メタンハイドレートを胚胎するコア試料の弾性波と比抵抗特性

抄録

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 samples were produced in the tri-axial cell of the experiment device. All core samples were saturated by brine just before the measurements. P-wave velocities of the artificial methane hydrate samples increased by 133 m/s at every 10% increase of MH saturation. S-wave velocities of those samples increased by 118 m/s at every 10% increase of MH saturation. Poisson's ratios calculated from the P- and S-wave velocities were in the range from 0.32 to 0.47. 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 by 0.54Ωm at every 10% increase of 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.