## 1 Introduction  
The mechanical properties of fluid-bearing porous rocks depend on pore fluid properties. From the poroelasticity viewpoint, there are two different regimes to be considered: (i) in the drained regime, fluids can flow freely so that the mechanical behavior will not be very different from that of a dry rock; whereas (ii) in the undrained regime, the no-flow condition will induce pore pressure build up when applied stresses lead to rock deformation. The concept of effective stresses implies that variations of pore fluid pressure will impact most of the mechanical and physical properties of porous rocks [1]. Apart from the purely mechanical effect of pore fluid pressure, physico-chemical fluid-rock interactions may introduce additional complexities. For example, water weakening is a well-known phenomenon which can affect both carbonate [2] and silicidastic rocks [3]. Many experimental studies have shown that the mechanical properties of porous rocks are lowered when the pore fluid is water. For example, [PERSON] et al. [4] compared the failure envelope of Liege Chalk saturated with air, water, and various water-glycol mixtures: they found that when saturated with water, the failure envelope is much smaller than for any other pore fluid. Water weakening and fluid-rock interactions were also responsible for the acceleration of the subsidence rate during and after water flooding operations in the Ekofisk oil field in North Sea [5]. Experimental studies on sandstones have shown that, like carbonate rocks, water weakening can significantly lower the mechanical strength and elastic moduli of siliciclastic rocks [6, 7]. Using a protocol intended to mimic water flooding operations in reservoir to increase the production, [PERSON] et al. [8] have shown that when dry or oil-saturated Sherwood sandstone samples are critically loaded, a small amount of water injected under low pressure is enough to trigger mechanical instabilities leading to failure. [PERSON] et al. [3] linked the drop in the critical pressure, i.e., the pressure at the onset of grain crushing and pore collapse under isotropic loading, for water-saturated sandstones to the decrease in the fracture surface energy at the fluid-solid interface. Indeed, a decrease in fracture surface energy will promote crack propagation at lower stresses compared to the dry rock, for example. Therefore, it seems important to relate mechanical weakening to variations of the solid-fluid interface properties for different pore fluids.  
The objective of this work is (i) to quantify the magnitude of water weakening effects in two chalks by measuring several mechanical properties (compressive and tensile strengths, elastic moduli, and fracture toughness) in laboratory experiments using different pore fluids, (ii) to characterize the surface properties in presence of different pore fluids using different techniques (atomic force microscopy, contact angle goniometry, and laser confocal microscopy), and (iii) to link the impact of water weakening on the mechanical properties to variations in the surface properties. Although other mechanisms have been proposed to explain water weakening in porous rocks, we show that the decrease in surface energy and the existence of repulsive forces are likely to be the dominant mechanism in our experiments.