Abstract
As an environment-friendly natural gas hydrate exploitation method, CO2 replacement method can not only achieve the purpose of mining natural gas hydrate, but also store the current greenhouse gas CO2 in the form of hydrate on the seabed, and maintain the stratum stability of hydrate deposit area. In order to improve the rate and efficiency of CH4-CO2 replacement reaction, researchers proposed to use CO2 contained gas mixture instead of pure CO2 to replace CH4 in natural gas hydrate. Based our previous work about CH4 hydrate recovery with 40% CO2 + 60% H2, in this study, the effect of gas concentration in gas phase on final CH4 recovery are investigated by implying different time interval of gas exchange operation. Experimental results show that The CH4 recovery efficiency is 10.41 when the gas exchange is continues through the whole replacement process, and CH4 recovery efficiency changes to 12.25, 32.24 and 28.86 when gas exchange operation is carried out every 12, 24, 36 h. Indicating that replaced CH4 needs to be discharged in time to avoid CH4 molecules being replaced to form hydrates again, and it is necessary to accurately control the time interval of gas exchange operation to avoid insufficient contact time between CO2 and H2 molecules and CH4 hydrate, which affects the final replacement efficiency. In addition, the mechanism of CO2 gas mixture containing small gas molecule such as H2, N2 are studied. The results indicate that when CO2 containing small molecules such as H2 and N2 displace CH4 hydrate, the existence of small molecules (H2, N2) can give rise to decompose the hydrate lattice and release CH4 gas. If the gas molecules (CO2, N2, H2, CH4) in the gas phase have enough driving force to enter the hydrate lattice and remain stability, CH4 hydrate will not decompose completely; If not, CH4 hydrate will be completely decomposed.
Highlights
Natural gas hydrate (NGH), which is widely distributed in continental margin and permafrost, is naturally formed when excess gas and water molecules exist in high and low temperature zones (Sloan and Koh, 2007; Chong et al, 2016)
Exchange Interval in CH4 Recovery gas from reservoirs is to use the driving potential based on temperature, pressure and chemical potential difference to change the equilibrium condition of NGH reservoir and decompose NGH (Li et al, 2016), including thermal stimulation (Wang et al, 2017; Li et al, 2008; Fitzgerald and Castaldi, 2013), depressurization (Yang et al, 2012; Zhao et al, 2013) and chemical inhibitor injection (Yuan et al, 2011; Javanmardi et al, 2013)
The researchers proposed the exploitation of CH4 hydrate with CO2 containing mixture (Koh et al, 2012; Sun et al, 2019; Tupsakhare and Castaldi, 2019; Wang et al, 2017) and the combined use of the above methods (Li et al, 2011; Kou et al, 2019; Kou et al, 2020; Wan et al, 2020) But there are still many problems, such as low efficiency due to large energy loss to surrounding stratum for thermal stimulation, obstacles of front propagation resulted from hydrate regeneration for depressurization, environmental issues and low productivity for inhibitor injection, and inability of monitoring CO2 utilization for CO2-CH4 replacement
Summary
Natural gas hydrate (NGH), which is widely distributed in continental margin and permafrost, is naturally formed when excess gas and water molecules exist in high and low temperature zones (Sloan and Koh, 2007; Chong et al, 2016). Exchange Interval in CH4 Recovery gas from reservoirs is to use the driving potential based on temperature, pressure and chemical potential difference to change the equilibrium condition of NGH reservoir and decompose NGH (Li et al, 2016), including thermal stimulation (Wang et al, 2017; Li et al, 2008; Fitzgerald and Castaldi, 2013), depressurization (Yang et al, 2012; Zhao et al, 2013) and chemical inhibitor injection (Yuan et al, 2011; Javanmardi et al, 2013) Besides these methods, CH4 recovery with CO2 injection into the NHG reserves was firstly proposed by Ohgaki et al (1996), and it has become a promising way to exploit CH4 from NGH reserves while sequestrating CO2 at the same time (Koh et al, 2012; Lee et al, 2013; Bo et al, 2014; Cha et al, 2015; Zhang et al, 2017). The decomposed gas was collected and each component concentration in the hydrate phase was determined by gas chromatography
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