Porosity Development Research Articles

Nanometer-scale porosity evolution and mechanical properties of additively manufactured 17-4PH stainless steel

Understanding of the microstructure and mechanical properties of additively manufactured (AM) 17-4PH stainless steel has matured significantly, but combined wrought-level strength and ductility has yet to be achieved. Identifying the microstructural obstacle(s) impeding this milestone is critical for many future applications and industries interested in this AM alloy. In this work, microstructure, mechanical properties, and fracture behavior is evaluated for AM 17-4PH built via the laser powder bed fusion technique (LPBF) and heat treated via a broad range of techniques to achieve wrought-level 17-4PH strength. The highest strength/ductility was achieved with a two-stage hot isostatic press (HIP) and solution anneal (SA) treatment followed by an aging heat treatment, but wrought-level ductility could not be matched. Persistent nano-porosity (0.06–0.23 μm) was identified and correlated to fracture surface dimpling (R2 = 0.73–0.85) indicating a possible role in this ductility limitation. Contextualization of this study's mechanical properties dataset within the literature suggests that this observation is endemic to the current state-of-the-art processing techniques for AM 17-4PH.

Tight carbonate reservoir evaluation case study based on neural network assisted fracture identification and analytic hierarchy process

Comprehensive evaluation of reservoirs is an important link in gas reservoir exploration and development. The evaluation of tight carbonate reservoirs often focuses on the characteristics of porosity and permeability, ignoring the important factor of fractures, also the quantitative evaluation of reservoirs is relatively few. It is difficult to identify fractures and evaluate the reservoir factors qualitatively and quantitatively. Herein, the sedimentary microfacies and microporosity of the tight carbonate reservoir of the Ma55 submember in the eastern Sulige area are comprehensively studied by casting thin section, rock physical property, and capillary pressure test data. The backpropagation (BP) neural network algorithm is used to identify and predict fractures. Finally, through the analytic hierarchy process, the above reservoir influencing factors are modeled and quantitatively analyzed for reservoir evaluation. The results show that the highest probability of fracture development in the central and northwest areas of the study area can reach 0.92. The accuracy of the BP neural network model in identifying cracks can reach 80%, which is reliable and effective compared with the conventional logging identification method. Reservoirs can be classified into four types according to their quality. The synthetic weights of porosity, permeability and fracture development probability are 0.2, 0.2 and 0.216 respectively, which are the three most important evaluation parameters. This study improves the accuracy of fracture identification and prediction of tight reservoirs in comprehensive reservoir evaluation, which provides guidance and scheme for more detailed exploration and development of tight carbonate reservoirs.

Dealloying-Derived Self-Supporting Nanoporous Zinc Film with Optimized Macro/Microstructure for High-Performance Solar Steam Generation.

Solar steam generation (SSG) is a promising technology for the production of freshwater that can help alleviate global water scarcity. Nanostructured metals, known for their localized surface plasmon resonance effect, have generated significant interest, but low-cost metal films with excellent water evaporation properties are challenging. In this work, we present a one-step dealloying route for fabricating self-supporting black nanoporous zinc (NP-Zn) films with a bicontinuous ligament/channel structure, using Al-Zn solid solution alloys as the precursors. The influence of alloy composition on the formation and macro/microstructure of NP-Zn was investigated, and an optimal Al98Zn2 was selected. Additionally, in situ and ex situ characterizations were conducted to unveil the dealloying mechanism of Al98Zn2 and phase/microstructure evolution of NP-Zn during dealloying, including the phase transition of Al(Zn) → Zn, significant volume shrinkage (89.8%), and the development of high porosity (81.3%). The nanoscale ligament/channel structure and high porosity endow the NP-Zn films with good broadband absorption and superior hydrophilicity and, more importantly, give them excellent SSG performance. The NP-Zn2 film displays high evaporation efficiency, superior stability, and good seawater desalination performance. The efficient SSG performance, material abundance, and low cost suggest that NP-Zn films have promising applications in metal-based photothermal materials for SSG.

