Elsevier

Applied Energy

Volume 243, 1 June 2019, Pages 266-273
Applied Energy

Dissociation characteristics of methane hydrates in South China Sea sediments by depressurization

https://doi.org/10.1016/j.apenergy.2019.03.160Get rights and content

Highlights

  • Methane hydrate production was assessed in South China Sea sediments.

  • Higher hydrate saturation increased the dissociation duration.

  • Dissociation duration barely affected the return time of sediment temperature.

  • Double depressurization is employed to increase the production efficiency.

Abstract

Marine methane hydrate is a considerable energy source for use in the near future. With great obstacles to spot production, researchers are focusing on the production characteristics of hydrates in various experimental systems, such as glass beads, clay, silica sand and so on. This study investigated the production behaviors of methane hydrate in the real South China Sea sediments using depressurization method. The hydrate saturations of the remolded hydrate-bearing sediments ranged from 10.10% to 23.76%. The results indicate that an 8% increase of hydrate saturation can prolong the dissociation duration under the same backpressure of 2 MPa by 120 min. In addition, the excess temperature drop caused by the depressurization may induce the unpredictable occurrence of hydrate reformation or icing; therefore, a double depressurization method that depressurizes to 2 MPa (second stage) after 20 min maintenance at 4 MPa (first stage) is employed in order to shorten the temperature drop in the first stage and increase the dissociation rate in the second stage. The results of this study are significant for the spot production of marine hydrates in order to achieve high efficient gas production.

Introduction

With the rapid development of urbanization and industrialization, the consumption of methane gas has continually increased over the recent decades. Methane hydrate (MH), which is also known as flammable ice, is a kind of crystalline compound that stores methane gas [1]. It is estimated that the proven organic carbon storage in MH reservoirs is twice the total amount of coal, oil and natural gas [2]. According to the assessment, the recoverable reserves of worldwide MHs are approximately 3,000 trillion cubic meters [3]. Flammable ice is thus regarded as an important source of energy for future decades [4], [5]. The locations of flammable ice reservoirs are mainly in permafrost and marine regions [6]. Marine MHs are widely found in the shallow sediments of the South China Sea (SCS) by Guangzhou Marine Geological Survey of China [7]. It is reported that the hydrates with a saturation of approximately 40% are mainly distributed in the shallow layers approximately 150 m below the sea floor.

In addition to frequent explorations in recent years, some countries including the USA, Japan and China have conducted several pilot exploitation projects of marine MH reservoirs. According to the results of the exploration and exploitation, the SCS hydrate-bearing sediments are found to be composed of silt clay [7]. In addition, the dissociation characterization of the hydrate-bearing sediments from the Pearl River Mouth Basin in the SCS show that the hydration number of marine MH is approximately 5.90, and methane accounts for at least 99.9% of the recovered gas [8]. The physical analysis of the SCS sediments indicates that the moisture ranges from 16.84% to 42.93%, and the density is approximately 3.19 g/cm3 [9]. Till now, there are still huge difficulties for MH exploitation in the muddy fine silt sediments [10].

By changing the pressure and temperature conditions to dissociate MHs, the exploitation methods of MH reservoirs include depressurization [11], [12], [13], thermal stimulation [14], [15], [16], and other unconventional or combined methods [17], [18], [19], [20]. Previously, a large number of experimental studies on hydrate dissociation were conducted [21], [22], [23]. There are many studies about methane hydrate dissociation via depressurization [24], [25], [26], [27], [28], [29]. Li’s research group conducted many experiments on methane hydrate dissociation using depressurization [30], [31], [32], [33]. Their findings on the five periods during the gas depressurization production process and the heat transfer law of sediments are significant for the investigation of the MH dissociation characteristics. Recently, Wang et al. [34] investigated the effects of icing on MH depressurization dissociation. In addition, Chong et al. [19], [35] researched the influence of backpressure on gas recovery in silica sand using depressurization.

Real marine sediments are rarely investigated as the hydrate carrier in laboratories [36]. In this study, the real sediments from the South China Sea were employed to remold the hydrate-bearing sediments. Furthermore, two depressurization methods, single depressurization and double depressurization, were employed to dissociate the methane hydrate with different saturations in the real marine sediments. The results of this study on the gas production and temperature change of hydrate-bearing sediments are significant for guiding the spot production of marine hydrates.

Section snippets

Apparatus and materials

Fig. 1 shows the sketch of MHs dissociation experimental system for real marine sediments [37]. The experimental system includes three customized pressure vessels, which are the reactor, the gas-liquid separator and the gas collector. The reactor with an inner volume of 1000 mL is used to form and dissociate hydrates and can be pressurized up to 10 MPa and cooled to −10 °C using a circulator (AD28R, PolyScience). The gas-liquid separator is connected to the reactor via a control valve (Fisher

The MH formation situation in real marine sediments

The real marine sediments are different from laboratory porous media, such as glass sands, silica sands, river sands and so on [39], [40], [41], [42]. It is hard to remold the hydrate-bearing sediments, which is the exact same as the marine conditions. Limited by the moisture content of sediments, the hydrates are formed at excess gas condition in this study. Hydrates are formed at different pressures in order to control the hydrate saturation. Due to the pressure limitation of the reactor, a

Conclusion

Real sediments from the South China Sea were employed in order to investigate the formation and dissociation characteristics of methane hydrate in this study. The remolded hydrate-bearing sediments with different hydrate saturations ranging from 10.10% to 23.76% were dissociated using depressurization. The dissociation duration under the same pressure will increase with the increase of the hydrate saturation. An 8% difference of the hydrate saturation can cause a 120 min increase of the

Acknowledgements

This study was financially supported by grants from the National Natural Science Foundation, China (51436003, 51822603 and 51576025), the National Key Research and Development Plan, China (2017YFC0307303 and 2016YFC0304001), the Fok Ying-Tong Education Foundation for Young Teachers in the Higher Education Institutions, China (161050) and the Fundamental Research Funds for the Central Universities, China (DUT18ZD403). In addition, we thank the anonymous reviewers for their suggestions in

Associated content

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

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