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In-situ thermal upgrading is used to tune the pore system in low-maturity oil shales. We introduce fractal dimension (D), form factor (ff) and stochastic entropy (H) to quantify the heating-induced evolution of pore morphological complexity and azimuthal disorder and develop a model to estimate the impact on seepage capacity via permeability. Experiments are conducted under recreated in-situ temperatures and consider anisotropic properties-both parallel and perpendicular to bedding. Results indicate that azimuthal distribution of pores in the bedding-parallel direction are dispersed, while those in the bedding-perpendicular direction are concentrated. D values indicate that higher temperatures reduce the uniformity of the pore size distribution (PSD) in the bedding-parallel direction but narrow the PSD in the bedding-perpendicular direction. The greater ff (> 0.7) values in the bedding-parallel direction account for a large proportion, while the dominated in the bedding-perpendicular direction locates within 0.2e0.7, for all temperatures. The H value of the bedding-parallel sample remains stable at ~0.925 during heating, but gradually increases from 0.808 at 25 ℃ to 0.879 at 500 ℃ for the beddingperpendicular sample. Congruent with a mechanistic model, the permeability at 500 ℃ is elevated ~1.83 times (bedding-parallel) and ~6.08 times (bedding-perpendicular) relative to that at 25 -Cdconfirming the effectiveness of thermal treatment in potentially enhancing production from lowmaturity oil shales. [ABSTRACT FROM AUTHOR]

Shale gas is a prospective cleaner energy resource and the exploration and development of shale gas has made breakthroughs in many countries. Structure deformation is one of the main controlling factors of shale gas accumulation and enrichment in complex tectonic areas in southern China. In order to estimate the shale gas capacity of structurally deformed shale reservoirs, it is necessary to understand the systematic evolution of organic pores in the process of structural deformation. In particular, as the main storage space of high-over-mature marine shale reservoirs, the organic matter pore system directly affects the occurrence and migration of shale gas; however, there is a lack of systematic research on the fractal characteristics and deformation mechanism of organic pores under the background of different tectonic stresses. Therefore, to clarify the above issues, modular automated processing system (MAPS) scanning, low-pressure gas adsorption, quantitative evaluation of minerals by scanning (QEMSCAN), and focused ion beam scanning electron microscopy (FIB-SEM) were performed and interpreted with fractal and morphology analyses to investigate the deformation mechanisms and structure of organic pores from different tectonic units in Silurian Longmaxi shale. Results showed that in stress concentration areas such as around veins or high-angle fractures, the organic pore length-width ratio and the fractal dimension are higher, indicating that the pore is more obviously modified by stress. Under different tectonic backgrounds, the shale reservoir in Weiyuan suffered severe denudation and stronger tectonic compression during burial, which means that the organic pores are dominated by long strip pores and slit-shaped pores with high fractal dimension, while the pressure coefficient in Luzhou is high and the structural compression is weak, resulting in suborbicular pores and ink bottle pores with low fractal dimension. The porosity and permeability of different forms of organic pores are also obviously different; the connectivity of honeycomb pores with the smallest fractal dimension is the worst, that of suborbicular organic pores is medium, and that of long strip organic pores with the highest fractal dimension is the best. This study provides more mechanism discussion and case analysis for the microscopic heterogeneity of organic pores in shale reservoirs and also provides a new analysis perspective for the mechanism of shale gas productivity differences in different stress–strain environments. [ABSTRACT FROM AUTHOR]

Mercury intrusion porosimetry (MIP) is widely used for coal pore structure characterization, however, the matrix compressibility (MC) can lead to overestimated measurement results. Determination of MC is crucial for revealing the influence of pore structure on coalbed methane (CBM) flow behavior. In this study, MIP and low temperature N2 adsorption (LT-N2A) were conducted on 15 coal samples from major coal-producing regions in Northern China. The MIP data were corrected using MC theory, and the effects of coal rank and pore structure on coal MC were analyzed. The influence of MC on fractal dimension was elucidated, and the sensitivity of three fractal models to MC was effectively evaluated. Finally, the impact of MC on the coalbed methane (CBM) exploitation was discussed. The results show that low-rank coals have higher MC than medium/high-rank coals, and the MC coefficient follows a cubic polynomial relationship with coal rank, with two inflection points located at 1.4–2.5%, respectively. Micropores and transition pores are the main contributors to MC, for corrected data, the pore volume of both types of pores decreases significantly. The corrected pore size distribution exhibits better agreement with the LT-N2A measurement results, particularly in peak position and size for pores between 5 and 50 nm. This suggests the potential of corrected MIP data to supersede the combined use of MIP and LT-N2A data. MC can lead to overestimation of the fractal dimension, with the thermodynamic model showing the lowest sensitivity to MC. After the microfractures in medium/high-rank coal are greatly compressed, the compressional deformation of micropores and transition pores begins to have a significant impact on the CBM transport. The research results are of great significance for deeply understanding the mechanism of CBM transport. [ABSTRACT FROM AUTHOR]

The pore structure of coal plays a key role in the effectiveness of gas extraction. Conventional hydraulic fracturing techniques have limited success in modifying the pore structure using clean fracturing fluid (CFF), and the stimulating effects of ultrasonic can enhance the effectiveness of CFF in modifying coal pore structures. To research the effects of ultrasonic stimulation on the pore structure of medium to high-rank coal when using CFF, this study employed mercury intrusion porosimetry (MIP) and low-temperature nitrogen adsorption (LT-N2A) methods to analyze the changes in pore structures after cooperative modification. The results indicate that the pore volume and surface area of medium to high rank coal exhibit an increase and followed by a decrease with increasing Ro,max values, while the average pore diameter and permeability demonstrate a decrease and followed by an increase with Ro,max. Although there are some variations in the results of MIP and LT-N2A analysis for different pore size ranges, the overall findings suggest that ultrasonic stimulation in conjunction with CFF effectively alters the coal pore structure. The most significant improvement was observed in coking coal, where pore volume increased by 22%, pore area decreased by 11% and tortuosity decreased by 47%. The improvement of lean coal is the smallest, the pore volume increases by about 7%, and the surface area decreases by about 14%. It is found that the modification of coal pore volume is mainly concentrated in transition pores and macropores. These research outcomes provide valuable insights into the application of ultrasonic technology in coalbed gas extraction. [ABSTRACT FROM AUTHOR]