Compressed earthen blocks using alluvial clays from Mbam: performance comparison using statistical analysis of cement vs heat-stabilized blocks

AbstractIn the present study, a comparison of the thermal-insulation and mechanical performances of cement and heat-stabilized compressed earthen blocks (CEBs) was carried out to determine the factors which influence those properties. The raw clays used consist mainly of kaolinite, orthoclase and quartz. The mechanical strength increased with increase in both the amount of cement added and the firing temperature. However, the responses are better for cement-stabilized CEBs. The thermal insulation of fired bricks is greater than that of cement-stabilized bricks. This difference was related to the decrease in porosity and the formation of continuous-surface. The decrease in thermal insulation is mainly related to the formation of continuous-surface in cement-stabilized CEBs, whereas in the fired CEBs, it is due to the modification of pore volume. The mineralogy of the raw clays is statistically correlated to porosity and continuous-surface development that were confirmed as the main factors in the modification of both the mechanical strength and the thermal insulation. In cement-stabilization, the decrease in insulation is due to the development of continuous surface, while for heat-stabilization, mineral transformations during the sintering reduced continuous-surface formation and the insulation was controlled by both radiation and reduced surface conduction. The influence of the mineralogy of the raw material shows that clay content favours the insulation in fired bricks obtained at T ≤ 1000°C, while sand contents favour densification. In contrast, clay contents reduce the mechanical response of cement-stabilized material due to limited cement–clay interactions. In general, the mechanical response is more favourable in cement stabilization, while thermal insulation is better in fired bricks.

Impact of aluminate cements on the durability and mechanical performance of strain-hardening cementitious composites

Strain-hardening cementitious composites (SHCCs) are durable, ductile, fiber-reinforced cement-based materials. While fly ash (FA) is often used to partially replace Portland cement in SHCCs to enhance material performance, it can lead to long setting times and slow strength development. To address these limitations, this study investigates the use of aluminate cement as partial replacements for Portland cement in FA-based SHCCs. Furthermore, the study aims to fill a significant knowledge gap by thoroughly examining the influence of aluminate cement on the workability, durability, and strain-hardening behavior of SHCCs. Aluminate cements encompass both calcium sulfoaluminate (CSA) cement and calcium aluminate cement (CAC), distinguished by their composition rich in calcium aluminate phases. This study investigated the use of CSA and CAC cements as partial replacements for Portland cement in SHCCs. Both fresh and hardened properties of the CSA- and CAC-SHCCs were examined, while their microstructures and chemical phases were investigated using SEM and XRD analyses. Results showed that CSA and CAC cements reduced fluidity, setting time, and drying shrinkage but increased porosity and early strength development for SHCCs. CSA-SHCCs showed enhanced impermeability due to the refinement of interior pore space through the hydration products. While all modified SHCCs retained tensile strain hardening behavior, the tensile performance degraded due to the hydration effects of ye'elimite and calcium aluminates. The reduced tensile strain capacity was elucidated using the theory of steady-state crack opening.

Dust-dust collisions in cometary comas: applications to comet 67P/Churyumov-Gerasimenko

Abstract Silica has emerged as a crucial component within inner comet comas. This work investigates silica dust aggregates and their interactions within cometary comas. We study the probability that aggregates in the size range 1-100 μm collide with each other in the coma and analyze the outcomes of such collisions by using the “Collision of Porous Aggregates” (CPA) Software, which incorporates mass, size, and porosity evolution of the dust population. Beginning with assumed initial distributions and physical properties for silica aggregates at the comet nucleus, we compute their collisional evolution from when they depart the nucleus until they traverse the coma. Using data of dust particles observed in the coma of comet 67P/Churyumov-Gerasimenko, we demonstrate that dust-dust collisions in cometary comas cannot be neglected. Our analysis yields final distributions in terms of mass, size, and porosity. To validate our findings, we compare them with in-situ measurements of 67P/Churyumov-Gerasimenko collected by the COSIMA (COmetary Secondary Ion Mass Analyser) instrument of the Rosetta mission. Our investigation reveals a notable agreement between our derived size distributions and the data acquired by COSIMA within the same size range. This study may be applied to any comet that presents a similar dust production as it approaches the Sun. The insights of this work may contribute to estimating other dust properties such as strength, absorption, reflectivity, and thermal conductivity and highlight the importance of considering dust-dust collisions when studying cometary comas and their evolution.

