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The displacement efficiency depends upon the ratio of the viscous to capillary forces or capillary number. In enhanced recovery processes, the interfacial tension between the oil and water is reduced to improve the capillary number.<ref name=pt10r37>Willhite, P. G., 1986, Waterflooding: Society of Petroleum Engineers Textbook Series No. 3, chap. 2.</ref>
The displacement efficiency depends upon the ratio of the viscous to capillary forces or capillary number. In enhanced recovery processes, the interfacial tension between the oil and water is reduced to improve the capillary number.<ref name=pt10r37>Willhite, P. G., 1986, Waterflooding: Society of Petroleum Engineers Textbook Series No. 3, chap. 2.</ref>
The vertical sweep efficiency is a function of the vertical heterogeneity (layering) and the mobility ratio (''M''). The mobility ratio defined here is the ratio of the [[relative permeability]] to water at ''S''<sub>or</sub> (''k''<sub>rw</sub>) to the relative permeability of the oil at ''S''<sub>wi</sub> multiplied by the oil-water viscosity ratio (''μ''<sub>o</sub>/''μ''<sub>w</sub>) as expressed in the following equation:
The vertical sweep efficiency is a function of the vertical heterogeneity (layering) and the mobility ratio (''M''). The mobility ratio defined here is the ratio of the [[relative permeability]] to water at ''S''<sub>or</sub> (''k''<sub>rw</sub>) to the relative permeability of the oil at ''S''<sub>wi</sub> multiplied by the oil-water [[viscosity]] ratio (''μ''<sub>o</sub>/''μ''<sub>w</sub>) as expressed in the following equation:
:<math>M = (k_{\rm rw}/k_{\rm ro}) \times (\mu_{\rm o}/\mu_{\rm w})</math>
:<math>M = (k_{\rm rw}/k_{\rm ro}) \times (\mu_{\rm o}/\mu_{\rm w})</math>