Changes In Pore Structure Research Articles

Hydrothermal conversion of oil shale: Synthetic oil generation and micro-scale pore structure change

In this work, the hydrothermal conversion of oil shale from Natih B Formation (Oman) was evaluated at different temperatures (300, 350 and 400 °C) and 2291.6 psi (close to the reservoir pressure) using both crushed and uncrushed core samples. The performance of hydrothermal conversion of oil shale was evaluated in terms of material balance (yield of synthetic oil and gas), composition of evolved gases, synthetic oil quality (SARA fractions, elemental analysis, alkanes distribtuion by GC, and aliphtics and aromatic structure change by GC–MS etc.), and the change in organic matter (OM) in the core by LF-NMR. In addition, the change in pore size and structure was also investigated by X-ray computed tomography. The results show that 350 °C gives the best performance for the hydrothermal conversion of oil shale to generate synthetic oil in terms of synthetic oil yield and quality. The high quality of synthetic oil is manifested by a much higher content of aliphatic hydrocarbons than aromatics; a higher amount of saturates and aromatics (light fractions) than resins and asphaltenes (heavy fractions); and a higher amount of light alkanes (especially C10-C20) with a lower amount of heavy alkanes (especial C26-C36). The hydrothermal conversion of OM was confirmed by LF-NMR technique. It was found that hydrothermal treatment induced a significant increase in pore size and porosity with the formation of connected pore channels, resulting in the transformation of oil shale from an extremely low permeable into a higher permeable porous medium. This change in rock properties benefits the subsequent injection of any fluids for the conversion and production of oil shale in field. These promising results technically prove the potential for the development of shale oil from Natih B Formation by hydrothermal conversion.

Moisture diffusion in cement pastes with hydrophobic agent

Hydrophobic treatment of cement pastes is most effective ways to resist the penetration of water with aggressive ions to improve durability of cement-based materials. In this paper, the influence of carboxylic acid ammonium salt integral addition on hydrophobicity of cement-based materials is analyzed. The effects of carboxylic acid ammonium salt on hydrophobicity, moisture diffusion and hydration products are investigated by experimental methods. The results of water vapor sorption isotherm show that the addition of carboxylic acid ammonium salt increases the pore volume. The simplified analytical hydrodynamic model of cone type pore and Young’s relationship is proposed to analyze the relation between characteristic parameters of pore and the permeability of cementitious materials. It can be concluded that the hydrophobicity results from the change of pore structure and the hydrophobic surface of cone region. These results provide the guidance to design the durability and to develop the new hydrophobic agent used in cementitious material.

Pore changes of slickwater-containing shale under supercritical CO2 treatment

Application of liquid CO2-slickwater hybrid fracturing technology can significantly improve the energy-enhancing efficiency and fracturing fluid flowback efficiency of shale reservoirs. However, supercritical CO2 (ScCO2) fluid and slickwater are inevitably retained in reservoirs after fracturing, resulting in limited understanding of the changes of shale pore structure, further impacting the evaluation of the gas adsorption and transportation behaviors as well as CO2 sequestration capacity in shale reservoirs. Therefore, the influences of the ScCO2-slickwater coupling effect on shale pore structure was analyzed by using X-ray diffraction, low-pressure nitrogen gas adsorption (N2GA), mercury intrusion porosimetry (MIP), field emission-scanning electron microscopy (FE-SEM), and fractal analysis. After ScCO2-slickwater treatment, the calcite and albite contents decreased, while the quartz and clay minerals contents increased in shale. The increase in number of micropores and mesopores and the reduction in number of macropores were observed by a combination of N2GA and MIP, which led to the total specific surface area (TSSA) and total pore volume (TPV) of the shale enlarged, while the average pore size reduced. Meanwhile, the fractal dimensions calculated by N2GA (D1, D2) and MIP (D3) were raised, indicating that pore surface roughness and structural complexity were strengthened. D1, D2 and D3 can roughly reflect fractal characteristic of micropores (<3.5 nm), mesopores (3.5–50 nm) and macropores (>50 nm), respectively. Thus, D1 and D2/D3 can be used to evaluate the gas adsorption and flow behaviors of shale, respectively. By analyzing the control experimental results of ScCO2-water and He-slickwater treatment and the FE-SEM images, shale pore changes after ScCO2-slickwater treatment can be explained by CO2 extraction and dissolution, secondary mineral precipitation and polyacrylamide adsorption. After treatment, the increase of micropores and mesopores as well as TSSA and TPV makes the gas-adsorption capacity of shale improved, showing that the coupling effect of ScCO2-slickwater can not only improve shale gas productivity, but also benefit geological storage of CO2.