Mechanical Properties of Irradiated U-10 wt. %Mo Alloy Degraded by Porosity Development

Abstract A plate-type nuclear fuel consisting of a solid monolithic foil of U-10 wt. %Mo is under development for use in the United States' high-performance research reactors. In support of developing this fuel, the fuel has been fabricated for the first time by a commercial fuel vendor and subsequently irradiated in a test reactor. This provides an opportunity to evaluate postirradiation mechanical properties of the commercially fabricated fuel. Four-point bend testing was conducted on the irradiated U-10Mo samples to generate the fuel material properties, including the modulus of elasticity and the bending strength. Although the material behaves in a brittle manner due to the accumulated porosity, a general trend of strength and modulus reduction was found as fission density increases. The data produced was evaluated using both Weibull statistics and a modulus degradation model with recommendations provided.

Development of geometry-driven quantitative prediction for shrinkage porosity in T-junction of steel sand castings

Shrinkage porosity poses a significant challenge in metal casting processes, impacting both productivity and energy efficiency, especially when dealing with components that are not accepted or reprocessed. Addressing this issue requires proactive measures, and predictive techniques play a crucial role in minimizing its occurrence. Among these methods, the Criterion Function stands out as a valuable empirical model extensively explored in the literature. By intricately linking solidification processes to the development of shrinkage porosity, the Criterion Function leverages key process parameters, including thermal gradient, molten metal velocity during solidification, and cooling rate, to offer predictive insights into the location and presence of porosity. However, a criterion function is needed that also considers the effect of geometric variations as well as the size of the defect (shrinkage porosity). In this study, a casting with three T-joints was taken as a benchmark shape to develop a geometry-based quantitative prediction model for plain carbon steel castings. Real experimental results were combined with solidification simulation results to produce reliable data, which were then used to extrapolate the results. The developed quantitative prediction model, which includes the effect of geometric changes, has been validated and proven effective in predicting shrinkage porosity.

Kinetics mechanism of pore pressure effect on CO2-water-rock interactions: An experimental study

Carbon capture, utilization and storage (CCUS) is an effective technology to reduce carbon dioxide (CO2) content in atmosphere, while due to the insufficient mastery of deterioration laws of reservoir rocks caused by CO2, the extensive application of carbon storage projects is hindered by the leakage risk. Therefore, it was aimed to study the kinetics mechanism of pore water pressure on CO2-water–rock interactions. First, the pressure effect on water–rock interactions was studied to obtain the relationship between ratio of solid–liquid contact area Ψ and pore pressure, and the controlling mechanism of Ψ on reaction rate ri was verified. On this basis, the pressure effect on CO2-water–rock interactions was further studied. Considering the effects on Ψ and solution acidity, the original CO2-applicable kinetics equation was modified to reduce the error of calculating reaction rate by 33.60% on average. Meanwhile, a forecast method was proposed for the reaction rate under different pressure conditions, and the average calculation error was 41.00% lower than that of original kinetics equation. After verifying the accuracy of these two theoretical methods to calculate reaction rate, the reliability to calculate porosity evolution process was further verified, and the calculation errors could be reduced by 29.52% and 31.81%, respectively.

Green synthesis of biomass-derived porous carbon with hierarchical pores and enhanced surface area for superior VOCs adsorption

The release of volatile organic compounds (VOCs) during the production, storage, and transportation of petroleum and its derivatives can be hazardous to human health and lead to secondary pollution. Addressing this challenge, this study introduces a sustainable and low-energy approach for synthesizing high-efficiency porous carbon materials, leveraging the abundant and renewable watermelon rinds as a carbon source. The carbonization and activation processes were optimized to minimize energy consumption while maximizing adsorptive performance. In this research, watermelon rinds were used as a source of carbon production, and melamine phosphate (FR-MP) was employed as a modifying agent. The FR-MP modified porous biomass-derived activated carbon (PWAC) was created by subjecting it to physical milling, carbonization, and activation. This approach not only utilized a waste biomass effectively but also reduced the overall carbon footprint of the production process. An examination of the effects of FR-MP dosage on the pore structure, surface chemical properties, and VOCs adsorption abilities of PWAC revealed that a higher FR-MP dosage resulted in a wider range of pore sizes within PWAC. The incorporation of FR-MP intensified the development of internal porosity and pore expansion in PWAC. Specifically, the optimized PWAC-0.3 exhibited a significant increase in specific surface area (3149 m²/g) and pore volume (1.76 cm³/g), enhancing adsorption capacities for acetone and n-hexane up to 1046 mg/g and 760 mg/g, respectively. Furthermore, the PWAC demonstrated exceptional recyclability, maintaining 97% of its original adsorption capacity after five cycles, underscoring its sustainable and economic viability. The highly advanced hierarchical ultra-meso-macroporous structure of PWAC significantly reduced the diffusion resistance of VOCs, guaranteeing swift mass transfer and desorption of VOCs. Consequently, the developed PWAC represents a novel, cost-effective, and environmentally-friendly approach for VOCs adsorption, aligning with the principles of green chemistry and low-energy consumption. This research not only provides a promising solution for VOCs mitigation but also highlights the potential of waste biomass in producing high-performance carbon materials.

