Relation Between Permeability and Pore-Size Distribution of Methane-Hydrate-Bearing Sediments
Authors: Minagawa Hideki , Nishikawa Yasunori , Ikeda Ikuko , Sakamoto Yasuhide , Miyazaki Kuniyuki , Takahara Naoya , Komai Takeshi , Narita Hideo 
Published: October 2008

Abstract
In this study, proton nuclear magnetic resonance (NMR) measurement combined with a permeability measurement system was performed to characterize methane-hydrate-bearing sediment based on pore-size distribution and permeability. Pore-size distribution of sediment measured by NMR was compared with that measured by mercury porosimetry. Furthermore, techniques developed in our previous work to convert NMR spectra were applied in order to obtain the pore-size distribution with higher spatial resolution.1 The change of pore-size distribution due to MH dissociation was precisely analyzed using these conversion techniques. The effective permeability of sediment with different effective porosities, which had been measured by water flow based on Darcy's law, was compared with the permeability calculated by NMR spectra based on the SDR model. The permeability calculated by NMR spectra is similar to the permeability measured by water flow, with a difference between them of less than a factor of 2. Moreover, the tendency of permeability change during MH dissociation measured by water flow is similar to that calculated by NMR spectra. This study will describe the relation between pore-size distribution, including porosity, and effective permeability.

Introduction
Methane gas hydrates in sediment are expected to be developed as a resource of 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 the natural gas-hydrate-bearing sediment, i.e., mainly methane-hydrate-bearing sediment. The exploitation of methane hydrate and methods of producing methane gas from methane hydrate, such as (1) the depressurization method,2,3 (2) the thermal stimulated method,2,3 and (3) the inhibiter injection method,4 have been proposed. In any method, gas permeability and water permeability in the methane-hydrate-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. The permeability of methane-hydrate-bearing sediment is considerably affected by several properties of sediment, i.e., pore-size distribution, porosity, cementing, MH production characteristics, and MH saturation. Furthermore, the permeability measurements of methane-hydrate-bearing sediments using gas or liquid flow could cause reformation and/or heterogeneous dissociation of methane hydrate. Therefore, the calculations of permeability using mercury porosimetry and proton nuclear magnetic resonance have been proposed as an alternative method.
Mercury porosimetry is a very powerful tool for measuring the pore-size distribution of reservoir rock and sandstone.5 Pore-size distributions are calculated by the relation between mercury intrusion volume and pressure, based on the Washburn equation.
In petrophysical applications, NMR is used in bore holes as a wire-line logging tool (the Shulumberger Combinable Magnetic Resonance Tool (CMR)) to measure the pore-size distribution for oil reservoir rock and/or sandy sediment layers containing methane hydrate.6,7 Pore-size distributions are calculated by relaxation time, and permeability can be estimated by the SDR (Schlumberger-Doll Research) model and the Timur-Coates model based on the Kozeny-Carmann approximation.6,7,8 This method can be used at high pressures but is limited to samples with water-saturated pores. In methane-hydrate-bearing sediment measurements, methane hydrate contains abundant hydrogen in both its water and methane fractions, but this hydrogen is invisible because our NMR apparatus is not sensitive to hydrogen in solids. Therefore when methane hydrate is formed in pores, the apparent NMR porosity decreases with methane hydrate saturation.6,7 It also reduces the integrated amplitude and changes the shape of the apparent pore-size distribution.