Pore-scale flow simulation on the permeability in hydrate-bearing sediments

Gas hydrate exploitation involves complex physical and chemical processes. Formation and dissociation of hydrate break the pressure-temperature phase equilibrium and cause change of pore-structure. The permeability of hydrate-bearing sediments plays a significant role for the fluid flow. The permeability change depends on the evolution of pore habit which is affected by hydrate phase behavior. Quantitative analyzing the relationship between pore structure and permeability still keeps a challenging work. To study the microscopic mechanism of hydrate effect on flow characteristic, the pore-scale simulation was carried out considering hydrate morphology and distribution. Five kinds of hydrate occurrence patterns including grain-coating, pore-filling, throat-filling, grain-cementing, and load-bearing in hydrate-bearing sediments are established. The permeability of hydrate-bearing sediments was obtained by solving the Navier-Stokes equation from the direct numerical simulation (DNS). The effect of factors related to the skeleton grain and hydrate distribution on permeability variation was analyzed. The results indicate that the permeability of hydrate-bearing porous media always changes with the variation of hydrate saturation. In most instance, the permeability reduction curve obeys exponential distribution. Permeability decreases as the hydrate saturation increases. The permeability variation is highly sensitive to the grain size, porosity, pore-throat size, hydrate occurrence pattern, and hydrate distribution morphology. From the perspective of the pore-scale, the formation and growth of hydrate cause the reconstruction and complication of pore structure in porous media. The permeability reduction caused by the hydrate formation is pronounced in the porous media with a larger initial pore-throat size. Considering different hydrate occurrence patterns and distribution morphology, the curve shape of the normalized permeability shows different trend. This new workflow has a potential application for analyzing the microscopic flow mechanism of hydrate-bearing sediments.

Experimental Research on the Influence of Microwave Radiation on Coal Permeability and Microstructure.

At present, there is limited information available about the effects of microwave radiation on desorption characteristics, microstructures, and functional groups of coal. This research focuses on the influence of microwave radiation on coal sample desorption and examines the changes in pore structures and oxygenic groups of different coal samples using liquid nitrogen adsorption, nuclear magnetic resonance, and X-ray photoelectron spectroscopy. Results prove that the methane desorption capacity and desorption rate are proportional to the increase in microwave energy; the initial dynamic diffusion coefficient is also proportional to microwave energy but negatively proportional to the attenuation coefficient. As a result of microwave radiation, the Brunauer–Emmett–Teller (BET) surface area, pore size, and Barret–Joyner–Halenda (BJH) pore volume decreased. The specific surface area of BET decreased and then increased as microwave energy increased, while the average pore size increased and then decreased. However, the change in the BJH cumulative adsorption pore volume was complicated. The microwave radiation decreases the volume and number of micropores while increasing the volume and number of medium pores. With the increase in microwave energy, the number and volume of micropores continue to decrease, while the number and volume of medium pores continue to increase. An increase in microwave energy increased the surface area of oxygenic groups with the increasing relative content of COO–, C–O, and C=O bonds; however, the relative content of C–C/C–H bonds decreased. These findings deepen the understanding of the antireflection effects of microwaves on coal.

