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Influence of sediment media with different particle sizes on the nucleation of gas hydrate
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ScienceDirect Natural Gas Industry B 5 (2018) 652e659 www.elsevier.com/locate/ngib
Research article
Influence of sediment media with different particle sizes on the nucleation of gas hydrate*,** Zhang Baoyoong a,b, Zhou Lihong a,b, Liu Changling c, Zhang Qiang a,b,*, Wu Qiang a,b, Wu Qiong a,b & Liu Chuanhai a,b a
College of Safety Engineering, Heilongjiang University of Science and Technology, Harbin, Heilongjiang 150022, China National Central Laboratory of Hydrocarbon Gas Transportation Pipeline Safety, Harbin, Heilongjiang 150022, China c Qingdao Institute of Marine Geology, Qingdao, Shandong 266071, China
b
Received 29 March 2018; accepted 25 May 2018 Available online 10 December 2018
Abstract In order to identify the effect of the particle size of sediment media on the nucleation of gas hydrate (hereinafter referred to as hydrate), we conducted gas hydrate nucleation kinetics experiments on six types of sediment media by using the high-pressure visualized reactor to measure the nucleation induction time of hydrates in sediment media with different particle sizes, based on the size of hydrate-bearing sediments in the Shenhu sea area of the South China Sea. Besides, the nucleation pattern of hydrates in sediment media was analyzed using the probability distribution function. Then, considering the effect of capillarity on the liquid-gas interfacial tension in the pores of sediments, a theoretical model for the pore radius of sediment-nucleation induction time of hydrates was established based on the Kashchiev model in combination with the Arrhenius equation. Finally, the calculation results of the theoretical model were compared with the experimental results. The following results were obtained. First, sediment media with larger particle sizes can help effectively shorten the nucleation induction time of hydrate and increase the concentration degree of induction time. Second, it is deduced from the theoretical model formula that, due to the interfacial tension of pores in the sediment, the nucleation induction time of hydrate increases and then decreases with the increase of sediment particle size, and there is a critical particle size. Third, there is a non-positive correlation between the sediment particle size and the change of hydrate formation difficulty. Within a certain range, the hydrate formation difficulty decreases gradually with the increase of sediment particle size. The research results provide a technical support for the exploration of marine gas hydrate deposits. © 2018 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Gas hydrate; Nucleation induction time; Sediment; Particle size; Pore; Capillarity; Induction time; Laboratory experiment
* Project supported by the National Special Project of Gas Hydrate “Testing Technology and Simulation Experiment of Gas Hydrate” (No.: DD20160216), the National Natural Science Foundation Project “Kinetics of gas separation based on hydrate method via Raman spectroscopy under temperature regulation condition” (No.: 51774123), China, the University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province “Raman spectra characteristics of gas hydration-separated products for high concentration CO2 gas”, China, and the Heilongjiang Natural Science Fund Project “The method of high concentration coalbed methane hydration curing rapidly and its strengthening mechanism”, China (No.: ZD2017012). ** This is the English version of the originally published article in Natural Gas Industry (in Chinese), which can be found at https://doi.org/10.3787/j.issn.10000976.2018.05.018. * Corresponding author. E-mail address: [email protected] (Zhang Q.). Peer review under responsibility of Sichuan Petroleum Administration.
https://doi.org/10.1016/j.ngib.2018.11.001 2352-8540/© 2018 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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1. Introduction Sediments, which act as the carrier of marine gas hydrate (hereinafter referred to as hydrate), have complex pore structures which also play an important role in controlling hydrate formation and occurrence. Sediment size, shape and composition affect the nucleation and growth of hydrate [1,2]. According to the distribution of natural gas hydrate saturation in sediments obtained from field explorations, it is known that hydrate stable zones have been found in different sea areas, and their sediment lithologies show great differences [3], which affects the saturation and occurrence of hydrates. There are two correspondences between the saturation of hydrates and particle size of sediments. It was discovered from the drilling samples of some marine sediments that the coarser the sediments, the higher the saturation of natural gas hydrates [4,5]. It was found in the drilling at Blake Ridge on the continental margin of the west coast of Atlantic (ODP 164) that the particle size of sediment in the stable zone of gas hydrates was generally relatively coarse [6]. The gas hydrates in the sediment core at Hydrate Ridge in the northeastern continental margin of the Pacific Ocean (ODP 204) shows that hydrates mainly occur in the particle size of 8e26 mm and 50e148 mm [7]. When natural gas hydrates were drilled in the Ulleung Basin (UBGH2), the testing results of the drilling deposits indicate that there was a correspondence between gas hydrate saturation and sand content, and the generation and accumulation of gas hydrates occurred more preferably [8]. At the U1326 and U1327 stations in Cascadia (IODP311), field survey confirmed gas hydrate zones with saturation greater than 50%, and they are deposited in sand-rich turbid areas as thick as 20 m [9]. However, some exploration results indicate that the enrichment zone of gas hydrates does not simply increase with the sediment particle size. In the NGHP voyage, it was found in the KeG Basin of India that some high-saturation hydrates exist in the fine sediments with fractures [10]. In the UBGH2 voyage, field survey of 6C-5P and 6C-6P revealed that the saturation (5e43%) of gas hydrates in the pores of partial sand-rich sediments was not correlated with particle size [11]. Luo et al. [12] studied the mechanism of gas hydrate accumulation in sediments, and discovered natural gas hydrates in the coarse volcanic ash layer according to the principle of “accumulation of gas hydrate by migration”. His results confirmed that the fluid in the permeable layer will transport the methane from the deeper gas source to the gas hydrate stabilization zone (GHSZ); through the “short distance” migration, the microscopic particles generate fine sediments in the GHSZ, and the dissolved methane diffuses into the adjacent sand layer and forms the accumulation of gas hydrate hereafter. The above-mentioned exploration results demonstrate that the formation of hydrate is a complex process of nucleation, growth, and occurrence. In order to explore the accumulation process of gas hydrate in sediments, it is necessary to determine the nucleation kinetics of hydrates. Field surveys indicate that the pore characteristics of sediments play an
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important role in the formation of hydrates, and have an important impact on the nucleation, growth and distribution of natural gas hydrates. In this paper, with reference to the particle size of sediment containing gas hydrates in the Shenhu sea area of South China Sea, the nucleation kinetics experiment of mixed gas hydrates with multiple components in sedimentary media were carried out. Then, the distribution of nucleation induction time of hydrate in sedimentary media with different particle sizes was measured and the theoretical model of sediment pore radius-hydrate nucleation induction time was established to elucidate the nucleation law of gas hydrate in sediment medium, thereby providing a basic law for the exploration of marine gas hydrate deposits. 2. Experimental system A visualized high-pressure reactor, the main experimental device, is combined with the high and low temperature test chamber, pressure increasing and releasing system (including air compressor, booster pump and high-pressure pipe), temperature and pressure measurement system (including temperature sensor and pressure sensor), and data acquisition system (including a data acquisition unit and an industrial computer) to form a hydrate experimental system (Fig. 1). The visualized high-pressure reactor is a fully transparent reaction kettle with a volume of 150 mL (temperature between 10 and 50 C, a pressure limit of 20 MPa), which can achieve a direct observation of formation, growth and decomposition process of hydrate. The high and low temperature test chamber can control the temperature of the system during the experiment. The data acquisition system can simultaneously record the temperature, pressure and other data in each process, and draw the temperature and pressure curves in real time to analyze the experimental process. The Guangzhou Marine Geological Survey (GMGS) implemented the project of “hydrate drilling in the China sea
Fig. 1. Diagram of a high-pressure hydrate experimental system.
