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Quaternary mudstone biogas reservoirs in the Qaidam Basin have shown great potential. However, complex pore structures with high clay contents and high heterogeneity limit the understanding of the storage and migration principles of these reservoirs. In this paper, HPMI and nitrogen adsorption experiments, in combination with NMR experiments under water saturation, centrifugation, various drying temperatures and other conditions, were adopted to determine the pore structure characteristics. Specifically, the reservoir space types and pore radius distribution characteristics were clarified. The cutoff values for different types of pores were identified based on the water-saturated mudstone NMR T2 spectra for full aperture distribution scales jointly characterized by mercury injection and nitrogen adsorption experiments. Furthermore, the three pore components and the saturation of different fluids were obtained. The research results indicate that the mudstone biogas reservoir has developed various reservoir spaces, and the pore size is primarily in the micronanometer range. The average total porosity reaches 27.28%, but the proportion of movable water pores is only 9.23% with poor fluid mobility, and the fluids in the pores are mostly capillary-bound water and clay-bound water. Among the different lithologies, argillaceous sand is more likely to become a good production layer. [ABSTRACT FROM AUTHOR]

To explore the influence of surfactant concentration on the pore structure and permeability of coal samples during the chemical enhancement of coalbed methane production, different kinds and different concentrations of surfactants were added to the chemical solution, and the coal samples were soaked. Methods such as low-field nuclear magnetic resonance testing (NMR), fractal theory, permeability testing, surface tension testing, and contact angle testing were employed to analyze the variation patterns of coal sample pore structure, fractal characteristics, and permeability, and to explore the correlation between surface tension, contact angle, and the degree of pore structure development. The results show that the increase in total porosity of coal samples, the increase in the seepage pore porosity, the decrease in Dt, and the growth rate of permeability increase with the increase in surfactant concentration, and are negatively correlated with the surface tension of the solution and the contact angle of the coal-solution interface, while the decrease in Ds is not significantly correlated with surfactant concentration, surface tension, or contact angle. In terms of the erosion effect of a chemical solution on coal samples, the influence of contact angle is greater than that of surface tension, while surface tension has the greatest impact on the development of adsorption pores. By adding different surfactants, the surface tension of the chemical solution and the contact angle of the coal-solution interface can be controlled, further promoting the erosion of coal samples, which is of positive significance for the chemical enhancement of coalbed methane production. [ABSTRACT FROM AUTHOR]

The cyclic hydraulic stimulation (CHS) has proven as a prospective technology for enhancing the permeability of unconventional formations such as coalbeds. However, the effects of CHS on the microstructure and gas sorption behavior of coal remain unclear. In this study, laboratory tests including the nuclear magnetic resonance (NMR), low-temperature nitrogen sorption (LTNS), and methane sorption isotherm measurement were conducted to explore changes in the pore structure and methane sorption characteristics caused by CHS on an anthracite coal from Qinshui Basin, China. The NMR and LTNS tests show that after CHS treatment, meso- and macro-pores tend to be enlarged, whereas micropores with larger sizes and transition-pores may be converted into smaller-sized micro-pores. After the coal samples treated with 1, 3, 5 and 7 hydraulic stimulation cycles, the total specific surface area (TSSA) decreased from 0.636 to 0.538, 0.516, 0.505, and 0.491 m²/g, respectively. Fractal analysis based on the NMR and LTNS results show that the surface fractal dimensions increase with the increase in the number of hydraulic stimulation cycles, while the volume fractal dimensions exhibit an opposite trend to the surface fractal dimensions, indicating that the pore surface roughness and pore structure connectivity are both increased after CHS treatment. Methane sorption isothermal measurements show that both the Langmuir volume and Langmuir pressure decrease significantly with the increase in the number of hydraulic stimulation cycles. The Langmuir volume and the Langmuir pressure decrease from 33.47 cm³/g and 0.205 MPa to 24.18 cm³/g and 0.176 MPa after the coal samples treated with 7 hydraulic stimulation cycles, respectively. The increments of Langmuir volume and Langmuir pressure are positively correlated with the increment of TSSA and negatively correlated with the increments of surface fractal dimensions. [ABSTRACT FROM AUTHOR]

Under the influence of geostress, the thick hard sandstone roof layer at the upper end of the mining area is prone to faults, rockbursts, and sudden collapses during deep coal mining. Blasting top-cutting and depressurization technology centered on over-the-top deep hole blasting is an important method for this type of dynamic disaster control. To characterize the pore structure of sandstone damaged by blast loads and the change in energy dissipation in different pressure environments, specimens with different degrees of blasting damage were measured via nuclear magnetic resonance (NMR) spectroscopy. A triaxial test loading device was used to carry out compression tests on undamaged, vibration-damaged, and blast-damaged specimens under different pressures. The effects of blast load and confining pressure on the porosity, pore structure, energy dissipation, and fracture morphology characteristics of sandstone specimens were analyzed. (1) The T2 spectral curves of the sandstone specimens were all bimodal, and the signal intensity of the main peak was much greater than that of the secondary peak. The peak T2 spectrum curves of the blast-damaged specimens and vibration-damaged specimens were 1.21 and 1.14 times greater than those of the undamaged specimens, respectively, and the mean values of the peak relative area were 1.11 and 1.20 times greater than those of the undamaged specimens, respectively. The number of internal pores in the specimens increased significantly after blast loading. (2) The pore structure distributions in the sandstone specimens were dominated by the proportions of micropores and small holes. The blast load changed the internal pore ratio structure of the specimen, transformed the small holes into medium holes and large holes, and increased the porosity of the specimen. (3) Blast load damage could reduce the energy absorption and energy storage effects of sandstone specimens and weaken roof sandstone and reduce the stress concentration. Furthermore, the impact of the blasting damage area was greater than that of the vibration damage area. (4) With increasing confining pressure, the fracture degree of the specimen first decreased and then increased. Due to the high-energy storage abilities of the undamaged specimens, the macroscopic rupture degrees of the undamaged specimens were greater than those of the specimens damaged by vibration and blasting. The blasting load will cause damage to the roof sandstone to achieve the effect of pressure relief, so as to reduce the occurrence of geological disasters such as roof fault and rock burst. [ABSTRACT FROM AUTHOR]

