| Citation: |
Xu-wen Qin, Jing-an Lu, Hai-long Lu, Hai-jun Qiu, Jin-qiang Liang, Dong-ju Kang, Lin-sen Zhan, Hong-feng Lu, Zeng-gui Kuang, 2020. Coexistence of natural gas hydrate, free gas and water in the gas hydrate system in the Shenhu Area, South China Sea, China Geology, 3, 210-220. doi: 10.31035/cg2020038
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Coexistence of natural gas hydrate, free gas and water in the gas hydrate system in the Shenhu Area, South China Sea
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Abstract
Shenhu Area is located in the Baiyun Sag of Pearl River Mouth Basin, which is on the northern continental slope of the South China Sea. Gas hydrates in this area have been intensively investigated, achieving a wide coverage of the three-dimensional seismic survey, a large number of boreholes, and detailed data of the seismic survey, logging, and core analysis. In the beginning of 2020, China has successfully conducted the second offshore production test of gas hydrates in this area. In this paper, studies were made on the structure of the hydrate system for the production test, based on detailed logging data and core analysis of this area. As to the results of nuclear magnetic resonance (NMR) logging and sonic logging of Well GMGS6-SH02 drilled during the GMGS6 Expedition, the hydrate system on which the production well located can be divided into three layers: (1) 207.8–253.4 mbsf, 45.6 m thick, gas hydrate layer, with gas hydrate saturation of 0–54.5% (31% av.); (2) 253.4–278 mbsf, 24.6 m thick, mixing layer consisting of gas hydrates, free gas, and water, with gas hydrate saturation of 0–22% (10% av.) and free gas saturation of 0–32% (13% av.); (3) 278–297 mbsf, 19 m thick, with free gas saturation of less than 7%. Moreover, the pore water freshening identified in the sediment cores, taken from the depth below the theoretically calculated base of methane hydrate stability zone, indicates the occurrence of gas hydrate. All these data reveal that gas hydrates, free gas, and water coexist in the mixing layer from different aspects.
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Keywords:
- Mixing layer /
- Gas hydrate /
- NMR logging /
- Sonic logging /
- Core analysis /
- Oil gas exploration engineering /
- Shenhu Area /
- South China Sea /
- China
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References
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Xu-wen Qin, Jing-an Lu, Hai-long Lu, Hai-jun Qiu, Jin-qiang Liang, Dong-ju Kang, Lin-sen Zhan, Hong-feng Lu, Zeng-gui Kuang, 2020. Coexistence of natural gas hydrate, free gas and water in the gas hydrate system in the Shenhu Area, South China Sea, China Geology, 3, 210-220. doi: 10.31035/cg2020038
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Figure 1.
The geological map of the study area. a–regional geological background and the location of the study area (marked with a red square); b–the relative positions of Well GMGS6-SH02 and Well GMGS5-SH17.
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Figure 2.
Pore model for NMR logging of reservoir sediments.
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Figure 3.
Flowchart for saturation prediction of gas hydrates and free gas in sediments based on sonic logging data and rock physics model.
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Figure 4.
Baselines of P-wave velocity (a) and S-wave velocity (b) predicted using the rock physics model.
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Figure 5.
Downhole logging curves of Well GMGS6-SH02.
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Figure 6.
Gas hydrate saturation and free gas saturation of Well GMGS6-SH02 estimated based on NMR and density logging.
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Figure 7.
Gas hydrate saturation and free gas saturation of Well GMGS6-SH02 predicted with rock physics model. a–gas hydrate saturation predicted by P-wave velocity; b–free gas saturation predicted by P-wave velocity; c–gas hydrate saturation predicted by S-wave velocity; d–free gas saturation predicted by S-wave velocity.
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Figure 8.
Gas hydrate saturation estimated from chloride ions in Well GMGS5-SH17.