[en] Subsidence of chalk oil reservoirs in North Sea is related to the chalk compaction induced by fluid depletion and by the water – chalk interaction. A constitutive model is developed in order to take into account these two effects. It is based on frictional – cap elastoplasticity and on the Barcelona unsaturated soil model. Oil – water – chalk interaction is modeled through the suction variable. The two saturating fluids flow is also developed. After implementation into a finite element code, these tools allow to simulate a waterflooding experiment on a chalk sample. The model appears to reproduce qualitatively and quantitatively the experimental results.