Enhancing gas recovery from natural gas hydrate reservoirs in the eastern Nankai Trough: Deep depressurization and underburden sealing

Depressurization is recognized as the most effective way to recover natural gas hydrate. However, the bottom-hole pressure is usually controlled above the quadruple point and few studies focus on reducing the pressure below that one. Even though some laboratory-scale studies have been performed, the application is not validated in the reservoir-scale production. Therefore, this study constructs a prediction model based on the field survey data in Japan to test the feasibility of deep depressurization (i.e., reducing the bottomhole pressure below the quadruple point) in the actual reservoir. The effect of ice formation on gas production is discussed in detail. Moreover, a method of deep depressurization combined with underburden sealing is innovatively proposed to further stimulate productivity and the different sealing effects are quantitatively analyzed. The results suggest that deep depressurization can effectively enhance gas production by forming ice and increasing driving force for hydrate dissociation, and the flowing channel blockage may be negligible. Underbuden sealing can further improve productivity by increasing pressure drop transfer under deep depressurization. A relatively higher sealing ratio corresponds to a higher gas recovery and a higher gas-to-water ratio, and the total gas production can increase by about 41.9% when the sealing ratio reaches 50.

Automated summary

This study constructs a prediction model based on field survey data in Japan to test the feasibility of deep depressurization in an actual reservoir below the quadruple point. The impact of ice formation on gas production is discussed and a method of deep depressurization combined with underburden sealing is proposed. The results show that deep depressurization can enhance gas production effectively and underburden sealing can further improve productivity.