Water Permeability Evolution With Faulting for Unconsolidated Turbidite Sand in a Gas-Hydrate Reservoir in the Eastern Nankai Trough Area of Japan

摘要

Gas hydrates in sediments are forecast to become an unconventional natural gas resource. In 2013 and 2017, an offshore natural gas production test in subseabed gas hydrate-bearing sand layers was carried out in the eastern Nankai Trough area of Japan. However, permeability evolution with progressive shearing of these faulted reservoirs has remained enigmatic. It is necessary to understand fault behavior under shear, as well as how normal stress affects permeability evolution. Here, we examine the relationship between fault development and permeability reduction in unconsolidated turbidite sandy sediment from Japan's eastern Nankai Trough area using a laboratory ring-shear experiments. At each stress condition, the permeability decreased significantly following shear displacements of ~0.2-0.5 m, gradually decreasing further with shear displacements up to 10 m. Permeability after shearing was reduced by 3 to 4 orders of magnitude compared to that before shearing. Grain crushing and compaction within the shear zone and surrounding host sediment are dependent on stress and mineralogy. The findings of the role of shear zone development in modifying fault permeability for turbidite sand are expected to contribute directly to rational evaluation of gas-hydrate reservoir, production simulations, and development strategies in future studies.