This research systematically investigates the influence of raw material particle size and calcium content on the geopolymerization process to gain insight into the physical and mechanical properties of geopolymer gels, including setting time, fluidity, pore structure, compressive strength, and leaching characteristics of encapsulated Cr3+ heavy metal ions. Utilizing a diverse range of particle sizes of metakaolin (MK; 3.75, 7.5, and 12 µm) and fly ash (FA; 18, 45, and 75 µm), along with varied calcium levels, this study assesses the dual impact of these factors on the final properties of both metakaolin- and fly-ash-based geopolymers. Employing sophisticated analytical techniques such as Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), and Nuclear Magnetic Resonance (NMR), the research meticulously documents alterations in chemical bonding, micro-morphology, and pore structures. Key findings reveal that reducing the size of MK and FA particles to 3.75 and 18 µm, respectively, enhances the compressive strength of their matrices by 128.37 and 297.58%, respectively, compared to their original values (63.59 and 33.87 MPa, respectively) at larger particle sizes. While smaller particle sizes significantly bolster compressive strength, they adversely affect slurry flow and reduce the leaching rates of Cr3+ from MK- and FA-based matrices, reaching 0.42 and 0.75 mg/L at 3.75 and 18 µm, respectively. Conversely, increased calcium content markedly enhances setting times and contributes to the formation of dense microstructures through the production of calcium aluminate silicate hydrate (C-A-S-H) gels, thus improving the overall curing performance and durability of the materials. These insights underline the importance of fine-tuning particle size and calcium content to optimize geopolymer formulations, offering substantial benefits for varied engineering applications and promoting more sustainable construction practices. [ABSTRACT FROM AUTHOR]

To improve the efficiency of coalbed methane and recoverability of reservoirs, enhanced fracturing technology is usually required to improve the low porosity and permeability status of coal reservoirs. As a feasible method for strengthening permeability, microwave–LN2 freeze–thaw (MLFT) cycles modify the microscopic pore structure of coal through the coupled effect of temperature stress changes, phase change expansion, and fatigue damage. 1H nuclear magnetic resonance combined with fractal dimension theory was used to characterize quantitatively the pore system and geometric features of coal. The geometric fractal model constructed using the T2 spectrum indicates that the fractal dimensions Dp and De have high fitting accuracy, demonstrating that percolation and effective pores exhibit good fractal characteristics. Dp and De are correlated negatively and positively, respectively, with the cyclic parameters. The relevance analysis shows that the NMR fractal method can reflect the pore–fracture heterogeneity of coal, which has a significant effect on the percentage of fluid migration space. This study reveals that MLFT cycles have significant enhancement effects on promoting the extension of multi-type pores structures within the coal matrix, as well as the connectivity and permeability of cracks. [ABSTRACT FROM AUTHOR]

In China, significant progress has been made in the exploration and development of coalbed methane (CBM) in medium- to high-rank coals. However, the exploration and development potential of CBM in low-rank coals in the Zhalainuoer coalfield is unknown. In this study, various testing methods were utilized, including low temperature N2/CO2 adsorption, field emission scanning electron microscopy, and nuclear magnetic resonance, to investigate the pore structure characteristics of low-rank coals in the Zhalainuoer coalfield, so as to further evaluate the occurrence space of CBM therein. The results revealed that the prevalent pore types in the low-rank coals were "ink bottle" shaped pores and semi-closed pores, and micropores provide the main specific surface area (SSA) and total pore volume (TPV). Moreover, there is no significant correlation between vitrinite content and fractal dimension, while pore SSA and TPV are correlated positively with D1 but negatively with D2. The coalification degree significantly impacts the pore characteristics of the coal reservoirs. With coalification degree increasing, the SSA and TPV of micropores and transition pores generally exhibited a pattern of initially decreasing and then increasing. These research findings establish a theoretical foundation for the exploration and development of CBM in the Zhalainuoer coalfield. [ABSTRACT FROM AUTHOR]

Copyright of Acta Materiae Compositae Sinica is the property of Acta Materiea Compositae Sinica Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Based on investigated mechanical properties of alkali residue-based foamed concrete (A-FC), this paper presents a comprehensive examination of the microstructural features of A-FC and their impact on compressive properties. A detailed investigation of the compressive performance and pore structure characteristics was conducted through compression tests and low-field nuclear magnetic resonance (NMR) tests. The results demonstrate that the primary pore size in A-FC falls within the microscale, and the pores can be categorized into three types—gel pores, capillary pores, and air voids. In addition, the T2 spectrum of A-FC exhibits three distinct relaxation peaks, each of which is attributed to different water components, i.e., adsorbed water, pore water, and free water. As the pore size, air void ratio, and porosity increase, the compressive strength of A-FC decreases. The main reasons for the increased in strain in low-strength A-FC include compression at the bottom, internal stress dissipation through the pore structure, and changes in the failure mode. [ABSTRACT FROM AUTHOR]

In this study, the pore structure and gas permeability of river sand concrete and manufactured sand concrete (mixed with various stone powders) are examined under dry and wet conditions. First, the pore structure of concrete was tested using the mercury-in-pressure (MIP), hydrostatic balance (HB), and nuclear magnetic resonance (NMR) methods. Second, the gas permeability of concrete was measured using the quasi-stationary flow method. Finally, a numerical model was developed based on the relationship between the pore structure and gas permeability. The results indicated that the gas permeability of dry concrete has a quadratic linear relationship with the porosity and volumetric characteristic pore size. The pore structure of partially water-saturated concrete can be corrected using a correction factor and the relaxation rate of the dry material. Finally, a random hierarchical bundle model using the HB–NMR method is proposed. The maximum relative deviation between the predicted and experimental values is 48.28%. [ABSTRACT FROM AUTHOR]

Copyright of Bulletin of the Chinese Ceramic Society is the property of Bulletin of the Chinese Ceramic Society Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Water reducer and water-repellent admixture are very important in improving the workability and durability of cement-based materials. However, the synergistic effect of the two types of admixtures has not been well investigated. In this study, polycarboxylate ether-based superplasticizer (PCE) and octyltriethoxysilane (OTS) were adopted as water reducer and water-repellent admixture, respectively. Their synergistic effect on the fluidity, compressive strength, and water absorption rate of cement-based materials was investigated. Particularly, the pore structure and hydration state of cement paste were analyzed using 1H Low-Field Nuclear Magnetic Resonance (1H LF NMR). The result showed that the fluidity of cement paste containing different dosages of PCE was reduced by 5–10 mm by incorporating 1% OTS, and the compressive strength at the early age of 3 d of mortar containing high PCE dosage of 0.25% decreased up to 15% by using 1% OTS. In contrast, the compressive strength of mortar containing 0.20% PCE was slightly enhanced by the addition of 1% OTS. 1H LF NMR analysis revealed that the combination of PCE and OTS would increase the pore size and total pore volume of cement paste, and more bleeding water would be generated at high PCE dosage. The intensity-weighted T2 values of the main peak ( T 2 ¯ ) implied that both PCE and OTS produced a retardation effect on cement hydration. However, the water absorption rate decreased by 46.6% despite the increase in pore size and total pore volume. The conflict phenomenon powerfully revealed that the internal hydrophobic treatment by OTS has been successfully achieved. Overall, the combination of 0.20% PCE and 1% OTS exerted a positive synergistic effect in improving the compressive strength and water-repelling ability of cement-based materials, which is meaningful for improving their durability and service life. [ABSTRACT FROM AUTHOR]

Abstract Quaternary mudstone biogas reservoirs in the Qaidam Basin have shown great potential. However, complex pore structures with high clay contents and high heterogeneity limit the understanding of the storage and migration principles of these reservoirs. In this paper, HPMI and nitrogen adsorption experiments, in combination with NMR experiments under water saturation, centrifugation, various drying temperatures and other conditions, were adopted to determine the pore structure characteristics. Specifically, the reservoir space types and pore radius distribution characteristics were clarified. The cutoff values for different types of pores were identified based on the water-saturated mudstone NMR T2 spectra for full aperture distribution scales jointly characterized by mercury injection and nitrogen adsorption experiments. Furthermore, the three pore components and the saturation of different fluids were obtained. The research results indicate that the mudstone biogas reservoir has developed various reservoir spaces, and the pore size is primarily in the micronanometer range. The average total porosity reaches 27.28%, but the proportion of movable water pores is only 9.23% with poor fluid mobility, and the fluids in the pores are mostly capillary-bound water and clay-bound water. Among the different lithologies, argillaceous sand is more likely to become a good production layer.

