Most phenomena in shale drilling instabilities have only been proved in laboratories, on reconstituted, outcrop or core samples. Partial saturation is evidenced for these samples, its origins explained and its consequences detailed. A model is proposed for the calculation of the apparent compressive strength of a rock from its pore size distribution and moisture content. Effects of capillary pressures are presented in various shale/fluid inhibition tests and some classical interpretations turned down. Last, a rehydration procedure is presented, which has been validated on a low permeability shale.