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areas” in the Shenhu area on the northern slope of South China Sea. Among the 8 drilling holes, 2 holes (SH2B and SH7B) obtained hydrate samples, and the particle size distribution of different sediment media was measured, including clay (particle size less than 4 mm), silt (particle size between 4 and 63 mm) and sand (particle size greater than 63 mm), with clay and siltstone in dominance. Meanwhile, the reservoir of hydrates contains mixed gas components [13,14]. In this experiment, 6 types of sediment media were studied and their compositions are shown in Table 1. And the mixture composed of multiple gases was selected for nucleation kinetics research. 3. Experiment and results 3.1. Experimental process The sediment was washed and dried; 80 g sediment was placed in the reactor and brine with a concentration of 3.5% was added; the mixture was stirred and held for 12 h, and then the brine above the sediment was removed to obtain naturally deposited sediment. The measured volume of natural sedimentary seawater was 25 mL and the height of sediment was 7.1 cm. Before the blank seawater control experiment started, the bottom of the reactor was filled with solid natural rubber to make the liquid level reach the above-mentioned average height, so that gas volume and seawater consumption were the same as that in sediment experiment. The experiment was carried out at an initial temperature of 13 C and an initial pressure of 6.3 MPa. When the high and low temperature test chamber was adjusted to the above initial temperature, air component was removed by means of gas displacement. It was found through experiments that when the reaction system reaches the condition of hydrate formation, hydrate is generated in the sediment, and its generation rate is high in the initial stage. Take Experiment No.6 of Medium A as an example, at the 22nd min., white ice crystal hydrates were formed in the sediment (Fig. 2-a); at the 26th min., ice spine hydrates appeared on the upper surface of the sediment (Fig. 2-b); at the 31st min., many massive hydrates appeared at the contact between the upper surface of the sediment and the wall of the reactor (Fig. 2-c); at the 42nd min., a large amount of hydrate was formed on the upper surface of the sediment (Fig. 2-d). In addition, 11 groups of repeated experiments showed the same phenomenon: hydrate was first generated in the sediment and formed along the sediment and the reactor
Fig. 2. Typical photographs of hydrates. (Medium A, Experiment No.6).
wall then on the upper surface of the sediment; it rapidly grew in a short time, and the pressure dropped significantly. Apart from Medium A, the other sediment media were white fine sand; therefore, the phenomenon of hydrate formation was similar macroscopically. In Experiment No.3 of Medium D, hydrates grew rapidly after generation. At the 38th min., speckle-like white ice crystal hydrate occurred in the upper sediment and above the reactor wall (Fig. 3-a). At the 63rd min., a large amount of massive hydrate appeared on the inner wall surface of the reactor above the sediment, and a large number of voids were created inside the sediment, indicating that a great quantity of hydrates were formed (Fig. 3-b). The hydrates were layered inside the sediment, with the volume gradually expanding. 4. Results and discussion The nucleation induction time of hydrates in sediments with different particle sizes and seawater was obtained in 11 groups of repeated experiments of 7 systems. For purpose of visual analysis, the experimental induction time of 7 systems was ranked in an ascending order and the average nucleation induction time was obtained (Table 2). The typical experimental pressure profile for hydrate formation in each system is shown in Fig. 4. The nucleation induction time of hydrates in sediments with different particle sizes and blank seawater system was
Table 1 Composition and median diameter data of sediment media. Medium
Clay content
Silt content
Sand content
Medium diameter/mm
A B C D E F
38.97% 42.33% 76.75% 62.78% 48.30% 36.46%
51.50% 49.83% 21.53% 35.92% 50.66% 59.82%
9.52% 7.84% 1.70% 1.30% 1.03% 3.70%
8.680 7.640 0.357 1.450 4.030 6.720
Note: The gas used in the experiment includes CH4 (70%), CO2 (5%), C3H8 (10%), N2 (12%) and O2 (3%).
Fig. 3. Typical photographs of hydrates. (Medium D, Experiment No.3).
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Table 2 Nucleation induction time of hydrates in sediments with different particle sizes and blank seawater system. Experiment No.
Nucleation induction time/min
1 2 3 4 5 6 7 8 9 10 11 Average/min
Medium A
Medium B
Medium C
Medium D
Medium E
Medium F
Blank seawater
9 11 12 14 15 15 18 20 21 23 29 17
15 18 20 25 27 28 31 33 38 42 49 29.6
8 41 79 120 129 166 167 196 198 294 296 154
18 31 38 50 55 63 73 78 81 118 123 66.2
15 18 18 20 22 22 70 80 94 120 175 59.5
7 8 11 12 13 13 14 17 17 31 91 21.3
13 17 20 25 27 28 31 33 37 49 71 31.9
determined by pressure change method, and the nucleation induction time distribution map of each system was plotted (Fig. 5). It can be seen from Fig. 5 that the nucleation induction time in each group of the 11 repeated experiments is randomly distributed within a certain range, and the height difference between histograms in the middle for the systems is small, with a smooth trend. This indicates that the nucleation induction time is concentrated within this range, but in a varying degree for each system. It is assumed that the nucleation induction time in each system conforms to the normal distribution, namely 2 t t
1 f ðtÞ ¼ pffiffiffiffiffiffi e 2ps
2s2
ð1Þ
where, t represents the nucleation induction time, min; t represents the average nucleation induction time, min; s is the standard deviation of nucleation induction time, min. The probability distribution of the nucleation induction time obtained in each repeated experiment in the system can be obtained. The calculation results and statistics of indexes are shown in Table 3. The probability distribution of
Fig. 4. Typical experimental pressure curves for hydrates in different systems.
