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
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.
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%
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
Fig.2
Fig.3
Fig.4
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