The hydration reaction mechanism of geopolymers is intimately linked to their composition and structure. Additionally, pore structure characteristics play a vital role in the properties of hardened geopolymer pastes, significantly influencing the material's strength, impermeability, and thermal conductivity. In this study, fly ash geopolymer (FAG) was synthesized by utilizing NaOH, Na2SiO3 , and low-calcium fly ash. The pore structure characteristics of FAGs were analyzed using mercury intrusion porosimetry (MIP), and the fractal dimension of FAGs was calculated using the Menger sponge model (Df) and a fractal model based on thermodynamic relationships (Ds). Fourier infrared spectroscopy (FTIR), thermogravimetric differential thermal (TG/DTA), and scanning electron microscopy secondary electron phase (SEM-SE) were used to test the composition of the hydration products of FAGs and their microscopic morphology. The results showed that after 90 days of maintaining ambient temperature, the porosity of FAG is between 20% and 30%, and the most available pore size is 10–50 nm. The fractal dimension calculated using the analytical model based on thermodynamic relations can more comprehensively determine the pore structure characteristics of FAG. The geopolymerization reaction process of fly ash particles can be categorized into four primary processes, forming an amorphous aluminosilicate gel as the hydration product. [ABSTRACT FROM AUTHOR]

Abstract The pore structure of coal plays a key role in the effectiveness of gas extraction. Conventional hydraulic fracturing techniques have limited success in modifying the pore structure using clean fracturing fluid (CFF), and the stimulating effects of ultrasonic can enhance the effectiveness of CFF in modifying coal pore structures. To research the effects of ultrasonic stimulation on the pore structure of medium to high-rank coal when using CFF, this study employed mercury intrusion porosimetry (MIP) and low-temperature nitrogen adsorption (LT-N2A) methods to analyze the changes in pore structures after cooperative modification. The results indicate that the pore volume and surface area of medium to high rank coal exhibit an increase and followed by a decrease with increasing R o,max values, while the average pore diameter and permeability demonstrate a decrease and followed by an increase with R o,max . Although there are some variations in the results of MIP and LT-N2A analysis for different pore size ranges, the overall findings suggest that ultrasonic stimulation in conjunction with CFF effectively alters the coal pore structure. The most significant improvement was observed in coking coal, where pore volume increased by 22%, pore area decreased by 11% and tortuosity decreased by 47%. The improvement of lean coal is the smallest, the pore volume increases by about 7%, and the surface area decreases by about 14%. It is found that the modification of coal pore volume is mainly concentrated in transition pores and macropores. These research outcomes provide valuable insights into the application of ultrasonic technology in coalbed gas extraction.

Abstract To quantitatively characterize the complexity of shale pore structures and their controlling factors in the Longmaxi Formation of Western Hubei, our study focused on the organic‐rich shale outcrops of the Longmaxi Formation in the Yidu‐Hefeng compound anticline. We conducted tests for shale organic content, maturity, and whole‐rock mineral composition, along with employing high‐pressure mercury injection and low‐temperature gas adsorption experiments. Utilizing the V‐S, FHH, and sponge models, we calculated the fractal dimensions of micro‐, meso‐, and macropores. In the Yidu‐Hefeng region, the Longmaxi Formation is characterized by calcium‐rich shales that are abundant in organic matter. Our analysis of samples revealed a total organic carbon (TOC) ranging between 1.04% and 4.24%, with an average of 2.5%. The Ro values fluctuate between 2.98% and 3.57%, with a mean value of 2.845%, indicating an over‐mature stage from early to late thermogenesis. Constituents such as quartz span from 39.8% to 51.3%, with a median of 44.3%, while feldspar oscillates between 3.8% and 12.4%, averaging at 8.48%. Clay minerals constitute 24.3% to 41.7% of the samples, with a mean of 34.16%. Shale porosity exhibits a segmented fractal nature. For instance, D1 varies from 2.1278 to 2.4056, with a mean of 2.2767; D2 fluctuates between 2.4995 and 2.7492, averaging at 2.6309; and D3 ranges from 2.6835 to 2.9427, centering around 2.8111. These variations indicate the intricacies of the macropore structure. Positive correlations between TOC and maturity with D1 and D2 are evident, whereas a negative association is observed with D3. The collaborative interplay between siliceous minerals and organics mirrors the relationship between the siliceous mineral content and its fractal dimensions, akin to TOC. Clay mineral transformations, due to accumulation and dehydration, predominantly contribute to macro‐porosity, weakly aligning negatively with D1 and D2 but positively with D3. Variations in carbonate and siliceous minerals and their role in primarily yielding dissolution macropores manifest a subtle negative link with D1 and D2 while enhancing D3. Pore volume correlates positively with D1 and D2, exhibits no conspicuous association with D3, and trends negatively. The compaction and transformation processes of clay minerals seem to favor the generation of macropores, mildly aligning positively with D3.

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]

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The characteristics of micro-pore structure are the focus of reservoir research, which is of great significance to the study of micro-heterogeneity and the exploration and development of the reservoir in the later stages. Taking the Chang 6 tight sandstone reservoir in the Yan'an-Ganquan area in the southern Ordos Basin as an example, through casting thin sections, scanning electron microscope analysis, X-ray diffraction and high-pressure mercury intrusion experiments, combined with fractal theory, the fractal characteristics of the target reservoir were determined, and the relationship between fractal dimension and reservoir physical properties, pore characteristics and mineral composition was discussed. The results show that the reservoir pores of the Chang 6 member in the study area are mainly intergranular pores and dissolution pores, which are ultra-low porosity and tight reservoirs. The fractal dimension of the reservoirs is generally greater than 2. 5, the pore structure is complex, and the sorting property is poor, strong homogeneity. The fractal dimension has a good correlation with the pore-throat characteristic parameters. The larger the fractal dimension, the uneven distribution of pores, poor connectivity, and weaker storage capacity and seepage capacity. The relationship between fractal dimension and mineral composition there is also a good correlation between them. Mineral composition and its content are the influencing factors that determine the complexity of pore throats, which in turn reflect the quality and pore structure characteristics of the reservoir. [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]