Fig. 5. Nucleation induction time of hydrates in different systems.
nucleation induction time in sediments with different particle sizes is shown in Fig. 6. It can be seen from Table 3 that, among the 11 groups of repeated experiments of Medium A, the shortest nucleation induction time is 9 min, the longest is 29 min, and the average time is 17 min; the nucleation induction time mainly distributes at 17 ± 5 min (in 12e22 min). Fig. 6-a shows the fitted normal distribution curve of Medium A in 11 groups of repeated experiments. The normal distribution probability of the nucleation induction time of 12e22 min is 78.49% (as shown in the shadow of the figure). This probability value involves 7 experiments. Compared with other media, Medium A shows the most concentrated range of nucleation induction time and the weakest randomness. For Medium C, the shortest nucleation induction time is 8 min, the longest is 296 min, and the average time is 154 min. Fig. 6-c illustrates the fitted normal distribution curve of Medium C in 11 groups of repeated experiments. The normal distribution probability of the nucleation induction time of 154 ± 5 min (149e159 min) is 8.30% (as shown in the shadow of the figure). By contrasting the results of 11 groups of repeated experiments, it is found that they are not included in
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Table 3 Nucleation induction time of hydrates in different systems. System
Shortest nucleation induction time/min
Longest nucleation induction time/min
Average nucleation induction time/min
Medium A Medium B Medium C Medium D Medium E Medium F Blank seawater
9 15 8 18 15 7 13
29 49 296 123 175 91 71
17 29.6 154 66.2 59.5 21.3 31.9
Time span/min
Time range/min
Frequency/times
Distribution probability
10
12e22 24.6e34.6 149e159 61.2e71.2 54.5e64.5 16.3e26.3 26.9e36.9
7 5 0 1 0 2 4
78.49% 63.67% 8.30% 12.10% 14.87% 27.90% 39.49%
Fig. 6. Probability distribution of nucleation induction time of hydrates in sediment media.
the probability range. Compared with other media, Medium C shows the most discrete range of nucleation induction time and the strongest randomness. The experimental results show that the nucleation induction time distribution of hydrates in Medium A and Medium B is concentrated, and that in Medium C, Medium D, Medium E and Medium F is relatively discrete. For Medium F, the shortest induction nucleation time is 7 min, the longest is 91 min, and the average time is 21.3 min. Fig. 6-f shows the fitted normal distribution curve of Medium F in 11 groups of repeated experiments. The normal distribution probability of the nucleation induction time of 16.3e26.3 min is 27.9% (as shown in the shadow of the figure), which involves 2 experiments. The average nucleation induction time of Medium F is greater than that of Medium A and smaller than that of other systems. Among the six sediment media, Medium F has the highest silt content, the lowest clay content and a higher sand content, indicating that the increase of sediment particle size facilitates the rapid formation of hydrates therein. As shown in Fig. 7, Medium C reveals the largest time span (288 min) between the longest and the shortest nucleation induction time, corresponding to the largest standard deviation. This indicates that the nucleation induction time in this range is the most discrete and the most random. In contrast, Medium A reveals the smallest standard deviation, suggesting
that the distribution of nucleation induction time is concentrated with the weakest randomness. From Medium A to Medium F, the standard deviation decreases with the increase of sediment particle size. It can be inferred that the distribution of nucleation induction time tends to concentrate with the increase of sediment particle size, and have a decreasing randomness. By comparing the average nucleation induction time and standard deviation of the six medium systems with that of the blank seawater system, it is found that Media A and Medium B are smaller than the blank seawater, and Medium C, Medium D and Medium E are larger than the blank seawater. It is indicated that Medium A and Medium B with larger particle size can effectively shorten the nucleation induction time, increase the concentration of nucleation induction time, and increase the probability of hydrate nucleation. 5. Theoretical model of pore diameter-nucleation induction time 5.1. Establishment of pore model The particle size of sediment affects the size of intergranular pore. It is assumed that the particle size of sediments is the same and the pore is formed among any random three
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5.2. Effect of sediment particle size on nucleation induction time The liquidegas interfacial tension in the pores of sediments under capillary action is expressed by Ref. [15]: F¼
rp hrg 2cosq
ð4Þ
where, F is the liquidegas interfacial tension, N/m; similarly, rp: the pore radius, m; h: the liquid level difference caused by capillary force, m; r: the density of the solution, kg/m3; g: the unit of gravity, N/kg; q: the contact angle, ( ). The effect of liquidegas interfacial tension on the surface Gibbs free energy is [16]: X G¼ ni ðmi þ FAsi Þ ð5Þ
Fig. 7. Standard deviation of nucleation induction time.
particles at the same horizontal position (Fig. 8-a). Water saturation in the pore is different. Gaseliquid contact is the cross section of the pore. The maximum area Spmax of pore cross section is the black triangle area (Fig. 8-b), and the minimum area Spmin is the green spherical triangle area (Fig. 8-c). For the sake of calculation convenience, the two areas are converted to the circles with the same area, then the expression between particle radius of sediment and pore radius is: rpmax ¼ 30:25 rg
ð2Þ
p0:5 rpmin ¼ 30:5 rg 2
ð3Þ
where, G is the surface Gibbs free energy, J; similarly, ni: the molar amount, mol; mi: the chemical potential of gas component, J/mol; Asi: the molar boundary area, m2/mol. Then the change in the chemical potential of the gas component (Dm) is: DG rp hrg As ð6Þ n 2cosq Kashchiev and Firoozabadi [17] analyzed the nucleation kinetics of hydrates in water-rich solutions and derived the heterogeneous nucleation rate of hydrate on the basis of Arrhenius equation: h 332i
Dm ¼
Dm kT
J ¼ Ae e
where, rpmax represents the maximum pore radius, mm; rg represents the particle radius of sediments, mm; rpmin represents the minimum pore radius, mm.
4b s V M 27kTðDmÞ2
ð7Þ
where, J is the nucleation rate, mol/(gs); similarly, A: the kinetics constant, mol/(gs); k: the Boltzmann constant, J/K; T: the reaction temperature, K; b: the shape parameter, dimensionless; s: the specific surface energy, J/m2; VM: the molar volume of gas, m3. When the amount of hydrate generated is constant, the nucleation induction time is inversely proportional to the hydration rate. Assuming that the gas per unit mass is completely reacted, According to Formula (7), the relation between nucleation induction time and nucleation rate in the pore is expressed as: # 1 " 3 s3 V 2 4b ( M ) 27kTðDmÞ2
Dm
t ¼ KJ 1 ¼ K Ae kT e
8 > <
¼ K Ae > :
2
!
