ABSTRACT:

A proton nuclear magnetic resonance (NMR) system combined with a permeability measurement system has been used to clarify the relation between permeability and methane-hydrate saturation of methanehydrate- bearing sediment with regard to effective pore size distribution. Pore-size distributions of sediments have been calculated using the relaxation time distribution of NMR-T2. Two different laboratory methods of growing gas hydrate in sediment cores have been employed to determine the relationship between hydrate saturation and permeability, a "conventional" approach called the connate water method and a dissolved-gas method. The permeability and pore-size distribution of sediment produced by two different methods differed from each other.

INTRODUCTION

Methane hydrates in sediment are expected to be developed as a resource for natural gas and have been studied as a possible future energy resource. In-situ dissociation of natural gas hydrate is necessary in order to commercially recover natural gas from natural-gas-hydratebearing sediment, i.e., mainly methane-hydrate-bearing sediment (Makogon 1966, 1981). The exploitation of methane hydrate (MH) and various methods of producing methane gas from methane hydrate, such as (1) the depressurization method (Sakamoto 2007a, 2007b), (2) the thermal-stimulation method (Sakamoto 2007a, 2007b), and (3) the inhibitor-injection method (Kawamura 2006) have been proposed. In any method, the gas permeability and water permeability of methanehydrate- bearing sediments are important factors for estimating the efficiency of methane-gas production. The sediment permeability is generally determined by measurement using gas or liquid flows. For example, the permeability of a methane-hydrate-bearing layer is measured by using gas or liquid flows for methane-hydrate-bearing sediment, which is explored using a pressure-temperature core sampler (PTCS). The permeability of methane-hydrate-bearing sediment is considerably affected by several properties of sediment, e.g., pore-size distribution, porosity, cementing, MH production characteristics, and MH saturation. As another method of estimating permeability, proton nuclear magnetic resonance (NMR) methods have been used.

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