Deformation and yield in high porosity outcrop chalk

Mechanically high porosity chalks behave as frictional materials, but with an end-cap reflecting pore collapse failure. Shear failure between the grains seems to be the basic failure mechanism, not only in compression tests at low confining pressures where shear bands are formed, but also in pore collapse where distributed shear failure will occur in the material. Also tensile failure seems to be initiated by a shear failure mechanism, and tensile strength can be predicted from the compressive yield criterion (Mohr-Coulomb). The mechanical properties of chalk are strongly dependent on the type of fluid in the pores. Water saturated chalk is considerably weaker than dry or oil saturated chalk. Chalk and fluids may interact through capillary forces and through surface physical/chemical reactions. Capillary forces will always be present when two immiscible fluids are present in the pore space. But such effects can only partly explain the experimental observations. There are however indications that the water weakening effect might be caused by repulsive forces generated by dipole-dipole interactions in the very narrow grain contact areas.