Abstract
The replacement of gas hydrate in clayey sediment by a CO2/N2 (20:80) gas mixture injection was experimentally studied to explore the influence of clay on the gas exchange behaviours in the gas hydrate. Clay (montmorillonite) and silica sand were mixed in three different proportions (clay mass ratios of 10%, 30% and 50%) to simulate the host sediments of natural gas hydrate while pure silica sand sediment was selected for comparison. Experimental results showed that clay hindered gas diffusion during the initial replacement stage and thus reduced the methane recovery rate. In the later stage, the gas exchange between CO2/N2 and methane in the hydrate structure might be subject to thermodynamic inhibition and geometric constraints of the clay interlayer. Moreover, the CO2 sequestration ratio was lowered significantly in the sediment with a 50% clay mass ratio. Therefore, it was determined that clay has an inhibitory effect on gas hydrate replacement by CO2/N2.
Highlights
Natural gas hydrate (NGH) is a nonstoichiometric, crystalline compound formed by water and light hydrocarbons under certain temperature and pressure conditions (Li et al, 2012; Sloan and Koh, 2007)
Direct component determination by gas chromatography (GC) is helpful for quantitative analysis of the gas phase in the reactor
These results demonstrate that CO2 sequestration capacity of silica sand sediment surpassed clayey sediment
Summary
Natural gas hydrate (NGH) is a nonstoichiometric, crystalline compound formed by water and light hydrocarbons (mainly CH4) under certain temperature and pressure conditions (Li et al, 2012; Sloan and Koh, 2007). Koh et al (2012) compared gas replacement in pure CH4 hydrates, NGH sediments and CH4 hydrates bearing clay (montmorillonite) by injecting a CO2 and CO2/N2 gas mixture. They found that the total recovered CH4 for the all three samples were nearly identical; the methane recovery rate was largest in the pure CH4 hydrate, followed by the NGH sediments and the CH4 hydrate-bearing clay. It is significant to completely reveal the CO2-CH4 replacement mechanism in hydrate-bearing clayey sediments and investigate the influence of clay content on gas exchange behaviours. The uncertainties of measured pressure and temperature are Æ0.01 MPa and Æ0.1 K, respectively
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