Research paperPermeability of sediment cores from methane hydrate deposit in the Eastern Nankai Trough

- Effective and absolute permeability were measured using natural sediment cores.
- Experimental results were compared to logging data.
- Effective permeability was in the range of 1-100 md for hydrate-bearing sediments.
- The change of permeability, potentially caused by gas production, was analyzed.

Effective and absolute permeability are among the most important factors affecting the productivity of hydrate-bearing sediments during gas recovery operations. In this study, effective and absolute permeability have been measured using natural sediment cores obtained from a methane hydrate reservoir in the Eastern Nankai Trough off the shore of Japan. The cores were recovered under pressure and shaped cylindrically with liquid nitrogen spray after rapid pressure release. The cylindrical core was inserted into a core holder for flooding tests in order to apply a near in situ effective stress. The effective permeability of water in the hydrate-bearing sandy sediment was 47 millidarcies (md) with a hydrate saturation of 70%. After hydrate dissociation, the absolute permeability was estimated to be 840 md. Other test results showed that the absolute permeability of the hydrate-free sediments was estimated to be tens of microdarcies for clayey sediments, tens of md for silty sediments, and up to 1.5 darcy for sandy sediments. Absolute permeability showed a strong correlation with sediment grain size in log-log plots. In addition, the effective permeability of hydrate-bearing sandy sediments and the absolute permeability of hydrate-free sandy sediments correlated with the effective porosity. We compared measured data to other experimental data using pressure cores recovered from the same well and wireline pressure tests from a well near the coring well. The results are consistent with each other. At this location, we found that the effective permeability for hydrate-bearing sandy sediments was in the range of 1-100 md, which was 2-3 orders of magnitude higher than conventional estimates. Finally, the change of permeability, potentially caused by depressurization-induced gas production, was analyzed. It was found that the high effective stress owing to depressurization and freshwater generation originating from hydrate dissociation caused reduction in absolute permeability.