Abstract
Natural gas hydrates samples are rare and difficult to store and transport at in situ pressure and temperature conditions, resulting in difficulty to characterize natural hydrate-bearing sediments and to identify hydrate accumulation position and saturation at the field scale. A new apparatus was designed to study the acoustic properties of seafloor recovered cores with and without hydrate. To protect the natural frames of recovered cores and control hydrate distribution, the addition of water into cores was performed by injecting water vapor. The results show that hydrate saturation and types of host sediments are the two most important factors that govern the elastic properties of hydrate-bearing sediments. When gas hydrate saturation adds approximately to 5–25%, the corresponding P-wave velocity (Vp) increases from 1.94 to 3.93 km/s and S-wave velocity (Vs) increases from 1.14 to 2.23 km/s for sandy specimens; Vp and Vs for clayey samples are 1.72–2.13 km/s and 1.10–1.32 km/s, respectively. The acoustic properties of sandy sediments can be significantly changed by the formation/dissociation of gas hydrate, while these only minorly change for clayey specimens.
Highlights
The combustion of traditional fossil fuel has caused serious environmental issues, such as global warming, leading to the growing demand for low-carbon energy
Natural gas hydrates are solid crystals, which is a special form of natural gas that is widely accumulated in subsea and permafrost zones [1]
It has been estimated that the gross organic carbon in the form of natural gas hydrates exceeds the reserves of all known fossil fuels on the earth, including coal, oil, and conventional gases [2,3]
Summary
The combustion of traditional fossil fuel has caused serious environmental issues, such as global warming, leading to the growing demand for low-carbon energy. Reported that methane hydrate can cement unconsolidated sediment when forming in an abundant gas environment, which would increase the acoustic velocity of hydrate-bearing sediments. Except for hydrate saturation and pore space occupants, the lithologic characteristics of host sediments, test conditions (burial depth) and hydrate distribution can affect the acoustic velocity of hydrate-bearing sediments [15,16]. To accommodate the need for better methods to obtain elastic properties, we propose a new water-adding method for the preparation of gas hydrate in natural recovered cores, and aim at studying the influence of lithologic characteristics of host sediments, test conditions, and pore space occupancy on the acoustic properties of hydrate-bearing sediments, which are necessary for geologic exploration and hydrate development
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