Evolution of bubbles in dissociated water and their enhancement mechanism for hydrate reformation

doi:10.12034/j.issn.1009-606X.225304

Abstract

Abstract:

Although depressurization is one of the most promising methods for natural gas hydrate extraction, it carries the risk of hydrate reformation within hydrate-bearing sediments and wellbores during hydrate extraction. This study investigated the formation and evolution of bubbles in water from hydrate dissociation and their effects on hydrate reformation. The results suggested that the hydrate dissociation process was accompanied by the generation of a significant number of bubbles, whereas methane-saturated water without prior hydrate formation did not produce bubbles after the same depressurization process. An increase in the dissociation pressure led to a higher number of bubbles in the dissociation water, a substantial proportion of which had diameters smaller than 5 μm. Under static conditions, bubbles in the dissociation water exhibited phenomena such as coalescence and size increase; however, the overall bubble population remained relatively stable. The presence of bubbles was found to promote hydrate reformation by providing nucleation sites and enhancing mass transfer. In the dissociation water that dissociated at 3 MPa, the water conversion efficiency during the reformation process increased by approximately 12% compared to that at atmospheric pressure. Furthermore, a higher reformation pressure resulted in a higher equilibrium water conversion. During hydrate reformation on the bubble surfaces, the bubbles underwent concave deformation, became translucent, and eventually, the encapsulated gas was completely converted into hydrates. Compared to pure water, the existence of sodium dodecyl sulfate (SDS) could significantly accelerate the hydrate formation. The resulting hydrate film exhibited a loose and porous structure, which thereby reduced mass transfer resistance. These findings not only confirm that microbubbles in the dissociation water are a critical factor affecting hydrate reformation but also provide theoretical support for controlling hydrate reformation during depressurization-based extraction.

Key words: hydrate, dissociation water, bubbles, reformation

摘要：

降压开采是天然气水合物开采中最具前景的技术之一，但开采过程中水合物沉积层与井筒均面临水合物二次生成风险。本实验研究了分解水中气泡的产生、演变规律及对水合物再生成的影响。结果表明，水合物分解过程会伴随大量气泡生成；而未生成水合物的甲烷饱和水，经过同样的降压过程后并无气泡产生。分解压力较高时，分解水中气泡数量更多，且存在大量直径小于5 µm的微气泡。静置过程中，分解水中气泡发生聚并、尺寸增大，但总体数量较为稳定。气泡可为水合物生成提供成核位点和传质通道，对水合物再生有促进作用；在3 MPa分解条件下，水合物再生过程的水转化率较常压分解提升约12%。此外，再生成压力越高，体系达到平衡时的水转化率越高。水合物在气泡表面再次生成时，气泡逐渐发生凹陷变形，最终呈现半透明化状态，气泡内的气体完全转化为水合物。与纯水体系相比，在分解水中添加表面活性剂十二烷基硫酸钠(SDS)，水合物的生成速率更快，且生成的水合物膜呈现出典型的蓬松多孔结构，这种结构减小了传质阻力。本研究结果证实，分解水中的微气泡是强化水合物再生成的重要因素，可为降压开采时水合物再生成行为的调控提供理论支撑。

关键词: 水合物, 分解水, 气泡, 再生成

CLC Number:

TE31

Siyang LI, Yujie ZHU, Yuzhou CHEN, Kang LIU, Peng XIAO, Changyu SUN. Evolution of bubbles in dissociated water and their enhancement mechanism for hydrate reformation[J]. The Chinese Journal of Process Engineering, 2026, 26(9): 1031-1041.

李思阳, 朱玉洁, 陈瑜洲, 刘康, 肖朋, 孙长宇. 分解水中气泡演变及其对水合物再生成的强化机理[J]. 过程工程学报, 2026, 26(9): 1031-1041.

Figures/Tables 16

Fig.1 Schematic diagram of the experimental apparatus

Table 1 Experimental conditions for hydrate dissociation

Runs	System	Dissociation pressure/MPa	Dissociation temperature/K
1	Pure water	3.0	276.15
2	Pure water	1.5	276.15
3	Pure water	0.1	276.15
4	0.01wt% SDS solution	3.0	276.15

Table 2 Experimental conditions for hydrate reformation

Runs	System	Dissociation pressure/MPa	Dissociation temperature/K	Reformation pressure/MPa	Reformation temperature/K
5	Pure water	3.0	276.15	4.0	273.15
6	Pure water	3.0	276.15	5.0	273.15
7	Pure water	3.0	276.15	7.3	273.15
8	Pure water	1.5	276.15	5.0	273.15
9	Pure water	0.1	276.15	5.0	273.15
10	Pure water	-	-	5.0	273.15
11	0.01wt% SDS solution	3.0	276.15	5.0	273.15
12	0.005wt% SDS solution	3.0	276.15	5.0	273.15
13	0.0025wt% SDS solution	3.0	276.15	5.0	273.15

Fig.2 Comparison of residual bubble population in the liquid phase under different dissociation conditions: (a) dissociation pressure of 3.0 MPa; (b) dissociation pressure of 1.5 MPa; (c) dissociation pressure of 0.1 MPa; (d) methane-saturated aqueous solution

Fig.3 Morphologies of bubbles at different locations in the liquid phase before and after 72 h under 3.0 MPa decomposition pressure

Fig.4 Typical morphology changes of bubbles in the liquid phase

Fig.5 Bubble morphologies in dissociated water under different dissociation pressures at high magnification: (a) 3.0 MPa; (b) 0.1 MPa

Fig.6 Comparison of the residual bubble population in pure water and SDS solution: (a) pure water; (b) SDS solution

Fig.7 Water conversion efficiency of hydrate reformation after hydrate dissociation at different pressures

Fig.8 Morphologies of hydrate reformation after hydrate dissociation under different pressures: (a) 3.0 MPa; (b) 0.1 MPa

Fig.9 Water conversion efficiency of hydrate reformation under different reformation pressures

Fig.10 Changes in hydrate morphology with time during the reformation process at various pressures: (a) 4.0 MPa; (b) 5.0 MPa; (c) 7.3 MPa

Fig.11 Changes in confocal microscopic images and the corresponding Raman spectra of hydrates with time during reformation at 7.3 MPa: (a) 30 min; (b) 1 h; (c) 2.5 h; (d) 5.3 h

Fig.12 Water conversion efficiencies of hydrate reformation in SDS solution and pure water

Fig.13 Changes in hydrate morphology during the reformation process in SDS solutions: (a) 0.0025wt% SDS solution; (b) 0.005wt% SDS solution; (c) 0.01wt% SDS solution

Fig.14 Changes in confocal microscopic images and the corresponding Raman spectra of hydrates with time during reformation in 0.01wt% SDS solution: (a) 5 min; (b) 2.2 h; (c) 5.2 h; (d) 25.3 h

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