Specific surface area

The specific surface areas obtained from N₂ adsorption (BET) vary between ~ 10 and 40 m²/g (Tab. 4-11). These values represent so-called external surfaces that characterise outer surfaces between mineral grains, but do not include internal surfaces in smectite clay minerals. Fig. 4-22 displays all N₂ isotherms obtained at Uni Bern, which exhibit the typical hysteresis between adsorption and desorption. When calculating fluid volume from the adsorbed gas volume, the total volume of N₂ adsorbed corresponds roughly to (with tendency to be larger than) the water-loss porosity and the physical porosity (Tab. 4-7), which gives confidence in the adsorption data. Tab. 4-11 also lists the energetic constants C obtained from the BET fits. There is a weak correlation between the N₂ BET surface areas and the physical porosity, as well as between the surface areas and the clay-mineral content (Fig. 4-23). The latter can be explained by the small particle sizes for clays, leading to large specific external surfaces.

Ciemat obtained also N2 isotherms for two of their subsamples (SLA 880.30 and SLA 929.40) (Tab. 4-12). While similar curves (Fig. 4-24) and similar BET surface areas were obtained for sample SLA 880.30 by Ciemat and Uni Bern, a larger discrepancy is observed for sample SLA 929.40. This is possibly related to heterogeneity at the cm to dm scale; the 5-cm subsamples for Ciemat were typically taken from the bottom of a 25 cm-core, those for Uni Bern from the other part. One should note, however, that both derived BET surface areas are in a reasonable range consistent with the general trend with the clay-mineral content (see Fig. 4-23).

Fig. 4-22: Full N2 ad- and desorption isotherms measured for Schlattingen samples at Uni Bern.

Fig. 4-23: BET specific surface areas versus physical porosity (left) and clay-mineral content (right).

Closed symbols: Uni Bern samples; open symbols: Ciemat samples.

Tab. 4-11: N2 BET data obtained at Uni Bern.

Sample ID Stratigraphy BET Error Total ads.

pore volume

error C value Error

m²/g m²/g ml/g ml/g – –

SLA 734.88 Effingen Member 19.44 0.97 0.0322 0.00161 117.8 11.8 SLA 742.48 Effingen Member 16.95 0.85 0.0301 0.00151 118.9 11.9 SLA 750.73 Effingen Member 17.69 0.88 0.0324 0.00162 115.5 11.5 SLA 758.79 Wutach Fm. 26.08 1.30 0.0760 0.00380 109.2 10.9 SLA 765.31 Variansmergel Fm. 11.35 0.57 0.0279 0.00140 46.2 4.6 SLA 768.62 Parkinsoni-Württemb. Beds 25.74 1.29 0.0393 0.00197 94.1 9.4 SLA 778.70 Parkinsoni-Württemb. Beds 18.51 0.93 0.0342 0.00171 99.2 9.9 SLA 787.33 Parkinsoni-Württemb. Beds 27.87 1.39 0.0482 0.00241 94.3 9.4 SLA 800.01 Humphriesioolith Fm. 25.90 1.29 0.0403 0.00202 111.9 11.2 SLA 812.11 Wedelsandstein Fm. 31.85 1.59 0.0489 0.00245 109.3 10.9 SLA 816.73 Wedelsandstein Fm. 31.50 1.58 0.0491 0.00246 101.4 10.1 SLA 823.53 Wedelsandstein Fm. 15.81 0.79 0.0346 0.00173 35.0 3.5 SLA 833.08 Opalinuston 17.00 0.85 0.0313 0.00157 101.9 10.2 SLA 844.56 Opalinuston 27.04 1.35 0.0461 0.00231 105.3 10.5 SLA 852.06 Opalinuston 24.00 1.20 0.0436 0.00218 96.7 9.7 SLA 860.77 Opalinuston 21.28 1.06 0.0401 0.00201 94.4 9.4 SLA 872.12 Opalinuston 16.42 0.82 0.0339 0.00170 96.9 9.7 SLA 880.30 Opalinuston 24.48 1.22 0.0538 0.00269 96.4 9.6 SLA 888.33 Opalinuston 25.09 1.25 0.0502 0.00251 96.3 9.6 SLA 898.31 Opalinuston 19.85 0.99 0.0452 0.00226 77.6 7.8 SLA 908.32 Opalinuston 23.07 1.15 0.0430 0.00215 108.1 10.8 SLA 915.67 Opalinuston 23.62 1.18 0.0447 0.00224 92.4 9.2 SLA 921.15 Opalinuston 20.86 1.04 0.0438 0.00219 84.9 8.5 SLA 929.40 Opalinuston 21.38 1.07 0.0417 0.00209 72.5 7.2 SLA 939.48 Opalinuston 28.12 1.41 0.0488 0.00244 104.3 10.4 SLA 949.45 Opalinuston 25.86 1.29 0.0533 0.00267 107.6 10.8 SLA 960.38 Posidonienschiefer 15.81 0.79 0.0437 0.00219 61.7 6.2 SLA 971.89 Obtusus Beds 28.28 1.41 0.0463 0.00232 112.6 11.3 SLA 981.04 Obtusus Beds 30.98 1.55 0.0469 0.00235 102.8 10.3 SLA 987.61 Psiloceras Beds 37.49 1.87 0.0539 0.00270 123.0 12.3

