Natural gas hydrates (NGHs), considered as an alternative energy source for the future with the gradually increasing energy consumption and the fossil fuels approaching depletion, widely exist in permafrost regions and sea-floor. Understanding the properties of hydrate-bearing sediments such as hydrate saturation, lithology, density, and permeability, is of importance for diverse energy and environmental issues, which contributes to the evaluation of the hydrate resources in situ and even the developing technology of naturally hydrate-bearing sediments without geo-hazards. The acoustic wave velocity is one of the significant geophysical properties, which can provide basis for the geological interpretation of seismic data of hydrate-bearing sediments.<br/>In this work, an experimental apparatus was used to measure P-wave velocity of gas-hydrate-bearing sediment. Keeping a low system temperature was purposed to synthesize hydrate-bearing sediments with homogeneous distribution. Then rising and subsequently cooling the system temperature made hydrate-bearing sediments with heterogeneous distribution to be synthesized. Methane was used as a free gas during hydrate formation process. Both hydrate concentration and distribution play an important role in investigating the acoustic of hydrate-bearing sediments. The influence of water migration as a result of gravity along with the rising temperature on acoustic properties of hydrate-bearing sediments was investigated in the hydrate formation process. The amplitude for the first arrival wave signal was also collected immediately during hydrate formation process. The results showed that the P-wave velocity of the homogeneous distribution hydrate-bearing sediment is faster than that of the heterogeneous for a same hydrate saturation. In this work, it is the first time to propose to divide hydrate formation process into a nucleation phase and a growth phase according to the growth rate for amplitude. It also macroscopically proves the memory effect occurring in the nucleation stage during the hydrate formation process in the porous sediments.

Acoustic properties of different distribution in hydrate-bearing sediments

256th National Meeting (2018)

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