Evolution of sandstone nanoscale pore structure and suspended particles under dissolution: Implications for pore clogging during uranium leaching

Investigation of the factors that control the development of pore structure in lacustrine shale: A case study of block X in the Ordos Basin, China

Evolution of pore structure and reactive transport parameters during acid leaching of sandstone uranium ore

The unsaturated permeability coefficient of granite residual soil (GRS) increases rapidly with rising moisture content, as the loss of matric suction enhances the continuity of the water phase within the soil pores. This can lead to slope instability and embankment collapse during rainfall. This study investigated the effects of wet-dry cycles on the hydraulic and microstructural evolution of GRS, introducing key innovations over prior research. First, microstructure changes were investigated using mercury intrusion porosimetry (MIP) tests, investigates the evolution of bimodal pore structure under cyclic wetting and drying. Second, the entire range of matric suction was comprehensively measured by integrating the pressure plate method (PPM), filter paper method (FPM), and vapor equilibrium method (VEM), capturing both low and high suction regimes comprehensively. Third, the Li model was applied to fit the bimodal SWCC across different wet-dry cycles, the unsaturated permeability coefficient was calculated using the Zhai model. The results indicate that the microstructure of GRS under different wet-dry cycles presents a clear bimodal pore size distribution (PSD), intra-aggregate pores peaked near 450 nm, while inter-aggregate pores ranged between 20,000–60,000 nm.. After six wet-dry cycles, the volume of the dominant intra-aggregate pores decreased by approximately 25%, while the larger inter-aggregate pores saw a reduction of about 15%, indicating a coarsening of the pore network. Meanwhile, there is a clear decrease in inter-aggregate pore distribution density. The combination of measurement methods can cover the entire matric suction range. The Li model is applied to fit the SWCC under different wet-dry cycles, and the correlation coefficient (R2) are all higher than 0.95. The unsaturated permeability coefficient of GRS exhibits a nonlinearly variation with saturation, in-creasing with the increase in saturation or the increase in wet-dry cycles. The unsaturated permeability coefficient of bimodal GRS was calculated based on the Zhai model and the lgk(s) and saturation can be expressed by a logarithmic function, with the correlation coefficient (R2) higher than 0.99 under different wet-dry cycles. The study contributes useful insights into the evolution of pore structure and hydraulic behavior of GRS under cyclic wetting and drying, which is important for slope stability and hydrological modeling in subtropical regions.

Underground coal thermal treatment (UCTT) is an emerging technology for cleaner coal utilization, with the residual in-situ pyrolytic char offering promising potential for CO2 storage. To elucidate the dynamic evolution of coal pore structures during underground thermal treatment, this study systematically examined changes in pore structure, fractal characteristics, and evolution patterns across a temperature range of 0-600 ℃ using low-temperature CO2 adsorption, low-temperature N2 adsorption, and mercury intrusion porosimetry. The results show that as temperature rises, the total pore volume initially decreases and then increases, whereas the total specific surface area increases continuously. Specifically, micropore (< 2nm) volume and specific surface area increase continuously; mesopores (2-50nm) volume and specific surface area gradually decrease; macropores (> 50nm) volume first decreases and then increases. With increasing temperature, the fractal dimensions D1 and D2, derived from nitrogen adsorption, initially decrease and then increase, whereas the D1 obtained from mercury intrusion porosimetry exhibits a continuous decline, indicating reduced surface roughness and complexity followed by recovery, except for macropores (> 140nm) whose complexity consistently decreases. Based on the thermal evolution characteristics of coal, the pore evolution model can be categorized into three stages: Ro < 0.54% (30-350 ℃), 0.54% < Ro < 1.39% (350-450 ℃), and Ro > 1.39% (450-600 ℃). A higher thermal treatment temperature (600 ℃) promotes the development of micropores and macropores, increases the total specific surface area, and enhances pore connectivity. These findings establish a theoretical basis for optimizing the operational parameters of UCTT.