Abstract To further raise the gas extraction efficiency of the tight sandstone, the liquid nitrogen (LN2) freezing-thawing cycles method can be employed to improve the permeability of the low-permeability reservoirs. Permeability is generally regarded as a macroscopic description of the pore structure and usually has functional relationship to pore structure properties. The permeability of the rock is closely related to the change of microscopic pore structure. The permeability of rock depends on how the subzero temperatures changed the microscopic pore structure of rock, but it has not yet been confirmed obviously. In this study, the nuclear magnetic resonance (NMR) technique was adopted to investigate the pore structure evolution law and permeability of the tight sandstone with different LN2 freezing-thawing cycles. The results demonstrate that the LN2 freezing-thawing cycles promotes pore development and micro-fracture connection, and enhances the pore connectivity. The proportion of meso-pores, macro-pores and micro-fractures in the sandstone samples increases significantly, which provides a channel for the sandstone gas flow and extraction. Total porosity and effective porosity of the samples present a trend of rapid increase as the number of LN2 freezing-thawing cycles increasing, while the residual porosity decreases as the number of LN2 freezing-thawing cycles increasing. Coates model, SDR model (mean T2 model) and PP model were used to calculate and evaluate the permeability of the samples subjected to different LN2 freezing-thawing cycles. Furthermore, PP model can provide a better permeability estimate than the classical Coates and SDR model.

Graphene oxide (GO) significantly enhances cement hydration. This study investigates the effects of GO on the early hydration and pore structure evolution of cement paste by examining its impact on the setting time and mechanical properties of cement-based materials. In situ monitoring of the hydration process and microstructural changes was conducted using low-field nuclear magnetic resonance technology. The findings reveal that GO's nucleation effect accelerates cement hydration, reducing the setting time and increasing both the hydration rate and degree. Moreover, hydration products utilize GO as a template, leading to adjusted morphologies and enhanced interlinking of C–S–H gels. This results in a marked reduction in porosity and a denser microstructure within the cement paste. Consequently, the early mechanical properties of the cement paste are improved, with a significant increase in flexural strength observed at 72 hours. These results suggest that GO, which is cost-effective, size-controllable and suitable for mass production, holds great potential for applications in cement-based materials. [ABSTRACT FROM AUTHOR]

Fatigue damage can occur in surface rock engineering due to various factors, including earthquakes, blasting, and impacts. The underlying cause for the variations in physical and mechanical properties of the rock resulting from impact loading is the alteration in the internal pore structure. To investigate the evolution characteristics of the pore structure under impact fatigue damage, red sandstone subjected to cyclic impact compression by split Hopkinson pressure bar (SHPB) was analyzed using nuclear magnetic resonance (NMR) technology. The parameters describing the evolution of pore structure were obtained and quantified using fractal methods. The development of the pore structure in rocks subjected to cyclic impact was quantitatively analyzed, and two fractal evolution models based on pore size and pore connectivity were constructed. The results indicate that with an increasing number of impact loading cycles, the porosity of the red sandstone gradually increases, the T2 cutoff (T2c) value decreases, the most probable gray value of magnetic resonance imaging (MRI) increases, the pores' connectivity is enhanced, and the fractal dimension decreases gradually. Moreover, the pore distribution space tends to transition from three-dimensional to two-dimensional, suggesting the expansion of dominant pores into clusters, forming microfractures or even macroscopic fissures. The findings provide valuable insights into the impact fatigue characteristics of rocks from a microscopic perspective and contribute to the evaluation of time-varying stability and the assessment of progressive damage in rock engineering. [ABSTRACT FROM AUTHOR]

Copyright of Bulletin of the Chinese Ceramic Society is the property of Bulletin of the Chinese Ceramic Society Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Coal Geology & Exploration is the property of Xian Research Institute of China Coal Research Institute and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Freeze–thaw cycles are frequently overlooked as a pivotal factor contributing to leakage and structural failures in clayey soil-impermeable barriers used in landfills or tailings repositories in regions subject to seasonal freezing. This investigation explores the recovery and residual strength properties of Jilin ball clay undergoing six freeze–thaw cycles, and assesses the pore structure characteristics through a series of nuclear magnetic resonance (NMR) tests. The results indicate that normal stress has a greater impact on peak recovery strength than dry density and rest periods. Cohesion increases earlier and more significantly during rest periods compared to internal friction angle. Although the pore diameter remains consistent within the micropores during the freeze–thaw cycles, the soil's structural integrity undergoes notable changes. The concluding analysis provides valuable insights for the construction and management of impermeable barriers in landfills or tailings repositories within seasonally frozen areas. [ABSTRACT FROM AUTHOR]

In this study, the pore structure and oil-water phase seepage characteristics of sandstone reservoirs of the Yanchang Formation in the Ordos Basin were deeply analyzed. Through scanning electron microscopy observation, oil-water phase percolation experiments and nuclear magnetic resonance (NMR) experiments on 30 core samples, the pore structure was categorized into uniform macroporous, striated, microfractured and dense types. The experimental results show that the uniform macroporous type pore structure shows the strongest seepage capacity, and its ratio of liquid-measured to gas-measured permeability is 0.724 on average, while the dense type pore structure has the weakest seepage capacity, and the ratio is 0.091 on average; NMR T2 spectral analysis reveals that the cores with different types of pore structure show significant characteristic differences in the water-driven oil drive process, especially in the substitution of oil in the macroporous and micrometer pore In particular, the oil substitution in large pores and micron pores is remarkable, that is, the oil-water seepage process and the response of seepage and suction of cores with different types of pore structure are obviously different, which is a better way to study the characteristics of oil-water seepage and distribution of oil and water; the uniform macroporous type and the striptype pore structure are more effective in oil-water two-phase seepage, and the microfracture type and the dense type are relatively poor. This paper provides a basis for the exploitation of tight reservoirs in the study area, which is of great theoretical and practical significance for optimizing the oilfield development strategy and improving the efficiency of oil and gas exploitation. [ABSTRACT FROM AUTHOR]

