Microscopic distribution and dynamic variation of water under stress in middle and high rank coal samples

Zhang, Junjian; Wei, Chongtao; Ju, Wei; Qin, Zhengyuan; Ji, Yukun; Quan, Fangkai; Hu, Yunbing

doi:10.1016/j.jngse.2020.103369

Cited by 28 publications

(17 citation statements)

References 44 publications

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“…According to the research object, there are two division schemes. In one division, T 2cutoff is taken as the cutoff point, and the slope is divided into fractal dimensions of movable water and bound water distribution. , In the other scheme, the T 2 values for small pores (adsorption pores) and large pores (seepage pores) are taken as the dividing point, and the slope is divided into fractal dimensions of small pores and large pores. ,,, …”

Section: Resultsmentioning

confidence: 99%

“…The other set q – D ( q ) is introduced from the perspective of the information theory and called the generalized fractal dimension. A detailed description can be seen in Zhang et al …”

Section: Experimental Samples and Methodsmentioning

confidence: 99%

“…Compared to other techniques, − nuclear magnetic resonance (NMR) has the advantage of rapidly and accurately characterizing a pore-facture system without damaging it; − it can be used to quantitatively analyze the full-scale pore size of the tight sandstone reservoir. In addition, water migration in the pore-fracture system is analyzed by centrifugal and saturation methods for one sample type; then, the fluid parameters such as irreducible, movable water saturation and T 2 cutoff values are obtained. − It has been widely adopted by many researchers to study the morphology, size, and porosity of the pore-fracture system. ,,,− …”

Section: Introductionmentioning

confidence: 99%

“…The higher complexity of the pore-throat structure makes the traditional Euclidean geometry difficult to describe quantitatively the pore structure . So, the fractal theory is adapted to study the pore-fracture complexity and heterogeneity of unconventional reservoirs using NMR relaxation time ( T 2 ) spectra. ,,,,,, Zhang and Weller described the principle of NMR fractal calculation and its relationship with the sandstone pore structure.…”

Section: Introductionmentioning

confidence: 99%

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Comparative Evaluation of Pore Structure Heterogeneity in Low-Permeability Tight Sandstones Using Different Fractal Models Based on NMR Technology: A Case Study of Benxi Formation in the Central Ordos Basin

Zhang

2020

Energy Fuels

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Based on the NMR T 2 spectra of all of the samples, the applicability of the single fractal model and the multifractal model for the quantitative investigation of pore diameter distribution heterogeneity is analyzed. Then, the relationships between pore structures, physical properties, and fractal dimensions calculated by each model are discussed. The following results have been achieved. (1) Model 1 can be used to study the heterogeneity of different fluid states (including saturation and bound water) or partial pore size distribution, and the calculated fractal dimension has the best correlation with permeability. This model is mainly suitable for type I samples with macropores developed. (2) Models 2, 3, and 4 can quantitatively analyze the heterogeneity of the overall pore diameter distribution. These calculated fractal dimensions have a good linear relationship with porosity, T 2cutoff, T 2gm, and other physical parameters, which are mainly suitable for type II, III, and IV samples with micropores developed. (3) There is a negative correlation between multifractal dimensions calculated by model 4 and single fractal parameters calculated by models 2 and 3. Micropores and macropores are the key pore sizes that affect the variation of single fractal parameters, while mesopores are the key size affecting the variation of multifractal parameters. (4) The selection of a reasonable T 2 spectrum range has become a key to determine the calculation accuracy of models 2 and 3. Above all, models 1 and 4 should be given priority in characterizing pore distribution heterogeneity of tight sandstone gas reservoirs.

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Section: Resultsmentioning

confidence: 99%

“…The other set q – D ( q ) is introduced from the perspective of the information theory and called the generalized fractal dimension. A detailed description can be seen in Zhang et al …”

Section: Experimental Samples and Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Comparative Evaluation of Pore Structure Heterogeneity in Low-Permeability Tight Sandstones Using Different Fractal Models Based on NMR Technology: A Case Study of Benxi Formation in the Central Ordos Basin

Zhang

2020

Energy Fuels

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“…The T 2 spectra of all coal samples at a water pressure of 15 MPa are shown in Figure 9. Zhang et al 15 and Zou et al 18 classified the pore-fracture system into adsorption pores (0−10 2 nm, T 2 < 2.5 ms), seepage pores (10 2 −10 4 nm, 2.5 ms < T 2 < 50 ms), and fractures (>10 4 nm, T 2 > 100 ms) using the T 2 spectrum and the response characteristics (relaxation time) of different pore diameters. This work refers to the division scheme given in the study of Yao et al, 17 with the classification of adsorption pores, seepage pores, and fractures based on the T 2 spectrum.…”

Section: Energy and Fuelsmentioning

confidence: 99%

Water Saturation and Distribution Variation in Coal Reservoirs: Intrusion and Drainage Experiments Using One- and Two-Dimensional NMR Techniques

