Gas hydrate (GH) is a strategic energy resource in China, but GH dissociation may cause marine geological hazards. The researchers in the group of soil mechanics in the Institute of Mechanics, CAS have established integrated experimental apparatus and analytical methods for the safety evaluation of GH exploration and exploitation since 2005.
Layered fracture and gas outburst were found in the experiments of heating- induced GH dissociation (J. Petro. Sci. Eng., 2011), and the related critical conditions were obtained for the two kinds of stratum failures. Stratum instabilities such as marine slide during hydrate dissociation were studied through a series of scaled and geotechnical centrifugal tests. This new type of fluid-solid coupling problem contains four physical effects: heat conduction, phase transformation, multiphase seepage, soil deformation and failure.
Layered fracture and gas outburst occur due to the stratum softening and the formation of excess pore pressure during GH dissociation, meanwhile the other reason is the development of GH dissociation zone leads to the decrease of the resistance and the increase of excess pore fluid pressure of the over layer. Layered fracture is filled with gas and water cracking the continuous stratum (J. Petrol. Sci. Eng., 2015); Gas erupts with soil and water as three phase flow under considerable gas expansion (Int. J. Numer. Anal. Met., 2015); Layered fractures or continuous layered fracture formed between the overlying stratum and hydrate dissociation stratum can lead to shallow marine slide with a large volume and small slope (Ocean Eng., 2015; 2016).
The thermal and mechanical parameters of the stratum during GH dissociation were obtained in the experimental platform (J. Heat Trans.-T. ASME, 2014; Mar. Petrol. Geol., 2015; Int. J. Offshore Polar, 2015), and the analytical models were presented to describe the critical conditions and the development of these three types of stratum instabilities (Fig. 2). These results can support the GH-related engineering in South China Sea.
Fig. 2 The analytical models for layered fracture, gas outburst and stratum slide
We thank for the support of National Natural Scientific Foundation of China, and projects of China Geology Survey and CNPC-CAS Cooperation.