Experimental study on 2-D acoustic characteristics of hydrate-bearing sediments

Key words:  natural gas hydrate  reservoir  2-D acoustic characteristics  hydrate saturation  simulation experiment

Abstract:

In order to obtain the hydrate distribution and 2-D acoustic characteristics of a reservoir, an experimental system was developed to measure the velocity structure of the hydrate-bearing sediment. It is found that when the hydrate saturation is less than 15％, the acoustic velocity in each layer increases rapidly, while with greater than 15％ saturation the acoustic velocity increases slowly. In the first stage of hydrate formation, the acoustic velocity in the coarse sediment is larger than that in the fine sediment. In the final hydrate formation, the acoustic velocities in different layers have little difference, and hydrates are almost evenly distributed in the reservoir. The 2-D hydrate distribution and its acoustic responses during the hydrate formation process show that in the longitudinal direction, the hydrate preferentially forms closely to gas source, thus the acoustic velocity is large in this area. In the transverse direction, the hydrate preferentially forms around the wall of high pressure cells, resulting in larger acoustic velocities than that in the internal sediments. The empirical formula of P-and S-wave velocity in hydrate-bearing sediments, as well as the correlation between the wave velocity ratio and the hydrate saturation are also obtained.