To quantitatively characterize the complexity of shale pore structures and their controlling factors in the Longmaxi Formation of Western Hubei, our study focused on the organic‐rich shale outcrops of the Longmaxi Formation in the Yidu‐Hefeng compound anticline. We conducted tests for shale organic content, maturity, and whole‐rock mineral composition, along with employing high‐pressure mercury injection and low‐temperature gas adsorption experiments. Utilizing the V‐S, FHH, and sponge models, we calculated the fractal dimensions of micro‐, meso‐, and macropores. In the Yidu‐Hefeng region, the Longmaxi Formation is characterized by calcium‐rich shales that are abundant in organic matter. Our analysis of samples revealed a total organic carbon (TOC) ranging between 1.04% and 4.24%, with an average of 2.5%. The Ro values fluctuate between 2.98% and 3.57%, with a mean value of 2.845%, indicating an over‐mature stage from early to late thermogenesis. Constituents such as quartz span from 39.8% to 51.3%, with a median of 44.3%, while feldspar oscillates between 3.8% and 12.4%, averaging at 8.48%. Clay minerals constitute 24.3% to 41.7% of the samples, with a mean of 34.16%. Shale porosity exhibits a segmented fractal nature. For instance, D1 varies from 2.1278 to 2.4056, with a mean of 2.2767; D2 fluctuates between 2.4995 and 2.7492, averaging at 2.6309; and D3 ranges from 2.6835 to 2.9427, centering around 2.8111. These variations indicate the intricacies of the macropore structure. Positive correlations between TOC and maturity with D1 and D2 are evident, whereas a negative association is observed with D3. The collaborative interplay between siliceous minerals and organics mirrors the relationship between the siliceous mineral content and its fractal dimensions, akin to TOC. Clay mineral transformations, due to accumulation and dehydration, predominantly contribute to macro‐porosity, weakly aligning negatively with D1 and D2 but positively with D3. Variations in carbonate and siliceous minerals and their role in primarily yielding dissolution macropores manifest a subtle negative link with D1 and D2 while enhancing D3. Pore volume correlates positively with D1 and D2, exhibits no conspicuous association with D3, and trends negatively. The compaction and transformation processes of clay minerals seem to favor the generation of macropores, mildly aligning positively with D3. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Engineering Geology / Gongcheng Dizhi Xuebao is the property of Journal of Engineering Geology 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.)

Foam concrete is characterized by lightweight, self-compacting and high flowability, thereby widely used as a subgrade bed filler. High-speed railway subgrades usually experience inhomogeneous deformation due to the occurrence of freezing-thawing cycles in seasonally frozen soil areas. It is essential to study the deformation behavior of foam concrete under the coupling effect of freezing-thawing cycles and dynamic loading. In this paper, dynamic triaxial tests were performed to study the accumulative deformation of the foam concrete under different numbers of freezing-thawing cycles, freezing temperatures, amplitudes and frequencies of dynamic loading. Based on the scanning electron microscopy (SEM) tests, the characteristics of the pore structure were analyzed quantitatively by introducing the directional distribution frequency and fractal dimension. The research results illustrate that the damage caused by freezing-thawing progress to the pore structure results in more significant deformation of the foam concrete subjected to dynamic loading. There exists an accumulative damage effect induced by the coupling action of long-term dynamic loading and freezing-thawing progress on the microstructure and mechanical properties of foam concrete. The development of the fractal dimension agrees with that of the accumulative strain, indicating a close connection between the microstructure and the dynamic behavior of foam concrete. The findings concluded in this study contribute to a sufficient understanding of the performance of foam concrete used as high-speed railway subgrade fillers subjected to seasonal freezing. [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.)

Copyright of Natural Gas Geoscience is the property of Natural Gas Geoscience 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.)

Taking the Longmaxi deep-marine shale gas reservoir in Zigong region as the research target, this paper aimed to characterize the nano-scale pore structure and investigate the reservoirs' heterogeneity based on fractal theory. By conducting a series of experimental studies, mainly including TOC, XRD, gas adsorption (N2 and CH4), we were able to clarify the main controlling factors for the heterogeneity of deep shale pore structure. Our results indicated that the deep marine shale possessed a significant amount of organic matter, as the average TOC value is 3.68%. The XRD analysis results show that quartz and clay were the main mineral types, and the total content of these two minerals averaged 77.5%. Positive correlations were observed between TOC and quartz, while TOC decreases as the clay mineral increases, this discovery indicating that quartz is biogenic. Based on FHH (Frenkele-Halseye-Hill) method, by using the LTNA adsorption isotherms, we took relative pressure P/P0 = 0.5 as the boundary, then two separate fractal dimension were deduced, D1 and D2 represent the fractal characteristics of small and large pores, respectively. Our study revealed that both D1 and D2 demonstrated positive correlations with N2 adsorption pore volume and adsorption specific surface area, while negatively correlated with the adsorption average pore diameter. Moreover, the two fractal dimensions showed positive associations with TOC and quartz and negative associations with clay. Additionally, D1 and D2 also demonstrated a positive correlation with Langmuir volume. The presence of micropores was found to significantly influence the formation of an irregular pore structure in shale. As the pore size decreased, the adsorption specific surface area increased, resulting in a more intricate pore structure, and the fractal dimension of the pores elevated, ultimately. This intricate structure is beneficial for the accumulation of shale gas. These research findings offer valuable insights for the comprehensive assessment of deep shale gas, and enrich our knowledge of enrichment mechanisms in deep shale gas reservoirs. [ABSTRACT FROM AUTHOR]

In order to study the influence of low temperature liquid nitrogen immersion treatment on the pore structure and fractal characteristics of middle grade coal in Huainan Mining Area, coal samples were treated with different liquid nitrogen immersion time, and the pore structure of coal was measured by mercury injection and liquid nitrogen adsorption method. The fractal theory was combined to analyze the development law and scale characteristics of coal pores under different immersion time from multiple angles. The results show that with the increase of liquid nitrogen immersion time, the total pore volume of coal increases from 198.089×10−3 cm3/g to 371.553×10−3 cm3/g, while the total specific surface area decreases from 4.984 m2/g to 4.496 m2/g, and the effect is significant. The adsorption pore decreases, the seepage pore increases, and the pore connectivity of the adsorption pore increases to form a larger level of pore, which gradually changes to the seepage pore. There was a negative linear correlation between D1 (fractal dimension of seepage pore) and D2 (fractal dimension of adsorption pore) and the immersion time of liquid nitrogen, and the influence of liquid nitrogen immersion on D1 was more significant than that of D2. D1 and D2 decreased gradually with the increase of liquid nitrogen immersion time, indicating that the structural complexity of internal pores decreased with the increase of liquid nitrogen immersion time, and the porosity and permeability of coal increased. Dc (comprehensive fractal dimension) decreases with the increase of average pore diameter and total pore volume of coal body, and increases with the increase of total specific surface area. With the increase of liquid nitrogen immersion time, the comprehensive fractal dimension of coal body decreases, the adsorption capacity of coal body is weakened, and the seepage capacity is enhanced, which is helpful to improve the coal-bed methane extraction effect of low permeability coal seam in Huainan Mining Area.