Pore structure characterization and fractal analysis have great significance for understanding and evaluating tight limestone reservoirs. In this work, the pore structure of tight limestone, low-temperature nitrogen adsorption (LTNA), and low-field nuclear magnetic resonance (NMR) are characterized, and the fractal dimension of the pore structure of tight limestone is discussed based on LTNA and NMR data. The results indicate that the pores of tight limestone have H3 and H4 types, the pore size distribution (PSD) of the H3 type is a wave distribution ranging from 2 to 10 nm, and the PSD of the H4 type is a unimodal distribution ranging from 2 to 10 nm. The transverse relaxation time (T2) spectrum of tight limestone shows a single peak (DF), double peak (SF), and triple peak (TF), and the ranges for the T2 spectra for micropores, mesopores, and macropores are 0.1 to 10 ms, 10 to 100 ms, and greater than 100 ms, respectively. The LTNA fractal dimension of tight limestone (DL) ranges between 2.4446 and 2.7688, with an average of 2.5729, and the NMR fractal dimensions of micropores (DNMR1), mesopores (DNMR2), and macropores (DNMR3) are distributed between 0.3744 and 1.1293, 2.4263 and 2.9395, and 2.6582 and 2.9989, respectively. Moreover, there is a negative correlation between DL and average pore radius, a positive correlation between DL and specific surface area, and a positive correlation between DNMR2 and DNMR3 and micropore content, while DNMR2 and DNMR3 are negatively correlated with the content of mesopores and macropores. [ABSTRACT FROM AUTHOR]

Controlling the extent of water invasion in the reservoir and mitigating its detrimental effects on gas well production and natural gas recovery have long been a challenging task in the efficient development of strongly heterogeneous edge water gas reservoirs. To elucidate the edge water invasion mechanism of strongly heterogeneous carbonate gas reservoirs, this study investigates the pore throat characteristics and fluid mobility from both qualitative and quantitative aspects, leveraging natural core observations, cast thin sections, scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR) tests with centrifuge experiments. A core-scale edge water invasion simulation experiment was conducted under online NMR monitoring to examine the dynamic gas production characteristics of the three types of reservoirs during the water invasion process and to elucidate the formation mechanism and distribution pattern of water-sealed gas. Research findings indicate that carbonate reservoirs typically exhibit a diverse range of pore types, including various types of fractures and cavities. Fractures significantly enhance reservoir connectivity, thereby increasing fluid mobility, but also lead to strong non-uniform water invasion. In contrast, cavities substantially improve the storage capacity of the reservoir and can retard the advancement of the water invasion front, thereby alleviating the adverse effects of water invasion. The ultimate recovery rates of fracture-type, cavity-type, and fracture-cavity cores in the water invasion simulation experiment were 29.81%, 64.87%, and 53.03%, respectively. Premature water breakthroughs in the reservoir can result in a large number of gases in matrix pores and even cavities being sealed by formation water, rendering them unrecoverable, which seriously impacts the gas recovery rate of the reservoir. [ABSTRACT FROM AUTHOR]

Copyright of Natural Gas Industry is the property of Natural Gas Industry Journal Agency and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Coal is a porous material containing pore structures and mineral components, exhibiting pronounced anisotropy and size effects. In order to investigate the influence of coal anisotropy and size effects on its failure characteristics, this paper proposes a simulation method for characterizing and reconstructing three-dimensionally the internal pores and mineral components of coal samples based on CT scanning, nuclear magnetic resonance, and X-ray diffraction. Specifically, we obtained simulation parameters of three-dimensional reconstruction models of coal matrix and mineral components through inverse laboratory uniaxial compression experiments, while simulated and analyzed the strength damage characteristics of coal bodies with different aspect ratios. The simulation results show that: ① During the loading process, the plastic zone first gradually expands and connects outward around the pores and mineral components. In terms of spatial distribution, the plastic zone expands vertically from the loading end to the interior in the early stage, and in the later stage, it expands horizontally from the surroundings to the interior. After the model is damaged, a "double truncated cone structure" is formed in the non-plastic zone. ② The increase of aspect ratio leads to an increase in the compressive strength(p) of coal samples, the strain(ζ) at yielding strength, and the elastic modulus(K), among which ζ and K increase linearly, while the margin of increase in p gradually decreases. ③ The total energy and elastic energy of coal sample loading increase exponentially, while the dissipated energy increases linearly. The increase of aspect ratio leads to an increase both in the accumulated elastic energy in the coal body and in the released energy during failure, which easily induce dynamic impact-related disasters. This study provide references for the reasonable selection of coal pillar size in impact mine pressure area. [ABSTRACT FROM AUTHOR]

The study of deep shale gas (>3500 m) has become a new research hotspot in the field of shale gas research in China. In this study, 16 representative deep shale samples were selected from different layers of the Wufeng–Longmaxi Formation in the Z-3 well in the western Chongqing area to conduct low-field nuclear magnetic resonance (NMR) tests, field-emission scanning electron microscopy (FE-SEM) observation, and fractal modeling. By comparing the differences in pore structure and their influencing factors in representative samples from different layers, the particularities of high-quality reservoirs have been revealed. The results show that the Z-3 well shales mainly develop micropores and mesopores, with pore sizes of 1 nm–200 nm. The fractal dimensions of bound fluid pores D1 (1.6895–2.3821) and fractal dimension of movable fluid pores D2 (2.9914–2.9996) were obtained from T2 spectra and linear fitting, and the pores were divided into three sections based on the NMR fractal characteristics. TOC content was one of the major factors affecting the gas content in the study area. The shale samples in the bottom S1l1-1 sub-layer with a higher TOC content have larger porosity and permeability, leading to enhanced homogeneity of the pore structure and favorable conditions for shale gas adsorption. A comparative understanding of the particularities of pore structure and influencing factors in high-quality reservoirs with higher gas content will provide the scientific basis for further exploration and exploitation of the Wufeng–Longmaxi Formation deep shale reservoirs in the western Chongqing area. [ABSTRACT FROM AUTHOR]

