Geomechanical property of gas hydrate-bearing zone

For the development of gas hydrate as possible future energy resources, the great effort has been paid for developing new seismic methods to delineate hydrate reservoir and its quantification. Not only for reservoir delineation but also for understanding gas hydrate system and geological history of hydrate sediment, we have made a geomechanical study for hydrate and non-hydrate sediment theoretically and experimentally using the well data and core samples acquired in Nankai Trough area. In first step of the study, we have tried to calculate the mechanical strength of the gas hydrate-bearing sediment based on the Coulomb-Mohr failure criterion using well-logging data. The result shows that the sediment below the gas hydrate-bearing layer is apparently mechanically weaker than gas hydrate-bearing layer. This result is consistent with our knowledge that caliper data shows the weakness of the well-bore wall. In second step, we measured the mechanical strength of the core samples recovered by PTCS at the Nankai Trough area. The measurement was made to the core samples, one of which is under in situ condition and another of which was done after dissociation. As a result, the strength of the gas hydrate-contained core with high saturation is about 4 times stronger than that of gas hydrate-dissociated core. This result is completely same as that of calculated strength using the well-logging data. These information will be important for reservoir monitoring while production and hazard prevention while drilling.