6 4
DGrp hrgA n 2cosq s kT
e
3 4b3 V 2 s3 M rp hrg 27kT DG n 2cosq As
1
7 2 5 9 > = > ;
ð8Þ
where, t represents the induction time, s; K represents the molar constant of hydrate nucleation, dimensionless. Fig. 8. Pore model.
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According to the calculation by Formula (8), there are three solutions to the equation of pore radius rp, in other words, there are three critical radii r1, r2 and r3 in the relationship between nucleation induction time and pore radius. When r1 < rp < r2, the nucleation induction time t decreases with the decrease of rg. When r2 < rp < r3, the nucleation induction time t increases with the decrease of rg. In the experimental range, for Medium A (D50 of 8.68 mm), Medium B (D50 of 7.64 mm), Medium F (D50 of 6.72 mm), Medium E (D50 of 4.03 mm), Medium D (D50 of 1.45 mm), and Medium C (D50 of 0.357 mm), the particle sizes gradually decrease, and the nucleation induction time generally increases. Therefore, it can be inferred that the pore radius of Media A to F is in the range of r2 < rp < r3, and the nucleation induction time t increases with the decrease of rg. It is indicated in the experiments that the distribution probability of the nucleation induction time is 78.49% for Medium A (concentration range of 12e22 min), 63.67% for Medium B (concentration range of 24.6e34.6 min), and 39.49% for blank seawater (concentration range of 26.9e36.9 min). For Media A to E, the nucleation induction time gradually decreases with the increase of particle size. The nucleation induction time of hydrate in Medium F is larger than that in Medium A and smaller than that in other systems. Media A and B reveal higher nucleation probability of hydrates and more regular distribution of nucleation induction time. In summary, the results obtained by the theoretical model of sediment pore radius-hydrate nucleation induction time match well with the field exploration results indicated in the literature [4,6,10]. When the sediment particle size is larger than a certain range, the nucleation induction time decreases with the increase of the particle size, that is, the hydrate may be preferentially formed in the sediment with larger particle size, and it is slightly affected by the disturbance of seawater environment, and exists in the sediment stably. When the particle size is smaller than a certain range, the nucleation induction time increases with the increase of particle size, that is, the hydrate may be more easily formed in sediments with smaller particle sizes. Affected by other factors, hydrate may migrate and accumulate in sediments with large particle sizes. When it is less affected by disturbances of seawater environment, hydrate may also occur stably in sediments with smaller particle sizes. Therefore, the hydrate saturation obtained by field exploration does not vary with the decrease of sediment particle size. Many factors affect the formation of hydrate. The sediment particle size affects the nucleation induction time and saturation of hydrate. Additionally, ion concentration, temperature and pressure of pore water affect the accumulation of hydrate. Based on the results of the study on the nucleation kinetics of hydrates, the effects of the abovementioned influencing factors on hydrate accumulation in marine sediments should be further studied. 6. Conclusions Both experimental and theoretical derivation results confirm the field exploration results, and the formation of hydrates in sediments is complex.
1) The experimental results show that the medium with larger particle size can effectively shorten the nucleation induction time of hydrate and increase the concentration of nucleation induction time. For Media A to F, as the particle size decreases, the nucleation induction time generally increases. Medium A and Medium B reveals a higher probability of hydrate nucleation and relatively concentrated distribution of nucleation induction time. 2) It is derived from the theoretical model and formula that the nucleation induction time of hydrate increases first and then decreases with the increase of particle size due to the influence of the surface tension of pores. There is a critical particle size, which corresponds to the longest nucleation induction time. 3) There is a non-positive correlation between the sediment particle size increase and the change of hydrate formation difficulty. Within a certain range, the hydrate formation difficulty decreases gradually with the increase of sediment particle size. References [1] Ben Clennell M, Hovland M, Booth JS, Henry P & Winters WJ. Formation of natural gas hydrates in marine sediments: 1. Conceptual model of gas hydrate growth conditioned by host sediment properties. J Geophys Res: Solid Earth 1999;104(B10):22985e3003. [2] Wu Nengyou, Huang Li, Su Zheng, Yang Shengxiong, Wang Hongbin, Liang Jinqiang, et al. A study of geological evaluation indicators for the exploitation potential of marine natural gas hydrates: theory and methodology. Nat Gas Ind 2013;33(7):11e7. [3] He Jiaxiong, Lu Zhenquan, Su Pibo, Zhang Wei & Feng Junxi. Source supply system and reservoir forming model prediction of natural gas hydrate in the deep water area of the northern South China Sea. Journal of Southwest Petroleum University (Science & Technology Edition) 2016;38(6):8e24. [4] Khlebnikov VN, Antonov SV, Mishin AS, Liang Meng, Khamidullina IV, Zobov PM, et al. Major factors influencing the generation of natural gas hydrate in porous media. Nat Gas Ind B 2017;4(6):442e8. [5] Zhang Hui, Lu Hailong, Liang Jinqiang & Wu Nengyou. The methane hydrate accumulation controlled compellingly by sediment particle at Shenhu, northern South China Sea. Chin Sci Bull 2016;61(3):388e97. [6] Kraemer LM, Owen RM & Dickens GR. Lithology of the upper gas hydrate zone, Blake Outer Ridge: a link between diatoms, porosity, and gas hydrate. In: Proceedings of the ocean drilling Program: scientific results, vol. 164. College Station: Texas A&M University; 2000. p. 229e36. [7] Wang Jiasheng, Gao Yuya, Li Qing, Yang Cuiping, Chen Qi, Wei Qing, et al. Particle size constraint on gas hydrate occurrence: evidence from sediment size during IODP311. Adv Earth Sci 2007;22(7):659e65. [8] Bahk JJ, Kim DH, Chun JH, Son BK, Kim JH, Ryu BJ, et al. Gas hydrate occurrences and their relation to host sediment properties: results from second Ulleung Basin gas hydrate drilling expedition, East Sea. Mar Petrol Geol 2013;47:21e9. [9] Riedel M, Collett TS & Malone M. Expedition 311 synthesis: scientific findings. In: proceedings of the integrated ocean drilling Program, vol. 311. Washington DC: Integrated Ocean Drilling Program Management International, Inc.; 2010. p. 2. [10] Shankar U & Riedel M. Assessment of gas hydrate saturation in marine sediments from resistivity and compressional-wave velocity log measurements in the Mahanadi Basin, India. Mar Petrol Geol 2014;58:265e77. [11] Ryu BJ, Collett TS, Riedel M, Kim GY, Chun JH, Bahk JJ, et al. Scientific results of the second gas hydrate drilling expedition in the Ulleung Basin (UBGH2). Mar Petrol Geol 2013;47:1e20.