The marine shale within the Sichuan Basin constitutes China's significant shale gas production, featuring old formation age, high degree of thermal evolution, multiple tectonic movements, and complex structural conditions. However, there are significant differences in the shale gas preservation conditions and reservoir quality in different areas, limiting future large-scale exploration and development. Pore structure significantly influences shale reservoir quality, gas content, and exploration of shale gas occurrence, migration, and enrichment mechanisms. The influence of structural-dominated preservation conditions on shale pore structures is essential to comprehend for effective shale gas exploitation. This study employs field-emission scanning electron microscopy in conjunction with other techniques (low-temperature N2 adsorption, low-temperature CO2 adsorption, and nuclear magnetic resonance) for detailed analyses of the pore structure across varied structural zones, revealing the influence of structural attributes, fault systems, depth of burial, and formation pressure on pore architecture, and examining the relationship between pore structure and shale gas preservation conditions. The results show that stable structural condition is conducive to the development and preservation of shale pores. Structural compression causes inorganic and organic pores to become narrow and elongated due to shrinkage, with a significant increase in microfractures. The porosity of shale with stable structural conditions exhibits markedly increased porosity compared to samples under structural compressions. Under conditions of similar TOC and mineral composition, the pore size distribution (PSD), pore volume (PV), and specific surface area (SSA) of shale after structural compression are significantly lower than those of samples with stable structural conditions. As the burial depth increases, the shale porosity shows a decreasing trend, but the decrease is limited. Burial depth significantly impacts the SSA and PV of high-TOC samples (3%-6%). As the burial depth increases, both SSA and PV show a significant decreasing trend. When the burial depth reaches 4000 m, SSA and PV tend to concentrate. The formation pressure coefficient is an important factor for the development and preservation of shale pores, and porosity is positively correlated with the formation pressure coefficient. Increased formation pressure coefficient indicates superior preservation conditions and enhanced pore development. [ABSTRACT FROM AUTHOR]

With the increase of coal mining depth, coal seam has the characteristics of complex pore structure and low porosity under the influence of ground stress, which limits the effect of coal seam water injection. Alternating hydraulic intrusion is an effective method to improve the permeability of low permeability coal seam. By applying low water pressure circulation, it can force the pore structure of coal to be damaged, which is helpful to change its pore structure. However, there is limited research on the evolution of the multi-sacale pore structure in coal under alternating hydraulic pressure using low-field nuclear magnetic resonance (LF-NMR) technology. In this study, we investigate the evolution of coal pore structure under alternating hydraulic pressure using LF-NMR. The pore size distribution (PSD), porosity, fractal dimension, and sensitivity of pores to alternating hydraulic pressure were analyzed. The results indicate that the pore volume of coal exhibits an increasing trend initially and then decreases with the increase of cycle times. An appropriate number of alternating cycles contribute to the development of the coal pore structure. Fractal dimension analysis reveals that the pore fractal dimension decreases first and then increases during the test, indicating a significant effect of alternating hydraulic pressure on the development of pores. Analysis of the sensitivity of pores to alternating hydraulic pressure shows a decreasing trend overall with an increasing cycle time, although the rate of decrease gradually slows non-significant with continued cycling. Mechanism analysis suggests that during the alternating hydraulic intrusion process, the development of new pores and the compaction of pores occur simultaneously under the hydraulic pressure. With the continuous loading of alternating hydraulic pressure, the development rate of new pores gradually becomes lower than the compaction rate of pores, which leads to the phenomenon that the pore structure of coal expands first and then compacts. This study provides theoretical support for the application of alternating water injection in coal reservoir engineering. [ABSTRACT FROM AUTHOR]

Copyright of Bulletin of the Chinese Ceramic Society is the property of Bulletin of the Chinese Ceramic Society Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The Upper Permian Longtan Formation in the Laoshan Uplift of the South Yellow Sea Basin hosts thick and distinctive tight sandstones. However, a comprehensive understanding of its pore structure and reservoir classification remains lacking. This study investigates the fully cored well, CSDP-2, utilizing thin section analysis, scanning electron microscopy, energy spectrum analysis, X-ray diffraction, high-pressure mercury intrusion, and nuclear magnetic resonance to characterize its petrophysical properties, pore space, and movable fluid characteristics. Additionally, fractal principles are further employed to examine reservoir heterogeneity and conduct a quantitative assessment, considering the complexity of tight sandstone pore structures. The findings reveal that the sandstones predominantly comprise feldspathic litharenites, with an average porosity of 1.567% and permeability of 0.099 mD, primarily containing intragranular pores. Two distinct sets of pores with significantly different sizes (r < 2 μm; r > 6 μm) were identified, displaying relatively high fractal dimensions and discrete distribution. Movable fluids primarily occupy pores with radii > 0.019 μm, reflecting pronounced overall heterogeneity. The reservoir was classified into three categories utilizing permeability, median radius, and movable fluid saturation as key evaluation parameters, with Class I representing a relatively high-quality reservoir. These findings advance our understanding of the pore development mechanism of tight sandstone reservoirs and provide geological evidence for further hydrocarbon exploration in this study area. [ABSTRACT FROM AUTHOR]

Rare earth resources in the weathered crust elution-deposited rare earth ore (WCEDRE-ore) are mainly recovered by in situ leaching process. A new understanding of the engineering properties modification of WCEDRE-ore as well as the occurring microscopic mechanism was presented when leaching using ammonium sulfate agent occurred. The soil–water chemical reactions in leaching had a significant impact on engineering properties (such as permeability and strength) of WCEDRE-ore, which determined the success or failure of in situ leaching process. To reveal the internal processes and effects of ammonium sulfate leaching on WCEDRE-ore, indoor tests including leaching tests, chemical analyses, triaxial tests and nuclear magnetic resonance (NMR) tests were conducted. The results show that: (1) Under the action of ion exchange and seepage, the dispersion, deposition and clogging of fine particles occurred repeatedly and alternately along the seepage direction. (2) During the leaching process, the total porosity of the ore samples rarely changed, but the migration of fine particles leads to the transformation of micropores, mesopores and macropores in weathered-ore soil samples as well as the change of pore connectivity. As a result, the permeability coefficient of the tested soil samples decreased significantly at the beginning of test and then rose slightly. (3) Increased salt content during the leaching process enhanced soil cementation and structure until the end of ion exchange. However, owing to the loose pore structure in the upper part of sample and the dense pore structure in the lower part of the tested specimen generated by particle migration, the strength of soil sample was weakened, which was conducive to the occurrence of shear failure in the upper part of sample. [ABSTRACT FROM AUTHOR]

Ground Granulated Blast-Furnace Slag (GGBS) and silica fume (SF) are frequently utilized in gel materials to produce environmentally sustainable concrete. The blend of the two components contributes to an enhancement in the pore structure, which, in turn, increases the mechanical strength of the material and the compactness of the pore structure and decreases the permeability, thereby improving the durability of the concrete. In this study, the pore structures of GGBS and SF blends are assessed using Nuclear Magnetic Resonance (NMR) and Mercury Intrusion Porosimetry (MIP) tests. These methodologies provide a comprehensive evaluation of the effect of GGBS and SF on the pore structure of cementitious materials. Results showed that the addition of SF and GGBS reduces the amount of micro-capillary pores (10 < d < 100 nm) and the total pore volume. The results indicate that the transport properties are related to the pore structure. The incorporation of SF reduced the permeability of the concrete by an order of magnitude. The pore distribution and pore composition had a significant effect on the gas permeability. The difference in porosity obtained using the MIP and NMR tests was large due to differences in testing techniques. [ABSTRACT FROM AUTHOR]

