Thermodynamics analysis and ice behavior during the depressurization process of methane hydrate reservoir

As potential alternative energy sources, natural gas hydrate has attracted international attentions. In order to optimize its exploitation process, the thermodynamics characteristics during hydrate dissociation were investigated in this study. Eleven reservoirs with three ratios of gas amounts in gas and hydrate phases (2.1, 2.9 and 4.6) were employed, and four exhaust rates (2.3, 4.7, 7.1 and 8.8 ln/min) were conducted to pressurize the reservoirs to 2.0 MPa. Hydrates dissociated at a constant rate during the depressurization stage, and the dissociation rate increased with higher depressurization rate, while the effect is enhancing with the increase of gas space. The temperature decreased with the dropped pressure in a similar trend, affected by both depressurization rate and hydrate saturation. During constant-pressure stage, the hydrate-bearing reservoir temperature kept at approximately −2.5 °C until ice formation, making the temperature increase to −1.5 °C. Ice formation can nearly quintuple the instantaneous rate of hydrate dissociation, while no obvious changes in hydrate dissociation before and after the end of ice melting. In addition, the induction time and existence duration of ice obey good stepwise spatial and temporal sequences. These results are great significant for pressure controlling and efficiency optimization of later period of methane hydrate exploitation.