Tab. 4-12: N2 BET data obtained at Ciemat.

Sample ID Stratigraphy BET Error Total ads.

pore volume

C value

m²/g m²/g ml/g – SLA 880.30 Opalinuston 21.96 0.06 0.0460 93.96

SLA 929.40 Opalinuston 32.12 0.04 0.0546 140.94

Fig. 4-24: Comparison of N2 ad- and desorption isotherms obtained for different subsamples from the same core by Ciemat and Uni Bern.

The top row graphs show linear axes, the bottom row logarithmic axes

Total and internal surface area

These data are compiled in Tab. 4-13. Total surface areas of the Ciemat samples obtained from water adsorption at a relative humidity of 0.75 (Fig. 4-25) vary between 56 m²/g and 129 m²/g, with a general tendency of increasing values with depth (with some exceptions). For compari-son, the plot shows also the total surface areas obtained at a relative humidity of 0.85, as well as the N₂ BET surface areas obtained at Uni Bern and (samples SLA 880.30 and SLA 929.40 only) Ciemat. The N₂ BET surface area is generally considered as the external surface area not including the internal (interlayer) surface area of smectites. The latter was calculated as the difference between the total and the N₂ BET surface area. As expected, the total surface area, the internal surface area, and the external surface area are correlated with the clay-mineral con-tent of the sample, as shown in Fig. 4-26. The internal surface areas account for 65 to 86 % (mean of 75 %) of the total surface areas.

Tab. 4-13: Total and internal surface areas for Ciemat samples.

Sample ID Stratigraphy Total surface

1 considered as less reliable than value obtained at rh 0.75

2 calculated as total surface area (rh 0.75) minus N2 BET surface area from Uni Bern

3 based on first order error propagation

4 N₂ BET surface area taken from closest sample

5 no value determined; set to maximum value of other samples

Fig. 4-25: Profiles of specific surface areas; total surface areas from Ciemat (from data at relative humidity of 0.75 and 0.85), N₂ BET surface areas from Uni Bern and (two samples) Ciemat.

Malm'Brauner Dogger'Opalinus ClayLias

0 20 40 60 80 100 120 140 160

Fig. 4-26: Correlations between total surfaca area (from relative humidity of 0.75) or internal surface area (total minus N2 BET surface area) and the clay-mineral content for the Ciemat samples.

The N2 BET surface areas from Uni Bern used to calculate the internal surface areas are also shown, as well as the two N2 BET surface areas measured at Ciemat.

4.2.5 Water activities

Water activities were determined by measurements of 2 to 3 subsamples (only 1 subsample in one case). This revealed considerable variability between water activity data of different sub-samples, without any trend. In the following, we report the average activities and the standard deviations obtained from the different subsamples as large circles and error bars, respectively;

the individual measurements are shown as small dots. Tab. 4-14 lists average water activities and their standard deviations. Fig. 4-27 displays the measured water activities against the sample depth. All measured activities except one are clearly below 1, with a majority (about half of the values) between 0.84 and 0.87. There is no obvious trend of the measured activities with depth. A larger variability (values between ~ 0.76 and 0.99) is observed in the upper part at 730 – 760 m below ground.

Fig. 4-27: Measured water activities plotted as a function of depth below the surface.

The small symbols show the values of the subsamples; the large symbols indicate mean values, the bars ± one standard deviation.

The water activity is a fundamental measure of the energy state of water in a system. It reflects interactions of water molecules with solutes, with solid surfaces and with interfaces to other fluids. In the following, we are briefly discussing the possible relevance of these three factors.

