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Frontiers in Energy

Front. Energy    2020, Vol. 14 Issue (3) : 433-442     https://doi.org/10.1007/s11708-020-0684-1
RESEARCH ARTICLE
Key issues in development of offshore natural gas hydrate
Shouwei ZHOU(), Qingping LI, Xin LV(), Qiang FU, Junlong ZHU
State Key Laboratory of Natural Gas Hydrate, CNOOC Research Institute Co. Ltd, Beijing 100028, China; Oil and Gas Reservoir Geology and Exploitation, Chengdu 610500, China
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Abstract

As a new clean energy resource in the 21st century, natural gas hydrate is considered as one of the most promising strategic resources in the future. This paper, based on the research progress in exploitation of natural gas hydrate (NGH) in China and the world, systematically reviewed and discussed the key issues in development of natural gas hydrate. From an exploitation point of view, it is recommended that the concepts of diagenetic hydrate and non-diagenetic hydrate be introduced. The main factors to be considered are whether diagenesis, stability of rock skeleton structure, particle size and cementation mode, thus NGHs are divided into 6 levels and used unused exploitation methods according to different types. The study of the description and quantitative characterization of abundance in hydrate enrichment zone, and looking for gas hydrate dessert areas with commercial exploitation value should be enhanced. The concept of dynamic permeability and characterization of the permeability of NGH by time-varying equations should be established. The ‘Three-gas co-production’ (natural gas hydrate, shallow gas, and conventional gas) may be an effective way to achieve early commercial exploitation. Although great progress has been made in the exploitation of natural gas hydrate, there still exist enormous challenges in basic theory research, production methods, and equipment and operation modes. Only through hard and persistent exploration and innovation can natural gas hydrate be truly commercially developed on a large scale and contribute to sustainable energy supply.
Keywords natural gas hydrate exploitation offshore      diagenetic and non-diagenetic hydrate      solid-state fluidization method      dessert in enrichment area      three-gas combined production on gas hydrate abundance     
Corresponding Author(s): Shouwei ZHOU,Xin LV   
Online First Date: 13 July 2020    Issue Date: 14 September 2020
 Cite this article:   
Shouwei ZHOU,Qingping LI,Xin LV, et al. Key issues in development of offshore natural gas hydrate[J]. Front. Energy, 2020, 14(3): 433-442.
 URL:  
http://journal.hep.com.cn/fie/EN/10.1007/s11708-020-0684-1
http://journal.hep.com.cn/fie/EN/Y2020/V14/I3/433
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Shouwei ZHOU
Qingping LI
Xin LV
Qiang FU
Junlong ZHU
Fig.1  Oil autonomous sampling by CNOOC.
Degree
1 2 3 4 5 6
Diagenesis Diagenetic hydrate Diagenetic hydrate Main diagenetic hydrate Non-diagenetic hydrate Non-diagenetic hydrate Non-diagenetic hydrate
Structural degree of rock skeleton Hydrate fills in skeleton voids, and rock skeleton is stable after complete decomposition Hydrate is basically filled in skeleton voids, and the deformation of rock skeleton is smaller after complete decomposition Hydrate is a part of skeleton, and the deformation of rock skeleton is larger after complete decomposition Hydrate constitutes the main body of skeleton, and basic deformation of rock skeleton after complete decomposition Hydrate is the skeleton. After complete decomposition, the rock skeleton collapses Pure hydrate
Particle size and cementing method >500 μm Coarse grains 500–250 μm
Medium grains		 250–100 μm
Fine grains		 100–50 μm
Coarse silts		 <5 μm
Fine silts		 –
Specific heat of rocks <800 800–1200 1200–1600 1600–2000 >2000
Hydrate saturation >0.6 0.6–0.5 0.5–0.3 0.3–0.1 <0.1 >0.9
Porosity >0.6 0.6–0.5 0.5–0.4 0.4–0.3 <0.3 -
Example Mesoyaha Buried depth: 700–800 m, thickness 84 m, porosity 16%–38%, the only commercial hydrate mining well Aichi Sea, Japan; the median particle size of the sample is 133.2 μm, accounting for 72.3% In the Shenhu sea area of the South China Sea, the particle size distribution of hydrate samples in the South China Sea is mainly between 0.221 and 174.55 microns, and below 40 μm, the particle size distribution reaches 83.25% Dongsha, South China Sea
Tab.1  Natural gas hydrate reservoir classification method from exploitation
No. Sample depth/mbsf1 Median diameter/μm D16/μm D25/μm D75/μm D84/μm
Topsoil – 5.303 1.999 2.752 9.484 12.7
D-100 100 21.72 4.941 8.202 42.21 55.77
D-120.2 120.2 14.44 3.636 5.948 30.36 41.62
D-120.5 120.5 13.01 3.192 5.132 28.74 39.60
D-122 122 12.56 3.146 5.02 27.88 38.24
Tab.2  Analysis of hydrate samples from a well in South China Sea by CNOOC
Exploitation modes Types of gas hydrates
Diagenetic hydrate Basic diagenesis (serious sand production) Non-diagenetic hydrate (mud-free sand cover, exposed seabed) Non-diagenetic hydrate
(with mud-sand cap)
Exploitation Decompression Decompression Solid fluidization Solid fluidization
Auxiliary method Heating, injection and carbon dioxide replacement, etc. Heating, injection and carbon dioxide replacement, etc. Seabed mining+ decompression Underground articulated suction crushing+ decompression+ injection chemical agent
Sand control mode Sand Control by Combining Gravel Filling Combining Pre-expansion GeoFORM sand Control system Underwater separation+ backfilling Underwater separation+ backfilling
Tab.3  Exploitation modes of different gas hydrates
Fig.2  CT image of gas hydrate samples from the South China Sea.
Fig.3  Comparison of natural gas reserves abundance.
Fig.4  Explanation of electric S-well test in the South China Sea.
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