## 5 Conclusions  
The impact of water weakening on the mechanical behavior of two chalks, the Obourg Chalk and the Ciply Chalk from the Mons Basin in Belgium, was investigated through different mechanical tests in the laboratory: uniaxial and triaxial compression tests, Brazilian tests, water injection tests, and semi-circular bending tests were conducted on samples saturated with air, water, or brine. From these experiments, we obtained the so-called wet-to-dry ratios, i.e., the ratio between a given property measured on a dry sample and the same property for the water-saturated sample, for the following mechanical properties: UCS, tensile strength, Young's modulus, mechanical strength under triaxial loading, critical pressure, fracture toughness in mode I, cohesion, and internal friction coefficient. For both chalks, and for any of these rock properties, water has a significant weakening effect, as the wet-to-dry ratios roughly ranged between 0.40 and 0.75. On average, the Ciply Chalk exhibits more water weakening than the Obourg Chalk. Some parameters like UCS, critical pressure, and Young's modulus have the highest water sensitivity compared to other properties. From the experiments on brine-saturated samples, a weakening effect is still present but the results are dependent on the fluid composition. Figure 17: Schematic drawing of the repulsive effect (**a**) in a water-saturated chalk, (**b**) in a brine-saturated chalk. For the sake of simplicity, the relative size of the water molecule with respect to Mg\({}^{2+}\) and Cl’ ions is not respected. As the fluid-rock interaction processes occur mainly at the fluid-solid interface, we tried to link our results to the surface properties of the chalks using three different techniques: AFM (atomic force microscopy), contact angle goniometry, and laser confocal microscopy. AFM and contact angle analysis allow one to estimate the surface energy of the fluid-rock interface, which can be compared to the \"fracture\" surface energy inferred from fracture toughness measurements. Our results show that the latter is about one order of magnitude higher that the former. However, in all cases the surface energy in the presence of water was reduced with respect to the dry rocks, which could explain the observed water weakening in both chalks. Another possible explanation is the existence of repulsive forces near the fluid-rock interface, the magnitude of which could be estimated from the adhesion forces provided by AFM. The higher surface energy decrease for Ciply Chalk is in good agreement with the stronger water weakening impact on the mechanical properties observed in this chalk. Conceptualization, D.G. and C.D. ; data curation, D.G. ; formal analysis, D.G. and R.I.; funding acquisition, C.D. ; investigation, D.G., C.D. and R.I.; methodology, D.G. and A.E.H. ; project administration, C.D. ; resources, D.G., C.D. and A.E.H. ; supervision, C.D. and A.E.H. ; validation, D.G., C.D., R.I. and A.E.H. ; visualization, D.G. ; writing--original draft, D.G. and C.D. ; writing--review and editing, D.G., C.D. and A.E.H. All authors have read and agreed to the published version of the manuscript. This work was funded by the GEO\({}^{2}\)FRI\({}^{2}\)SK project (Geophysical and Geotechnical Impact of Fluid-Rock Interactions for Risk Assessment in Chalk Formations) granted by the Initiative d'Excellence Paris Seine at CY Cergy Paris Universite. The data set supporting this work has been made available at [[https://doi.org/10.6084/m9.figshare.16918519](https://doi.org/10.6084/m9.figshare.16918519)]([https://doi.org/10.6084/m9.figshare.16918519](https://doi.org/10.6084/m9.figshare.16918519)) (accessed date 1 December 2021). Part of the experimental results on Obourg Chalk was obtained by [PERSON], currently lab analyst at Stratum Reservoir, during a Master internship at Cergy Paris Universite in 2020. Many thanks to [PERSON], [PERSON], and [PERSON] for technical help with the experiments ran at the i-Mat Microscopy and Analyses Platform at CY Cergy Paris Universite.