the dynamic process of counter-current imbibition for a natural reservoir sandstone core with an all-face-open boundary condition was monitored using magnetic resonance imaging (MRI). A series of images and relaxation time T1 spectra were acquired. The movement of water spontaneously entering the core sample while oil escapes, the spatial distribution of oil and water, and the in situ saturation change of oil and water in porous media can be accurately detected using MRI. MRI assists the direct evaluation of the basic mechanisms of imbibitions. Experimental results suggest the remaining oil was trapped in some large pores because of the capillary pressure, and the oil recovery in some large-pore regions is lower than that in some small-pore regions at the end of imbibition. Experimental findings show a close agreement between conventional material balance and oil recovery determined from MRI. The in situ oil recovery data agree well with the empirical models. The observations from MRI images could provide test cases to enable the development of mathematical models and to facilitate the evaluation of the proposed imbibition mechanisms. Copyright © 2016 John Wiley & Sons, Ltd.

Effect of Nano‐Porosity on High Gain Permeable Metal‐Base Transistors

As one type of vertical thin‐film transistors, permeable metal‐base transistors (PMBTs) with a permeable metal film embedded between two semiconductor layers have been proposed for high gain current amplifier. In principle, compared with conventional bipolar transistors, PMBTs should have a higher speed and are easier to fabricate compatible with flexible and printed electronics. However, functional PMBTs are not realized due to low current gains (&lt;50) and lack of output current saturation. In this paper, making use of the nano‐textured surface of an organic semiconductor, we are able to fabricate devices with permeable metal base films having a pore size of about 20 nm and achieve current gains up to 476 with output current saturation. Correlations between the nano‐scale porosity and the charge transmission/amplification behaviors in the device are explained with characterization of the metal base porosity. From our device simulation results, the small pore size is essential to achieve current saturation in the device due to the potential‐pinning effect in the small pore regions. Finally, using a similar strategy, we also demonstrate a high gain (=260) solution‐processed metal oxide‐based PMBTs with output current saturation.

Two-Region Extended Archie's Law Model for Soil Air Permeability and Gas Diffusivity

The air permeability (k a ) and soil gas diffusion coefficients (D p ) are controlling factors for gas transport and fate in variably saturated soils. We developed a unified model for k a and D based on the classical Archie's law, extended by: (i) allowing for two-region gas transport behavior for structured soils, with the natural field moisture condition (set at -100 cm H 2 O matric potential [pF 2]) as the reference (spliced) point between the large-pore (drained pore diameter ≥30 μm at pF ≤ 2) and the small-pore (subsequently drained pores 2) regions, and (ii) including a percolation threshold, set as 10% of the total porosity for structureless porous media or 10% of the porosity in the large-pore region for structured soils. The resulting extended Archie's law with reference point (EXAR) models for k a and D were fitted to the measured data. For both structureless and structured porous media, Archie's saturation exponent (n) was higher for D p than for k a , indicating higher water blockage effects on gas diffusion. For structured soils, the saturation exponent for the large-pore region (n 1 ) was lower than for the small-pore region (n 2 ). Generally, n 1 values of ∼1 for k a and 2 for D p and n 2 values of 4/3 for k a and 7/3 for D described the data well. Two reference-point expressions for k a at pF 2 were also developed and tested together with existing models for D at pF 2 against independent data across soil types. The best-performing reference-point models were a k a model based on the classical Kozeny equation and the Moldrup D p model.

