USBM wettability index

The U.S. Bureau of Mines (USBM), developed by Donaldson et al. in 1969, is a method to measure wettability of petroleum reservoir rocks. In this method, the areas under the forced displacement Capillary pressure curves of oil and water drive processes are denoted as $A1$ and $A2$ to calculate the USBM index.

$USBM=log{\frac {A_{\mathit {1}}}{\ A_{\mathit {2}}}}$

USBM index is positive for water-wet rocks, and negative for oil-wet systems.

Bounded USBM (or USBM*)

The USBM index is theoretically unbounded and can vary from negative infinity to positive infinity. Since other wettability indices such as Amott-Harvey, Lak wettability index and modified Lak are bounded in the range of -1 to 1, Abouzar Mirzaei-Paiaman highlighted the bounded form of USBM (called USBM*) as a replacement of the traditional USBM as

$USBM*={\frac {A_{\mathit {1}}-A_{\mathit {2}}}{\ A_{\mathit {1}}+A_{\mathit {2}}}}$

USBM* varies from -1 to 1 for strongly oil-wet and strongly water-wet rocks, respectively.

See also

• Wetting
• Amott test
• Lak wettability index