Study on the decomposition characteristics of natural gas hydrate formed in porous media with different saturations

The exploitation of natural gas hydrate (NGH) is of great significance to alleviate the world energy crisis and solve the problem of natural gas resource shortage in China. Facts prove that depressurization method is a relatively simple, safe, and effective method for the exploitation of natural gas hydrates. However, there is a lack of in-depth understanding of the decomposition characteristics of hydrates with high hydrate saturation under depressurization. Therefore, the formation and decomposition experiments of NGH were carried out in the porous medium (quartz sand) system. The decomposed characteristics of NGH under three different saturation conditions (38.1%, 42.1%, 46.4%) were studied, and the decomposition pressures were 2 MPa and 3 MPa, respectively. The results show that the hydrate decomposition rate is quite high corresponding to 2 MPa, and the simulated reservoir temperature drops below the freezing point, that will cause ice blocking to stop further decomposition of hydrate in the actual production of NGH. When the decomposition pressure is 3 MPa, from 0 h to 2 h, the gas production rate decreases with the increase in hydrate saturation, and from 2 h to the end of the experiment, the gas production rate increases with the increase of hydrate saturation. In the depressuring stage, the reservoir temperature (T₁, T₂, T₃) of NGH with different saturations drops rapidly to a similar minimum. In the constant pressure stage, the reservoir temperature of NGH with different saturations recovers to rise. The higher the saturation, the greater the reservoir temperature fluctuation and longer the time to return to the experimental temperature. In order to achieve high-efficiency exploitation, measures have to be taken to prevent ice blocking. The hydrate decomposition pressure should be set at 3 MPa, and the reservoir with a high hydrate saturation should be set for a longer decomposition time to increase CH₄ recovery rate and ensure the efficiency of exploitation.