Effect of the Presence of HCl on Simultaneous CO2 Capture and Contaminants Removal from Simulated Biomass Gasification Producer Gas by CaO-Fe2O3 Sorbent in Calcium Looping Cycles

This study investigated the effect of HCl in biomass gasification producer gas on the CO2 capture efficiency and contaminants removal efficiency by CaO-Fe2O3 based sorbent material in the calcium looping process. Experiments were conducted in a fixed bed reactor to capture CO2 from the producer gas with the combined contaminants of HCl at 200 ppmv, H2S at 230 ppmv, and NH3 at 2300 ppmv. The results show that with presence of HCl in the feeding gas, sorbent reactivity for CO2 capture and contaminants removal was enhanced. The maximum CO2 capture was achieved at carbonation temperatures of 680 °C, with efficiencies of 93%, 92%, and 87%, respectively, for three carbonation-calcination cycles. At this carbonation temperature, the average contaminant removal efficiencies were 92.7% for HCl, 99% for NH3, and 94.7% for H2S. The outlet contaminant concentrations during the calcination process were also examined which is useful for CO2 reuse. The pore structure change of the used sorbent material suggests that the HCl in the feeding gas contributes to high CO2 capture efficiency and contaminants removal simultaneously.

CHEMICAL AND STRUCTURAL CHANGES IN POPLAR WOOD UPON STEAM TREATMENT AT CONSTANT TEMPERATURE AND PRESSURE CONDITIONS FOR DIFFERENT TIME INTERVALS

Technology advancement has helped in the development of high-throughput equipment for the analysis of raw material in paper industries. In this research, we have used some advanced techniques to analyze the pore size, structural and chemical changes, and cellulose crystallinity of poplar wood pretreated with steam at constant temperature and pressure conditions for different treatment time. Samples were analyzed by the nitrogen adsorption test, Fourier-transform infrared spectroscopy – attenuated total reflectance (FTIR–ATR), X-ray diffraction (XRD), and field scanning electron microscopy (FE-SEM). Slit-shaped pores were formed, with a diameter of 2.12 nm, after 30 minutes of treatment. FTIR results revealed the degradation of the lignin skeleton through the formation of guaiacyl and syringyl units and deformation in the cellulose and hemicelluloses structure. The crystallinity index (CI) increased upon steam treatment for up to 15 min, but after that, a drop in the CI was observed. The crystallite thickness (d200) increased after 15 min of treatment, due to the rearrangement of cellulose chains. However, a further increase in steam treatment duration to 30 min resulted in a decline of d200, followed by an increase in the cellulose II crystalline region and d020. The steam treatment duration of 15-30 min was found to be a critical time interval, which led to increases in the number of mesopores, CI, d200, and the cellulose II region in the poplar wood.

Change mechanism of pore structure and filling efficiency during injection production of sandstone underground gas storage

Ten sandstone samples were chosen to conduct rock pore structure change law experiment before and after injection and production and the NMR fluid test experiment, the changes of pore size, permeability, and space filling degree were compared and analyzed. According to the findings, certain authigenic minerals (authigenic quartz and feldspar) in sandstone pores may come off after repeated rounds of high-intensity fluid erosion, obstructing the seepage channel. The composition and amount of clay minerals vary little due to gas–water interaction displacement, although some banded clay particles readily break off (polyhydrate kaolinite). The cracks produced by certain brittle rock particles can enhance the rock's porosity and permeability by about 20% and 10%, respectively. The fluid wettability of the rock changes as the operating cycle of subterranean gas storage rises, the gas saturation degree in the pores gradually increases, and the water gradually declines. Gas injection and production tended to attain equilibrium after 10 injection production cycles, and gas loss steadily decreased. It can be considered the node for the stable operation of subterranean gas storage at the present moment. As a result, high-intensity and numerous cycles of injection and production for sandstone underground gas storage may expand storage space, improve permeability, and progressively improve the gas-water ratio, all of which are beneficial to the stable and efficient operation of underground gas storage.

