Enhanced CH4 recovery from hydrate-bearing sand packs via CO2 replacement assisted thermal stimulation method

Abstract

Natural gas exploitation from natural gas hydrate (NGH) reservoir by thermal stimulation combined with CO2 replacement is a promising method to enhance the natural gas recovery as well as sequestrate CO2 permanently. However, the operation scheme of thermal stimulation is crucial for the efficiency of both gas recovery and CO2 sequestration. In this study, a novel CO2–CH4 replacement assisted thermal stimulation method was proposed to enhance both CH4 recovery and CO2 sequestration simultaneously. Experiments were conducted in the CH4 hydrate unstable zone with pressure and temperature range from 1.97 to 3.22 MPa and 273.63–276.17 K, respectively. One step, continuous and stepwise thermal stimulation schemes with and without CO2 replacement were applied to evaluate the effect of different operation schemes on gas recovery and CO2 sequestration. The results indicated that the CO2–CH4 replacement process significantly increased the CH4 recovery percentage by 8.21%–34.79%. Overall, higher pressure and temperature were able to provide higher CO2–CH4 exchange driving force, which leaded to higher CH4 recovery ratio and CO2 sequestration efficiency. Among three thermal stimulation schemes, the stepwise thermal stimulation showed the best CH4 recovery of 62.60% and CO2 sequestration efficiency of 71.23% on average. The maximum CH4 recovery percentage of 69.51% and CO2 sequestration efficiency of 88.29% were obtained at 3.21 MPa and 276.13 K with stepwise thermal stimulation. The mechanism of this recovery enhancement can be concluded to that CH4 hydrate decomposition rate was enhanced by repeatedly imposed heat fluctuation in stepwise thermal stimulation and eventually the CH4–CO2 replacement process in hydrate was promoted. By comparison with published data, the CH4 recovery efficiency in this work increased by 19.2% on average. We expected that the proposed combination method could be applied in the late stage of NGH reservoir exploitation to obtain an enhanced CH4 gas production yield with sequestration of CO2 simultaneously.