The characteristics of shale reservoirs play a crucial role in the enrichment and exploitation of shale gas. The Upper Permian Dalong Formation in the northern Sichuan Basin is a significant marine-derived high-quality hydrocarbon source rock that requires further detailed investigation. This study conducted qualitative and quantitative analyses on the pore structure of deep shale reservoirs within the Permian Dalong Formation in northeastern Sichuan. Various methods such as high-resolution field emission scanning electron microscopy, CO2 adsorption, N2 adsorption, and high-pressure Hg injection measurements were employed. Fractal fitting using the V-S model based on CO2 adsorption, the FHH model based on N2 adsorption, and the fractal geometry model based on high-pressure Hg injection was utilized to characterize the pore structure of deep shale. The findings revealed that deep shale reservoirs in the Dalong Formation in northeastern Sichuan Basin contain abundant nano-scale organic pores and limited inorganic pores. The development of organic pores exhibits strong heterogeneity influenced by maceral differences and organic matter distribution. Similar to Longmaxi Formation shale, the pores in the Dalong Formation are predominantly micropores and mesopores, comprising over 90% of the total pore volume and primarily influenced by organic matter abundance. The research on fractal dimension characteristics suggests that macropores in deep shale exhibit stronger heterogeneity compared to mesopores and micropores. Due to their smaller size and minimal diagenetic impact, micropores and mesopores have simpler structures with lower fractal dimensions. On the other hand, macropores display significant heterogeneity due to their larger size, wide distribution range, diverse genesis, and susceptibility to diagenesis. Due to their abundant reservoir space and strong self-similarity, micropores and mesopores in deep shale are conducive to the occurrence, seepage and exploitation of shale gas.

This study aims to investigate the influence of corn cob particles on pore structure of mortar. By using NMR technology, the pore parameters of fast-hardening sulfoaluminate cement mortar (SAC) and ordinary Portland cement mortar (PO) were measured and compared. The influence of corncob particles on the mechanical properties of eco-mortar was evaluated by uniaxial compression test. The results indicate that medium and large pores of both SAC and PO increases with corn cob particle, as well as the porosity. The fractal dimensions of harmful and multiple harmful pores are negatively correlated with the volume fraction of corn cob particles. The addition of corn cob particles reduced the overall fractal dimension of the mortar and decreased the complexity of internal pores. The probability density function of grayscale values transitions from being “thin and tall” to “short and wide”, with an increasing proportion of high grayscale values. This leads to an increase in the proportion of large pores within the mortar, resulting in a continuous deterioration of pore structure. The addition of corn cob particles greatly reduces the strength of SAC and PO, but the peak stress of SAC is higher than that of PO. When corn cob particle content is 30 wt% and 50 wt%, the peak stress of SAC is increased by 23.37% and 14.25%, respectively, compared with the peak stress of PO.

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]

This paper examines sandstones of varying particle sizes within the coal-bearing strata in the Fugu area of northern Shaanxi. Employing high-temperature heat treatment, the study performs tests at different temperatures (25 ~ 600 ℃) on rock samples under both oxygen and nitrogen environments. Utilizing nitrogen adsorption tests combined with fractal dimension analysis, we investigate the pore structure and fractal characteristics of sandstones with varying particle sizes in coal-bearing formations. The results indicate that the total pore volume (aperture range 0.35 ~ 500nm), specific surface area, and average pore diameter of sandstone increase with temperature. Moreover, smaller particles result in more pronounced changes in the sandstone's pore structure. At the same time, in comparison to the oxygen environment, sandstone treated in a nitrogen environment undergoes multiple occurrences of molecular layer nitrogen adsorption, leading to a higher degree of pore damage. In addition, the fractal dimension of sandstone pores decreases with rising temperature. Under high-temperature conditions, the pores undergo a transformation from the initial irregular crack structure to a homogenized pore structure. The weakening of non-uniformity contributes to the decrease in fractal dimension. [ABSTRACT FROM AUTHOR]

Marine shales from the Niutitang Formation and marine–continental transitional shales from the Longtan Formation are two sets of extremely important hydrocarbon source rocks in South China. In order to quantitatively compare the pore complexity characteristics between marine and marine–continental transitional shales, the shale and kerogen of the Niutitang Formation and the Longtan Formation are taken as our research subjects. Based on organic petrology, geochemistry, and low-temperature gas adsorption analyses, the fractal dimension of their pores is calculated by the Frenkel–Halsey–Hill (FHH) and Sierpinski models, and the influences of total organic carbon (TOC), vitrinite reflectance (Ro), and mineral composition on the pore fractals of the shale and kerogen are discussed. Our results show the following: (1) Marine shale predominantly has wedge-shaped and slit pores, while marine–continental transitional shale has inkpot-shaped and slit pores. (2) Cylindrical pores are common in organic matter of both shale types, with marine shale having a greater gas storage space (CRV) from organic matter pores, while marine–continental transitional shale relies more on inorganic pores, especially interlayer clay mineral pores, for gas storage due to their large specific surface area and high adsorption capacity (CRA). (3) The fractal characteristics of marine and marine–continental transitional shale pores are influenced differently. In marine shale, TOC positively correlates with fractal dimensions, while in marine–continental shale, Ro and clay minerals have a stronger influence. Ro is the primary factor affecting organic matter pore complexity. (4) Our two pore fractal models show that the complexity of the shale in the Longtan Formation surpasses that of the shale in the Niutitang Formation, and type I kerogen has more complex organic matter pores than type III, aiding in evaluating pore connectivity and flow effectiveness in shale reservoirs. [ABSTRACT FROM AUTHOR]

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A systematic study of the pore structure characteristics of Lower Cambrian shales in the southeastern Upper Yangtze Platform, was conducted using organic geochemistry, mineralogy, nitrogen adsorption, physical property analysis, and scanning electron microscopy. The results indicate that: 1) The Total organic carbon (TOC) content shows a strong correlation with quartz and clay minerals. Shales with low TOC content and rich in clay minerals primarily exhibit slit-shaped and narrow slit-like inter-clay particle pores with pore size distribution is dominated by mesopores and macropores. Shales with high TOC content predominantly feature narrow slit-like and ink bottle-shaped pores with pore size distribution dominated by micropores and mesopores. 2) Shale pore structures vary significantly under different gas content and preservation conditions. Shales under favorable preservation conditions exhibit a relatively "high porosity, low permeability, and high gas content" pattern, with well-developed organic pores and a strong pore-permeability correlation. In contrast, shales under unfavorable preservation conditions appear dense, with excessively developed fractures increasing both average pore size and local permeability. The pore-permeability correlation is weak, presenting a relatively "low porosity, high permeability, and low gas content" pattern. 3) TOC content plays a crucial role in controlling pore structure, showing overall positive correlations with pore volume, specific surface area, and porosity, and negative correlations with pore size. High TOC content enhances shale plasticity, resulting in lower pore diameters. Factors such as compaction and unfavorable preservation conditions lead to the shrinkage, collapse, and closure of some narrow pore throats, negatively impacting pore volume, specific surface area, brittleness, and fractal dimension, exhibiting a negative correlation with TOC content. 4) The pore structure of Lower Cambrian shales is complex, with fractal dimensions D1 and D2 exhibiting negative correlations with average pore size and positive correlations with TOC, specific surface area, and total pore volume. A high D1 value indicates well-preserved nanoscale pore surface structures with low complexity, suggesting minimal alteration by external fluids and better shale gas preservation. D1 serves as an indicator for shale gas content and preservation conditions. D2 shows better correlations with various pore structure parameters, making it suitable for characterizing pore structures. [ABSTRACT FROM AUTHOR]

