Carbonate reservoir wettability is usually studied with quantitative methods such as Amott‑Harvey, USBM (US Bureau of Mines) and sessile drop contact angle. These methods are used both to determine the original wettability state of the reservoir core and to study how different enhanced oil recovery strategies change it.
The wettability of the reservoir has gained a lot of interest since it is recognized as one of the key factors affecting enhanced oil recovery. In carbonate reservoirs, wettability alteration is often the main mechanism for improving oil recovery, and the original wettability state of the rock is therefore important to know.
The Amott-Harvey method uses a combination of spontaneous and forced imbibition measurements of initially oil saturated cores. Displacement of both water by oil as well as oil by water is tested. The data is used to calculate the displacement by oil index which is a ratio between water volume displaced by spontaneous oil imbibition alone and the total volume displaced by oil imbibition and forced displacement.
Prior to measurements, the preparation of the core sample is done so that it is in irreducible water saturation, i.e. at the lowest water saturation possible to achieve. In the first measurement step the core is placed into the water-filled tube and the spontaneous imbibition of water over a period of time, at least 10 days, is measured. Then, the core is placed in a flow cell and water is forced through the sample. The additional oil recovery is recorded. Now the sample is in residual oil saturation point meaning the state where, as much oil is recovered as possible with water. Next, the process is repeated so that the spontaneous imbibition of oil is measured followed by the forced flow.
The result is the Amott-Harvey index which is a difference between the water and oil ratios and varies from + 1 (strongly water-wet) to – 1 (strongly oil-wet).
The USBM test was developed by Donaldson in 1969. As the Amott – Harvey test, it measures the average wettability of the core. The USBM test compares the amount of work required to replace one fluid with the other. The method is based on forced imbibition of fluid inside the core which is achieved by centrifuge spins with stepwise-increasing speed.
As the Amott-Harvey method, The USBM starts with the irreducible water saturated core inside a water-filled tube. After periods at several spin rates, the sample reaches residual oil saturation, and it is placed in an oil-filled tube for other series of measurements. The produced curve is similar to Amott-Harvey, but now the areas between each of the capillary-pressure curves and the zero capillary -pressure line are calculated. The logarithm of the ratio of the water -increasing to oil-increasing areas give the USBM wettability index. The practical measurement results are typically in a range of +1 (strongly water-wet) to + 1 (strongly oil – wet). The USMB method is faster than Amott-Harvey since the spontaneous imbibition is not done but must be corrected because the centrifuge induces a nonlinear capillary-pressure gradient in the sample.
Sessile drop contact angle measurements offer a simple method to study reservoir wettability. One of the advantages of the method is also that it can be conducted at elevated temperatures and high pressures which mimics the reservoir conditions. It can also be used to study the effect of other experimental conditions such as brine chemistry on wettability.
In practice, a drop of liquid, typically crude oil, is placed on a core sample surrounded by brine. As contact angle is commonly presented as a drop of heavier liquid in lighter phase (eq. water in air or water in oil), the contact angle ranges for water-wet, intermediate – wet and oil – wet are as presented in figure below.
For more information about the three methods, please see our article on comparison of oil reservoir wettability measurement techniques.
Most commonly used quantitative methods to study reservoir wettability are Amott‑Harvey, USBM (US Bureau of Mines) and sessile drop contact angle. Amott‑Harvey and USBM provide wettability indices for reservoir core plugs, while sessile drop contact angle gives direct information on how water and oil interact with the rock surface and can be used on smaller samples or under a wider range of test conditions.
The choice of method depends on the available core material, the expected wettability state and how closely the measurements should mimic reservoir conditions. Amott‑Harvey and USBM tests are useful when full core plugs are available and an averaged wettability index is needed, while contact angle measurements are well suited for studying local wetting behaviour, surface treatments and the effect of different brines or crude oils on smaller rock samples.
Yes. Sessile drop contact angle measurements and interfacial tension measurements can be performed at elevated temperatures and high pressures to better mimic reservoir conditions.
If you would like to see how these methods are applied in practice and how contact angle and interfacial tension measurements can support enhanced oil recovery studies, you are welcome to join our free webinar “Measuring wettability and interfacial tension for EOR optimization”.
In this 60‑minute session on May 12, 2026, Susanna Laurén from Biolin Scientific walks through how wettability and interfacial tension are measured under relevant conditions, shares practical tips for reliable data, and shows examples of how laboratory results can be used in EOR optimization.
.png?width=500&name=oil%20&%20gas%20(1).png)
Susanna is an Application Scientist at Biolin Scientific. In her PhD thesis, she developed fabrication methods for a new type of inorganic-organic polymers. Microfabricated polymer chips were utilized as tool for biomolecule separation in analytical chemistry.
