|Wettability is a major factor controlling the pressure, flow and distribution of different fluids in a reservoir. The wettability of a core will control capillary pressure, relative permeability and electrical properties. Therefore it is important that the preferential wettability state of the reservoir should be determined before starting a special core analysis program.
The coring process and invasion of non-native fluids (drilling mud) can alter the wettability of the cores before the core arrives in the laboratory. In the laboratory, the rock can be resaturated with the reservoir fluids and stored at elevated temperature and pressure for a certain time (aging) to allow the wettability to be restored.
- Amott wettability: relies on spontaneous imbibition of the wetting fluid. From this, the Amott-Harvey Displacement Index is calculated to express wettability with a single number. The Amott wettability test is insensitive near neutral wetting conditions.
- USBM: measures the wettability by determination of drainage and imbibition multi-speed capillary pressure curves using an ultra-centrifuge. Comparison of the areas under the drainage and imbibtion Pc curves gives an insight of the wettability characteristics. This technique is more sensitive for neutral wetting conditions.
These wettability tests can be performed on fresh / native, clean and restored state core samples, at ambient or elevated temperatures.