Relationship between nanoscale deformation of coal structure and metamorphic-deformed environments

During the leaching process of the ion-adsorbed rare earth (RE) ore, the pore structure evolution of the ore body plays a vital role in the seepage of the leaching solution. To investigate the evolution of the pore structure during the leaching process, experiments have been carried out with remodelled RE ore samples based on the physical characteristics of in situ ores. The seepage rate difference between deionized water leaching solution and 2% NH4Cl leaching solution during the active leaching period was analysed. The porosity and the dynamic pore size evolution of pore structures in the ore body are discussed. Results indicate that along with ion exchange between the RE ore and the leaching solution, the porosity of the sample remains constant and the pore structure shows a decreasing trend in the first part and an increasing trend in the second part. Specifically, during the ion exchange process, the number of minimal pores (0–5 µm), small pores (5–10 µm) and medium pores (10–25 µm) increases significantly and the number of medium–large pores (25–60 µm), large pores (60–120 µm) and mega pores (greater than 120 µm) decreases dramatically. Along with the completion of the ion exchange process, the evolution of porous structure shows an opposite trend. The mechanism study reveals that the evolution of pore structure is induced by the difference of ionic strength in the leaching solution during the ion exchange process, where the RE ore microparticles will be absorbed or desorbed on to the solid phase.

To clarify the evolution of thermal maturity and pore structure in continental organic-rich shales, calcareous shales of the Liaohe Basin (China) were pyrolyzed, and examined using Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), nitrogen sorption isotherms, and nuclear magnetic resonance (NMR) spectroscopy. The increase in Raman G‒D band separation and decrease in Raman ID/IG ratio with increasing thermal maturity indicate that these parameters provide superior thermal maturity indicators. This is also confirmed by the good linear correlation of G‒D band shifts and ID/IG with vitrinite reflectance (VR) and maximum temperature (Tmax), respectively. The relative detection accuracy (DA), sampling requirements (SR), sample preparation (SP), detection time (DT), and equipment requirement (ER) of VR, G‒D band shifts, ID/IG, Tmax, FTIR, and XPS indicate that Raman analysis is a simple, quick, and robust method to evaluate thermal maturity. The moderate SR, complex SP, and long DT suggest that VR and Tmax are less widely applicable for characterizing thermal maturity. The FTIR and XPS techniques provide semi-quantitative maturity indicators with poor DA and high ER. Pores observed within organic matter and minerals under SEM indicate that an increase in thermal maturity not only affects the development of organic pores but is also beneficial for the formation of mineral dissolution pores, such as those formed during the transformation of Na0.95Ca0.16Al1.16Si2.84O8 to Na0.84Ca0.02Al1.02Si2.98O8, a process confirmed by XRD. The BET and NMR data also indicate that the development of pore structure is closely related to the evolution of thermal maturity in calcareous shale. During the initial stage, primary pores are filled by bitumen generated from kerogen; this leads to a decrease in transition pores, mesopores, and shale porosity, and reduced pore connectivity. Then, secondary nanoscale pores, transition pores, and mesopores increase with increasing thermal maturity. The peak in secondary porosity is consistent with the liquid hydrocarbon production rate peak, a process that increases shale porosity and leads to improved pore connectivity. The dissolution of minerals induced by organic acids may also contribute to this secondary porosity. With a further increase in thermal maturity, secondary porosity at the microscale is further developed, while transition pores and mesopores collapse, resulting in reduced pore connectivity. The poor pore connectivity that occurs at both low and high VR values may be more conducive to the preservation of shale oil and gas. This study is significant for research into the evolution of thermal maturity and pore structure in continental organic-rich shales.

Zhundong coal can significantly reduce the preparation temperature of activated carbon (AC) due to the high contents of alkali and alkaline earth metals (AAEMs) present in it. Moreover, because of its lower operating temperature and the presence of carbon matrix, Zhundong coal can effectively inhibit the release of AAEM during the preparation of AC. For these reasons, the preparation of AC from Zhundong coal is a promising approach for the clean utilization of Zhundong coal. Accordingly, this study was aimed to investigate optimum conditions for the preparation of AC from Zhundong coal. For this purpose, at first, Raman spectroscopy was used to determine the conditions for an optimal carbonization process using a coal sample; then, the evolution of the pore structure of AC under different conditions was examined by small-angle X-ray scattering (SAXS) and the N2 adsorption analyser. Furthermore, environmental scanning electron microscopy (ESEM) was performed to analyze the surface morphology of AC. Finally, by dividing the activation process into gas–solid diffusion and activation reactions, a mechanism for the evolution of pore structure during the preparation of AC was proposed. The results showed that the char with an amorphous structure and less graphite-like carbon, which was obtained by heating Zhundong coal from room temperature to 600 °C at 5 °C min−1 under the protection of N2 and then maintaining it at this temperature for 60 min, is suitable for the subsequent activation process. At low temperatures, the diffusion of H2O was dominant in the activation process, and the weak gas–solid reaction resulted in poor development of the pore structure; on the other hand, the CO2 activation reaction mainly occurred on the surface of the char due to the poor diffusion of CO2, and then, the produced pores could improve the diffusion of CO2; this led to significant development of the pore structure. With an increase in temperature, the H2O diffusion reaction was enhanced, and the pore structure of AC was completely developed; however, the diffusion of CO2 reduced with an enhancement in the CO2 activation reaction, leading to the consumption of carbon matrix by CO2 gasification instead of pore formation by the CO2 activation reaction. Therefore, proper utilization of the unique characteristics of H2O and CO2 during pore formation is important to control the activation process.