Zhang

Chang

et al. 2022

Energy Fuels

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Determining water occurrence in pore-fracture systems under specific water saturation is of great significance to reveal the correlation between the water content and porosity/permeability of coal reservoirs. In this work, simulation experiments of water intrusion and drainage are used to study the micro-occurrence and migration of water using NMR T 2 and T 1 −T 2 techniques and discuss the influence of pore-fracture system structure parameters on water micro-occurrence. Meanwhile, water distribution heterogeneity in the pore-fracture system is clarified by single-and multifractal theories. The results show that (1) the vacuum saturation method without pressure is unsuitable for high-rank coal samples with micropore development, and water saturation variation leads to a change in significant permeability when water saturation is greater than the critical value, which is related to the coal rank and degree of fracture development; (2) the single-fractal theory can characterize the heterogeneity of water and pore size distribution under static conditions; however, multifractal analyses have a stronger applicability in characterizing water distribution heterogeneity under dynamic conditions; and (3) multifractal parameters have a good correlation with coal sample characteristics such as the water volume in pores and fractures. In the process of centrifugation, both D −10 −D 0 and D −10 −D 10 parameters from fractal analyses decrease linearly with a decrease in water content in coal samples, indicating that water distribution heterogeneity in pore-fracture systems decreases with an increase in centrifugal force; and (4) T 2 and two-dimensional spectra in the same coal sample should be comprehensively analyzed as they can quantitatively identify the amount of water migration at different saturation stages.

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Single and multi‐fractal dimension variation of tight sandstone by using centrifuge T₂ spectral curve

Feng,

Li,

Zhang

et al. 2023

Greenhouse Gases

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Pore‐fracture structure distribution heterogeneity (PFSH) affects dynamic variation of porosity‐permeability of tight sandstone reservoirs, then restricting gas production performance. A fractal model by low‐field nuclear magnetic resonance technology (LF‐NMR) has been used in the quantitative characterization of PFSH. Among some literature, PFSH was studied by using a saturated T2 spectrum. However, there are few studies on fractal characteristics of T2 spectral morphology in a centrifugal state and its influence on porosity‐permeability parameters. In this paper, 30 tight sandstone samples were collected from Taiyuan Formation in Qinshui Basin. Then LF‐NMR technology was used to analyze PFSH, and sample types were divided by using T2 spectra difference under saturated and centrifugal conditions. Meanwhile, single (model 1 and 2) and multi‐fractal model are adopted to calculate fractal parameters of saturated and centrifugal T2 spectra, and then a difference in fractal parameters under different water conditions was compared. Correlation between different fractal parameters, pore structure and T2 cut‐off value are studied, and a mathematical prediction model for T2 cut‐off value by using fractal and pore structure parameters are established. The results are as follows. (1) All the samples are divided into four types A/B/C/D. For example, the type A sample is characterized by a single peak of T2 spectrum and T2 value is less than 10 ms, which indicates that this type belongs a smaller‐pore developed. Type B sample is characterized by a single peak of T2 spectrum and T2 value is10–100 ms, which indicates that this type belongs to mesopore developed. (2) In saturated state (DS), PFSH of type A sample by using model 1 and 2 is stronger than that of type B, followed by type C and D. Then the multifractal model shows that PFSH of type B sample is stronger than that of other sample types. Correlation between fractal dimension calculated by using single fractal and pore structure parameters is stronger than that of multifractal dimension. (3) T2 spectrum in centrifugal state has fractal characteristics (Di), and there are certain correlation Di with Ds. Therefore, a mathematical prediction model for T2 cut‐off value by using fractal and pore structure parameters is established. © 2023 Society of Chemical Industry and John Wiley & Sons, Ltd.

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Microscopic distribution and dynamic variation of water under stress in middle and high rank coal samples

Cited by 28 publications

References 44 publications

Comparative Evaluation of Pore Structure Heterogeneity in Low-Permeability Tight Sandstones Using Different Fractal Models Based on NMR Technology: A Case Study of Benxi Formation in the Central Ordos Basin

Comparative Evaluation of Pore Structure Heterogeneity in Low-Permeability Tight Sandstones Using Different Fractal Models Based on NMR Technology: A Case Study of Benxi Formation in the Central Ordos Basin

Water Saturation and Distribution Variation in Coal Reservoirs: Intrusion and Drainage Experiments Using One- and Two-Dimensional NMR Techniques

Single and multi‐fractal dimension variation of tight sandstone by using centrifuge T₂ spectral curve

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Microscopic distribution and dynamic variation of water under stress in middle and high rank coal samples

Cited by 28 publications

References 44 publications

Comparative Evaluation of Pore Structure Heterogeneity in Low-Permeability Tight Sandstones Using Different Fractal Models Based on NMR Technology: A Case Study of Benxi Formation in the Central Ordos Basin

Comparative Evaluation of Pore Structure Heterogeneity in Low-Permeability Tight Sandstones Using Different Fractal Models Based on NMR Technology: A Case Study of Benxi Formation in the Central Ordos Basin

Water Saturation and Distribution Variation in Coal Reservoirs: Intrusion and Drainage Experiments Using One- and Two-Dimensional NMR Techniques

Single and multi‐fractal dimension variation of tight sandstone by using centrifuge T2 spectral curve

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Single and multi‐fractal dimension variation of tight sandstone by using centrifuge T₂ spectral curve