Pore-scale analysis of coal structure and mechanical properties evolution through liquid nitrogen thermal shock

Liquid nitrogen fracturing is one of the potential feasible technologies for improving the stimulation efficiency of coalbed methane (CBM) reservoirs. At present, the visualization of pore-throat connectivity and microscopic seepage characteristics in coal rocks under liquid nitrogen thermal shock is still lack of studying. Hence, the influence of liquid nitrogen thermal shock on the micro-nano pore structure and mechanical property of coal rocks are not understood clearly. In order to provide theoretical basis for the stimulation behavior of liquid nitrogen fracturing in coal beds, this paper investigates the change of micro-nano pore structure and mechanical property of coal rocks before and after liquid nitrogen treatment means of CT scanning and atomic force microscope (AFM). In addition, the influence of liquid nitrogen thermal shock on the seepage routes of coal rock are revealed. The following research results can be obtained. First, the number and scales of pores in the coal increase after liquid nitrogen thermal shock. In this experiment, porosity is increased by 200%, micro-fracture is dominant and its volume proportion is increased to 90.0% from 7.7% before liquid nitrogen treatment. Second, the three-dimensional pore structure reconstruction model obtained by CT shows that after the liquid nitrogen treatment, the number, total length and total volume of throats in the coal rock are increased by 170%, 140% and 130% and the pore connectivity is improved greatly. Third, after liquid nitrogen treatment, the sample's absolute permeability is improved significantly. In this experiment, the absolute permeability of coal after liquid nitrogen treatment is 77 times higher than that before liquid nitrogen cooling. The micro-fractures induced by thermal stress are the main percolation routes in coal after liquid nitrogen cooling. Fourth, pores and fractures are newly formed on both matrix and mineral domains, and the surface roughness is increased. In the meantime, the elastic modulus in matrix and mineral domains of coal drops, and the average elastic modulus drops by 81% and 91%, respectively. In conclusion, liquid nitrogen thermal shock leads to the increase of microscopic defects in coal and the deterioration of mechanical property. Liquid nitrogen fracturing is expected to be a new kind of efficient and green CBM reservoir stimulation technology.

Study on Durability and Pore Characteristics of Concrete under Salt Freezing Environment.

The macroscopic mechanical properties and frost resistance durability of concrete are closely related to the changes in the internal pore structure. In this study, the two-dimensional and three-dimensional ICT (Industrial Computerized Tomography) pore characteristics of C30 concrete specimens before and after freezing and thawing in clean water, 5 wt.% NaCl, 5 wt.% CaCl2, and 5 wt.% CH3COOK solution environments are obtained through concrete frost resistance durability test and ICT scanning technology. The effects of pore structure changes on concrete frost resistance, durability, and compressive strength mechanical properties after freezing and thawing cycles in different salt solution environments are analyzed. This paper provides new means and ideas for the study of concrete pores. The results show that with the increase in the freezing and thawing times, the concrete porosity, two-dimensional pore area, three-dimensional pore volume, and pore number generally increase in any solution environment, resulting in the loss of concrete compressive strength, mortar spalling, and the decrease in the relative dynamic elastic modulus. Among them, the CH3COOK solution has the least influence on the concrete pore changes; the NaCl solution has the greatest influence on the change in the concrete internal porosity. The damage of CaCl2 solution to concrete is second only to the NaCl solution, followed by clean water. The increase in the concrete internal porosity from high to low is NaCl, CaCl2, clean water, and CH3COOK. The change in the pore volume of 0.1 to 1 mm3 after the freeze–thaw cycle is the main factor for reducing concrete strength. The test results have certain guiding value for the selection of deicing salt in engineering.

