Thermophysical Properties of Methane Hydrate-bearing Sediments Recovered from Nankai Trough Wells Under Vertical Stress Conditions

Morita, Hiromitsu (National Institute of Advanced Industrial Science and Technology (AIST)) | Muraoka, Michihiro (National Institute of Advanced Industrial Science and Technology (AIST)) | Yamamoto, Yoshitaka (National Institute of Advanced Industrial Science and Technology (AIST))

OnePetro 

This paper measures the thermophysical properties of natural methane hydrate (MH)-bearing sediments recovered from the Nankai Trough, Japan. The thermal conductivity, thermal diffusivity, and specific heat of the sample under vertical stress (VS) loading were measured by the hot-disk transient method. The thermal conductivity of the sediments increased with increasing VS. The specific heat and thermal diffusivity have a constant value independent of VS. After MH dissociation, the thermal conductivity and the specific heat dropped significantly, and the thermal diffusivity was increased. In addition, the thermal conductivity, specific heat, and thermal diffusivity were calculated by an estimation model.

INTRODUCTION

Methane hydrate (MH) is expected to be developed as an unconventional natural gas source, replacing existing fossil fuels. MH is a crystalline solid in which cages of hydrogen-bonded water molecules enclose the methane gas molecules. MH is stable in a high-pressure/low-temperature environment. A large amount of MH is known to exist in permafrost on land and in sedimentary layers beneath the seabed (Sloan and Koh, 2007).

The collected seismic data for oil and gas exploration show a wide distribution of bottom-simulating reflections (BSRs) under the seafloor in the Nankai Trough region near the Japan Sea coast. BSRs indicate the lower limit of gas hydrate stability zone in a vertical profile. In 1999, the first Nankai Trough methane hydrate exploration well was drilled. In early 2004, the Japan Ministry of Economy, Trade, and Industry drilled a multiwell from Tokaioki to Kumano-nada (Tsuji et al., 2009). The core was recovered using a pressure-temperature core sampler, which maintained the in-situ condition of 16 excavation sites at water depths ranging from 720 to 2,030 m in the same year. Recovered core analysis confirmed that the MH-bearing sediments in the Nankai Trough area are pore-filling-type hydrates (Fujii, Nakamizu, et al., 2009; Fujii, Saeki, et al., 2009).