Wettability is a tendency of the liquid to spread on a solid surface. In enhanced oil recovery wettability plays an important role as that determines the interactions between the solid (rock) and the liquids in the reservoirs (crude oil, brine). Wettability has been recognized as one of the key parameters controlling the remaining oil-in-place.
Reservoirs are characterized by their wettability
Rocks can be classified as water-wet, mixed-wet or oil-wet. When the rock is water-wet, water is preferentially in contact with the mineral when oil is the surrounding phase. The rock is said to be oil-wet when oil is the liquid in contact. The state in between is called intermediate- or mixed-wet. Contact angles are used as a measure of wettability and thus contact angle ranges for different wetting states can be defined.
From oil-wet to water-wet by wettability alteration
Wettability alteration is an effective approach to enhanced oil recovery. The purpose of the wettability alteration is to change the wetting state of the rock to be more water-wet. Several factors, including crude oil, brine, and rock composition as well as pressure and temperature affect the wettability.
Different approaches have been taken to adjust the wettability of the reservoirs. For carbonate reservoir that is oil-wet, the main methods include CO2 injections and chemical flooding with polymers. Alternative methods are constantly developed due to the increasing importance of the carbonate reservoirs as our source of oil in the future.
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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.
Unconventional oils, such as heavy oil, extra heavy oil, and bitumen, normally exist tightly on host solids such as rocks, sands and clay minerals. Successful liberation of unconventional oil from solids is essential for effective recovery.