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ScienceDirect Natural Gas Industry B 5 (2018) 652e659 www.elsevier.com/locate/ngib
Research article
Influence of sediment media with different particle sizes on the nucleation of gas hydrate*,** Zhang Baoyoong a,b, Zhou Lihong a,b, Liu Changling c, Zhang Qiang a,b,*, Wu Qiang a,b, Wu Qiong a,b & Liu Chuanhai a,b a
College of Safety Engineering, Heilongjiang University of Science and Technology, Harbin, Heilongjiang 150022, China National Central Laboratory of Hydrocarbon Gas Transportation Pipeline Safety, Harbin, Heilongjiang 150022, China c Qingdao Institute of Marine Geology, Qingdao, Shandong 266071, China
b
Received 29 March 2018; accepted 25 May 2018 Available online 10 December 2018
Abstract In order to identify the effect of the particle size of sediment media on the nucleation of gas hydrate (hereinafter referred to as hydrate), we conducted gas hydrate nucleation kinetics experiments on six types of sediment media by using the high-pressure visualized reactor to measure the nucleation induction time of hydrates in sediment media with different particle sizes, based on the size of hydrate-bearing sediments in the Shenhu sea area of the South China Sea. Besides, the nucleation pattern of hydrates in sediment media was analyzed using the probability distribution function. Then, considering the effect of capillarity on the liquid-gas interfacial tension in the pores of sediments, a theoretical model for the pore radius of sediment-nucleation induction time of hydrates was established based on the Kashchiev model in combination with the Arrhenius equation. Finally, the calculation results of the theoretical model were compared with the experimental results. The following results were obtained. First, sediment media with larger particle sizes can help effectively shorten the nucleation induction time of hydrate and increase the concentration degree of induction time. Second, it is deduced from the theoretical model formula that, due to the interfacial tension of pores in the sediment, the nucleation induction time of hydrate increases and then decreases with the increase of sediment particle size, and there is a critical particle size. Third, there is a non-positive correlation between the sediment particle size and the change of hydrate formation difficulty. Within a certain range, the hydrate formation difficulty decreases gradually with the increase of sediment particle size. The research results provide a technical support for the exploration of marine gas hydrate deposits. © 2018 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Gas hydrate; Nucleation induction time; Sediment; Particle size; Pore; Capillarity; Induction time; Laboratory experiment
* Project supported by the National Special Project of Gas Hydrate “Testing Technology and Simulation Experiment of Gas Hydrate” (No.: DD20160216), the National Natural Science Foundation Project “Kinetics of gas separation based on hydrate method via Raman spectroscopy under temperature regulation condition” (No.: 51774123), China, the University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province “Raman spectra characteristics of gas hydration-separated products for high concentration CO2 gas”, China, and the Heilongjiang Natural Science Fund Project “The method of high concentration coalbed methane hydration curing rapidly and its strengthening mechanism”, China (No.: ZD2017012). ** This is the English version of the originally published article in Natural Gas Industry (in Chinese), which can be found at https://doi.org/10.3787/j.issn.10000976.2018.05.018. * Corresponding author. E-mail address: [email protected] (Zhang Q.). Peer review under responsibility of Sichuan Petroleum Administration.
https://doi.org/10.1016/j.ngib.2018.11.001 2352-8540/© 2018 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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1. Introduction Sediments, which act as the carrier of marine gas hydrate (hereinafter referred to as hydrate), have complex pore structures which also play an important role in controlling hydrate formation and occurrence. Sediment size, shape and composition affect the nucleation and growth of hydrate [1,2]. According to the distribution of natural gas hydrate saturation in sediments obtained from field explorations, it is known that hydrate stable zones have been found in different sea areas, and their sediment lithologies show great differences [3], which affects the saturation and occurrence of hydrates. There are two correspondences between the saturation of hydrates and particle size of sediments. It was discovered from the drilling samples of some marine sediments that the coarser the sediments, the higher the saturation of natural gas hydrates [4,5]. It was found in the drilling at Blake Ridge on the continental margin of the west coast of Atlantic (ODP 164) that the particle size of sediment in the stable zone of gas hydrates was generally relatively coarse [6]. The gas hydrates in the sediment core at Hydrate Ridge in the northeastern continental margin of the Pacific Ocean (ODP 204) shows that hydrates mainly occur in the particle size of 8e26 mm and 50e148 mm [7]. When natural gas hydrates were drilled in the Ulleung Basin (UBGH2), the testing results of the drilling deposits indicate that there was a correspondence between gas hydrate saturation and sand content, and the generation and accumulation of gas hydrates occurred more preferably [8]. At the U1326 and U1327 stations in Cascadia (IODP311), field survey confirmed gas hydrate zones with saturation greater than 50%, and they are deposited in sand-rich turbid areas as thick as 20 m [9]. However, some exploration results indicate that the enrichment zone of gas hydrates does not simply increase with the sediment particle size. In the NGHP voyage, it was found in the KeG Basin of India that some high-saturation hydrates exist in the fine sediments with fractures [10]. In the UBGH2 voyage, field survey of 6C-5P and 6C-6P revealed that the saturation (5e43%) of gas hydrates in the pores of partial sand-rich sediments was not correlated with particle size [11]. Luo et al. [12] studied the mechanism of gas hydrate accumulation in sediments, and discovered natural gas hydrates in the coarse volcanic ash layer according to the principle of “accumulation of gas hydrate by migration”. His results confirmed that the fluid in the permeable layer will transport the methane from the deeper gas source to the gas hydrate stabilization zone (GHSZ); through the “short distance” migration, the microscopic particles generate fine sediments in the GHSZ, and the dissolved methane diffuses into the adjacent sand layer and forms the accumulation of gas hydrate hereafter. The above-mentioned exploration results demonstrate that the formation of hydrate is a complex process of nucleation, growth, and occurrence. In order to explore the accumulation process of gas hydrate in sediments, it is necessary to determine the nucleation kinetics of hydrates. Field surveys indicate that the pore characteristics of sediments play an
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important role in the formation of hydrates, and have an important impact on the nucleation, growth and distribution of natural gas hydrates. In this paper, with reference to the particle size of sediment containing gas hydrates in the Shenhu sea area of South China Sea, the nucleation kinetics experiment of mixed gas hydrates with multiple components in sedimentary media were carried out. Then, the distribution of nucleation induction time of hydrate in sedimentary media with different particle sizes was measured and the theoretical model of sediment pore radius-hydrate nucleation induction time was established to elucidate the nucleation law of gas hydrate in sediment medium, thereby providing a basic law for the exploration of marine gas hydrate deposits. 2. Experimental system A visualized high-pressure reactor, the main experimental device, is combined with the high and low temperature test chamber, pressure increasing and releasing system (including air compressor, booster pump and high-pressure pipe), temperature and pressure measurement system (including temperature sensor and pressure sensor), and data acquisition system (including a data acquisition unit and an industrial computer) to form a hydrate experimental system (Fig. 1). The visualized high-pressure reactor is a fully transparent reaction kettle with a volume of 150 mL (temperature between 10 and 50 C, a pressure limit of 20 MPa), which can achieve a direct observation of formation, growth and decomposition process of hydrate. The high and low temperature test chamber can control the temperature of the system during the experiment. The data acquisition system can simultaneously record the temperature, pressure and other data in each process, and draw the temperature and pressure curves in real time to analyze the experimental process. The Guangzhou Marine Geological Survey (GMGS) implemented the project of “hydrate drilling in the China sea
Fig. 1. Diagram of a high-pressure hydrate experimental system.