Pore structural characteristics and methane adsorption capacity of transitional shale of different depositional and burial processes: a case study of shale from Taiyuan formation of the Southern North China basin

Pore structural characteristics and methane adsorption capacity are two significant aspects affecting shale gas potential, but the impact of deposition and burial processes on these two aspects is not clear. Hence, the shale samples of Taiyuan Formation deposited continuously and experienced multi-stage tectonic uplift in Fuyang-Bozhou area of Southern North China Basin were collected in this study. Based on the total organic carbon content analysis, mineral composition determination, low-pressure CO2 and N2 adsorption, high-pressure methane adsorption and argon ion polishing-field emission scanning electron microscope observation. The impact of depositional and burial processes variation on shale reservoir physical properties and adsorption performance is studied. The results display that the pore types of shale samples which were continues deposited and experienced multi-stage tectonic uplift have no obvious differences, while the pore volume as well as specific surface area (SSA) of micropores and mesopores of shale samples under multi-stage tectonic uplift are larger significantly. Meanwhile, the roughness of shale pores increases also. The decrease of loading pressure caused by multi-stage tectonic uplift may be the main factor for the pore structure changes of shale sample. Compared with the continuous deposited samples, the shale samples under multi-stage tectonic uplift have stronger methane adsorption capacity, which is relevant to the greater SSA of micropores as well as mesopores. This study provides an example and new revelation for the influence of depositional and burial processes on shale pore structure and methane adsorption capacity.

Factors influencing the evolution of shale pores in enclosed and semi-enclosed thermal simulation experiments, Permian Lucaogou Formation, Santanghu Basin, China

The microscopic pore structure of organic-rich shales is an important factor affecting the enrichment of shale gas; however, the characteristics of pore structure changes during thermal evolution are unclear, and the factors influencing the pore evolution remain controversial. High-temperature and high-pressure semi-enclosed and enclosed pyrolysis experiments were conducted in the low-mature oil shale of the Middle Permian Lucaogou Formation in the Santanghu Basin, Xinjiang, to reveal the evolution characteristics of shale pores from the low-mature to high- and over-mature stages. The results show that in the low-mature to mature stages, retained oil/residual bitumen has a clogging effect on shale pores in both systems, and the extra pressure destroys the structure of the mesopores and macropores. However, in mature to high- and over-mature stages, the pore pressure generated by the cracking of oil and gas transforms some micropores to mesopores and macropores in both systems. In semi-closed systems, large amounts of pre-oil and hydrocarbons are discharged periodically, which is conducive to the development of kerogen pores; however, the pore contents of solid bitumen and pyrobitumen are relatively small. In other cases, organic pores in solid bitumen and pyrobitumen in enclosed systems are more developed.

Pore structure changes induced by hydrate dissociation: An example of the unconsolidated clayey-silty hydrate bearing sediment reservoir in the South China Sea

The natural gas hydrates (NGHs) in the South China Sea are found in unconsolidated clayey-silty sediments, featured by very weak mechanical strength and low permeability. During gas extraction from such unstable, low permeability reservoirs, hydrate dissociation can result in distinct changes in pore properties and flow capability. Thus, it is extremely important to investigate the pore characteristics and gas flow mechanisms in such reservoirs. In this study, five sediment samples including two from the hydrate bearing layer (HBL) and three from the underlying layer (ULL), were taken from an exploration well in the Shenhu area, South China Sea. The pore morphology, pore specific surface area (SSA), total pore volume, porosity, and pore size distribution of these samples have been analyzed to compare the reservoir physical properties of the HBL (after the hydrate has dissociated and disappeared) with the ULL. Results from scanning electron microscopy reveal that the HBL samples without the hydrate present feature larger grain sizes, larger pore apertures and better connectivity than ULL samples. Results of NMR analysis and low temperature N2/CO2-adsorption show that both HBL and ULL samples have high total porosities (42.4%–48.8%) and high SSA (21.17–30.09 m2/g), while the results from centrifugal experiments demonstrate that the producible porosity (defined as the pore volume available for hydrocarbon emplacement) of the HBL samples is about twice (26.6%–30.3%) that of the ULL samples (15.5%–19.6%). The pores in the two sample types are dominated by intra-particle pores of 80 nm, and inter-particle and inter-aggregate pores of 0.2–3 μm. The above differences between HBL and ULL can be used to analyze the pore structure changes occurring as a result of the dissociation of NGHs in the HBL. We assume that on the one hand, formation of NGHs reduces pore pressure, thus resulting in higher effective stress, then compacts the pore space and reduces the total porosity. On the other hand, the release of high-pressure gas during hydrate dissociation could increase the pore aperture and improve pore connectivity, and thus increase the producible porosity of the HBL reservoir. It is also assumed that the flow capacity in the methane production zone in the HBL can be improved by the NGHs dissociation, suggesting that not only the hydrate saturation but also the pore structure changes should be considered when estimating the permeability during the gas production process.

