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doi:10.2204/iodp.proc.314315316.205.2011

Introduction

When analyzing deformation processes in accretionary complexes like Nankai, one has to take into account several timescales. One important timescale is a result of the competition between two kinetics, one related to the eventual pore pressure build-up linked to pore fluid trapping during tectonic loading of the subduction zone and the other related to the ability of the pore fluid to flow out of the system, thus leading to pore pressure dissipation and avoiding any effective confining stress decrease that would enhance unstable slip of the system. The fluid flow is controlled by the hydraulic diffusivity of the rock, a function of both permeability and specific storage. Permeability measurements on samples from the Nankai accretionary complex have been previously performed without pressure confinement (Taylor and Fisher, 1993) or at low confining pressure (<1 MPa) (Gamage and Screaton, 2003; Karig, 1993). Measurements of permeability at effective confining pressure of 1–5 MPa give lower values (Byrne et al., 1993). More recently, Bourlange et al. (2004) reported permeability measurements on three samples from Ocean Drilling Program Leg 190 performed in the 0.2–2.5 MPa range in a triaxial cell with the main purpose of approaching in situ stress conditions. Overall, their results indicate that permeability decreases from 10^–18 to 10^–19 m² with effective confining pressure increasing from 0.2 to 1.5 MPa. When the effective pressure is then increased from 1.5 to 2.5 MPa, permeability is roughly constant (~1 × 10^–19 to 4 × 10^–19 m²), indicating a threshold pressure beyond which fracture closure is stopped. However, measurements at low effective pressure were too dispersed to yield a precise general relationship between pressure and permeability, and thus crack geometrical parameters.

In this report, we present permeability measurements performed at 1–30 MPa in a hydrostatic cell with the main goal of refining this relationship, thus giving some new insights in the pressure dependence of microstructural characteristics of samples having various lithologic origins.

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