Unconsolidated sand reservoirs containing heavy oil play a significant role in hydrocarbon resources, characterized by high porosity and permeability alongside abundant movable fines. During production, these fines can detach and migrate with the reservoir fluids, causing pore plugging and reduced productivity. Visualizing and quantitatively evaluating the evolution of pore structure caused by fines migration under various influencing factors at the microscale is fundamental for devising effective prevention and mitigation measures. This study employs on-line NMR experiments and CFD-DEM simulations to investigate fines migration processes and their effects on physical properties and pore structure at the pore scale. Results indicate that fines migration initiates the formation of a preferential network of migration pathways. The evolution of pore structure demonstrates zonal characteristics along the flow direction, with fines plugging and residual accumulation primarily occurring in the middle/rear section of the core. As the core’s skeleton porosity decreases, fines plugging intensifies; however, at high injection velocity, new dominant flow channels may emerge, leading to a transition from a single-peak to a double-peak T2 spectrum. Below the critical velocity (0.5–1 mL/min), an increase in flow velocity exacerbates severe fines plugging. Conversely, above the critical velocity, an increase in flow velocity results in a more pronounced enhancement of permeability.

Porous structure is an important component in C/C composites, which directly affects the fatigue behavior of materials. Therefore, it is necessary to discuss the evolution of porous structure in C/C composites under the fatigue loading. In present work, the character of porous structure in original C/C composites was summarized and the evolution of porous structure after fatigue loading was analyzed. The positive effect of the porous structure evolution after fatigue loading on the reinforcing behavior of fatigue was proposed as well, which could provide a basis for further studies on the fatigue mechanisms of C/C composites.

Porous structure is an important component in carbon/carbon(C/C)composites,which directly af- fects the fatigue behavior of materials.Therefore,it is necessary to discuss the evolution of porous structure in C/C composites under the fatigue loading.In this paper,the character of porous structure in original C/C composites is sum- marized and the evolution of porous structure after fatigue loading is analyzed.The positive effect of the porous struc- ture evolution after fatigue loading on the reinforcing behavior of fatigue is proposed as well,which provids a basis for further studies on the fatigue mechanisms of C/C composites.

Lacustrine shale oil in the Chang 7 Member of the Ordos Basin is controlled by a multi-scale pore–throat system in which oil occurrence, spontaneous imbibition, and pore-structure evolution are tightly coupled. In this study, nitrogen adsorption and micro-computed tomography (μCT) were employed to characterize pore-size distribution and connectivity, whereas nuclear magnetic resonance (NMR) T2 relaxation was utilized to classify oil occurrence states, and X-ray diffraction (XRD) and total organic carbon (TOC) analyses were performed to determine mineralogical and organic compositions. Spontaneous imbibition experiments were conducted at 60 °C and subsequently extended to temperature–pressure sequence tests. The Chang 7 shale exhibits a stratified pore system in which micropores, mesopores, and macropores jointly define a three-tier “micropore adsorption–mesopore confinement–macropore mobility” pattern. As pore size and connectivity increase, the equilibrium imbibed mass and initial imbibition rate both rise, while enhanced wettability (contact angle decreasing from 81.2° to 58.7°) further strengthens capillary uptake. Temperature elevation promotes imbibition, whereas increasing confining pressure suppresses it, revealing a “thermal enhancement–pressure suppression” behavior. μCT-based network analysis shows that imbibition activates previously ineffective pore–throat elements, increasing coordination number and connectivity and reducing tortuosity, which collectively represents a capillary-driven structural reconfiguration of the pore network. When connectivity exceeds a threshold of about 0.70, the flow regime shifts from interface-dominated to channel-dominated. Building on these observations, a multi-scalecoupling framework and a three-stage synergistic mechanism of “pore-throat activation–energy conversion–structural reconstruction” are established. These results provide a quantitative basis for predicting imbibition efficiency and optimizing capillary-driven development strategies in deep shale oil reservoirs.

