There are three commonly used wettability measurement techniques for oil reservoir characterization; Contact angle, Amott-Harvey, and USBM. The techniques are already explained in our previous blog post, so here the focus is on the comparison.
Surface wettability vs. average wettability of the core
The main difference is that the contact angle measures the wettability of the surface whereas both Amott-Harvey and USBM give the average wettability of the whole core sample. The contact angle gives an angle that is related to the wettability of the core. Amott-Harvey and USBM give wettability indexes.
Although measurement time is not always the most critical factor it can be meaningful in some measurements. Especially in surfactant screening contact angle measurements are powerful as they can be performed in a relatively short time.
Amott-Harvey is the most time consuming of the three as it realizes partly on spontaneous imbibition to the core. This step typically takes at least 10 days. The USBM method is faster since the spontaneous imbibition is not included but the whole measurement is conducted with the forced imbibition using a centrifuge.
Measurements at reservoir conditions
When enhanced oil recovery methods are studied the wettability measurements should be performed at the reservoir conditions. The interfacial properties are dependent on the atmospheric conditions and thus also the wettability is affected.
Wettability can be studied with the contact angle as measurements at high temperatures and even under pressure are possible with commercial instrumentation. For Amott-Harvey and USBM measurement, there are no instruments in the market that support measurements at reservoir conditions.
Reservoir types can cause limitations
Reservoir type can cause challenges for wettability measurements. Contact angle measurements are done on top of the surface which makes it possible to use practically on all reservoir cores. Amott-Harvey, on the other hand, is not really suitable for shales and other tight reservoirs as spontaneous imbibition is not possible. Although USBM is done with forced imbibition, the centrifugal force needed for tighter reservoirs is too high for most of the centrifuge.
Different enhanced oil recovery methods are used to alter the wettability of the reservoir rock. To study the wettability alteration at the reservoir conditions, an instrument where the measurements can be done at high pressures and temperatures are needed.