Dynamic coupling between cementation and porosity evolution of the Chang 8 tight sandstone reservoir, Ordos Basin: Insights from in-situ microanalysis

Understanding cement paragenesis is crucial to the study of the porosity evolution in low-permeability sandstone reservoirs. Carbon and oxygen isotope analysis with micro-drilling sampling, electron probe microanalysis, and laser Raman spectroscopy microanalysis was used to clarify the origin of cements and their relationship with reservoir quality of the Chang 8 tight sandstone reservoir in the Longdong area of the Ordos Basin. The predominant cements are chlorite, illite, kaolinite, quartz, and carbonate, which occur in 3 forms: grain rim, overgrowth, and filling of intergranular pore and intragranular dissolved pore. The quantitative paragenesis of the cements is established based on the formation temperature of cements obtained through fluid inclusion, oxygen isotope, chemical composition and Raman parameters. The main source of carbon for carbonate cements is the decarboxylation of organic matter in the adjacent mudstone interlayers. The conversion of smectite to illite and the dissolution of feldspar and previous carbonates provide Ca2+, Mg2+, and Fe3+ ions for carbonate cements and authigenic chlorite, and the authigenic chlorite rim continuously grows from the grain surface outward. The porosity reduction by the cements mainly occurs during the main hydrocarbon filling period (80–120 °C), and the reservoir belongs to the type of “hydrocarbon accumulation prior to significant porosity reduction”. This study provides a new approach for studying the temporal and spatial evolution of clastic rock diagenesis.

Elemental and mineralogical mechanisms controlling the storage and flow performances of tight conglomerates

This study aims to shed light on the elemental and mineralogical mechanisms controlling the storage and flow performances of tight conglomerates. This is achieved through the application of Field Emission Scanning Electron Microscopy (FE-SEM), Energy Dispersive Spectrometer (EDS), TESCAN Integrated Mineral Analyzer (TIMA), and geophysical logging data.The findings suggest that augmenting the Mg and CaO content in silicates can markedly enhance the storage capacity of tight conglomerates within the Upper Wuerhe Formation. Chlorite exerts a positive influence on the evolution of reservoir porosity, whereas an escalation in Na content does not foster the development of reservoirs with superior physical attributes. A reduction in the chemical index of alteration (CIA) facilitates the creation of a more expansive pore space, while an elevation in the index of component variation (ICV) augments porosity and diminishes mud content, albeit it may also curtail oil content. It is pertinent to note that an increase in ICV is not invariably advantageous. The emergence of orthoclase results in a decrease in porosity and an enhancement in permeability, whereas the progression of kaolinite adversely affects reservoir porosity.The conclusion may yield significant insights for hydrocarbon exploration in these regions, as well as in reservoirs exhibiting analogous lithologies.

