13/08/2026
Understanding Reservoir Wettability & The Amott Index Method
1. What is Wettability?
Wettability is the tendency of a solid surface (like reservoir rock) to preferentially attract or be covered by one fluid rather than another in a multiphase system (e.g., oil and water).
Analogy: Think of a paper towel dipping into water—it absorbs the water naturally without any external pressure because it is water-wet.
In petroleum engineering, wettability dictates fluid distribution inside pore spaces, capillary pressure, and ultimate oil recovery.
2. How Do We Measure It? (The Amott Index Method)
The Amott Test quantitatively evaluates wettability by measuring the amount of fluid spontaneously imbibed (naturally drawn in) compared to the amount displaced by forced injection.
The procedure determines two individual indices:
Amott Water Index (Iw):
Iw = {Sw(B) - Sw(A)}/{Sw(C) - Sw(A)}
Amott Oil Index (Io):
Io = {So(D) - So(C)}/{So(E) - So(C)}
Where:
Sw(A): Initial water saturation before spontaneous water imbibition.
Sw(B): Water saturation after spontaneous water imbibition.
Sw(C): Water saturation after forced water injection.
So(C): Oil saturation after forced water injection 1 - Sw(C).
So(D): Oil saturation after spontaneous oil imbibition.
So(E): Oil saturation after forced oil injection.
3. Overall Amott Index (IA) & Classification
The overall Amott Index (IA) is the difference between the water and oil indices:
IA = Iw - Io
Depending on the resulting value of IA, the rock's wettability state is defined as:
Wettability StateAmott Index Range (IA)
Water-wet+0.3 to +1.0
Intermediate / Neutral-wet-0.3 to +0.3
Oil-wet -1.0 to -0.3
Reference
Alyafei, N., Fundamentals of Reservoir Rock Properties, 2nd Edition.
