## 3 Results
### Strength and Elastic Weakening Evidenced through Standard Mechanical Testing
#### 3.1.4 Yielding Envelopes  
In addition to the test at 1.5 MPa confining pressure described in the previous section, more triaxial tests in a wide range of confining pressures were conducted in order to characterize completely the failure envelope of both chalks, for different pore fluids. We plotted in Figure 10 the stress state at yielding (i.e., departure from elasticity) in the so-called Q-P plot with on the \(x\)-axis the mean effective stress \(\mathrm{P}=(\sigma_{1}+2\sigma_{3})/3-\mathrm{p}_{f}\) and Q is the differential stress \(\mathrm{Q}=(\sigma_{1}-\sigma_{3})\). In doing so, we obtained the yielding envelope. For the dry tests the pore fluid was at atmospheric pressure and for all the other tests the pore pressure was 1 MPa or 0.5 MPa (for one test only). The data set for the dry and water-saturated Obourg Chalk was already presented in our previous work [9]: the large shrinkage of the envelope for the water-saturated rock was clearly linked to water weakening effects in Obourg Chalk. New results are presented in Figure 10 from experiments on oil-saturated Obourg Chalk samples. The oil that was used in the triaxial tests was MarcolTM 52, a light oil with density 0.83. Compared to the dry rock, the yielding envelope for the oil-saturated rock is slightly smaller, meaning that even oil has a small weakening effect on the mechanical properties of Obourg Chalk. We also added on the same plot the results from a multistage experiment on a single dry sample: in such a test (i) the sample is first loaded with an isotropic stress \(\sigma_{1}=\sigma_{3}\), (ii) the axial stress is increased until yielding providing one point on the Q-P plot, (iii) the confining pressure is increased and the axial stress is decreased to achieve again isotropic loading, and (iv) the loading cycle proceeds again [21; 22]. With such a protocol, we could get the complete yielding envelope from one single sample for the dry Obourg Chalk (open symbols in Figure 10), which is very close to the \"classical\" one inferred from multiple triaxial tests. This gave us confidence to apply the multistage protocol to the Ciply Chalk, as we had only a limited volume of rock to prepare samples. Figure 10 shows that the water-saturated Ciply Chalk is much weaker than the dry rock. Whereas the yielding envelope of the dry Ciply Chalk is slightly larger than for Obourg Chalk, the one for the water-saturated Ciply Chalk has shrunk much more. As already mentioned, water weakening processes seem to be more intense in Ciply Chalk. To quantify this effect, we can compare the critical pressures P* which mark the onset of yielding by pore collapse under isotropic loading (Q = 0); indeed, it was shown in previous studies that the critical pressure is a scaling parameter for the actual shape of the failure envelopes because it anchors the envelope on the \(x\)-axis in the Q-P plot [23]. For Obourg Chalk we found P*(dry) = 14.1 MPa, P*(oil) = 12.4 MPa, and P*(water) = 7.1 MPa. For Ciply Chalk we found P*(dry) = 16 MPa and P*(water) = 5.9 MPa. Figure 10: Yielding envelopes obtained from the yield stresses for both chalks saturated with different fluids. Solids symbols are for conventional triaxial tests, open symbols are for data obtained using a multistage loading approach on a single sample. The \(y\)-axis scale is the same for both plots. critical pressures, this yields wet-to-dry ratios WDR(P*) of 0.50 for Obourg Chalk and 0.37 for Ciply Chalk reflecting the important shrinking of the yielding envelopes. A Mohr-Coulomb type of approach was also used to characterize failure; this could be conducted for Obourg Chalk only, as all triaxial tests were conducted until the samples failed, but not for Ciply Chalk for which only the yield stresses were obtained from multi-stage loading (Figure 10). In Figure 11, we plotted all the Mohr circles representative of the stress state at failure for the triaxial tests conducted in the brittle field at low effective confining pressures (below 2 MPa) on the dry and water-saturated Obourg Chalk samples.