ABSTRACT

In-situ dissociation process of gas hydrate by partial-oxidation and heating through acid or oxidant injection into the reservoir has been proposed. However, it was only evaluated by simulator based on the calorific value of acid. In this study, we carried out gas hydrate dissociation experiments in laboratory scale, by acid injection for the evaluation of this process. From the results, we clarified that the equilibrium temperature shifts to low temperature. In the case of acid injection, the equilibrium temperature decreased from 6.6 °C to 3.5 °C. The gas production rate was enhanced to more than twice due to the effects of exothermic reaction and inhibitor.

INTRODUCTION

Gas hydrates are crystalline substances containing water and gas molecules. Host water molecule forms cage structure and guest gas molecule (M) is trapped inside this cage, with the general formula of Mn(H2O)p and stable under the low-temperature and high-pressure conditions. Formation conditions are different depending on the guest molecule. Methane hydrate (MH) is one of gas hydrates. In this case, methane as guest molecule is included inside the cage formed from water as host one. The large amount of MH exists in permafrost region and submarine sediments all over the world (Sloan, 1990). The temperature of seabed is about 4 °C corresponding to that of sea water. Considering the equilibrium pressure of MH, MH can stably exist on the region ranging from the sea bottom to the depth of 500 m or more (Yamamoto, 2013). MH is expected as one of the new energy resources in the future, because the amount of hydrocarbon resources in MH are larger than total amount of oil, natural gas and coal resources worldwide (Lee et al., 2001).

However, since MH is a solid material without mobility, it is necessary to dissociate MH to methane gas and water in site for gas recovery. The dissociation methods are classified into depressurization, thermal stimulation, and inhibitor injection (Sloan, 1990). Currently, depressurization is expected to be the most effective method from the viewpoint of its energy efficiency and gas productivity (Kurihara et al., 2009). However, since MH dissociation is an endothermic reaction, the dissociation by depressurization progresses, causes the reservoir temperature decreases so as to reach as equilibrium again (Sakamoto et al., 2005). Therefore, MH dissociation process stagnates due to this temperature decrease and MH existing in the reservoir cannot be completely dissociated only by depressurization method. In order to further enhance the gas production rate, secondary recovery by thermal stimulation is needed to be applied. In the case of application of thermal stimulation, complete dissociation can be performed as long as the supply of heat in to the reservoir is continued (Kurihara et al., 2009). However, in hot water injection as general heating process, the heat loss of water during the transport process before injection into the reservoir is considered to be large. The injected energy becomes higher in the hot water injection process, as a result, the amount of energy injection has the possibility to exceed the final recoverable energy. Hence, in order to enhance MH dissociation, several methods to supply heat into MH reservoir as a secondary recovery after depressurization have been investigated (Kaneko et al., 2017).

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