Estimation of the In-Situ Permeabilities of Nankai Trough Hydrate Bearing Sediments From Pressure Temparature Core Samplar (PTCS) Core Measurements
Authors: Suzuki Kiyofumi , Ebinuma Takao , Narita Hideo 
Published: October 2008

As permeability is important for every hydrocarbon production, it will be important for natural gas production from methane hydrate bearing layers. Also, it is very important to design the tools for future production when we are going to apply depressurization method on methane hydrate bearing sediments.

Several samples of Nankai Trough methane hydrate zone were sampled using pressure temperature core sampler. A few of them were not disturbed so that they were enough to measure relationship between permeability and porosity, and the others had given enough information of methane hydrate bearing layers. Features of Nankai Trough methane hydrate bearing layers are that methane hydrate is as a pore-filling matrix of sediments, host sediments consist with turbiditie and they are member of sand and mud layers which are not consolidated.

Every samples had been compressed as K0 condition until overburden pressure before permeability measurement. For natural sediments, each sample was trimmed by a lathe from frozen core, which had been stored in liquid nitrogen to prevent methane hydrate dissociation, was settled in tri-axial compression apparatus. Also, permeability of several artificial samples which grain size distribution and mineral composition were simulated for Tb section of Nankai Trough natural turbidite were measured.

Result of measurements, permeability of turbidite Tb section has correlation with void percentage, which is defined as true void volume percentage within sediments, not but only porosity of host sediments. Moreover, permeability is influenced by grain size distributions of sample intervals. These results imply that permeability is influenced by not only hydrate saturation nor porosity, but also by grain size distribution of each layer. The results will help to estimate permeability from interpreted well logging data.

Introduction

Since 1995, field surveys have been done by Ministry of Economy, Trade and Industry (MITI) to grasp nature of methane hydrate bearing sediments in Nankai Trough region along the south-eastern margin of Japan (Figure 1). Result of seismic researches of study area, there were bottom simulating reflector (BSR) and other several features, which implied lying methane hydrate in subsurface sediments. On these grounds, MITI promoted the drilling project "MITI Research well Off-Tokai to Kumano-nada" in 2004 and methane hydrate bearing marine sediments were sampled by using Pressure Temperature Core Sampler (PTCS).

The PTCS had kept in-situ pressure of cored level so as not to dissociate methane hydrate within sediments. When the PTCS was opened on research vessel, the methane hydrate within cores was not dissociated by self preservation effect of dissociation on core surface. As well as methane hydrate dissociation on core surface, it caused heat absorption, hence, surround sediments showed sub-zero temperature, i.e. they were methane hydrate bearing sandy sediments. Each part of sediments that showed sub-zero temperature was chopped and was stored in vacuum bottles together with liquid nitrogen. They had been kept cool under methane hydrate stability condition not to change their physical properties. Then, other sediments that did not show sub-zero temperature were stored in refrigerator. Both of them were stored stable until experiments for measuring permeability.