MATERIALS AND METHODS

Sediment permeability is most commonly measured by one of three methods (Bryant et al., 1975, 1981). Falling-head permeameters are accurate only when permeabilities are high. Constant-head permeameters are in common use but are difficult to use for fine-grained materials. Most measurements made on deep-sea sediments have been made by oedometer compaction tests, in part because high backpressures can be used to ensure complete water saturation. However, oedometer tests suffer from significant systematic errors when sample diameters are smaller than several inches (Olson, 1986). A fourth method for measuring permeabilities in "tight" materials such as crystalline rocks and shales is the transient-pulse test (TPT) pioneered by Brace et al. (1968) and refined by Sutherland and Cave (1980) and Trimmer (1981). A schematic diagram of the experimental apparatus is shown in Figure F1.

Briefly, the transient-pulse method consists of placing a cylindrical sample between two pore-fluid reservoirs at equal pressure. The pressure on the "upstream" side is increased by an increment, P. The pressure difference between the upstream and downstream reservoirs (P_u – P_d, respectively) is then recorded for a suitable period of time as it decays toward a new equilibrium value, P_f . The time-dependent pressure decay depends on the permeability of the sample:

P_u – P_d = pe^–t, (1)

where

= (kA/µL)[(1/V_u) + (1/V_d)], (2)

and

k = material permeability,

µ = fluid viscosity,

= fluid compressibility,

A = sample cross-sectional area,

L = sample length,

V_u = upstream reservoir volume, and

V_d = downstream reservoir volume.

is found by linear regression from

ln(P_u – P_d) = ln(p) – t, (3)

and the permeability is given by

k = (µL/A)[(1/V_u) + (1/V_d)]^–1. (4)

TPT has been used to measure permeabilities as low as 5 x 10^–21 m² in Wilcox shale with a precision of 1%–3%; sample-to-sample variability is ~10% (Kwon et al., 2001).

As shown in Figure F2, sediments from three sedimentary units were recovered at Site 1179 (Kanazawa, Sager, Escutia, et al., 2001):

Unit I (0–223.5 mbsf) is a radiolarian-bearing diatom ooze, composed of diatoms (40%–60%), radiolarians (10%–35%), and clay (illite) with small amounts of quartz. The dominant particle size is silt.

Unit II (223.5–246.0 mbsf) is a clay- and diatom-rich radiolarian ooze. The dominant constituents are radiolarians (40%–60%), diatom frustules (13%–17%), and clays (20%–35%).

Unit III (246.0–283.5 mbsf) is composed of brown pelagic clay (clay content = 75%–99%). The clays are typically aggregates cemented by zeolites. Unit III contains little, if any, biogenic material.

Twenty samples were collected from the sediment column at Site 1179 for permeability studies. The sample set includes 17 samples from Unit I, 2 samples from Unit II, and 1 sample from Unit III. The samples are half rounds with a diameter of ~6.5 cm and a thickness of 5 cm. On the ship, the samples were packed wet, wrapped in foil, and then coated with wax and placed in refrigerated storage. They were shipped in a refrigerated van to the Gulf Coast core repository at the Ocean Drilling Program (ODP) at Texas A&M University in College Station, Texas. They remained in refrigerated storage until the permeability measurements were made. We chose six of these samples for the permeability measurements: four from Unit I, one from Unit II, and one from Unit III.

Samples were prepared for the permeability measurements by using a standard cork punch to cut a core 1.9 cm in diameter and trimming the ends to form right circular cylinders ~2 cm long. The samples were then immersed in salt water under vacuum for 2 days to ensure that they were water saturated. For the TPT measurements, porous stones were placed between the ends of the sample cylinder and a pair of pistons with axial pore pressure ports, and the entire assembly was jacketed with a heat-shrinkable polyolefin tube, as shown in Figure F1. A seal between the jacket and the pistons was achieved by tying Nychrome wires over grooves in the pistons.

After the permeability measurements were completed, the samples were sent to Core Labs, Inc., for laser grain size analysis. Measurements of bulk density, grain density, and porosity were made on samples from adjacent parts of the core.

We found that the procedure was quite sensitive to the effects of small variations of temperature, which affect reservoir volumes and, hence, pressures. The source of the problem proved to be air circulation in the laboratory. We insulated the apparatus by packing the instrument housing with crumpled paper and draping the entire apparatus with a polyethylene sheet. We further reduced the influence of temperature variations on the measurements by limiting the duration of each test.

Sample permeabilities were measured by first increasing the confining pressure and pore pressure in small increments (to avoid deforming the sample) to a differential pressure that approximates the in situ conditions for the sample. Then the pressure in the upstream reservoir was raised by a few kiloPascal (kPa) (typically ~15% of the confining pressure), and the differential pressure between the upstream and downstream reservoirs (P_u – P_d) was recorded using a Validyne differential pressure transducer until the differential pressure decayed to 20%–30% of its initial value. The permeability of each sample was then estimated by fitting equation 3 to the data, to determine , and then calculating k from equation 4, as described above.