Influence of pore structure and chemical properties of supported molybdenum catalysts on their performance in upgrading heavy coal liquids

In the hydroprocessing of solvent-refined coals, both the pore structure and the chemical properties of the catalysts affect the conversion of the heavy materials. Increasing median pore diameter (MPD) of unimodal Ni-Mo/Al 2 O 3 catalysts in the relatively small pore region (up to 150 A) enhanced the conversion of both asphaltene and preasphaltene, but further increasing the MPD up to 730 A mainly promoted preasphaltene conversion. In the runs of the isolated fractions, however, conversions increased with MPD up to 290 A for asphaltene and up to 730 A for preasphaltene

Unsaturated solute transport through a forest soil during rain storm events

The unsaturated transport of Br − through two distinct pore classes of an undisturbed pedon was investigated as a function of time and profile depth for a series of intermittent rain events. Water movement was monitored with tensiometers and a neutron probe and solute was collected in coarse and fine fritted glass solution samplers maintained at 2 and 10 kPa suction, respectively. The coarse samplers collected soil solution held at tensions of 0 to 2 kPa and are believed to have monitored predominately macropore and mesopore channels (large pores). The fine samplers are thought to have collected soil solution from predominately matrix mesopores (small pores) which are assumed to be approximately the lower pore size limit for gravitationally mobile water in this soil. The rate of tracer movement through large and small pores was dependent on the intensity and duration of a particular rain event as well as the antecedent soil water content. Small pores retained water and solutes longer and had a greater operational storage for ions than large pores. The data suggested that during the vertical flux of water through the soil profile, solutes were transported by convection and diffusion from small-pore regions to large-pore regions via hydraulic and concentration gradients, respectively, with small pores being a major source for solute transport in large pores. Because of physical and chemical nonequilibrium during rain events, solutes diffused from micropores (believed to have matric potentials > 10 kPa) into large and small pores. Solutes which were rapidly transported to greater depths through large pores increased the solute reserves in adjacent small pores at the same depths through a reversal of the mechanism described above. Solute reserves in small pores were also fed by significant vertical transport processes through this pore class.

On the structures of X- and Y-type zeolites. Structural changes within the cavities of Na-Y during dehydration and contrasts with Na-X systems

Samples of dehydrated and partially dehydrated Na-Y were examined by X-ray diffraction methods revealing the progressive structural changes which occur as water is removed and the different behaviour compared with Na-X. The differences between Na-Y and Na-X reflect the reduced Al content of Na-Y and the consequential lower average capability of each framework oxygen atom for balancing cation charges. In the small-pore region of Na-Y, the total number of Na atoms remains relatively constant during dehydration (ca. 15 per unit cell compared withca. 18 in Na-X); unlike Na-X, Na-Y, has no site I atoms. Significant dehydration of the small-pore region, and the change from low to high site II occupancy, do not occur until the total water content of the sample is less than that which pertains under atmospheric conditions. In the 12-ring regionn of Na-Y, [Na(H2O)2]+ units are observed at an intermediate level of dehydration, possibly linked by water molecules astride the site III region to networks in adjacent 12-rings. There is no build-up, as in Na-X, of Na at site III, and site IIB (=III′) is (at least partially) occupied by H2O rather than Na. Further dehydration progressively removes H2O but there is little rearrangement of Na atom positions, except the build-up in site II which accounts for much of the loss of Na from the mobile phase.

On the structures of X- and Y-type zeolites. Structural changes within the cavities of Na-X during dehydration

Samples of partially dehydrated and dehydrated Na-X were examined structurally by X-ray diffraction methods, revealing the progressive structural changes which occur as water is removed. In general, the total number of Na atoms in the small pore region remains unchanged by dehydration (ca. 18 per unit cell), as does the total number (non-mobile) in the 12-ring and site III regions (ca. 39). The site II population, however, is more than doubled by dehydration, from about 12 to about 25 Na, accounting for most of the loss from the mobile phase. The 12-ring sites, which in hydrated samples appear to comprise pairs of centrosymmetrically related [Na(H2O)2]+ units, rearrange during dehydration, with site III becoming an important location of Na atoms. At intermediate levels of dehydration, the remaining localized water molecules in the 12-ring region are found in a variety of associations with Na atoms, including perhaps as [Na(H2O)5]+ units whose Na atom occupies site III. In a sample containing H3O+ ions as well as Na+ as counter ions, site II was found to have a very low occupancy.