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 A 1 {\displaystyle A1} and A 2 {\displaystyle A2} to calculate the USBM index.
U S B M = l o g A 1 A 2 {\displaystyle 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
U S B M ∗ = A 1 − A 2 A 1 + A 2 {\displaystyle 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
References