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areas” in the Shenhu area on the northern slope of South China Sea. Among the 8 drilling holes, 2 holes (SH2B and SH7B) obtained hydrate samples, and the particle size distribution of different sediment media was measured, including clay (particle size less than 4 mm), silt (particle size between 4 and 63 mm) and sand (particle size greater than 63 mm), with clay and siltstone in dominance. Meanwhile, the reservoir of hydrates contains mixed gas components [13,14]. In this experiment, 6 types of sediment media were studied and their compositions are shown in Table 1. And the mixture composed of multiple gases was selected for nucleation kinetics research. 3. Experiment and results 3.1. Experimental process The sediment was washed and dried; 80 g sediment was placed in the reactor and brine with a concentration of 3.5% was added; the mixture was stirred and held for 12 h, and then the brine above the sediment was removed to obtain naturally deposited sediment. The measured volume of natural sedimentary seawater was 25 mL and the height of sediment was 7.1 cm. Before the blank seawater control experiment started, the bottom of the reactor was filled with solid natural rubber to make the liquid level reach the above-mentioned average height, so that gas volume and seawater consumption were the same as that in sediment experiment. The experiment was carried out at an initial temperature of 13 C and an initial pressure of 6.3 MPa. When the high and low temperature test chamber was adjusted to the above initial temperature, air component was removed by means of gas displacement. It was found through experiments that when the reaction system reaches the condition of hydrate formation, hydrate is generated in the sediment, and its generation rate is high in the initial stage. Take Experiment No.6 of Medium A as an example, at the 22nd min., white ice crystal hydrates were formed in the sediment (Fig. 2-a); at the 26th min., ice spine hydrates appeared on the upper surface of the sediment (Fig. 2-b); at the 31st min., many massive hydrates appeared at the contact between the upper surface of the sediment and the wall of the reactor (Fig. 2-c); at the 42nd min., a large amount of hydrate was formed on the upper surface of the sediment (Fig. 2-d). In addition, 11 groups of repeated experiments showed the same phenomenon: hydrate was first generated in the sediment and formed along the sediment and the reactor
Fig. 2. Typical photographs of hydrates. (Medium A, Experiment No.6).
wall then on the upper surface of the sediment; it rapidly grew in a short time, and the pressure dropped significantly. Apart from Medium A, the other sediment media were white fine sand; therefore, the phenomenon of hydrate formation was similar macroscopically. In Experiment No.3 of Medium D, hydrates grew rapidly after generation. At the 38th min., speckle-like white ice crystal hydrate occurred in the upper sediment and above the reactor wall (Fig. 3-a). At the 63rd min., a large amount of massive hydrate appeared on the inner wall surface of the reactor above the sediment, and a large number of voids were created inside the sediment, indicating that a great quantity of hydrates were formed (Fig. 3-b). The hydrates were layered inside the sediment, with the volume gradually expanding. 4. Results and discussion The nucleation induction time of hydrates in sediments with different particle sizes and seawater was obtained in 11 groups of repeated experiments of 7 systems. For purpose of visual analysis, the experimental induction time of 7 systems was ranked in an ascending order and the average nucleation induction time was obtained (Table 2). The typical experimental pressure profile for hydrate formation in each system is shown in Fig. 4. The nucleation induction time of hydrates in sediments with different particle sizes and blank seawater system was
Table 1 Composition and median diameter data of sediment media. Medium
Clay content
Silt content
Sand content
Medium diameter/mm
A B C D E F
38.97% 42.33% 76.75% 62.78% 48.30% 36.46%
51.50% 49.83% 21.53% 35.92% 50.66% 59.82%
9.52% 7.84% 1.70% 1.30% 1.03% 3.70%
8.680 7.640 0.357 1.450 4.030 6.720
Note: The gas used in the experiment includes CH4 (70%), CO2 (5%), C3H8 (10%), N2 (12%) and O2 (3%).
Fig. 3. Typical photographs of hydrates. (Medium D, Experiment No.3).
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Table 2 Nucleation induction time of hydrates in sediments with different particle sizes and blank seawater system. Experiment No.
Nucleation induction time/min
1 2 3 4 5 6 7 8 9 10 11 Average/min
Medium A
Medium B
Medium C
Medium D
Medium E
Medium F
Blank seawater
9 11 12 14 15 15 18 20 21 23 29 17
15 18 20 25 27 28 31 33 38 42 49 29.6
8 41 79 120 129 166 167 196 198 294 296 154
18 31 38 50 55 63 73 78 81 118 123 66.2
15 18 18 20 22 22 70 80 94 120 175 59.5
7 8 11 12 13 13 14 17 17 31 91 21.3
13 17 20 25 27 28 31 33 37 49 71 31.9
determined by pressure change method, and the nucleation induction time distribution map of each system was plotted (Fig. 5). It can be seen from Fig. 5 that the nucleation induction time in each group of the 11 repeated experiments is randomly distributed within a certain range, and the height difference between histograms in the middle for the systems is small, with a smooth trend. This indicates that the nucleation induction time is concentrated within this range, but in a varying degree for each system. It is assumed that the nucleation induction time in each system conforms to the normal distribution, namely 2 t t
1 f ðtÞ ¼ pffiffiffiffiffiffi e 2ps
2s2
ð1Þ
where, t represents the nucleation induction time, min; t represents the average nucleation induction time, min; s is the standard deviation of nucleation induction time, min. The probability distribution of the nucleation induction time obtained in each repeated experiment in the system can be obtained. The calculation results and statistics of indexes are shown in Table 3. The probability distribution of
Fig. 4. Typical experimental pressure curves for hydrates in different systems.
