If compression stresses are high enough, grains can crush filling the pore space. Pore collapse may happen in nature due to rock burial and also in petroleum engineering during reservoir depletion. In both cases effective stresses increase in all directions. In long and thin reservoirs, depletion does not cause strains in all directions but predominantly in the vertical direction (Figures and 3.19). This type of deformation is called “uniaxial-strain” condition. When effective stress goes over the yield stress ( in Fig. 4.16), significant plastic irrecoverable deformations occur and may decrease permeability. High compression combined with shear can lead to grain crushing at shear and compaction bands resulting in permeability much lower than that of the original rock matrix.