The tight tuff reservoir is an unusual type of unconventional reservoir with strong heterogeneity. However, there is a lack of research on the microscopic pore structure that causes the heterogeneity of tuff reservoirs. Using the Chang 7 Formation in Ordos Basin, China as a case study, carbon-dioxide gas adsorption, nitrogen gas adsorption and high-pressure mercury injection are integrated to investigate the multi-scale pore structure characteristics of tuff reservoirs. Meanwhile, the fractal dimension is introduced to characterize the complexity of pore structure in tuff reservoirs. By this multi-experimental method, the quantitative characterizations of the full-range pore size distribution of four tuff types were obtained and compared in the size ranges of micropores, mesopores and macropores. Fractal dimension curves derived from full-range pores are divided into six segments as D1, D2, D3, D4, D5 and D6 corresponding to fractal characteristics of micropores, smaller mesopores, larger mesopores, smaller macropores, medium macropores and larger macropores, respectively. The macropore volume, average macropore radius and fractal dimension D5 significantly control petrophysical properties. The larger macropore volume, average macropore radius and D5 correspond to favorable pore structure and good reservoir quality, which provides new indexes for the tuff reservoir evaluation. This study enriches the understanding of the heterogeneity of pore structures and contributes to unconventional oil and gas exploration and development. [ABSTRACT FROM AUTHOR]

This study investigated the influence of movable oil on the pore structure of various shale types, analyzing 19 shale samples from Well X in the Mahu Sag of the Junggar Basin. Initially, X-ray diffraction (XRD) analysis classified the shale samples. Subsequently, the geochemical properties and pore structures of the samples, both pre and post oil Soxhlet extraction, were comparatively analyzed through Total Organic Carbon (TOC) content measurement, Rock-Eval pyrolysis, and nitrogen adsorption experiments. Additionally, fractal theory quantitatively described the impact of movable oil on the pore structure of different shale types. Results indicated higher movable oil content in siliceous shale compared to calcareous shale. Oil extraction led to a significant increase in specific surface area and pore volume in all samples, particularly in siliceous shale. Calcareous shale predominantly displays H2–H3 type hysteresis loops, indicating a uniform pore structure with ink-bottle-shaped pores. Conversely, siliceous shale exhibited diverse hysteresis loops, reflecting its complex pore structure. The fractal dimension in calcareous shale correlated primarily with pore structure, exhibiting no significant correlation with TOC content before or after oil extraction. Conversely, the fractal dimension changes in siliceous shale samples do not have a clear correlation with either TOC content or pore structure, suggesting variations may result from both TOC and pore structure. [ABSTRACT FROM AUTHOR]

The degree of irregularity and complexity of the pore structure are comprehensively reflected in the fractal dimension. The porosity of coal was determined by its fractal dimension, where a larger dimension indicates a lower porosity. Fractal theory and the Frenkel–Halsey–Hill (FHH) model were applied to explore the variation rules of concentration on functional groups and pore structure in this study. Combined with infrared spectroscopy (FTIR) and low-temperature nitrogen adsorption, a starch-polymerized aluminum sulfate composite fracturing fluid was prepared, which plays an important role in methane adsorption and permeability of coal samples. The test results showed that, compared with the original coal, the pore volume and specific surface area of each group of coal samples were reduced, the average pore diameter was initially enlarged and then declined, and fractal dimension D1 dropped by 5.4% to 15.4%, while fractal dimension D2 gained 1.2% to 7.9%. Moreover, the nitrogen adsorption of each group of coal samples was obviously lower than the original coal, and the concentration of starch-polymerized aluminum sulfate solution existed at a critical optimal concentration for the modification of the coal samples, and the nitrogen adsorption reached a minimum value of 0.6814 cm3/g at a concentration of 10%. The novel composite solution prepared by the combination of starch and flocculant in this paper enhanced the permeability of the coal seam, which is of great significance in improving the efficiency of coalbed methane mining. [ABSTRACT FROM AUTHOR]

Taking the Longmaxi deep-marine shale gas reservoir in Zigong region as the research target, this paper aimed to characterize the nano-scale pore structure and investigate the reservoirs’ heterogeneity based on fractal theory. By conducting a series of experimental studies, mainly including TOC, XRD, gas adsorption (N2 and CH4), we were able to clarify the main controlling factors for the heterogeneity of deep shale pore structure. Our results indicated that the deep marine shale possessed a significant amount of organic matter, as the average TOC value is 3.68%. The XRD analysis results show that quartz and clay were the main mineral types, and the total content of these two minerals averaged 77.5%. Positive correlations were observed between TOC and quartz, while TOC decreases as the clay mineral increases, this discovery indicating that quartz is biogenic. Based on FHH (Frenkele-Halseye-Hill) method, by using the LTNA adsorption isotherms, we took relative pressure P/P0=0.5 as the boundary, then two separate fractal dimension were deduced, D1 and D2 represent the fractal characteristics of small and large pores, respectively. Our study revealed that both D1 and D2 demonstrated positive correlations with N2 adsorption pore volume and adsorption specific surface area, while negatively correlated with the adsorption average pore diameter. Moreover, the two fractal dimensions showed positive associations with TOC and quartz and negative associations with clay. Additionally, D1 and D2 also demonstrated a positive correlation with Langmuir volume. The presence of micropores was found to significantly influence the formation of an irregular pore structure in shale. As the pore size decreased, the adsorption specific surface area increased, resulting in a more intricate pore structure, and the fractal dimension of the pores elevated, ultimately. This intricate structure is beneficial for the accumulation of shale gas. These research findings offer valuable insights for the comprehensive assessment of deep shale gas, and enrich our knowledge of enrichment mechanisms in deep shale gas reservoirs.

