Abstract
Current technologies for production of natural gas hydrates (NGH), which include thermal stimulation, depressurization and inhibitor injection, have raised concerns over unintended consequences. The possibility of catastrophic slope failure and marine ecosystem damage remain serious challenges to safe NGH production. As a potential approach, this paper presents air-driven NGH recovery from permeable marine sediments induced by simultaneous mechanisms for methane liberation (NGH decomposition) and CH4-air or CH4-CO2/air replacement. Air is diffused into and penetrates NGH and, on its surface, forms a boundary between the gas and solid phases. Then spontaneous melting proceeds until the chemical potentials become equal in both phases as NGH depletion continues and self-regulated CH4-air replacement occurs over an arbitrary point. We observed the existence of critical methane concentration forming the boundary between decomposition and replacement mechanisms in the NGH reservoirs. Furthermore, when CO2 was added, we observed a very strong, stable, self-regulating process of exchange (CH4 replaced by CO2/air; hereafter CH4-CO2/air) occurring in the NGH. The proposed process will work well for most global gas hydrate reservoirs, regardless of the injection conditions or geothermal gradient.
Highlights
Nondestructive natural gas hydrate recovery driven by air and carbon dioxide Hyery Kang1*, Dong-Yeun Koh1* & Huen Lee[1,2]
Thermal stimulation causes vacillation between natural gas hydrates (NGH) formation and dissociation temperatures (DT), depressurization shifts the reservoir pressure to much lower than NGH-formation pressure (DP), and inhibitor injection alters the chemical environment (Dm) of the NGH to inhibit the stable formation of gas hydrates[9]
The use of pure air contributes to methane production by NGH melting, but the key issue arises as to whether replacement can occur when we further extend the systems with concentrated methane hydrates or injection of CO2-enriched air (CO2/air)
Summary
Nondestructive natural gas hydrate recovery driven by air and carbon dioxide Hyery Kang1*, Dong-Yeun Koh1* & Huen Lee[1,2]. The currently proposed technologies for marine NGH production are generally derived from standard methods used in conventional oil and gas industries These take advantage of the driving potential of differences in temperature, pressure and chemical potential, which cause significant changes in the conditions within NGH reservoirs. Thermal stimulation causes vacillation between NGH formation and dissociation temperatures (DT), depressurization shifts the reservoir pressure to much lower than NGH-formation pressure (DP), and inhibitor injection alters the chemical environment (Dm) of the NGH to inhibit the stable formation of gas hydrates[9] These methods are all dissociation-based technologies, which can raise serious concerns about repercussions such as slope failure and seabed ecosystem destruction[10]. In a field production test on the Alaska North Slope in 2012, the CO2/N2 injection method was adopted as the main process and was successfully performed[3]
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