## 3 Results
### Strength and Elastic Weakening Evidenced through Standard Mechanical Testing  
#### 3.1.1 Uniaxial Compression Tests  
In total, 100 uniaxial compression tests were conducted on Obourg chalk for different fluid contents: 10 tests on dry samples (after drying them in an oven at 60 \({}^{\circ}\)C for 48 h), 5 tests on distilled-water-saturated samples, and the rest on samples saturated with several aqueous solutions. Among these we used brines with different concentrations (10, 30, and 50 g/L) of NaCl (20 samples), MgCl\({}_{2}\) (20 samples), NaHCO\({}_{3}\) (15 samples), Na\({}_{2}\)SO\({}_{4}\) (15 samples), and CaCl\({}_{2}\) (15 samples). All the results for the UCS are plotted in Figure 8 vs. the sample porosity. First, let us discuss the large difference in UCS values for the dry samples compared to the water-saturated samples. Systematically larger values are found for the UCS of dry samples compared to the water-saturated samples. For the dry samples the average \begin{table}
\begin{tabular}{c c c c c c} \hline
**Free Surface Energy (m)/m2)** & **DW** & **DIO** & **Form** & **DMSO** & **Glyc** \\ \hline Dispersive component \(\gamma^{\text{(D)}}\) & 21.8 & 48.5 & 39.5 & 36.0 & 34.0 \\ \hline Polar component \(\gamma^{\text{(P)}}\) & 51.0 & 2.3 & 18.7 & 8.0 & 30.0 \\ \hline Total free energy & 72.8 & 50.8 & 58.2 & 44.0 & 64.0 \\ \hline \end{tabular}
\end{table} 
Table 2: Dispersive and polar components of the free surface energy for the 5 fluids used in our experiments. The total free energy is the sum of both components. DW = distilled water/DIO = Diiodomethane/Form = Formamide/DMSO = Dimethyl Solifoxide/Glyc = Glycerol. Figure 8: UCS values for the Obourg chalk vs. porosity for 100 samples with different fluid content. value is \(\text{UCS}^{(\text{dry})}=7.0\) MPa with standard deviation 1.3 MPa, and for the water-saturated samples the average value is \(\text{UCS}^{(\text{sat})}=3.6\) MPa with standard deviation 0.6 MPa. This shows that there is a strong weakening effect on the compressive strength, which can be characterized by the wet-to-dry ratio \(\text{WDR}(\text{UCS})=0.51\). Furthermore our results show that the UCS is strongly controlled by the porosity even in the very narrow range corresponding to our sample set: the UCS clearly decreases when the porosity increases for both the dry and water-saturated samples, with a much steeper slope for the former compared to the latter. For the brine-saturated samples, we did not find any systematic trend with the salt concentration; for this reason, we plotted in Figure 8 all the results with a single symbol per salt regardless the concentration. Sodium chloride has no effect on UCS as we found similar results as for the water-saturated samples. In contrast for magnesium chloride and sodium sulphate, the UCS values fall in between the values for dry and water-saturated samples, meaning that these salts have some inhibiting effect on water weakening in Obourg Chalk. The results for [PERSON]'s modulus are also very different for dry and water-saturated samples: we found \(\text{E}^{(\text{dry})}=5.5\) GPa with standard deviation 1.4 GPa and \(\text{E}^{(\text{sat})}=3.2\) GPa with standard deviation 1.3 GPa. Therefore, water has also an elastic weakening effect which can be characterized by the wet-to-dry ratio \(\text{WDR}(\text{E})=0.58\), which is comparable to the UCS ratio. For the Ciply chalk, five samples were tested until failure to get the UCS and [PERSON]'s modulus in dry conditions, five additional dry samples provided [PERSON]'s modulus but not the UCS, and four water-saturated samples were tested until failure. For the dry samples, the average UCS was \(\text{UCS}^{(\text{dry})}=4.4\) MPa with standard deviation 1.8 MPa, and [PERSON]'s modulus was \(\text{E}^{(\text{dry})}=1.2\) GPa with standard deviation 0.5 GPa. For the water-saturated samples, we found \(\text{UCS}^{(\text{sat})}=1.7\) MPa with standard deviation 0.14 MPa and \(\text{E}^{(\text{sat})}=0.50\) GPa with standard deviation 0.23 GPa.