Exploration and development effects of Lucaogou Formation laminated shale are not ideal in the Santanghu Basin, and the reasons are not clear. Therefore, in this study, laminated and massive shales were systematically sampled, and experiments of cryogenic adsorption of nitrogen and multi‐dimensional nuclear magnetic resonance comparison were carried out. The results show that Lucaogou Formation shale is rich in volcanic ash and low content of clay, which can be divided into tuff, dolomite, tuffaceous dolomite, dolomitic tuff, and other lithologic types. The laminar structure is developed in tuffaceous dolomite and dolomitic tuff. The pores of Lucaogou Formation shale with a diameter of more than 10 nm can be used as an effective storage space for movable oil. The oil saturation and mobile oil ratio of laminated shale are lower than that of massive shale. Movable oil is controlled by carbonate content, and the higher the content, the higher the proportion of movable oil. Millimetre laminae are rich in organic matter and have poor interlaminar pore connectivity, which leads to strong adsorption capacity for oil. Therefore, the migration of oil between laminae is blocked, and oil mainly distributes or migrates in the bedding plane between laminae in laminated shale, which cannot be used as favourable reservoirs. [ABSTRACT FROM AUTHOR]

The pore size distributions of an expansive soil under different simulated rainfall conditions were studied using nuclear magnetic resonance (NMR) techniques. Four sets of treatments with different rainfall intensities (light, moderate, heavy, and rainstorm) and durations (0.5 d, 1 d, 2 d, and 3 d) were designed to analyze the effects of rainfall on the microstructure of the expansive soil. Results show that with increasing rainfall duration, micropores gradually form and develop, enlarge, and interconnect in the soil, eventually forming stable seepage channels. Under light and moderate rain conditions, the proportion of micropores gradually decreases, while the proportions of mesopores and macropores gradually increase. Under heavy and rainstorm conditions, the proportion of micropores sharply decreases and then stabilizes, while the proportions of mesopores and macropores increase. With increasing rainfall intensity, the dominant pore size and porosity both initially increase and then stabilize. A quantitative relationship model between pore size, porosity, and rainfall conditions is established and the fitting effect is good. This study shows that rainfall alters the microstructure of expansive soils, which stabilizes after dynamic equilibrium. This provides a theoretical basis for predicting and controlling the engineering behavior of expansive soils under different rainfall conditions. [ABSTRACT FROM AUTHOR]

Copyright of Petroleum Geology & Oilfield Development in Daqing is the property of Editorial Department of Petroleum Geology & Oilfield Development in Daqing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

With the gradual depletion of natural sand due to over-exploitation, alternative building materials, such as manufactured sand aggregate (MSA), have attracted much attention. In order to interpret the evolution of pore structure and fractal characteristics in MSA mortar over long-term water saturation, the 1H low-field nuclear magnetic resonance (LF-NMR) relaxation method was used to investigate the temporal evolution of the pore structure in five single-graded MSA mortars and synthetic-graded mortars with small amplitudes in particle size. MSA presents a fresh rock interface characterized by a scarcity of pores, which significantly reduces the porosity of the mortar. The surface-to-volume ratio (SVR) is employed for characterizing the MSA gradation. Through an analysis of parameters, such as total porosity, pore gradation, pore connectivity, and pore fractal dimension of mortar, a correlation model between pore structure parameters and aggregate SVR is constructed. The fractal characteristics of pores and their variations are discussed under three kinds of pore gradations, and the correlation model between fractal dimension and porosity is established. These results demonstrate the high impermeability and outstanding corrosion resistance of synthetic-graded mortar. The fractal model of the pore structure evolution of MSA mortar has a guiding effect on the pore distribution evolution and engineering permeability evaluation of MSA mortar in engineering. [ABSTRACT FROM AUTHOR]

Experimental methods, including mercury pressure, nuclear magnetic resonance (NMR) and core (wateroil) displacement, are used to examine the effects of high-multiple water injection (i.e. water injection with high injected pore volume) on rock properties, pore structure and oil displacement efficiency of an oilfield in the western South China Sea. The results show an increase in the permeability of rocks along with particle migration, an increase in the pore volume and the average pore throat radius, and enhanced heterogeneity after high-multiple water injection. Compared with normal water injection methods, a high-multiple water injection is more effective in improving the oil displacement efficiency. The degree of recovery increases faster in the early stage due to the expansion of the swept area, and the transition from oil-wet to water-wet. The degree of recovery increases less in the late stage due to various factors, including the enhancement of heterogeneity in the rocks. Considering both the economic aspect and the production limit of water flooding, it is recommended to adopt other technologies to further enhance oil recovery after 300 PV water injection. [ABSTRACT FROM AUTHOR]

The macroscopic mechanical test of C40 aeolian sand concrete was carried out. Combined with scanning electron microscope (SEM) and nuclear magnetic resonance (NMR) technology, the internal relationship between pore structure and mechanical properties of aeolian sand concrete was discussed, and the influence mechanism was revealed. The regression analysis was used to establish the pore structure-strength model under two factors. The results show that with the increase of aeolian sand replacement rate, the microstructure of aeolian sand concrete is compact first and then loose, the porosity and harmless pore ratio decrease first and then increase, and the compressive strength increases first and then decreases. When the replacement rate is 20%, the strength is the highest. The appropriate amount of aeolian sand improves the aggregate gradation of concrete, refines the pores, reduces the initial damage of the loading surface, and enhances the macroscopic performance. The established pore structure-strength model with harmless pore ratio and harmless porosity as dependent variables can better predict the strength of concrete. [ABSTRACT FROM AUTHOR]

The Qinghai–Tibet Plateau is the main permafrost area in China. Concrete structures constructed on permafrost are affected by the early negative-temperature environment. In particular, the negative-temperature environment seriously affects the strength growth process and the frost resistance of concrete (FRC). Therefore, this study considered the influence of the gas content, water–binder ratio (w/b), age, and other factors on the strength variation law and FRC under −3 °C curing conditions. Nuclear magnetic resonance (NMR) was used to analyze the pore structure of concrete before and after freeze–thaw cycles (FTCs). The results showed that the compressive strength of the concrete (CSC) under −3 °C curing was only 57.8–86.4% of that cured under standard conditions. The CSC under −3 °C curing showed an obvious age-lag phenomenon. The FRC under −3 °C curing was much lower than that under standard curing. The porosity of the concrete under −3 °C curing was greater, with a higher percentage of harmful and multi-harmful pores than that under standard curing. The concrete properties deteriorated primarily because curing at −3 °C hindered the hydration reaction compared with standard methods. This hindrance resulted in diminished hydration development, weakening the concrete's structural integrity. Under both curing conditions, when the gas content was between 3.2% and 3.8%, the frost resistance was the best. This is because a gas content within this range effectively enhances the internal pore structure, therefore relieving the swelling pressure caused by FTCs. Based on the freeze–thaw damage (FTD) model proposed by previous authors, a new model for the CSC under −3 °C curing reaching that of the concrete under standard curing for 28 d was established in this study. This advanced model was capable of accurately assessing the FTD of concrete structures in permafrost regions. Finally, the life expectancy of concrete in Northwest China was predicted. The life of the concrete reached 46.9 years under standard curing, while the longest life of the concrete under −3 °C curing was only 12.9 years. Therefore, attention should be paid to constructing and curing concrete structures in cold environments. [ABSTRACT FROM AUTHOR]

