In a strict sense, in a rock-oil-water system, wettability refers to a measure of the preference that the rock has for either the oil or water.
In essence, wettability is used to for the wetting preference of the rock; and does not necessarily refer to the fluid that is in contact with the rock at any given time.
If it is so, whether the (purely horizontal and open to atmospheric pressure as against the real complex three-dimensional pore-network under very high pressure and temperature) solid rock surface is simultaneously exposed to both oil and water - during the wettability measurement at the laboratory-scale?
Dear Kumar,
First of all, we should focus on the concept of wettability alteration. The rock in oil reservoirs, naturally, has oil-wet, water-wet, or mixed-wet conditions. Some fluids, like chemical solutions, are injected into those reservoirs. Thus, the wettability of the rock is altered, and fluids have crucial impacts on the wetting state of the rock. In this respect, wettability alteration is measured by different methods at the bench scale, including the Amott method, the USBM method, and the sessile drop technique.
In all mentioned methods, both oil and fluid are employed to measure wettability alteration. For instance, in the sessile drop technique, a drop of oil is placed on the rock surface in the presence of fluid. Therefore, The rock, oil, and fluid are in contact simultaneously, and the contact angle between the oil droplet and the rock surface is analyzed.
Dear Kumar let's give another explanation which answers your question. Wettability is described as "the tendency of a fluid to spread on a solid surface while the fluid and the solid surface is also in contact with another immiscible fluid".
And YES, the solid surface is exposed to both immiscible fluids during the lab-scale measurements as well as in the reservoir.
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