Mechanism of Unfrozen Water Content Evolution during Melting of Cryogenic Frozen Coal Body Based on 2D NMR
Abstract
:1. Introduction
2. Coal Sample and Experiment
2.1. Coal Sample Preparation
2.2. Experiment Method
- (1)
- Vacuum full of water machine: BSJ intelligent vacuum full of water machine.
- (2)
- Nuclear magnetic resonance core analyzer: Niumag MR-60 nuclear magnetic resonance core analyzer, the number of single sampling points (TD) was 1024, the cumulative sampling times (NS) was 32 times, the echo time (TE) was 0.233 ms, and the number of echoes (NECH) was 6000, the hydrogen proton resonance frequency was 21.3 MHz and the diameter of the probe coil was 60 mm.
3. Experimental Results and Analysis
3.1. T2 Spectrum Test Results
3.2. T1-T2 Spectrum Test Results
4. Evolution Mechanism of Unfrozen Water Content in Pore
4.1. Governing Equation of Water Ice Transition in Pore
4.2. Effect of Pore Temperature on Unfrozen Water Content
4.3. Effect of Potential Energy between Pore Wall and Unfrozen Water on Unfrozen Water Content
5. Conclusions
- (1)
- The two-dimensional NMR T1-T2 spectra reflected the unfrozen water space within the pore space. According to the T1-T2 spectra at different temperatures, the unfrozen water content could be obtained in ascending order with increasing temperature.
- (2)
- In the melting process of frozen coal sample, the pore structure was composed of coal pore and ice pore. As the melting progresses, the small pores formed by the ice gradually disappeared, the number of small pores decreased and the number of intermediate pores increased.
- (3)
- The unfrozen water content was affected by temperature and pore size. The change of pore pressure with temperature directly affected the melting point of ice. The melting of coal samples started from the small pores until the temperature rose to a certain degree and the large pores began to melt.
- (4)
- Under the influence of intermolecular potential energy between water molecules and coal wall in the pore, the ice melting point decreased in small pore. Since the smaller the pore is, the larger the intermolecular potential energy is, a more significant effect occurs on the ice melting point reduction. This is the key reason why the ice in the small pores melted first during the melting process of frozen coal. In this paper, we only theoretically analyzed the effect of pore pressure on pore ice melting point and unfrozen water content under a single pore size condition. Due to the uneven pore size distribution of real coals, the control mechanism of pore size and distribution on unfrozen water content in coals with different degrees of metamorphism will be discussed in our future studies.
- (5)
- In this paper, it was found that the order of pore ice melting was small pores first and then medium-large pores, and the slow ice melting in medium-large pores will undoubtedly increase fracturing time and reduce the permeability-enhancement effect in the coal seam. At the same time, previous tests found that the freezing sequence was medium-large pores first and then small pores. Therefore, we believe that when applying low-temperature freeze-thaw fracturing technology, rapid freeze-thawing should be adopted to improve the freeze-thawing efficiency of medium-large pores, avoiding the waste of energy and time by freezing and thawing of small pores, so as to achieve the purpose of high-efficiency permeability enhancement and coalbed methane production increase. For example, rapid cyclic freezing and thawing or rapid hot and cold alternation can be adopted to fully reduce the consumption of liquid nitrogen, energy and time, as well as to promote the low-cost and high-efficiency application of this technology.
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Coal Specimens | Proximate (wt %) | Ro,max (%) | Maceral Composition (vol %) | ||||||
---|---|---|---|---|---|---|---|---|---|
Mad | Aad | Vad | FCad | V | I | E | M | ||
Lignite | 6.16 | 6.94 | 28.97 | 57.93 | 0.62 | 45.76 | 48.27 | 1.03 | 4.94 |
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Liu, T.; Zhang, X.; Qin, L.; Lin, B.; Mu, M.; Yang, W.; Lv, S.; Li, J. Mechanism of Unfrozen Water Content Evolution during Melting of Cryogenic Frozen Coal Body Based on 2D NMR. Appl. Sci. 2024, 14, 5182. https://doi.org/10.3390/app14125182
Liu T, Zhang X, Qin L, Lin B, Mu M, Yang W, Lv S, Li J. Mechanism of Unfrozen Water Content Evolution during Melting of Cryogenic Frozen Coal Body Based on 2D NMR. Applied Sciences. 2024; 14(12):5182. https://doi.org/10.3390/app14125182
Chicago/Turabian StyleLiu, Tong, Xian Zhang, Lei Qin, Baiquan Lin, Miao Mu, Wei Yang, Shiyin Lv, and Jiawei Li. 2024. "Mechanism of Unfrozen Water Content Evolution during Melting of Cryogenic Frozen Coal Body Based on 2D NMR" Applied Sciences 14, no. 12: 5182. https://doi.org/10.3390/app14125182
APA StyleLiu, T., Zhang, X., Qin, L., Lin, B., Mu, M., Yang, W., Lv, S., & Li, J. (2024). Mechanism of Unfrozen Water Content Evolution during Melting of Cryogenic Frozen Coal Body Based on 2D NMR. Applied Sciences, 14(12), 5182. https://doi.org/10.3390/app14125182