Fig. 5. Nucleation induction time of hydrates in different systems.
nucleation induction time in sediments with different particle sizes is shown in Fig. 6. It can be seen from Table 3 that, among the 11 groups of repeated experiments of Medium A, the shortest nucleation induction time is 9 min, the longest is 29 min, and the average time is 17 min; the nucleation induction time mainly distributes at 17 ± 5 min (in 12e22 min). Fig. 6-a shows the fitted normal distribution curve of Medium A in 11 groups of repeated experiments. The normal distribution probability of the nucleation induction time of 12e22 min is 78.49% (as shown in the shadow of the figure). This probability value involves 7 experiments. Compared with other media, Medium A shows the most concentrated range of nucleation induction time and the weakest randomness. For Medium C, the shortest nucleation induction time is 8 min, the longest is 296 min, and the average time is 154 min. Fig. 6-c illustrates the fitted normal distribution curve of Medium C in 11 groups of repeated experiments. The normal distribution probability of the nucleation induction time of 154 ± 5 min (149e159 min) is 8.30% (as shown in the shadow of the figure). By contrasting the results of 11 groups of repeated experiments, it is found that they are not included in
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Table 3 Nucleation induction time of hydrates in different systems. System
Shortest nucleation induction time/min
Longest nucleation induction time/min
Average nucleation induction time/min
Medium A Medium B Medium C Medium D Medium E Medium F Blank seawater
9 15 8 18 15 7 13
29 49 296 123 175 91 71
17 29.6 154 66.2 59.5 21.3 31.9
Time span/min
Time range/min
Frequency/times
Distribution probability
10
12e22 24.6e34.6 149e159 61.2e71.2 54.5e64.5 16.3e26.3 26.9e36.9
7 5 0 1 0 2 4
78.49% 63.67% 8.30% 12.10% 14.87% 27.90% 39.49%
Fig. 6. Probability distribution of nucleation induction time of hydrates in sediment media.
the probability range. Compared with other media, Medium C shows the most discrete range of nucleation induction time and the strongest randomness. The experimental results show that the nucleation induction time distribution of hydrates in Medium A and Medium B is concentrated, and that in Medium C, Medium D, Medium E and Medium F is relatively discrete. For Medium F, the shortest induction nucleation time is 7 min, the longest is 91 min, and the average time is 21.3 min. Fig. 6-f shows the fitted normal distribution curve of Medium F in 11 groups of repeated experiments. The normal distribution probability of the nucleation induction time of 16.3e26.3 min is 27.9% (as shown in the shadow of the figure), which involves 2 experiments. The average nucleation induction time of Medium F is greater than that of Medium A and smaller than that of other systems. Among the six sediment media, Medium F has the highest silt content, the lowest clay content and a higher sand content, indicating that the increase of sediment particle size facilitates the rapid formation of hydrates therein. As shown in Fig. 7, Medium C reveals the largest time span (288 min) between the longest and the shortest nucleation induction time, corresponding to the largest standard deviation. This indicates that the nucleation induction time in this range is the most discrete and the most random. In contrast, Medium A reveals the smallest standard deviation, suggesting
that the distribution of nucleation induction time is concentrated with the weakest randomness. From Medium A to Medium F, the standard deviation decreases with the increase of sediment particle size. It can be inferred that the distribution of nucleation induction time tends to concentrate with the increase of sediment particle size, and have a decreasing randomness. By comparing the average nucleation induction time and standard deviation of the six medium systems with that of the blank seawater system, it is found that Media A and Medium B are smaller than the blank seawater, and Medium C, Medium D and Medium E are larger than the blank seawater. It is indicated that Medium A and Medium B with larger particle size can effectively shorten the nucleation induction time, increase the concentration of nucleation induction time, and increase the probability of hydrate nucleation. 5. Theoretical model of pore diameter-nucleation induction time 5.1. Establishment of pore model The particle size of sediment affects the size of intergranular pore. It is assumed that the particle size of sediments is the same and the pore is formed among any random three
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5.2. Effect of sediment particle size on nucleation induction time The liquidegas interfacial tension in the pores of sediments under capillary action is expressed by Ref. [15]: F¼
rp hrg 2cosq
ð4Þ
where, F is the liquidegas interfacial tension, N/m; similarly, rp: the pore radius, m; h: the liquid level difference caused by capillary force, m; r: the density of the solution, kg/m3; g: the unit of gravity, N/kg; q: the contact angle, ( ). The effect of liquidegas interfacial tension on the surface Gibbs free energy is [16]: X G¼ ni ðmi þ FAsi Þ ð5Þ
Fig. 7. Standard deviation of nucleation induction time.
particles at the same horizontal position (Fig. 8-a). Water saturation in the pore is different. Gaseliquid contact is the cross section of the pore. The maximum area Spmax of pore cross section is the black triangle area (Fig. 8-b), and the minimum area Spmin is the green spherical triangle area (Fig. 8-c). For the sake of calculation convenience, the two areas are converted to the circles with the same area, then the expression between particle radius of sediment and pore radius is: rpmax ¼ 30:25 rg
ð2Þ
p0:5 rpmin ¼ 30:5 rg 2
ð3Þ
where, G is the surface Gibbs free energy, J; similarly, ni: the molar amount, mol; mi: the chemical potential of gas component, J/mol; Asi: the molar boundary area, m2/mol. Then the change in the chemical potential of the gas component (Dm) is: DG rp hrg As ð6Þ n 2cosq Kashchiev and Firoozabadi [17] analyzed the nucleation kinetics of hydrates in water-rich solutions and derived the heterogeneous nucleation rate of hydrate on the basis of Arrhenius equation: h 332i
Dm ¼
Dm kT
J ¼ Ae e
where, rpmax represents the maximum pore radius, mm; rg represents the particle radius of sediments, mm; rpmin represents the minimum pore radius, mm.
4b s V M 27kTðDmÞ2
ð7Þ
where, J is the nucleation rate, mol/(gs); similarly, A: the kinetics constant, mol/(gs); k: the Boltzmann constant, J/K; T: the reaction temperature, K; b: the shape parameter, dimensionless; s: the specific surface energy, J/m2; VM: the molar volume of gas, m3. When the amount of hydrate generated is constant, the nucleation induction time is inversely proportional to the hydration rate. Assuming that the gas per unit mass is completely reacted, According to Formula (7), the relation between nucleation induction time and nucleation rate in the pore is expressed as: # 1 " 3 s3 V 2 4b ( M ) 27kTðDmÞ2
Dm
t ¼ KJ 1 ¼ K Ae kT e
8 > <
¼ K Ae > :
2
!
