-
k = (K x µ)/(
x g),
Constant-flow permeability tests were used to estimate hydraulic conductivity values for selected core samples from ODP Sites 1173 and 1174 from the lower Shikoku Basin. The constant-flow approach pumps fluid into and out of the sample, and the resulting hydraulic gradient is measured. The constant-flow permeability tests were conducted using the Trautwein Soil Testing Equipment Company's DigiFlow K (Figs. F2, F3).
The equipment consists of the cell (to contain the sample and provide isostatic effective stress) and three pumps (sample top pump, sample bottom pump, and cell pump). Bladder accumulators were used to allow deionized water as the fluid in the pumps while using an idealized solution of seawater (25 g NaCl + 8 g MgSO4 per liter of water) as the permeant throughout the sample. ASTM designation D 5084-90 (1990) was used as a guideline for general procedures.
The Leg 190 core samples were stored in plastic core liners and sealed with wax to prevent moisture loss. The samples were stored in water in the refrigerator at 4°C until immediately prior to sample preparation. Immediately before testing, cores were trimmed on both ends using a wire saw or a utility knife, depending on the lithology of the core, to fit within the flexible wall membrane. The samples had a minimum diameter of 2 in and sample heights varied from 2.5 to 3.8 in. The ends of every sample were trimmed to provide freshly exposed surfaces. After placing the sample within the flexible membrane, the samples were fitted with filter paper and saturated porous disks and the sample diameter and height were measured. 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 to fully saturate the sample. Flow lines were flushed with deionized water to remove trapped air bubbles. The sample was backpressured at ~0.28 MPa (40 psi) in order to fully saturate the sample. Backpressure was achieved by concurrently ramping the cell pressure and the sample pressure to maintain a steady effective stress. Saturation was verified by measuring the ratio of change in pore water pressure in the porous material to the change in the confining pressure (ASTM, 1990). Once the sample reached saturation, the cell fluid pressure was increased while the sample backpressure was maintained, thus increasing the effective stress on the sample. The maximum stress that the cell is able to sustain is ~1.03 MPa (150 psi), limiting the maximum effective stress to ~0.75 MPa (110 psi). Once the target effective stress was achieved, cell pressure and backpressure were maintained. The sample was allowed to equilibrate for at least 4 hr and generally overnight. Throughout the testing, vertical sample displacement and change in cell fluid volume were monitored.
Once the target effective stress level was achieved, a brief constant-pressure gradient test was conducted to select an appropriate flow rate for the subsequent constant-flow tests. During the constant-flow tests, flow rates were maintained by the top and bottom pumps, one on each end of the sample, ensuring that the volume of the sample is unchanged. During the permeation step, the head gradient was monitored to assure that gradients were not excessive (ASTM, 1990). Since fluid pressure in the closed hydraulic system is affected by temperature changes, testing was conducted within a closed cabinet with a fan to keep the internal temperature uniform. Temperature was monitored throughout the testing phase. For highly consolidated samples with low hydraulic conductivities, the Harvard apparatus PHD 2000 syringe pump was used to conduct very low flow rates, instead of the top and bottom pumps. The PHD 2000 consists of two syringes that can simultaneously pump fluid in and out of the sample at a specified flow rate.
Two to three constant-flow tests were performed at each effective stress level. Once permeability values were obtained, cell pressure was increased and the sample was allowed to equilibrate overnight at the new effective stress. At least two different effective stress steps were performed for each sample. If the permeability of the sample decreased significantly during permeation from the first effective stress step to the second effective stress step, then an extra effective stress step was performed.
Assuming hydrostatic pore fluid pressures, estimated depths at effective stress equal to 0.75 MPa at Sites 1173 and 1174 were 80 and 100 meters below seafloor (mbsf), respectively. Thus, applied effective stress levels used in the laboratory are likely to be well below in situ values. However, previous permeability studies (e.g., Bolton and Maltman, 1998; Bolton et al., 2000) have shown that the largest decrease in permeability occurred as effective stress were increased from 0 to 0.1 MPa; subsequently, permeabilities remained relatively constant. Therefore, we used the permeability results from the highest effective stress, and the values presented in this study represent the maximum in situ vertical permeabilities.
Using these measurements, the specified flow rate, Q (in cubic meters per second), and the pressure difference that was monitored by the testing equipment, hydraulic conductivity, K, values were calculated for each sample using Darcy's Law:
where
These conductivity values were then converted to permeability (in square meters) using the following equation:
x g),
where
= density (1027 kg/m3), and