First, the salinity of the porewater affects the water activity, with increasing salinity leading to decreasing activity. The activity of seawater is about 0.98, so this means that for most natural waters the effect of the salinity on the water activity remains in a rather narrow range. From the maximum porewater ion concentrations obtained by squeezing of Schlattingen samples, an approximate equivalent NaCl concentration of ~ 0.23 M can be calculated. Such a concentration would lead to a water activity in the order of 0.99. Next, in clayey samples, the water activity may also be influenced by the interaction of the charged surfaces with the porewater (matrix surface effect). A negative correlation with the clay-mineral content or the specific surface areas would be expected if this effect were dominant. Finally, a partial saturation of a sample, which leads to capillary interactions at gas-water-solid interfaces, can lower the water activity strongly.

Desaturation can occur when the sample is exposed to ambient or dry air. It may also result from decompression of samples brought to the surfaces as a result of outgassing of dissolved gases, or as a result of a volume change, if the samples are in contact with a gas phase, but not with a water phase at the time of decompression. Pressure and temperature changes can further complicate the situation. The reduction of the water activity is in any case related in a non-linear way to the saturation state of the sample.

In order to check for potential causes for the measured reduced water activities, we plotted the values as a function of different variables: the clay-mineral and calcite content (Fig. 4-28) and the total surface area (Fig. 4-29a), considered as indicator of the matrix surface effect, and the apparent water saturation (Fig. 4-29b) as an indicator of capillary effects. The apparent water saturation 'S_w' was estimated as the water-loss porosity divided by the physical porosity of the samples. No clear trend was observed in any of these plots. The measured activities seem to be about independent of the clay-mineral content and the specific surface area. Two samples with a

Malm'Brauner Dogger'Opalinus ClayLias

0.75 0.8 0.85 0.9 0.95 1

low clay-mineral content and a large calcite content (but not a third one) exhibit the largest differences between the activity measurements of the two subsamples. A very weak tendency of lower water activities with lower apparent saturation appears in Fig. 4-29b. But note the very large error bars calculated for 'Sw' from first-order error propagation. Apparent water saturations larger than 1 were calculated for some samples. These values, which are physically impossible, also demonstrate the large errors associated with the calculated apparent water saturation.

Equally, no trends of the measured water activities with the water-loss porosities or the physical porosities were evident (data not shown).

Fig. 4-30a plots the measured water activities together with the calculated apparent water saturations as a function of depth, Fig. 4-30b the activities together with the clay-mineral content as a function of depth. Except for the upper, mostly calcite rich samples, the apparent water saturation appears to decrease with depth. This could represent an effect of the decom-pression of the samples, with larger decomdecom-pression effects expected for the samples from larger depth and with higher clay-mineral contents. There is, however, no concomitant decrease of the water activity with depth, as would be expected for samples of similar or even larger clay-mineral content.

(a) (b)

Fig. 4-28: Measured water activities plotted (a) vs. the clay-mineral content and (b) vs. the calcite content of the samples.

(a) (b)

Fig. 4-29: (a) Water activities a_w plotted against S_BET surface area; (b) water activities plotted against the apparent water saturation 'S_w', calculated as water-loss porosity divided by physical porosity.

(a) (b)

Fig. 4-30: (a) Water activities aw and apparent water saturations 'Sw' versus depth; (b) water activities aw and clay-mineral content versus depth.

In summary, water activities mostly between 0.80 and 0.95 were measured, with ca. 50 % of the data between 0.84 and 0.87. A water activity of 0.85 corresponds to a water potential of about – 22 MPa. The measured water activities do not clearly correlate with the content of clay mine-rals, the surface area, or the sample depth, and only very weakly with the apparent water satura-tion. Accordingly, the reduced water activity values could not be clearly attributed to a single cause. The salinity of the porewater is certainly not responsible for values lower than about 0.98 – 0.99, and it appears thus likely that the values are influenced by a slight desaturation of the samples, as it may occur during decompression, and possibly also by interactions of water molecules with charged surfaces. The influence of pressure and temperature changes compared to in-situ conditions on the measured water activity of a sample is another point that deserves some further investigation. On balance, the low activities do not necessarily compromise the quality of the obtained petrophysical and porewater data.

Malm'Brauner Dogger'Opalinus ClayLias

0.2 0.4 0.6 0.8 0.9 1

700

750

800

850

900

950

1000

aw a aw b aw c aw avrg Clay min

Clay-mineral content (-) or Water activity a_w (-)

Depth (m)

Tab. 4-14: Water activity data.