6 4
DGrp hrgA n 2cosq s kT
e
3 4b3 V 2 s3 M rp hrg 27kT DG n 2cosq As
1
7 2 5 9 > = > ;
ð8Þ
where, t represents the induction time, s; K represents the molar constant of hydrate nucleation, dimensionless. Fig. 8. Pore model.
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According to the calculation by Formula (8), there are three solutions to the equation of pore radius rp, in other words, there are three critical radii r1, r2 and r3 in the relationship between nucleation induction time and pore radius. When r1 < rp < r2, the nucleation induction time t decreases with the decrease of rg. When r2 < rp < r3, the nucleation induction time t increases with the decrease of rg. In the experimental range, for Medium A (D50 of 8.68 mm), Medium B (D50 of 7.64 mm), Medium F (D50 of 6.72 mm), Medium E (D50 of 4.03 mm), Medium D (D50 of 1.45 mm), and Medium C (D50 of 0.357 mm), the particle sizes gradually decrease, and the nucleation induction time generally increases. Therefore, it can be inferred that the pore radius of Media A to F is in the range of r2 < rp < r3, and the nucleation induction time t increases with the decrease of rg. It is indicated in the experiments that the distribution probability of the nucleation induction time is 78.49% for Medium A (concentration range of 12e22 min), 63.67% for Medium B (concentration range of 24.6e34.6 min), and 39.49% for blank seawater (concentration range of 26.9e36.9 min). For Media A to E, the nucleation induction time gradually decreases with the increase of particle size. The nucleation induction time of hydrate in Medium F is larger than that in Medium A and smaller than that in other systems. Media A and B reveal higher nucleation probability of hydrates and more regular distribution of nucleation induction time. In summary, the results obtained by the theoretical model of sediment pore radius-hydrate nucleation induction time match well with the field exploration results indicated in the literature [4,6,10]. When the sediment particle size is larger than a certain range, the nucleation induction time decreases with the increase of the particle size, that is, the hydrate may be preferentially formed in the sediment with larger particle size, and it is slightly affected by the disturbance of seawater environment, and exists in the sediment stably. When the particle size is smaller than a certain range, the nucleation induction time increases with the increase of particle size, that is, the hydrate may be more easily formed in sediments with smaller particle sizes. Affected by other factors, hydrate may migrate and accumulate in sediments with large particle sizes. When it is less affected by disturbances of seawater environment, hydrate may also occur stably in sediments with smaller particle sizes. Therefore, the hydrate saturation obtained by field exploration does not vary with the decrease of sediment particle size. Many factors affect the formation of hydrate. The sediment particle size affects the nucleation induction time and saturation of hydrate. Additionally, ion concentration, temperature and pressure of pore water affect the accumulation of hydrate. Based on the results of the study on the nucleation kinetics of hydrates, the effects of the abovementioned influencing factors on hydrate accumulation in marine sediments should be further studied. 6. Conclusions Both experimental and theoretical derivation results confirm the field exploration results, and the formation of hydrates in sediments is complex.
1) The experimental results show that the medium with larger particle size can effectively shorten the nucleation induction time of hydrate and increase the concentration of nucleation induction time. For Media A to F, as the particle size decreases, the nucleation induction time generally increases. Medium A and Medium B reveals a higher probability of hydrate nucleation and relatively concentrated distribution of nucleation induction time. 2) It is derived from the theoretical model and formula that the nucleation induction time of hydrate increases first and then decreases with the increase of particle size due to the influence of the surface tension of pores. There is a critical particle size, which corresponds to the longest nucleation induction time. 3) There is a non-positive correlation between the sediment particle size increase and the change of hydrate formation difficulty. Within a certain range, the hydrate formation difficulty decreases gradually with the increase of sediment particle size. References [1] Ben Clennell M, Hovland M, Booth JS, Henry P & Winters WJ. Formation of natural gas hydrates in marine sediments: 1. Conceptual model of gas hydrate growth conditioned by host sediment properties. J Geophys Res: Solid Earth 1999;104(B10):22985e3003. [2] Wu Nengyou, Huang Li, Su Zheng, Yang Shengxiong, Wang Hongbin, Liang Jinqiang, et al. A study of geological evaluation indicators for the exploitation potential of marine natural gas hydrates: theory and methodology. Nat Gas Ind 2013;33(7):11e7. [3] He Jiaxiong, Lu Zhenquan, Su Pibo, Zhang Wei & Feng Junxi. Source supply system and reservoir forming model prediction of natural gas hydrate in the deep water area of the northern South China Sea. Journal of Southwest Petroleum University (Science & Technology Edition) 2016;38(6):8e24. [4] Khlebnikov VN, Antonov SV, Mishin AS, Liang Meng, Khamidullina IV, Zobov PM, et al. Major factors influencing the generation of natural gas hydrate in porous media. Nat Gas Ind B 2017;4(6):442e8. [5] Zhang Hui, Lu Hailong, Liang Jinqiang & Wu Nengyou. The methane hydrate accumulation controlled compellingly by sediment particle at Shenhu, northern South China Sea. Chin Sci Bull 2016;61(3):388e97. [6] Kraemer LM, Owen RM & Dickens GR. Lithology of the upper gas hydrate zone, Blake Outer Ridge: a link between diatoms, porosity, and gas hydrate. In: Proceedings of the ocean drilling Program: scientific results, vol. 164. College Station: Texas A&M University; 2000. p. 229e36. [7] Wang Jiasheng, Gao Yuya, Li Qing, Yang Cuiping, Chen Qi, Wei Qing, et al. Particle size constraint on gas hydrate occurrence: evidence from sediment size during IODP311. Adv Earth Sci 2007;22(7):659e65. [8] Bahk JJ, Kim DH, Chun JH, Son BK, Kim JH, Ryu BJ, et al. Gas hydrate occurrences and their relation to host sediment properties: results from second Ulleung Basin gas hydrate drilling expedition, East Sea. Mar Petrol Geol 2013;47:21e9. [9] Riedel M, Collett TS & Malone M. Expedition 311 synthesis: scientific findings. In: proceedings of the integrated ocean drilling Program, vol. 311. Washington DC: Integrated Ocean Drilling Program Management International, Inc.; 2010. p. 2. [10] Shankar U & Riedel M. Assessment of gas hydrate saturation in marine sediments from resistivity and compressional-wave velocity log measurements in the Mahanadi Basin, India. Mar Petrol Geol 2014;58:265e77. [11] Ryu BJ, Collett TS, Riedel M, Kim GY, Chun JH, Bahk JJ, et al. Scientific results of the second gas hydrate drilling expedition in the Ulleung Basin (UBGH2). Mar Petrol Geol 2013;47:1e20.
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