Surface relaxation and permeability of cement pastes with hydrophobic agent: Combining 1H NMR and BET

The carboxylic acid ammonium salt is mixed into the concrete matrix to resist water with corrosive ions to improve concrete durability. The 1H NMR and BET experimental methods were applied to evaluate the effect of carboxylic acid ammonium salt on pore surface and structure. The results show that the surface relaxation of cement pastes containing hydrophobic agent is reduced to 60% of reference sample and excellent consistency exists between surface relaxation and water adsorption. The analytical Kozeny equation is utilized to predict the permeability of cement pastes containing different amounts of carboxylic acid ammonium salt. Results show that the reduction trend changes with carboxylic acid ammonium salt amount due to the fact that the addition of hydrophobic agent changes the pore structure. It can be concluded that the generation of the flaky crystals results in the change of pore structure and pore surface properties which can increase the water resistance ability of pore in cement paste mixing with hydrophobic agent. These results reveal the mechanism of carboxylic acid ammonium salt integral addition on hydrophobicity of cementitious materials and provide the guidance to improve the hydrophobic agent application properties.

Differences in Petrophysical and Mechanical Properties Between Low- and Middle-Rank Coal Subjected to Liquid Nitrogen Cooling in Coalbed Methane Mining

Abstract Exploring the damage differences between different coal rank coal reservoirs subjected to liquid nitrogen cooling is of great significance to the rational development and efficient utilization of coalbed methane (CBM). For this purpose, the mechanical properties, acoustic emission (AE) characteristics, and energy evolution law of lignite and bituminous coal subjected to cooling treatment were investigated based on Brazilian splitting tests. Then, pore structure changes were analyzed to reveal the differences in microscopic damage between lignite and bituminous coal after a cooling treatment. The results showed that compared with bituminous coal, the pore structure of lignite coal changed more obviously, which manifested as follows: significant increases in porosity, pore diameters, and pore area and a larger transformation from micropores and transition pores to mesopores and macropores. After the cooling treatment, the thermal damage inside lignite and bituminous coal was 0.412 and 0.069, respectively. Thermal damage reduced the cohesive force between mineral particles, leading to the deterioration of the macroscopic physical and mechanical properties. Simultaneously, denser acoustic emission ringing counts and larger accumulated ringing counts were observed after cooling. Moreover, the random distribution of thermal damage enhanced the randomness of the macrocrack propagation direction, resulting in an increase in the crack path tortuosity. With more initial defects inside coal, a more obvious thermal damage degree and wider damage distribution will be induced by cooling treatment, leading to more complicated crack formation paths and a higher fragmentation degree, such as that of lignite coal.

Investigation of Sound Absorption Properties of Heat-Treated Indonesian Momala (Homalium foetidum (Roxb.) Benth.) and Korean Red Toon (Toona sinensis (A. Juss.) M. Roem.) Cross Sections

