P0 is applied, the differential pressure P(t) decays exponentially as a function of time, t:
Permeabilities in this study were measured using a pulse decay method (Brace et al., 1968; Jouniaux et al., 1994, 1995). The pulse is a small step change (0.1–0.3 MPa) of differential fluid pressure imposed between pressure vessels connected at the ends of the sample. When a pressure pulse P0 is applied, the differential pressure P(t) decays exponentially as a function of time, t:
P(t) = 2 P0 V2/(V1 + V2) e–mt,
where
Plotting the decay curve in terms of ln[P(t)] vs. time t yields a straight line having a slope m, and the permeability k can be determined by
where
Two GDS pressure controllers (50 cm3 internal volume) in standby mode were used as constant-volume pressure vessels during pulse decay measurements. Forty-one measurements were performed with the pulse decay method at various levels of effective confining pressure (confining pressure – pore pressure) from 0.34 to 2.4 MPa.
We discarded measurements when an exponential could not be fitted to the data. The largest deviations from the ideal exponential curve correlate with room temperature variations and are attributed to the thermal expansion of water within the pressure controllers (or of the pressure controllers themselves). The pulse decay method requires a small pulse initial pressure difference (10%) compared to the effective pressure. Because the pressure pulse is at least 0.1 MPa (limited by the precision of the pressure gauges), measurements performed at effective confining pressure <1 MPa do not follow this condition. These measurements were retained when the exponential fit was good but yielded scattered results (Fig. F2).
For comparison, two permeability measurements were performed from steady-state flow under a constant pressure gradient P (0.1–0.28 MPa) with GDS pressure controllers in locked pressure mode. Time needed for these measurements is about twice the time needed using the pulse decay method. Permeability was calculated using Darcy's law:
P).
Measurements were performed on three samples (diameter = 25 mm) from Ocean Drilling Program (ODP) Leg 190 (Moore, Taira, Klaus, et al., 2001; Moore et al., 2001) cored either in the vertical direction (v) or in the horizontal direction (h) (Table T1). The overburden stress was calculated by integrating the bulk density with depth. The effective overburden stress range is estimated assuming an overpressure ratio between 0 and 0.42 (maximum overpressure ratio determined for the underthrust sediments by Screaton et al. [2002]).