Micro- and nano-scale pores develop in shale reservoirs, and the associated pore structure controls the occurrence state, gas content, seepage capacity, and micro-migration and accumulation mechanisms of shale gas. For this study, we mainly conducted tests, using field emission-scanning electron microscopy, of the isothermal methane adsorption of powder-sized samples under high temperatures (60–130 °C) and pressures (0–45 MPa), along with methane-saturated nuclear magnetic resonance tests of plug-sized samples under different temperatures (60–100 °C) and pressures (0–35 MPa). These samples were from Longmaxi shale cores from strata at different burial depths from the Zhaotong, Weiyuan, and Luzhou areas. As the burial depth increases, organic pores transform from complex networks to relatively isolated and circular pore-like structures, and the proportion of organic matter-hosted pores increases from 25.0% to 61.2%. The pore size is influenced by the pressure difference inside and outside the pores, as well as the surface tension of organic matter in situ. As the burial depth increases to 4200 m, the main peak of the pore size first increases from 5–30 nm to 200–400 nm and then decreases to 50–200 nm. This work establishes an NMR method of saturated methane on plug-sized samples to test the free gas content and develop a prediction model of shale reservoirs at different burial depths. The gas content of a shale reservoir is influenced by both burial depths and pore structure. When the burial depth of the shale gas reservoir is less than 2000 m, inorganic pores and microfractures develop, and the self-sealing ability of the reservoir in terms of retaining shale gas is weak, resulting in low gas content. However, due to the small pore size of organic pores and the low formation temperature, the content of adsorbed gas increases, accounting for up to 60%. As the burial depth increases, the free gas and total gas content increase; at 4500 m, the total gas content of shale reservoirs is 18.9 m3/t, and the proportion of free gas can be as high as 80%. The total gas content predicted by our method is consistent with the results of the pressure-holding coring technique, which is about twice our original understanding of gas content, greatly enhancing our confidence in the possibility of accelerating the exploration and development of deep shale gas. [ABSTRACT FROM AUTHOR]

Acid treatment has been proven to effectively reduce the fracture initiation pressure in reservoirs, making it a viable method for stimulating deep tight sandstone reservoirs. However, the underlying mechanism of how sandstone pore structure and mechanical properties vary with acid treatment time remains unknown. Understanding this mechanism is crucial for studying the fracture initiation and propagation in tight sandstone reservoirs. In this study, to quantitatively analyze the changes in pore structure and mechanical properties of sandstone samples treated with acid for different times (0 h, 1 h, 2 h, 4 h, 6 h, 12 h, 24 h, 48 h, 72 h, 120 h, and 168 h), nuclear magnetic resonance and uniaxial compression tests were employed to examine these changes. The results revealed that the number of mesopores and macropores increased rapidly after 6 h and 48 h of acid treatment, respectively. Additionally, the permeability showed a significant increase after 24 h of acid treatment. This can be attributed to the gradual dissolution of intergranular cement, leading to the formation of new pores that connect with the existing pore network. The peak strength of sandstone undergoes stepwise changes with acid treatment time, which can be divided into three stages: stage I (0–6 h), stage II (12–72 h), and stage III (120–168 h). Under the influence of acid treatment and uniaxial compression, the damage and damage change rate of sandstone treated with acid for 24 h reached the minimum. Moreover, a variation mechanism for sandstone after acid treatment was proposed. [ABSTRACT FROM AUTHOR]

Rock mass subjected to acidic groundwater corrosion may cause great property alteration. In this work, the effects of microstructure on change of the pore structure and mechanical property under acid corrosion were investigated. Two types of limestone, which are microstructurally different but have almost same mineral compositions, were corroded in HCl and Na2SO4 mixed solutions with different pH values (pH = 3, 4, 5, 6, and 7). A series of approaches including uniaxial compression, Brazilian splitting, and nuclear magnetic resonance (NMR) tests were carried out to examine the strength, brittle index, porosity, and pore distribution. The mechanical properties (uniaxial compressive strength, elastic modulus, tensile strength, and brittleness index) decreased as the pH value decreased. In addition, the larger the initial microstructure, the faster the degradation rate of mechanical parameters except for the brittleness index. Moreover, NMR results revealed that the porosity increment of Group 2 (large initial microstructure) was larger than that of Group 1 (small initial microstructure). Under acid corrosion, the reason for the increasing porosity in Group 1 was the increased proportion of the internanopore, while it was transition from the supernanopore to the submicropore for Group 2. Furthermore, the brittleness index decreased as porosity increased and the proportion of the internanopore decreased. Finally, the degradation mechanisms of pore and mechanical properties were discussed. [ABSTRACT FROM AUTHOR]

Geological sequestration of carbon dioxide (CO2) is one of the key methods to attain peak CO2 emissions and carbon neutrality in unworkable or abandoned coal seams. To study displacement characteristics and effect of geological sequestration of CO2 in water-saturated coal seams, the pore structure of coal samples from three different mines was first investigated by nuclear magnetic resonance technique. Then, the CO2 was injected into water-saturated coal sample, and helium was used as the control group. Next, the CO2-injected coal sample was sealed with water. Finally, water was returned by unloading pressure. The results showed that the pores of the three types of coals were mainly micropores and transition pores. Moreover, the fractal dimension of the pores was distributed in the range of 2.236–2.655. The displacement experiment found that the water displaced by CO2 and helium was mainly movable water in macropores, but there were significant differences in the displacement characteristics of the two. On the whole, the amount of water displaced by CO2 was more than that by helium, and the difference between the water volume displaced by CO2 and helium was reflected mainly in the adsorption of CO2 on the coal surface. CO2 occupied the adsorption sites on the coal matrix surface, which reduced the interaction between water and coal matrix surface and improved the displacement efficiency. In contrast, helium did not show adsorption capacity on the coal surface. In the water injection stage, water can only enter the larger pores due to the retention and adsorption of CO2 in the coal. The water saturation of the coal sample after the return drainage increased by 10.24% compared with that before water injection, indicating that the water injected into the coal sample was not discharged completely. This study provides an experimental basis for investigating the characteristics of pore structure in coal, CO2 displacement in water-saturated coal seams, and CO2 geological sequestration. [ABSTRACT FROM AUTHOR]