This study investigates the effects of heat treatment time and presence of an air back cavity on the sound absorption performance of Indonesian momala (Homalium foetidum (Roxb.) Benth.) and Korean red toon (Toona sinensis (A. Juss.) M. Roem.) cross sections. To examine the porous characteristics of the two species before and after heat treatment, gas permeability, pore size, and porosity analyses were conducted. Additionally, the sound absorption coefficient was measured based on various heat treatment times and air back cavity sizes. The results showed that, with heat treatment at 210 °C for 6 h, the gas permeability improved by 4.3% for the momala and 38.5% for the red toon, the maximum pore size was improved by 5.25% in the momala and 26.0% in the red toon, and the through-pore porosity improved by 22.7% for the momala and 117.0% for the red toon. Due to these pore structure changes, the noise reduction coefficient (NRC) of the heat-treated momala improved by 6.8%. When a 3-cm air back cavity was applied to the heat-treated momala, the NRC was improved to 92.5%. Similarly, when the same air back cavity was applied to the heat-treated red toon, the NRC was improved to 190.7%. This study demonstrated that an increase in pore size and through-pore porosity by heat treatment triggered an increase in the sound absorption coefficient. Additionally, when an air cavity was applied, the sound absorption coefficient of both heat-treated wood species was increased at low frequency. From the results of this study, we expected that heat-treated momala and red toon cross-sections can be utilized as eco-friendly ceiling materials with sound absorption function.

Study on the Influence of Liquid Nitrogen Cold Soaking on the Temperature Variations and Seepage Characteristics of Coal Samples with Different Moisture Contents

Due to its advantages such as environmental friendliness and remarkable permeability enhancement effect, the technology of liquid nitrogen cold soaking (LNCS) cracking coal has become a hot spot in the research on coal seam permeability enhancement in recent years. The frost heave force generated by water-ice phase transformation and the temperature stress are the main mechanisms of LNCS cracking water-containing coal. This paper focuses on the effect of LNCS on the temperature variations and seepage characteristics of coal. To further this purpose, the temperature measurement test and the permeability test were conducted on coal samples with different moisture contents under LNCS, respectively. In addition, by comparing the computer tomography test results of coal samples before and after LNCS, the internal pore structure changes of coal samples were further analyzed from a three-dimensional perspective. The test results show that the coal sample with a higher moisture content consumes a shorter time to reach internal temperature equilibrium and experiences faster temperature changes. LNCS can enhance coal permeability, and the growth rate of permeability increases exponentially with the increase of moisture content. After the LNCS treatment, the dried coal sample is mainly sprouting new pores on the basis of primary pores; in contrast, for water-containing coal samples, new pores are sprouted while primary pores will penetrate each other spatially to form a fracture network. In the process of LNCS, moisture has a significant effect on the seepage characteristics of coal, so appropriately increasing the moisture content of the coal seam conduces to achieving a better permeability enhancement effect.

Carbon Corrosion in Polymer Electrolyte Fuel Cells: A Complex Interplay between Morphological Changes and Electrochemical Performance

Due to the high gravimetric energy density of hydrogen, the focus of implementation of polymer electrolyte fuel cells (PEFCs) has shifted from light duty passenger vehicles to heavy duty vehicles such as buses, trucks, locomotives and marine vessels1. A mechanistic understanding of degradation is therefore necessary to improve durability and efficiency. During start-up and shut-down (SUSD) of PEFC systems, the catalyst (Pt nanoparticles embedded on carbon support) undergoes local potentials ∼ 1 - 1.5 V caused by a combination of fuel (H2) starvation, mixed fuel region and cell reversal2. This leads to a series of degradation phenomenon including reduction in cathode catalyst layer (cCL) thickness and porosity, loss in electrochemical surface area (ECSA), ionomer degradation and loss in electrical contact, therefore resulting in severe performance loss2. The convoluted relationship between these individual degradation mechanisms, their chronology and their effects on electrochemical performance are yet unresolved.In this study, the complex interplay between morphological changes due to carbon corrosion and its effects on the electrochemical performance were analyzed using a combination of detailed electrochemical characterization, spectroscopy, and electron microscopy techniques. Commercially available catalyst coated membranes were subjected to the Department of Energy carbon corrosion accelerated stress test (AST) protocol. It was found that carbon corrosion in PEFCs can be divided into two phases. In the first phase (∼500 cycles), disordered domains of the carbon containing Pt nanoparticles oxidized faster with a severe loss of cCL thickness. Pt nanoparticles detached from the support and became electrically isolated and hence electrochemically inactive. Electrochemical surface area decreased rapidly in this phase with little change in the cCL pore structure. Both kinetic and mass transport overpotentials (due to decrease in roughness factor) increased. In the second phase (remaining 1500 cycles), a severe pore structure collapse was observed (see Figure 1). Porosity, mean pore size, and the pore connectivity decreased considerably. Pressure independent oxygen mass transport resistance showed anomalous increase. The oxygen-reduction reaction kinetics were impeded, reflected by an increase in the Tafel slope. This was caused due to severe mass transport limitation, even at low current densities of 0.1 A.cm-2 in oxygen environment owing to pore structure collapse. In addition, the AST did not change the concentration of oxides on carbon, however, the overall water management in the cCL deteriorated as the pores lost percolation. Ionomer degradation was observed in the second phase. Sheet capacitance and sheet resistance also decreased with carbon corrosion. Figure 1. a) 3D reconstructions of the cCL using FIB-SEM micrographs, b) porosity as a function of thickness. Scale bar is 1 µm.