Coupling control of organic and inorganic rock components on porosity and pore structure of lacustrine shale with medium maturity: A case study of the Qingshankou Formation in the southern Songliao Basin

Shale components affect organic–inorganic diagenesis, control the formation and evolution of pore structure, and lead to heterogeneity of shale reservoirs. Therefore, revealing the coupling control of shale components on the porosity and pore structure is crucial for selecting the dominant lithofacies. In this study, the lacustrine shale with medium maturity in the first member of the Qingshankou Formation (Qing1 Member) in the Changling Sag, southern Songliao Basin, China, was analyzed. Using pressurized sealed original shale samples and 2-D nuclear magnetic resonance (2D-NMR) analysis, the effective porosity of shale was accurately determined. The pore structure was characterized via experiments such as low-temperature nitrogen adsorption, NMR, and scanning electron microscope (SEM) analyses. In addition, the joint influences of the shale rock components, sedimentary structure, and maturity on the porosity and pore structure were investigated. Finally, the favorable reservoirs in the lacustrine shale with medium maturity and their formation conditions were identified. Studies have shown that for lacustrine shale with medium maturity: (1) a moderate total organic carbon (TOC) content (between 1% and 2.5%) is a key factor for improving the effective porosity and the proportions of mesopores (100–1000 nm) and small-pores (30–100 nm). This is related to the generation of organic acids and the cracking of organic matter into pores. An excessive TOC content can lead to increased plasticity of the rock and filling of the pores by bitumen. (2) The porosity and pore structure are controlled by the organic matter and inorganic minerals as well as sedimentary structure. Organic matter has dual impacts on the porosity improvement related to clay minerals, promotion of clay conversion and filling of clay-related pores, and the ratio of the clay content to the TOC (Vsh/TOC) can reflect the degree of influence of organic matter, the greater the Vsh/TOC ratio is, the greater the effective porosity and proportion of micropores are. (3) Organic matter and felsic minerals jointly effect the improvement of the effective porosity and larger pores (>30 nm), which is related to their good compression resistance, organic pore retention, and dissolution-enhanced porosity effect. The product of the TOC and felsic mineral content (Vsi*TOC) can reflect the degree of their coupled effect. For moderate TOC contents, a higher felsic mineral content, and moderate shell laminae (Vca>10%) are conducive to the development of the effective porosity and larger pores. (4) The organic medium-rich argillaceous laminated felsic shale and organic-medium argillaceous laminated clayey shale have a higher effective porosity and higher proportion of larger pores, making them the dominant lithofacies for shale oil sweet spots.

Integration of diagenesis, porosity evolution, and oil emplacement in lacustrine tight sandstone reservoirs: A review with illustrative cases from the major oil-bearing basins in China

Integration of diagenesis, porosity evolution, and oil emplacement in lacustrine tight sandstone reservoirs: A review with illustrative cases from the major oil-bearing basins in China

Influences of burial process on diagenesis and high-quality reservoir development of deep–ultra-deep clastic rocks: A case study of Lower Cretaceous Qingshuihe Formation in southern margin of Junggar Basin, NW China

Taking the Lower Cretaceous Qingshuihe Formation in the southern margin of Junggar Basin as an example, the influences of the burial process in a foreland basin on the diagenesis and the development of high-quality reservoirs of deep and ultra-deep clastic rocks were investigated using thin section, scanning electron microscope, electron probe, stable isotopic composition and fluid inclusion data. The Qingshuihe Formation went through four burial stages of slow shallow burial, tectonic uplift, progressive deep burial and rapid deep burial successively. The stages of slow shallow burial and tectonic uplift not only can alleviate the mechanical compaction of grains, but also can maintain an open diagenetic system in the reservoirs for a long time, which promotes the dissolution of soluble components by meteoric freshwater and inhibits the precipitation of dissolution products in the reservoirs. The late rapid deep burial process contributed to the development of fluid overpressure, which effectively inhibits the destruction of primary pores by compaction and cementation. The fluid overpressure promotes the development of microfractures in the reservoir, which enhances the dissolution effect of organic acids. Based on the quantitative reconstruction of porosity evolution history, it is found that the long-term slow shallow burial and tectonic uplift processes make the greatest contribution to the development of deep–ultra-deep high-quality clastic rock reservoirs, followed by the late rapid deep burial process, and the progressive deep burial process has little contribution.