The evolution of pore structure during in situ underground exploitation of oil shale directly affects the diffusion and permeability of pyrolysis products. In this study, on the basis of mineral analysis and thermogravimetric results, in combination with the low-pressure nitrogen adsorption (LPNA) and mercury intrusion porosimetry (MIP) technique, the evolution of pore structure from 23 to 650 °C is quantitatively analyzed by simulating in situ pyrolysis under pressure and temperature conditions. Furthermore, based on the experimental results, we analyze the mechanism of pore structure evolution. The results show the following: (1) The organic matter of Fushun oil shale has a degradation stage in the temperature range of 350–540 °C, and there is no obvious temperature gradient between decomposition of kerogen and the secondary decomposition of bitumen. The thermal response mechanisms of organic matter and minerals are different in each temperature stage, and influence the change of pore structure. (2) Significant changes occur in pore shape at 350 °C, where thermal decomposition of kerogen begins. The ink-bottle pores are dominant when the temperature is less than 350 °C, whereas slit pores dominate when the temperature is greater than 350 °C. (3) The change in pore structure of oil shale is much less significant from 23 to 350 °C. The pore volume, porosity, and specific surface area (SSA) of samples increase rapidly with temperature varying from 350 to 600 °C. The variation of each parameter is dissimilated from 600 to 650 °C: the porosity and pore volume increases with a small gradient from 600 to 650 °C, and SSA decreases significantly. (4) The lithostatic pressure does not cause change in the evolution discipline of the pore structure, but the inhibitory effect on the pore development is significant.

The study focuses on the evolution of porous structure and texture of silica/alumina xerogels during calcination in the temperature range from 500 to 1200°C. The xerogel was prepared via sol-gel method using subcritical drying. The silica/alumina xerogels were examined using transmission electron microscopy–energy dispersive spectroscopy (TEM–EDS), Brunauer Emmett Teller–Barrett Joyner Halenda (BET–BJH), differential scanning calorimetry (DSC), and Fourier transform infrared (FTIR) spectroscopy. SiO2 primary particles of size about 10 nm are connected with each other to form a porous xerogel structure. Alumina is uniformly distributed over the xerogel volume. The changes of textural characteristics under heat treatment of samples are radical; the specific surface area and pore size attain their maximum at 500–700°C. The heat treatment of samples causes dehydroxylation of the xerogel surface, and at 1200°C the sample is sintered, loses mesoporosity, and its specific surface area reduces considerably down to 78 m2/g.

Carbon dioxide (CO2) storage in sandstones is vital for enhancing oil/gas recovery and reducing CO2 emissions. The introduction of CO2 into sandstone reservoirs leads to chemical reactions between CO2 and minerals present in sandstone, which changes the pore structure of the sandstone reservoir. Herein, tight sandstone samples from the Coal-Measure Strata of the Shanxi Formation in the Huxiang area, Henan Province, were selected for simulation in this experimental study under supercritical CO2 (ScCO2)–H2O treatment in reservoir conditions. Further, mercury intrusion porosimetry and low-pressure nitrogen adsorption/desorption methods were used to analyze the evolution of the pore structures of tight sandstones, and the mechanism of pore structure evolution was discussed. The results show that pore volumes and specific surface areas in the micropores and transitional pores decreased after the ScCO2–H2O treatment, while those in the mesopores and macropores increased. In the micropores and transitional pores, some of the pores changed from open pores and ink-bottle-shaped pores to semi-closed pores after the ScCO2–H2O treatment, and the pore morphology became narrower, which might have deteriorated the pore connectivity. A pore structure evolution model of ScCO2–H2O-treated tight sandstones was proposed. The evolution of pore structure is a result of the synergistic effect of pore enlargement caused by mineral dissolution and secondary mineral precipitation, which together play a controlling role in pore structure evolution. This study is conducive to understanding the pore structure evolution under ScCO2–H2O treatment and implementing CO2 storage and enhancing oil/gas recovery in sandstone reservoirs.