Numerical analysis of hydrate exploitation performances of depressurization combined with thermal fluid injection in marine hydrate reservoir with impermeable boundary

Abstract

The combination of depressurization and thermal fluid injection (D + Ti) is a very promising method to realize the efficient development of marine natural gas hydrate (NGH). However, the performance of D + Ti for hydrate production in the reservoir with impermeable boundaries has received little attention. In this work, the hydrate exploitation performances of D + Ti in NGH reservoir with impermeable overlying layer or impermeable underlying layer (Oimp + Up, Op + Uimp, Oimp + Uimp) were numerically investigated. The reservoir with permeable overlying layer and underlying layer (Op + Up) was used for comparison. Moreover, the effect of thermal fluid injection temperature (Ti) and injection pressure (Pi) on hydrate decomposition and gas production behaviors in the reservoir with Oimp + Uimp or Op + Up were studied in detail. The results indicate that the hydrate decomposition potential of the reservoir with Op + Up, Oimp + Up or Op + Uimp is similar, which is significantly lower than that of the reservoir with Oimp + Uimp. The presence of impermeable layer in reservoir can enhance gas production and reduce water production, especially when both overlying and underlying layers are all impermeable. The gas to water ratio and energy ratio of the reservoir with Oimp + Uimp are 5.0–7.4 times and 5.5–8.6 times that of the other three reservoirs, respectively. On the other hand, the improvement of Pi increase on hydrate decomposition and gas production is far superior to the improvement caused by Ti rise. Moreover, the increase of Pi and Ti has a more significant improvement on gas production in the reservoir with Oimp + Uimp than that in the reservoir with Op + Up. The maximum improvement of gas production in the reservoir with Oimp + Uimp is 69.6%. These results can provide theoretical support for the exploitation of marine NGH by D + Ti.