Diagenetic Impact on High-Pressure High-Temperature Reservoirs in Deep-Water Submarine Fan Sandstone of Qiongdongnan Basin, South China Sea

The diagenetic evolution of sandstone is very complicated under the conditions of high temperatures and pressures in deep-water, deep-buried regimes, which have great influence on reservoir quality. This study investigates the typical reservoir target of Neogene deep-water, submarine-fan sandstones under high-temperature, high-pressure regimes in the Qiongdongnan Basin, South China Sea. Utilizing a thin section, scanning electron microscope (SEM), mineral geochemistry combined with burial history evolution, complex diagenetic events, and main controlling factors of the sandstone in the Neogene Meishan Formation were determined. The results show that the evolution of sandstone reservoirs is initially controlled by depositional framework compositions and subsequently modified by eogenetic and mesogenetic alterations during progressive burial. Eogenetic alterations mainly include the following: (1) mechanical compaction; (2) dissolution of feldspar; (3) low-Fe calcite cementation. Mesogenetic events were identified as the following: (1) dissolution of feldspar; (2) ferroan calcite and ankerite formation; (3) precipitation of quartz and clay mineral. Mechanical compaction is greatly influenced by the original depositional framework composition, and sandstone samples enriched in high contents of detrital clay matrix always experienced extensive mechanical compaction. Different phases of carbonate cement during different diagenetic regimes lead to continuous destruction on reservoir porosity. The dissolution of unstable feldspar minerals during eogenetic and mesogenetic environments leads to the development of secondary porosities and would enhance the quality of the reservoir. Overpressure formation is pervasively developed owing to early disequilibrium compaction and subsequent natural gas charging. Only well-sorted sandstones with low contents of detrital clay matrix could resist early mechanical compaction, lead to ample residual original porosities, and then undergo extensive mineral dissolution to generate sufficient secondary porosities. Subsequently, these porosities would be effectively protected by overpressure formation. Poor-sorted sandstones with high contents of detrital clay matrix would experience strong mechanical compaction and extensive destruction of original porosities. Thus, these sandstones are difficult to have significant dissolution and are unable to be effectively protected by overpressure formation. Therefore, the interplay between the original framework composition and the corresponding diagenetic pathways coupled with overpressure formation would result in strong reservoir heterogeneity for the deep-buried sandstones during progressive burial.

The role of thermal cycling in micro-cracking development and flow alteration: Advanced insights from CT and lab studies

The recent interest in geothermal energy, nuclear waste management, and coal gasification has led to an in-depth examination of how granite rock behaves under subterranean reservoir conditions. Our research delves into the intricate microstructural and fluid dynamics of the Australian Harcourt granite. We emphasize its reactions to repeated temperature changes, cycling between 25 and 900 °C over 1–15 cooling episodes. To achieve this, we utilised X-ray computed tomography (CT) to visualize and reconstruct the heat-treated samples. Subsequently, we measured the porosity, identifying both total and directly interconnected spaces within these samples post-thermal treatment. Using avizo 2021.2, we created a pore network model (PNM) based on the connected porosity data to shed light on how fluids move through these heat-affected samples.The data highlighted that total porosity stayed below 0.5% up to 500 °C, with no directly connected porous regions detected. However, beyond this temperature, interconnected porosity began to appear, intensifying with increasing temperatures and more cooling cycles. The PNM allowed us to visualize and quantify specific details like pore and throat dimensions, their volumes, their interconnectedness, and the lengths of their connecting channels. Notably, these properties became more pronounced with escalating temperatures and cooling episodes, largely because of the fractures caused by the heat. Furthermore, we used AVIZO to analyse the absolute permeability of the heat-treated samples, providing a detailed understanding of their permeation characteristics. We also ran a set of rigorous lab tests to assess the impact of both confining pressure (ranging from 10 MPa to 40 MPa) and temperature (from 22 to 250 °C) on the fluid movement in the heat-treated rock. The experiments confirmed that increasing confining pressure non-linearly decreased Darcy's permeability. A rise in temperature further reduced permeability, mainly due to the narrowing of flow channels from mechanical and thermal stresses.Overall, the study provides a holistic understanding of porosity evolution and the subsequent fluid movement within thermally quenched rock samples under various reservoir conditions, providing valuable insights for energy and waste management applications.

