Previous   |    Next

doi:10.2204/iodp.proc.322.208.2013

Experimental results

All permeability experimental results are summarized in Tables T2 and T3 and presented in Figures F3 and F4 (see PERM in “Supplementary material” for all data files). Nomenclature is provided in Table T4.

CRS consolidation and flow-through permeability experiments allow us to define the range of permeability of sediments at Sites C0011 and C0012 with respect to depth and porosity. We separate discussions based on type of experiment (CRS or flow-through), lithologic unit, and depth trends. To define the depth trends, we assume a log-linear relationship between vertical permeability and depth:

where

• k_v = vertical permeability (m²),

• B = permeability model constant,

• C = permeability model constant, and

• z = depth (mbsf).

B and C are determined by a best-fit model to the permeability-depth data.

CRS consolidation results

CRS consolidation experiments provide estimates of permeability at Site C0012 from 115.44 to 255.31 mbsf. Based on our CRS experiments, estimates of in situ k_v at Site C0012 range from 1.12 × 10^–17 to 2.54 × 10^–18 m² (Table T2; Figs. F3, F4). CRS-based estimates of in situ k_h at Site C0012 range from 1.66 × 10^–17 to 6.55 × 10^–18 m². All calculated horizontal permeabilities were higher than vertical permeabilities at equivalent depths, giving permeability ratios (k_v/k_h) of 0.38–0.99 (Table T2). These permeability estimates document a general decrease with increasing depth (Fig. F3), and a best-fit of the data to Equation 4 yields B = –0.0046 and C = –16.4, with an excellent goodness of fit (R² = 0.99). Permeability also shows a general decrease with decreasing porosity (Fig. F4).

Flow-through permeability experiments

Flow-through permeability experiments were used to estimate permeability at Site C0012 from 273.51 to 473.11 mbsf and at Site C0011 from 366.76 to 687.15 mbsf.

At Site C0012, in situ k_v estimates range from 6.09 × 10^–19 to 2.86 × 10^–19 m² (Table T3; Figs. F3, F4). The one in situ k_h estimate at Site C0012 from flow-through tests was 6.54 × 10^–19 (Table T2; Figs. F3, F4). The calculated horizontal permeability was higher than vertical permeability at an equivalent depth, giving a permeability ratio (k_v/k_h) of 0.58 (Table T3). These permeability estimates document a general decrease with increasing depth (Fig. F3), and a best-fit model of the data to Equation 4 yields B = –0.0011 and C = –18.0; however, the goodness of the fit is low (R² = 0.46). Permeability also shows a general decrease with decreasing porosity (Fig. F4). It should be noted that there is a step-decrease in permeability at Site C0012 moving from 255.23 to 273.51 mbsf (Fig. F3). This decrease corresponds to a change from CRS to flow-through experiments, which was influenced by the stiffness of the samples during trimming; however, there is not a significant change in porosity over this interval (Fig. F4). Therefore, this change in permeability warrants further investigation, as it could be a material property or an experimental artifact.

At Site C0011, in situ k_v estimates range from 7.03 × 10^–17 to 1.38 × 10^–18 m² (Table T3; Figs. F3, F4). One data point (Test PERM072; Table T3) appears to be anomalously high (k_v = 7.03 × 10^–17 m²) and may be the result of significant sample disturbance (specimen showed cracks after experiment), so we neglect it in depth and porosity trend discussions. Permeability estimates document a general decrease with increasing depth (Fig. F3). A best-fit model of the data to Equation 4 yields B = –0.0010 and C = –17.2; however, the goodness of the fit is low (R² = 0.31). Permeability also shows a general decrease with decreasing porosity (Fig. F4).

Using flow-through data at Sites C0011 and C0012, we compare permeability at equivalent depths and in the same lithologic units. At equivalent depths (below 350 mbsf), vertical permeability determined by flow-through experiments at Site C0011 is greater than vertical permeability determined by flow-through experiments at Site C0012 (Fig. F3). In lithologic Units III and IV, flow-through-estimated permeability at Site C0011 is greater than at Site C0012 (Fig. F3; Table T3).

These experimental results provide laboratory-based permeabilities for the interpreted in situ porosity. Ideally laboratory-based studies can provide the in situ porosity and permeability; however, for Sites C0011 and C0012, these data should be considered high-end estimates of in situ porosity and permeability because of significant sample disturbance. Sample disturbance is interpreted from three primary approaches:

1. The laboratory-estimated porosities at in situ conditions are similar to those determined by shipboard analyses (Fig. F2), and the shipboard measurements were interpreted as influenced by sample disturbance imparted by the drilling and coring processes.

2. Sample trimming for these experiments was extremely difficult. In many instances we had to make two or three attempts (each on a new specimen) to trim a competent specimen for experiments. Samples often broke during cutting and trimming. Incorrectly trimmed or visibly disturbed specimens were not used in experiments.

3. One specimen (Test PERM072; Table T3) showed cracks at the end of the experiment. As experimental stresses were low, these cracks could have been preexisting and reopened during the experiment.

We cannot accurately characterize the amount of disturbance on any of the specimens, but we emphasize that because of disturbance there are likely flow paths that were not present in situ. Also, as the flow-through experiments did not achieve in situ effective stress conditions, they may have higher than in situ porosity and permeability. Therefore our estimated permeability is a high-end estimate (upper limit) of the in situ conditions at any depth.

The presence of sample disturbance and its impact on permeability also suggest that all experimental samples should be carefully selected to minimize disturbance and that drilling data and reports should be used to look for drilling issues that may have been noted during the drilling and coring process. Such integration of drilling and experimental results cannot remove disturbance but can allow choosing samples with minimum disturbance and promote awareness of the potential problems with permeability estimates.

Top of page   |    Previous   |    Next