Wettability from NMR

Principle

The traditional interpretation of NMR logs implicitly assumes that the rock is fully waterwet. However, if this condition is not true, the oil will have contact with a fraction of the rock surface, and thus gets an accelerated relaxation. As a rule of thumb, a fully oil-wet condition would cause the oil to be shifted to approximately the position of the water peak (in reality roughly a factor 3 higher because the oil has a lesser surface relaxivity). The sensitivity is largest for lighter oils and smaller pores. This is a condition that is often encountered in carbonates.

The NMR wettability index, NWI, is defined in analogy with the Amott scale as +1 for waterwet, 0 for neutral/intermediate, to -1 for oilwet. It is obtained from a simple NMR log (T1 or T2), either by wireline or LDW.

My publications can be found HERE.

Benefits

The NMR method offers a unique option to know the wettability condition of a reservoir already within hours of reaching TD in the first well in a new field. It is not only much faster than core analysis, it also provides a continuous profile over the entire reservoir section(s).

Also in SCAL studies this method can be used, e.g. to monitor wettability changes during flooding, as it does not interfere with the fluid saturations and distribution.

Changing wettability from fully waterwet (NWI = 1) to neutral (NWI = 0) and oilwet causes the oil peak to shift to shorter relaxation time. This modeling allows a robust assessment of wettability from an actual NMR log.