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doi:10.2204/iodp.proc.314315316.205.2011

Experimental procedure

Permeability measurements were performed on two sets of samples from Sites C0001 (Expedition 315) and C0006 (Expedition 316) at various depths (Fig. F1). For each depth level, two cylindrical specimens (20 mm in diameter and 15–20 mm in length) were drilled out of the initial cores in a vertical direction (Fig. F2), one for porosity determination and the other for permeability measurement. The porosity of the unstressed samples was measured by using the triple-weighing method: successive measurements of the preserved originally saturated, saturated immersed, and dry specimen masses lead to the determination of the connected porosity. Table T1 summarizes porosity data for the tested samples and Figure F3 illustrates the porosity depth dependence. This dependency compares well with porosity data obtained onboard and derived from resistivity logs (see the “Expedition 315 Site C0001” [Expedition 315 Scientists, 2009] and “Expedition 316 Site C0006” [Expedition 316 Scientists, 2009] chapters).

Permeability measurements were carried out at room temperature in a 200 MPa hydrostatic pressure cell equipped with a pore fluid pressure circuit (Fig. F4). The entire apparatus was thermally regulated to keep pressures constant in the absence of imposed pressure changes. The samples were isolated from the confining pressure fluid by a Viton jacket clamped on the end pieces connected to the pore fluid circuit. Pore fluid and confining pressures (P_P and P_C, respectively) were controlled separately. During the experiments, an effective pressure (P_C – P_P) of at least 1 MPa was maintained on the sample to ensure uniform contact of the jacket with the specimen and to avoid any leaking. All the experiments were run on samples preserved with their original pore fluids and in a saturated state.

The initial pressure conditions for all samples were P_C = 3 MPa and P_P = 2 MPa in order to be able to compare their hydraulic conductivities. Once the pressures were constant, permeabilities were measured using a pulse decay method (Bernabé, 1987; Brace, 1984). After closing the isolating valve between the upstream and downstream pore pressure circuits, a small step change of differential pore fluid pressure ΔP_P = P_up – P_down was imposed in the upstream pore pressure section. Both pressures were then free to return to equilibrium through the sample. When the compressive storage in the sample is much smaller than the compressive storage in the pore fluid circuits, the differential pore pressure decay ΔP_P is approximately exponential and the decay time is inversely proportional to the permeability, as shown by the following equations (Hsieh et al., 1981):

and

where

• t = time,

• A = section area,

• L = sample length,

• µ = pore fluid viscosity (10^–3 Pa·s at 20°C),

• k = permeability,

• C_u = compressive storage of the upstream pore pressure circuit, and

• C_d = compressive storage of the downstream pore pressure circuit.

C_u and C_d are defined as the ratios of the change of fluid volume to the corresponding pore pressure variation (C = δV/δP_P), which are physical constants of the apparatus and have been experimentally determined as C_u = 3.957 × 10^–9 m³/MPa and C_d = 4.828 × 10^–9 m³/MPa. For large timescales, Hsieh et al. (1981) have shown that the pore pressure decay with time has a form similar to Equation 1. This similarity is due to the fact that at large timescales the effect of storativity in the sample itself becomes negligible. In Figure F5 we show an example of pore pressure evolution with time (Fig. F5A) and the resulting differential pore pressure decay (Fig. F5B). As one can see, a single exponential law fits the data well over the entire experimental time span, indicating that neglecting the storativity in the sample may be a valid assumption. The permeability value is then deduced from the slope of the exponential fitting curve by using Equation 2.

This measurement method was used for all samples at increasing levels of effective confining pressure (P_C – P_P) from 3 to 30 MPa, with 2 MPa increase steps for P_C and a constant P_P of 2 MPa. Because the pulse decay method requires a small initial pore pressure difference (10%) compared to the equilibrium pore pressure, we applied an initial 0.5 MPa positive pulse to the upstream pore circuit, but we restricted our analysis to the final 0.2 MPa portion of the ΔP_P decay curve.

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