Displacement efficiency defines the fraction of the initial pore-saturating fluid displaced in a the piston sweep by invaded fluid:

E_D = 1- \frac{V_{HC, \ LEFT}}{V_{HC, \ INITITAL}} = \frac{1-s_{wi}-s_{orw}}{1-s_{wi}}

where  – inititial water in oil pay,  – residual oil to water sweep.