Effect of solid solution treatment on the kinetics of hydrogen porosity evolution and mechanical properties in Al–Cu–Li alloys

The evolution kinetics of hydrogen porosity during solid solution treatment (SST) of vacuum-cast Al–Cu–Li alloys and the effect of porosity defects on the mechanical properties of the alloys were investigated. It has been found that porosity at 6h SST was dominated by nucleation and its evoluton of fractions has a linear correlation with time, APS = (0.05 ± 0.06)＋(0.11 ± 0.01) t. However, the growth of porosity at 12 h is evidenced by a decrease in the number of pores and an increase in the size of large pores, expanding inter-porosity distance due to Ostwald ripening. Three dimensional quasi-in situ X-ray CT quantification of porosity demonstrate that the hydrogen concentration at the interface in the early stages of the solid solution was not adequate to balance its surface tension and vacancies were created by cross-diffusion, reducing the interficial energy for porosity nucleation. The effects of porosity on the tensile strength satisfies lnσ-lnσ0 = αlnfp, and those detrimental hydrogen porosities can be predicted by a multi-scale model which take into account hdyrogen diffusion during vacuum solution treatment.

Correlation of the permeability and porosity development of carbon/carbon composites during pyrolysis

A cost-effective approach to manufacture Carbon/Carbon composites is to infiltrate carbon fiber preforms with a polymer precursor and pyrolyze it to form the carbon matrix. During pyrolysis, the microstructure changes due to decomposition reactions creating a high degree of porosity consisting of interconnected network of voids and cracks. To densify, the pyrolyzed composite is infiltrated with polymer precursor to fill the porous network. The cycle of pyrolysis followed by infiltration is repeated until the porosity is minimized and the desired density is achieved. The permeability, porosity and viscosity of the polymer govern the infusion pressure and time for the re-infiltration steps.In this work, the permeability of benzoxazine-based Carbon/Carbon composites was measured for different degradation levels with three distinctly different pyrolyzing schedules. The samples were characterized systematically at increasing pyrolyzing temperatures until full conversion of the precursor into carbon matrix was achieved. The porosity of the material was evaluated by X-ray micro-computed tomography and by pycnometry. The through the thickness permeability of the composite was measured via a pulse-decay experiment after each heating step in the pyrolysis cycle. In this experiment pressure decay with time is recorded as air is evacuated from the cavity through the connected pathways of the porous network within the sample. The experimental correlation between the permeability and interconnected porosity of the microstructure is established. It is shown that the pulse-decay test with air effectively characterizes the permeability of the pyrolyzed composite.

Field-scale numerical investigation of a hybrid gas production method from the oceanic hydrate accumulation at site NGHP-02-09 in the Krishna-Godavari Basin, and the associated geomechanical responses

The objectives of this study are to investigate (a) the technical feasibility of gas production from a gas hydrate accumulation at Site NGHP-02-09 in the Krishna-Godavari Basin, India Ocean using a hybrid production method involving a combination of depressurization and thermal stimulation, as well as (b) the associated geomechanical system response when considering both a simplified and a full geomechanical model. This is an extension of the earlier numerical study of Moridis et al. (2019a) of the site using cylindrical (single-well) 2D systems, which indicated (a) the main source of the produced gas to be CH4 dissolved in the water rather than CH4 from hydrate dissociation because of the presence of a high-permeability water-bearing zone that short-circuits the depressurization process and (b) the possibility of adverse geomechanical issues caused by significant displacements that could not be adequately investigated and resolved by the one-way coupling scheme invoked in that study. The proposed hybrid production plan involves two vertical wells in a 3D Cartesian domain: an injection well of warm ocean (saline) water and a production well at an appropriate distance, with the expectation that the hydrate dissociation caused by the depressurization at the production well can be augmented by the thermal effect of the injected warm water (as well as by its salinity), thus mitigating the effects of the high-permeability water zone. The results of this study that uses high-resolution 3D grids indicate that (a) the hybrid production scheme does not appear to offer any advantage over the simple single-well depressurization method of the earlier study, (b) the majority of the produced gas originates from CH4 exsolution from the water rather from hydrate dissociation, and (c) consideration of full geomechanics leads to generally slightly higher dissociation and fluid production because of the evolution of lower porosities and permeabilities, but also to the disappearance of the gas phase after 62 days of production, after which time CH4 from hydrate dissociation is fully dissolved in the water before production. Thus, the hybrid scheme investigated here appears ineffectual in enhancing hydrate dissociation and gas production, and the hydrate deposits at the Site NGHP-02-09 do not appear to be a promising production target under the conditions in this study, in agreement with the earlier conclusion of Moridis et al. (2019a).