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doi:10.2204/iodp.proc.322.210.2013

Methods

Permeability tests were conducted using the Trautwein Soil Testing Equipment Company’s DigiFlow K (Fig. F2). The equipment consists of a cell (to contain the sample and provide isostatic effective stress) and three pumps (sample top pump, sample bottom pump, and cell pump). Deionized water was used as the fluid in the pumps while an idealized solution of seawater (25 g NaCl and 8 g MgSO₄ per liter of water) permeated the sample. Pressure is transmitted from the deionized water in the top and bottom pumps to the permeant across rubber membranes in two interface chambers (Fig. F2).

The retrieved core samples from Expeditions 322 and 333 were stored in plastic core liners and sealed in aluminum bags during the expedition after sampling to prevent moisture loss. The sealed samples were stored in the refrigerator at 4°C until immediately prior to sample preparation. All testing was conducted with flow in the vertical direction (along the axis of the core). The permeability-testing apparatus accommodates the whole-round core. As a result, disturbance of the sample is minimal relative to testing during which plugs or subsamples are removed from the core. During preparation, the samples were carefully inspected for fracturing, disturbance, or signs of moisture loss (e.g., color change or cracking on outer surfaces). Two samples from Expedition 333 were not tested because of core damage. To provide freshly exposed surfaces, cores were trimmed on both ends using a wire saw or utility knife, depending on core properties. After trimming the ends of the sample, the diameter and height of the sample were measured. Sample diameters ranged from 5.7 to 6.5 cm, and sample heights varied from ~5.4 to 9.9 cm. The sample was then placed in a rubber membrane and fitted with saturated porous disks on both ends. The porous disks have been tested in the permeameter to ensure that they do not reduce flow and impact permeability calculations. Tests indicate that permeability of the disks is significantly >10^–14 m².

The sample was placed in the cell, which was filled with deionized water so that the membrane-encased sample was completely surrounded by fluid. A small confining pressure of ~0.03 MPa (5 psi) was applied, and flow lines were flushed to remove any trapped air bubbles. After flushing the flow lines, the sample was backpressured to ~0.28 MPa (40 psi). Backpressure was achieved by concurrently ramping the cell pressure and the sample pressure to maintain a steady effective stress of 0.03 MPa. Because the whole-round samples were sealed immediately after cutting the core liner, the samples were expected to be near saturation prior to testing. Backpressuring at 0.28 MPa (40 psi) for ~24 h is sufficient to ensure full saturation under these conditions (ASTM, 1990). After backpressure, the cell fluid pressure was increased while the sample backpressure was maintained, thus increasing the effective stress on the sample. This effective stress both consolidates the sample and pushes the flexible membrane against the sample to prevent flow bypassing the sample. Previous results from this laboratory have been consistent with results from fixed-wall consolidation cells (see fig. 6 in Skarbek and Saffer, 2009).

Once the target effective stress was achieved, the sample was allowed to equilibrate for at least 12 h and generally 24 h. Throughout testing, inflows and outflows to the cell fluid were monitored to assess changes in sample volume. Sample data were recorded every 1 min. Because fluid pressure in the closed hydraulic system can be affected by temperature changes, testing was conducted within a closed cabinet with a fan to keep the internal temperature uniform. The temperature was maintained at ~30°C (±1°C) during flow tests and consolidation steps. As many as three flow tests were performed at each effective stress level, with flow direction varied between tests. Flow tests were run by specifying pressures of the top and bottom pump and allowing flow rates into and out of the sample to equilibrate with time. Equilibrium was indicated by consistency between inflow and outflow rates. Pump pressure transducer calibration indicates errors <0.004%.

We used the measured flow rate, cross-sectional area of the sample, and the head difference between the top and bottom of the sample to calculate the hydraulic conductivity using Darcy’s law:

where

• Q = measured flow rate (in cubic meters per second),

• K = hydraulic conductivity (in meters per second),

• A = cross-sectional area of the sample (in square meters),

• Δh = difference in head across the sample (in meters), and

• Δl = length of the sample (in meters).

Hydraulic conductivity values were then converted to permeability (k; in square meters) using the following equation:

where

• ρ = fluid density (1023 kg/m³),

• g = the gravitational constant (9.81 m/s²), and

• µ = viscosity (0.000857 Pa.s).

The density value was estimated for a temperature of 30°C and a salinity of 33 kg/m³ (Haynes, 2012). Assuming a reasonable water compressibility, density change because of the applied pressure is minor (<0.1%). The viscosity value was obtained from a synthesis of previous relationships (Sharqawy et al., 2010) for water at a temperature of 30°C and salinity of 33 kg/m³. A 1 h interval of stable flow rates was averaged for the permeability calculations, and the standard deviation of the permeability during that interval was calculated to assess uncertainty. Fluctuations in the calculated permeability are likely caused by slight temperature variations. The resulting volume changes would cause temporary changes in measured flow rates. The time interval was selected based on where inflow best matched outflow, indicating steady-state conditions, and where the standard deviation of permeability was minimized.

For every sample, as many as three effective stress steps were performed, with effective stress conditions ranging from 0.14 to 0.55 MPa. The corresponding porosity for each effective stress was calculated using the change in volume of fluid (mL) contained in the cell during each consolidation step. Total sample volume (V_T(0)) was calculated using the following equation:

where

• r = radius of the core sample, and

• h = height of the sample.

Initial porosities (n₀) for volume calculations were obtained from shipboard moisture and density results of samples that were taken immediately adjacent to each whole-round sample collected for permeability testing. We assumed that the porosity of the sample at the end of backpressure is similar to the n₀ of the sample because of the small change in effective stress (0.03 MPa).

Using n₀, the volume of voids before testing (V_V(0)) was calculated:

Volume of solids (V_s) was calculated using the following equation:

Using the difference of cell volumes between two consecutive steps (e.g., cell volume at backpressure and cell volume at first consolidation), the change in volume of water in the cell (ΔV_T(1)) was calculated. The new total volume of the sample (V_T(1)) after pore spaces were reduced during the consolidation process was determined by subtracting the change in cell volume at the end of the consolidation step (ΔV_T(1)) from the total sample volume (V_T(0)):

Using the calculated new total volume of the sample (V_T(1)), the new porosity at the end of the consolidation is calculated. The new porosity (n₁) at the end of the consolidation is

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