Fundamentals of Fluid Flow in Porous Media
Capillary Pressure and Wettability
Wettability is measured, in practical terms, through the relative saturation changes in spontaneous drainage and imbibition. The ratio of the displaced volume through a spontaneous displacement to the volume displaced by the sum of the spontaneous and forced displacement is called the Amott Index. For a water/oil system, there can be an Amott Index for oil and an Amott Index for water. Figure 2‑82 shows the Amott ternary wettability diagram, where the different wettability indexes are presented. The log of the area under the drainage curve to the area under the imbibition curve is called the USBM Index. A water-wet system should have an Amott Index for water close to 1, an Amott Index for oil close to zero and a relatively large positive USBM Index. An oil-wet system should have an Amott index to water close to 1, an Amott index to oil close to zero and a relatively large negative USBM index. Centrifuge technology is often used for the determination of the capillary pressure and the wettability characteristics of a given reservoir, this technology will be covered in greater detail in the next chapter.
Figure 2-82: Amott Ternary Wettability Diagram