ABSTRACT:

In-situ dissociation of natural gas hydrate is necessary to commercially recover natural gas from natural gas hydrate sediment. Thermal stimulation is considered an effective dissociation method, along with depressurization. In this study, we examined the efficiency of electrical heating of the hydrate core for gas production. To ensure safety and to avoid explosions, we investigated electrical heating of xenon gas hydrate sediment, instead of methane hydrate sediment. Alternating current (AC) heating with depressurization and additional electrode heating of hydrate sediment saturated with electrolyte solution was confirmed to enable gas production from sediment with less electric power.

INTRODUCTION

Methane hydrate deposited in sea floor sediment and permafrost is considered an unconventional methane resource (Collet, 1998). To commercially recover natural gas from natural gas hydrate sediment, in-situ dissociation of natural gas hydrate is necessary. The exploitation of methane hydrate and production methods of methane gas from methane hydrate (e.g., depressurization (Sakamoto, 2007a), thermal stimulation (Sakamoto, 2007b), and inhibiter injection (Kawamura, 2006)) have been proposed. With all methods, gas permeability and water permeability in the methane hydrate sediments are important factors to estimate the efficiency of production of methane gas. For depressurization, the gas production rate increased with increasing pressure drawdown. However, large pressure drawdown caused the sediment to cool to equilibrium temperature in proportion to equilibrium dissociation pressure because of the endothermic reaction of methane hydrate dissociation. The gas production rate then decreased as the sediment temperature decreased (Kamata, 2005a). For hot water of low temperature injection as a thermal-stimulated method, the pressure near the methane hydrate decomposed region increased, and decomposition of methane hydrate advanced on the equilibrium pressure and temperature. However, when the water temperature was above 20oC higher than the equilibrium temperature, the temperature and pressure in the sample fluctuated in the equilibrium phase (Kamata, 2005b).

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