Shale gas is a prospective cleaner energy resource and the exploration and development of shale gas has made breakthroughs in many countries. Structure deformation is one of the main controlling factors of shale gas accumulation and enrichment in complex tectonic areas in southern China. In order to estimate the shale gas capacity of structurally deformed shale reservoirs, it is necessary to understand the systematic evolution of organic pores in the process of structural deformation. In particular, as the main storage space of high-over-mature marine shale reservoirs, the organic matter pore system directly affects the occurrence and migration of shale gas; however, there is a lack of systematic research on the fractal characteristics and deformation mechanism of organic pores under the background of different tectonic stresses. Therefore, to clarify the above issues, modular automated processing system (MAPS) scanning, low-pressure gas adsorption, quantitative evaluation of minerals by scanning (QEMSCAN), and focused ion beam scanning electron microscopy (FIB-SEM) were performed and interpreted with fractal and morphology analyses to investigate the deformation mechanisms and structure of organic pores from different tectonic units in Silurian Longmaxi shale. Results showed that in stress concentration areas such as around veins or high-angle fractures, the organic pore length-width ratio and the fractal dimension are higher, indicating that the pore is more obviously modified by stress. Under different tectonic backgrounds, the shale reservoir in Weiyuan suffered severe denudation and stronger tectonic compression during burial, which means that the organic pores are dominated by long strip pores and slit-shaped pores with high fractal dimension, while the pressure coefficient in Luzhou is high and the structural compression is weak, resulting in suborbicular pores and ink bottle pores with low fractal dimension. The porosity and permeability of different forms of organic pores are also obviously different; the connectivity of honeycomb pores with the smallest fractal dimension is the worst, that of suborbicular organic pores is medium, and that of long strip organic pores with the highest fractal dimension is the best. This study provides more mechanism discussion and case analysis for the microscopic heterogeneity of organic pores in shale reservoirs and also provides a new analysis perspective for the mechanism of shale gas productivity differences in different stress–strain environments.

The effect of different acids on the pore structure and fractal characteristics of micropores and mesopores was determined with the help of low-temperature liquid nitrogen adsorption, X-ray diffraction, and the Frenkel–Halsey–Hill (FHH) model by using Yuwu coal as a sample and placing it in acidic environments, such as HF, HCl, HNO3, and CH3COOH. The results show that the acidization effects of HF and CH3COOH are separately dominated by the micropore and mesopore formation effects, while HCl and HNO3 mainly play their roles in expanding mesopores. After acidization, the surface fractal dimensions D1 and D1′ of micropores and mesopores in coal are always negatively correlated with the total specific surface area SBET, specific surface area Smic of micropores, and specific surface area Smes of mesopores. After being acidized by HF, D2 is negatively correlated with the total volume Vtot and the corresponding micropore volume Vmic, while acidization with HCl and HNO3 leads to the opposite result. After being acidized by CH3COOH, D2 has a negative correlation with Vtot and a positive correlation with Vmic. The structural fractal dimensions D2′ of mesopores in samples acidized by HF and CH3COOH are positively correlated with both the volume Vtot and mesopore volume Vmes, while it is the opposite for samples acidized by HNO3. D2′ of coal samples acidized by HCl is negatively correlated with Vtot while positively correlated with Vmes. [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.)

Although freeze–thaw damage to rocks has been extensively studied in recent decades, the quantitative correlation between the microscopic pores and macroscopic physico-mechanical properties of rocks under freeze–thaw action has not yet been determined. In this study, the pore structure variation in three sandstones during the freeze–thaw process was continuously measured by the mercury intrusion porosimetry (MIP) test. The pore size distribution can be characterized by a multimodal exponential function. With increasing freeze–thaw cycles, the characteristic pore radii of the nanopores (d ≤ 5 × 10−5 mm), micropores (5 × 10−5 mm ≤ d ≤ 0.1 mm), and macropores (0.1 mm ≤ d ≤ 1 mm) increase. However, the volumetric fractions of nanopores and micropores decrease continuously during freeze–thaw cycles because a considerable number of nanopores and micropores develop into macropores. The fractal dimension of the pore structure gradually decreases as the number of freeze–thaw cycles increases. This implies that the pore size distribution gradually tends to be more homogeneous and that the complexity of the pore structure decreases. Finally, the relationship between microscopic pore structure parameters and macroscopic properties (porosity, P-wave velocity, and uniaxial compressive strength) was established. Through the analysis of microscopic structure evolution and loss of macroscopic properties, it can be concluded that the growth of macropores may play a dominant role in freeze–thaw damage and strength loss. This study can provide a better understanding of the macro-microscopic freeze–thaw damage mechanism of sandstones. [ABSTRACT FROM AUTHOR]

Large amount of inhalation of the radon exacerbates the risk of cancer in the human body, and human beings also pay more attention to the problem of environmental pollution. With the increasing mining of underground mineral resources, coal production has increased significantly, but at the same time, it has also increased the number of underground goaf, and it has also seriously increased the potential danger of spontaneous combustion in the goaf. The influence of thermal effect on pore structure and radon exhalation characteristics was studied by means of relevant measurement. The outcomes confirmed that the radon exhalation characteristics of fly ash increased linearly with the increase of temperature and then decreased exponentially. At 400 °C, the radon exhalation rate of fly ash is the highest, which is 8.41 Bq m−2 h−1, 2.11 times that of fly ash at normal temperature. This is closely related to the change in pore structure of fly ash after heat treatment. The research results in this study are significant for assessing the radiation risk of radon in fly ash. [ABSTRACT FROM AUTHOR]

In order to clarify the characteristics and main controlling factors of the pore structure of tuffs in the Urho Formation of the Dongdaohaizi depression, the pore size of full-size pores of tuffs was characterized based on low-temperature carbon dioxide adsorption, nitrogen adsorption and high-pressure mercury pressure experiments. The tuff mudstone mainly existed conical tube pores, conical plate pores with open ends and narrow parallel plate pores. Pore size has the multi-peak distribution, the total pore volume of the tuff mudstone ranges from 0. 005 4 to 0. 047 8 cm³ / g, with a large of various types of pores. The pore volume of the tuffs is mainly provided by mesopores, which ranges from 0. 004 2 to 0. 041 3 cm³ / g with the average of 0. 026 cm³ / g and accounts for 75. 10%, followed by micropores and macropores. Micropores range from 0. 000 6 to 0. 005 5 cm³ / g with the mean of 0. 003 1 cm³ / g, and macropore change from 0. 000 6 to 0. 005 3 cm³ / g with the average of 0. 002 8 cm³ / g and account for 11. 5% and 13. 4% respectively. The specific surface area of tuffaceous mudstone is mainly provided by micropores and mesopores. The tuff microporous fractal dimension Dm ranges from 2. 59 to 2. 97, mesopore fractal dimension D1 from 2. 23 to 2. 55 and mesopore fractal dimension D2 from 2. 62 to 2. 83, reflecting the pore strong inhomogeneity of the tuffaceous mudstone. The TOC and clay minerals are the main controlling factors for pore in the tuffs, with quartz and feldspar being less important. [ABSTRACT FROM AUTHOR]

