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The sediment type in the “Shenhu” sea area of the South China Sea is mud sediment, with poor reservoir physical conditions, and the seabed is loose with a small pressure window. These conditions pose heightened safety challenges for the extraction of hydrates in this region. This study develops a multiphase flow model that accounts for the endothermic decomposition of hydrates and employs the finite difference method for its solution. Utilizing this model, the multiphase flow characteristics during depressurization extraction at a specific well in the “Shenhu” area are investigated. Building on this, the study analyses the impact of varying pump rates and geothermal gradients on multiphase flow properties, hydrate yield, and engineering safety. Based on the analytical findings, recommendations are proposed to balance hydrate production with engineering safety, effectively mitigating potential engineering accidents during depressurization extraction. The outcomes of this research offer technical guidance for the commercial exploitation of hydrates in the “Shenhu” area of the South China Sea, laying a foundation for future related studies.

Recently, drilling wells have encountered rich gas hydrates in fine-grained sediments in the northern South China Sea. Gas hydrate in fine-grained sediments is very heterogeneous, and its physical properties are different from those of oil and gas reservoirs. The reliability of the classical logging saturation evaluation models established for diagenetic reservoirs is questionable. This study used four wells in GMGS3 and GMGS4 to evaluate the effects of the application of three typical methods for evaluating saturation with different principles in the unconsolidated fine-grained sediments: nuclear magnetic logging, sigma logging, and the Archie formula. It was found that the value of the lithologic capture cross-section in sigma logging and the rock's electrical parameters in the Archie formula affect the accuracy of the model. Therefore, to obtain a reliable saturation value for fine-grained sediments, an innovative method for the calculation of resistivity and acoustic time is proposed to estimate gas hydrate saturation based on logging data, which is most consistent with the results of core analysis. The overall relative error of the verification well was 5.87%, whereas that of the density NMR logging method was 56%, showing that the accuracy of the newly proposed resistivity DT logging method's saturation formula was significantly improved. Finally, a new model-based cross chart was developed, which can rapidly differentiate gas saturation during drilling. [ABSTRACT FROM AUTHOR]

The second gas hydrate expedition of the Guangzhou Marine Geological Survey drilled 13 sites and recovered a large amount of gas hydrate in the east of the Pearl River Mouth basin of the South China Sea. In this study, we examine three of these sites (sites 05, 08, and 16) using multichannel seismic data, LWD logging data, and multi-beam echo sounder data. The geophysical data suggest and core samples confirm that gas hydrates occur near the BSR at all three sites, whereas near-surface gas hydrates exist at sites 08 and 16 with high fluid flux. We present a conceptual model for the formation and accumulation of gas hydrates in our study area, controlled by the activity of dissolved and free gas-rich fluid. Interpreted abundant gas sources below the BSR contribute to the generation of overpressure, resulting in the movement of gas-rich fluid. Dissolved gas-rich fluid forms pore filling hydrates near the BSR, and the relatively deep hydrate layer thickens over time by making its way through the growing hydrate layer and adding to the top, if the fluid is supplied sufficiently. Free gas-rich fluid is more favorable to generate near-surface nodules or massive hydrates, which could be stable in natural conditions. As sediments deposit, the relatively shallow hydrates are buried deeper and keep their morphologies. Furthermore, gas hydrate formation and stability imply that gas hydrates may be increasingly considered as a potential energy resource. [ABSTRACT FROM AUTHOR]