Abstract

In order to clarify physical properties of methane hydrate-bearing sediments, seismic wave velocities and electric resistivities of natural and artificial samples were measured in relation to methane hydrate saturation. First, a laboratory experiment system was developed so that P- and S-wave velocities and electric resistivities were measured simultaneously under the condition of in situ pressure and temperature. Natural methane hydrate samples were prepared from the stored cores kept in liquid nitrogen at a low temperature. Artificial methane hydrate bearing sand samples were produced in the tri-axial cell of the experiment device. All core samples were saturated by brine just before the measurements. P- and S-wave velocities of the artificial methane hydrate samples increased with increased saturation of methane hydrate. These measurement results agreed well with those obtained by the previous study on the relationship between MH saturation and seismic wave velocities of the wireline logging around the Eastern Nankai Trough area offshore Japan. In addition, it was found that the relationship was explained by one of the rock physics models for the MH bearing sediments as Matrix-support type. Electric resistivities of the same samples as those used to measure the seismic velocities increased with increased MH saturation, though the resistivities run up as the saturation was more than 50 %. These also agreed well with those obtained from the wireline logging.

Introduction

In order to provide useful information to estimate the in situ status of methane hydrate, it is required to clarify the physical properties of sand samples containing methane hydrate, and to evaluate the relationship between hydrate saturation and seismic velocity and resistivity. First, we developed a laboratory experiment system that can generate artificial methane hydrate bearing sand samples. The system is also used to measure seismic velocity and electric resistivity under the condition of in situ pressure and temperature. Then, we measured seismic veolosity and electric resisrivity of both natural and artificial methane hydrate bearing samples by the developed system. Then the experiment results are compared with those obtained by well logging conducted around the Eastern Nankai Trough area offshore Japan. This paper describes the outline of the developed system, laboratory experiment procedures, and the experiment results.

Loboratory experiment system

We, first, developed a laboratory measurement system for seismic velocity and electric resistivity of core samples containing methane hydrate under the condition of in situ pressure and temperature. Since the natural core samples that can be used for the experiments were not so many, we decided to generate artificial methane hydrate bearing sand samples in the laboratory. To do this, we developed a new experiment system which has the following features:

  • Size of core sample: 50 mm (diameter) × 100 mm (height)

  • Confining presshure: 25 MPa (max)

  • Pore pressure: 20 MPa (max)

  • Controllable temperature: 0°C - 20°C

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