The stability of the dump is related to the safe operation and economic benefit of the mine, and the mechanical properties of its constituent soils are important basic data to evaluate its stability, while the changes in the mechanical properties are influenced by its internal structure. Nuclear magnetic resonance and scanning electron microscope techniques are used to obtain the pore size distribution and microscopic image characteristics of coarse-grained soil samples under the influence of freeze–thaw, respectively. Based on the fractal theory, the variation law of fractal dimension and box dimension of different apertures is analyzed and studied. In order to select reasonable pore structure parameters, linear fitting correlation analysis was used to determine the large pore ratio and medium pore fractal dimension as the model construction parameters. The correlation model between the dual pore structure parameters and uniaxial compressive strength (UCS) of coarse-grained soils was established. The results show that, with the increase of freeze–thaw cycles, the distance among the particles in the coarse-grained soil samples is increased, the connectivity among the pores is enhanced, the proportion of large pore, surface porosity, and box dimension are all increasing gradually. The proportion of macropores in the samples increased gradually, the medium and large pores showed good fractal characteristics. At the same time, the strength correlation model based on pore structure parameters can also achieve a good prediction of coarse-grained soil UCS. [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]

Coal reservoirs have a complex pore structure, which is accompanied by energy changes during the adsorption of methane gas. To study the heterogeneity of energy changes during the pore and methane adsorption processes in coal reservoirs, fractal dimension analysis was employed to analyze the heterogeneity of coal pores. The heterogeneity of energy changes during the isothermal adsorption process was analyzed using adsorption potential and surface free energy theory. The results show that sample PY-1, PB-1, and PB-2 mainly develop micropores, while sample PY-2 mainly develops macropores, and the pore distribution of different samples is significantly different. Micropores are the main sites for methane adsorption, but not the determining factor affecting methane adsorption. Coal􀆳s pore structure and inherent properties exert a significant influence on the heterogeneity of methane adsorption. As the vitrinite content increases, the complexity of pore structure and the fractal dimension of pores also increase, leading to enhanced heterogeneity. Consequently, methane adsorption exhibits higher uptake, accompanied by more pronounced variations in adsorption potential and surface free energy. [ABSTRACT FROM AUTHOR]

To study the fractal characteristics of the pore structure and the main controlling factors of coal measure shale in the Wuxiang block in Qinshui Basin, gas adsorption (CO2 and N2), mercury intrusion porosimetry (MIP), total organic carbon (TOC) content, and X-ray diffraction (XRD) experiments were carried out. The fractal dimensions of the micropores, mesopores, and macropores were computed by combining the V-S, FHH, and MENGER models. The results show that the fractal dimension increases with the increase in pore size; so, the macropore structure is the most complex. The effects of the TOC content, mineral fractions, and pore structure on the fractal dimensions were analyzed. The results showed that the TOC content certainly correlated with the mesopore fractal dimension, and the R2 is 0.9926. The pore volume and specific surface area show an obvious positive correlation with the macroporous fractal dimension, and their R2 values are 0.6953 and 0.6482, indicating that the macroporous pore structure of coal shale in the study area is more complex. There is a significant positive correlation between kaolinite and the macropore fractal dimension, and the R2 is 0.7295. Therefore, the organic carbon and kaolinite contents and the pore structure parameters are the most important factors affecting the fractal dimension characteristics. [ABSTRACT FROM AUTHOR]

The pore structure of coal has a great influence on the exploitation of coalbed methane. The primary coal and tectonic coal of the Faer Coal Mine and Wenjiaba Coal Mine in Guizhou, China, were selected for low-temperature liquid nitrogen and high-pressure mercury injection experiments. Based on the Frenkel–Halsey–Hill fractal model and multifractal model, combined with nano-CT system, the difference between primary structural coal and tectonic coal and their relationships with fractal dimension were analyzed. According to the results, coal can increase its pore volume and surface area by tectonic action. From the perspective of pore volume and surface area, micropores occupy the main part. According to the pore volume and pore surface area of micropores, that of primary structural coal was lower than that of tectonic coal. For the studied coal samples, their pores have fractal characteristics and complex structure. It can be seen from the fractional dimension that that of tectonic coal was higher than that of primary structure coal, indicating that the microscopic pore structure of coal tends to be complex after tectonic action. According to the results of multifractal and nano-CT analysis, the Hausdorff dimension of tectonic coal was slightly smaller than that of primary structure coal, but the multifractal parameter value was higher than that of primary structure coal. After tectonic action, the connected pores of coal increased by 142.86% compared with the connected pores of primary coal, and the adsorption capacity was further improved. It is further proved that the structure can affect the adsorption and desorption capacity by changing the distribution of pore structure. [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.)

Both CT and Avizo software were used to explore the effect of particle gradation on the evolution characteristics of pore structure and seepage paths in weathered crust elution-deposited rare earth ores during leaching. The results showed that the pore areas in four kinds of ore samples before leaching were mainly concentrated in 104–107 μm2, whose pore quantities accounted for 96.89%, 94.94%, 90.48%, and 89.45%, respectively, while the corresponding pore volume only accounted for 30.74%, 14.55%, 7.58%, and 2.84% of the total pore volume. With the decrease of fractal dimension, the average pore throat length increased, but pore throat quantities, the average pore throat radius and coordination number decreased. Compared with that before leaching, the change degree of pore structure during leaching increased with the fractal dimension decreasing. For example, the reduction rate of the average coordination number of ore samples was 14.36%, 21.30%, 28.00%, and 32.90%, respectively. Seepage simulation results indicated that seepage paths were uniformly distributed before leaching while the streamline density and seepage velocity increased with the fractal dimension decreasing. Besides, the phenomenon of the streamline interruption gradually reduced during leaching while preferential seepage got more obvious with the decrease of the fractal dimension.

The durability of concrete is determined by its pore structure and permeability. The present work examines the effect of pore structure and porosity on the permeability of concrete after nearly a century natural carbonation. The samples were cored from a concrete building built in 1931. X-ray diffraction analysis, phenolphthalein dye test, Mercury Intrusion Porosimetry (MIP) were conducted to ascertain the composition, the carbonation depth, the permeability, the porosity and pore size distribution of samples. The experimental results revealed that the front of core samples was completely carbonated. The fractal dimension was calculated using the Mandelbrot model to characterize the pore structure. The combined harmless and less harmless pore proportions exhibited a linear correlation with the fractal dimension. A relationship between permeability and fractal dimension is established. The increase in the fractal dimension resulted in a gradual decrease in the oxygen diffusion coefficient. The diffusion coefficient tends to plateau when the fractal dimension exceeds a critical value of the combined pore proportion. While the pore proportion of concrete was higher than 14.64%, there was no significant effect on the permeability. The results of this study can serve for future assessments of concrete durability.