Copyright of Acta Sedimentologica Sinica is the property of Acta Sedimentologica Sinica Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Objective In recent years, new discoveries have been made in the He 1 Member of the Hangjinqi Gas Field. And the initial gas production of some wells by fracturing can reach 10×104 m3/d, which shows that the He 1 Member has great exploration potential. However, due to the strong heterogeneity of the He 1 Member, the gas production mechanism of the He 1 Member is not clear at present, which restricts its efficient development. To accurately and quantitatively characterize the microscopic pore structure and movable fluid characteristics of tight gas sandstone reservoirs. Methods In this paper, taking the tight sandstone reservoir in the He 1 Member of the Shangshihezi Formation in the Hangjinqi Gas Field as an example, NMR and CT tests were used to study the response characteristics and fluid identification ability of different types of pores in tight sandstone. Results The research shows that the porosity of the test samples is mainly distributed in 1.7%-10%, and the gas permeability is mainly distributed in 0.1×10-3-1.4×10-3 μm2, which belongs to the typical low-porosity and low-permeability porous tight sandstone reservoir. According to the T2 relaxation time curve of the saturation component before centrifugation, the pore type of the reservoir in the He 1 Member is bimodal (mainly the left peak, the right peak is not obvious), including 3 subtypes: micropore-small pore type, small pore-mesopore type, micropore-small pore-mesopore type. The T2 relaxation time intervals corresponding to the above three subtypes of pore types are 0.1-10 ms, 1-100 ms, and 0.1-100 ms, respectively. The results of 3D CT scans show that the micropore-mesopore reservoir has the best physical properties, followed by the micropore-small pore-mesopore type, and the micropore-small pore type reservoirs have relatively poor physical properties. The T2 cut-off values of the tested samples were mainly distributed between 1 and 14 ms, with an average value of 6.11 ms. There is a certain negative correlation between the T2 cut-off value and the movable fluid percentage of the rock samples. The movable fluid porosity and permeability have a very good positive correlation, reflecting that the amount of movable fluid is significantly affected by the reservoir permeability and the number of throats. The movable water saturation in the tight sandstone reservoirs of He 1 Member in the study area is mainly distributed at 4%-9%, with an average value of 5.8%. Conclusion Overall, the original movable water saturation of the He 1 Member is low and has great development potential.

Hydraulic fracturing technology significantly enhances the productivity of shale oil and gas reservoirs. Nonetheless, the infiltration of fracturing fluid into shale formations can detrimentally affect the microscopic pore structure, thereby impairing the efficacy of hydraulic stimulation. In this study, nuclear magnetic resonance (NMR) technology was utilized to conduct high-pressure soaking tests on shale specimens treated with EM30+ + guar gum mixed water and CNI nano variable-viscosity slickwater, where various concentrations of a drag reducer were utilized. Additionally, the differences in porosity, permeability, mineral composition, and iron ion concentration before and after the measurements were compared, which were used to analyze the influence on the shale's microscopic pore structure. It features a reduction in the total pore volume after the interaction with the fracturing fluid, with the pore-throat damage degree, porosity damage degree, and permeability damage degree ranging from 0.63% to 5.62%, 1.51% to 6.84%, and 4.17% to 19.61%, respectively. Notably, EM30+ + guar gum mixed water exhibits heightened adsorption retention, alkaline dissolution, and precipitation compared to CNI nano variable-viscosity slickwater, rendering it more deleterious to shale. Moreover, higher concentrations of drag reducers, such as EM30+ or CNI-B, predominantly result in damage to the shale's micropores. Shale compositions characterized by lower content of quartz and elevated proportions of clay minerals and iron-bearing minerals showcase augmented mineral dissolution and precipitation, consequently intensifying the shale damage. The hydration expansion of mixed-layer illite/smectite profoundly diminishes the core permeability. Consequently, the mechanisms underpinning the damage inflicted on shale's microscopic pore structure primarily involve fracturing fluid adsorption and retention, mineral dissolution, and precipitation, such as clay minerals and iron-containing minerals. [ABSTRACT FROM AUTHOR]

The Xihu Sag in the East China Sea Basin is located at the edge of the eastern Chinese continent and has great exploration potential. In recent years, the development of low-permeability and tight sandstone gas has become an important area of exploration and development in the Huagang Formation (E3h) of the Xihu Sag. The tight sandstone reservoir in the Xihu Sag is characterized by serious heterogeneity, high water saturation, low resistivity, and a complex gas–water relationship. Because of these characteristics of tight sandstone reservoirs, it is difficult to perform an evaluation of them. In this work, a bimodal Gaussian density function was constructed using the data of high-pressure mercury intrusion (HPMI) and nuclear magnetic resonance (NMR); this approach was used to analyze the pore structure parameters. The reservoirs were divided into four types using the fitting parameter η, and the rock electric parameters that correspond to different pore structures were quite different. When combined with the log response equation of η with acoustic interval transit time (AC), density (DEN), and natural gamma (GR) logging curves, an evaluation method of gas-bearing properties that was based on the characteristics of the pore structure was established. Compared with the water saturation test of the sealing core, it was found that the water saturation calculated by the classification of the pore structure was more accurate than that obtained by the conventional method, and the error was less than 8.35%, which proves that this method is feasible and effective. The findings of this study can help provide a better understanding of the distribution characteristics of gas and water in tight sandstone and provide help for tight gas exploration and development. [ABSTRACT FROM AUTHOR]

The study of deep soil mechanics is the basis of deep shaft construction. Exploring the nonlinear permeability mechanism of deep confined aquifers in depth is the prerequisite and foundation for carrying out calculations of the hydrophobic consolidation settlement of thick alluviums and preventing and controlling deep-well-damage disasters. Against the background of shaft damage caused by hydrophobic consolidation settlement of the bottom aquifer of thick alluviums, a joint HPLTC-HPPNP (high-pressure long-term consolidation and high-pore-pressure nonlinear permeability) test was carried out on the bottom aquifer of thick alluviums based on the ETAS test system. This paper studied the evolution law of the permeability coefficient ( k v ) of bottom aquifers under different heads of confined water, confining pressures ( σ r ), permeability hydraulic gradients (i) and loading–unloading methods. The internal pore structure characteristics of clayey sand were obtained by using low-field nuclear magnetic resonance (NMR) technology to explore the clayey sand's nonlinear permeability micro-mechanism. The research results showed that the bottom aquifer seepage volume ( Δ Q i ) under high stress is affected by the head pressure difference and pore water dissipation, and k v decreases with an increasing σ r according to the power function relationship. The influence of the hydraulic gradient (i) on k v is significantly influenced by σ r . When σ r   < 4 MPa, k v decreased with an increasing i , and when σ r   > 4 MPa, k v increased with an increasing i first, then decreased, before then tending to be stable. Under different stress states, the T2 spectrum of clayey sand showed a bispectrum peak type, and the adsorbed water content decreased linearly with an increasing σ r , while the capillary water decreased according to the power function. The content of capillary water in the permeable pores plays a key role in the permeability of clayey sand, and it has a power function relationship with σ r . The research results of this paper provide a good experimental method for the study of deep soil permeability characteristics and parameter determination, provide a theoretical basis for deep alluvial hydrophobic consolidation and settlement, and further make up for the shortcomings of existing deep soil mechanics in permeability characteristics. [ABSTRACT FROM AUTHOR]