Effect of Ammonia-Soda Residue on the Strength and Chloride-Resistance Performance of Steel Slag-Granulated Blast Furnace Slag-Based Concrete after Immersion in Artificial Seawater.

Ammonia-soda residue (ASR) is the main solid waste generated from soda manufacturing and is hard to reuse due to its complex chemical composition. This study investigated the influence of ASR content on the strength and chloride-resistance capacity of concrete based on basic oxygen furnace slag and ground blast furnace slag. The hydration and chloride resistance mechanisms were analysed by comparing the hydrate products and pore structural changes. The results showed that adding ASR had the greatest impact on early strength. ASR-introduced chloride ions may participate in the hydration process to generate Friedel’s salt and decrease ettringite. The optimum pore distribution appeared when the ASR-to-desulphurisation gypsum ratio was 2:3 because of the introduction of nucleation sites and the decrease of C–S–H gels. The two chloride resistance-capacity measurements were affected differently by the ASR content. The apparent chloride diffusion coefficient was mainly affected by the percentage of pores that were larger than 10 nm. However, electric flux increased when ASR increased due to the influence of introduced chloride. The crystallisation pressure of Friedel’s salt decreases the strength of concrete with ASR content after high-concentration artificial-seawater immersion. The significant chloride-resistance property provided an alternative use for the concrete containing ASR.

Impact of de-ionized water on changes in porosity and permeability of shales mineralogy due to clay-swelling

Hydraulic fracturing is widely applied for economical gas production from shale reservoirs. Still, the swelling of the clay micro/nano pores due to retained fluid from hydraulic fracturing causes a gradual reduction of gas production. Four different gas-bearing shale samples with different mineralogical characteristics were investigated to study the expected shale swelling and reduction in gas permeability due to hydraulic fracturing. To simulate shale softening, these shale samples were immersed in deionized (DI) water heated to 100 °C temperature and subjected to 8 MPa pressure in a laboratory reactor for 72 hours to simulate shale softening. The low-temperature nitrogen adsorption and density measurements were performed on the original and treated shale to determine the changes in micro and nano pore structure. The micro and nano pore structures changed, and the porosity decreased after shale treatment. The porosity decreased by 4% for clayey shale, while for well-cemented shale the porosity only decreased by 0.52%. The findings showed that the initial mineralogical composition of shale plays a significant role in the change of micro and nano pores and the pore structure alteration due to retained fluid from hydraulic fracturing. A pore network model is used to simulate the permeability of shale used in this study. To define pore structure properties, specific factors such as porosity, pore size, pore throat distribution, and coordination number were used. Furthermore, the anisotropy characteristics of shale were integrated into the model via a coordination number ratio. Finally, the change in permeability due to shale softening was determined and compared with untreated with the progress of shale softening. The simulation showed that the permeability of Longmaxi shale could decrease from 3.82E–16 m2 to 4.71E–17 m2 after treatment.