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.

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.

Nano-clay has the potential to improve the properties of cement-based materials. However, the effectiveness of this improvement is influenced by the dispersion of the nano-clay. The effects of different nano-clay dispersion techniques on cement-based material properties and pore structure complexity were studied. The samples were prepared using manual and mechanical dispersion methods. The mechanical properties of the specimens were evaluated, and the pore characteristics of the cement-based materials were analysed using mercury intrusion porosimetry. The study investigated the effect of the dispersion method on the nano-clay dispersion. The complexity of the pore structure was evaluated using a fractal model, and the relationship between the fractal dimension, mechanical properties, and pore structure was analysed. The findings indicate that mechanical dispersion results in better dispersion than manual dispersion, and the mechanical properties of mechanical dispersion are superior to those of manual dispersion. Nano-clay particles can improve the internal pore structure of cement materials. Through mathematical calculation, the surface fractal dimension is between 2.90 and 2.95, with good fractal characteristics. There is a good correlation between the surface fractal dimension and the mechanical properties. The addition of nano-clay can reduce the complexity of the pore structure, and the fractal dimension has an excellent linear relationship with the pore structure. [ABSTRACT FROM AUTHOR]

In order to study the effect of temperature on the pore structure of lignite, liquid nitrogen adsorption and scanning electron microscopy (SEM) were used to analyze the change of pore structure of Indonesian lignite after heating, and the relationship between fractal dimension and pore structure of lignite after heating was analyzed. The effect of drying temperature on the pore structure of lignite was investigated by effective moisture diffusion coefficient and X-ray diffraction (×RD). The results show that between 40 and 160 ℃, with the increase of drying temperature, the N2 adsorption and desorption of Indonesian lignite shows a trend of first increasing and then decreasing. The fractal dimensions D1 and D2 of some lignites have a linear relationship with the specific surface area and average pore diameter, respectively, and the fractal dimension D3 has a linear relationship with the average pore diameter. As the drying temperature increases, the moisture transfer resistance decreases and the effective moisture diffusivity of lignite ( D e f f ) increases accordingly. There is a linear positive correlation between D e f f and the average pore diameter. The increase of the lignite aromatic system and the decrease of the crystallite stacking height have certain effects on the pore structure of lignite. [ABSTRACT FROM AUTHOR]

This study investigated the combined effects of carbon nanotubes (CNTs) and rubber particles on the durability, and pore structure of concrete. Three concentrations (0.05, 0.1, and 0.2% by cement weight) of CNTs were incorporated into crumb rubber concrete (CRC) containing rubber content at 5, 10, and 15%, respectively. The water absorption by immersion, residual compressive strength after immersion, chloride penetration, scanning electron microscope (SEM), and mercury intrusion porosimetry (MIP) tests were performed. The segmented fractal dimension was calculated to evaluate the scale-dependent fractal characteristics of the pore surface of the CNTs reinforced CRC (CNTCRC). The results showed that CNTs and rubber particles significantly reduced concrete's water absorption and chloride penetration by 21.1 and 32.3% compared to ordinary concrete. In addition, the incorporation of rubber particles mainly increased the coarse capillary pores and macropores of concrete. The fractal dimension has significant scale-dependent characteristics. Furthermore, unlike ordinary concrete, permanent non-fractal behavior was only observed in the middle capillary pores of CRC with/without CNTs. The addition of CNTs alleviated the coarsening and deterioration of pore structure caused by rubber particles. Overall, 0.1% CNTs improved the pore structure, permeability, and residual strength of CRC when there was more integration. [ABSTRACT FROM AUTHOR]

The development of pore structure in coal reservoirs determines the adsorption performance of coal gas. The pore structure of three coal samples with different degrees of metamorphism, namely long-flame coal, coking coal and anthracite, was tested by low-temperature liquid nitrogen adsorption experiments. The pore structure was quantitatively characterized based on fractal theory, and the effect of the pore structure on the methane adsorption properties of coals with different degrees of metamorphism was analyzed in depth in combination with the methane isothermal adsorption experiments. The results showed that the degree of metamorphism and the pore fractal dimension D1 showed a “bathtub” variation, and a linear negative correlation with D2; while the specific surface area and pore volume of the coal sample were positively correlated with the adsorption constant a. The larger specific surface area and pore volume, the stronger the adsorption capacity of coal. With the increase of pore fractal dimension D1, the adsorption constant a presents an approximate linear growth trend. It can be seen that the less smooth the pore structure of coal, the larger the specific surface area will be, so that the ultimate methane adsorption capacity of coal will also increase.

Marine shales from the Niutitang Formation and marine–continental transitional shales from the Longtan Formation are two sets of extremely important hydrocarbon source rocks in South China. In order to quantitatively compare the pore complexity characteristics between marine and marine–continental transitional shales, the shale and kerogen of the Niutitang Formation and the Longtan Formation are taken as our research subjects. Based on organic petrology, geochemistry, and low-temperature gas adsorption analyses, the fractal dimension of their pores is calculated by the Frenkel–Halsey–Hill (FHH) and Sierpinski models, and the influences of total organic carbon (TOC), vitrinite reflectance (Ro), and mineral composition on the pore fractals of the shale and kerogen are discussed. Our results show the following: (1) Marine shale predominantly has wedge-shaped and slit pores, while marine–continental transitional shale has inkpot-shaped and slit pores. (2) Cylindrical pores are common in organic matter of both shale types, with marine shale having a greater gas storage space (CRV) from organic matter pores, while marine–continental transitional shale relies more on inorganic pores, especially interlayer clay mineral pores, for gas storage due to their large specific surface area and high adsorption capacity (CRA). (3) The fractal characteristics of marine and marine–continental transitional shale pores are influenced differently. In marine shale, TOC positively correlates with fractal dimensions, while in marine–continental shale, Ro and clay minerals have a stronger influence. Ro is the primary factor affecting organic matter pore complexity. (4) Our two pore fractal models show that the complexity of the shale in the Longtan Formation surpasses that of the shale in the Niutitang Formation, and type I kerogen has more complex organic matter pores than type III, aiding in evaluating pore connectivity and flow effectiveness in shale reservoirs.