Mechanical behaviour of chalk reservoirs: numerical modelling of water sensitivity effects

Chalk is the constituent material of numerous oil reservoirs in North Sea. The mechanical behaviour of a saturated chalk has been largely studied. However, different aspects of its behaviour are not yet well understood: material characteristics depend on the saturating fluids and chalk response is time-dependent. This paper proposes the PASACHALK numerical model: an elasto-plastic constitutive law is presented, which reproduces the different plastic mechanisms of the chalk (pore collapse and shear failure) and the influence of pore fluids. The water sensitivity of this soft rock is explained by the existence of suction effects in chalk. Finally, a simulation of a hypothetical reservoir is proposed to show the response of the elasto-plastic model during depletion phase and water injection phase.