CO2 is one of the most important greenhouse gases that cause global warming. For this reason, CO2 sequestration, or storage, has become increasingly interesting. Deep saline aquifers and depleted hydrocarbon reservoirs offer possible storage sites for CO2. For reliable sequestration, it is important to prevent the leakage of CO2 from storage site also in the long term. The success of a storage project is dependent on an understanding of the processes that take place at the reservoir. The storage capacity is the function of pore-throat size, interfacial tension between CO2 and brine and contact angle in between CO2, brine and mineral (rock) surface.
The CO2-brine interfacial tension determines the storage capacity
Capillary forces in the cap rock prevent the upward migration and escape of the stored CO2. Interfacial tension between the brine and CO2 is the main parameter determining the capillary forces. Brine composition and concentration, as well as pressure and temperature, affect IFT between brine and CO2. For this reason, it is important to carry out the measurements at the reservoir temperatures and pressures. Usually, interfacial tension decreases as the pressure and temperature increases. This limits the amount of CO2 stored in the reservoir. It should be kept in mind that, in depleted hydrocarbon reservoirs, the determined breakthrough pressures for natural gases, such as methane, are usually much higher than for carbon dioxide. This is becauce CO2 - brine interfacial tensions are much lower that any hydrocarbon - brine IFT.
The CO2-brine- mineral contact angle affects the threshold pressure
Besides CO2-brine IFT, wettability, i.e. the contact angle of CO2 on the mineral surface in the presence of brine plays a role. Most of the minerals present in reservoir rocks are generally assumed to be water wet. Yet, there are some indications that the wettability changes from water wet to intermediate or oil wet as the temperature and pressure increases. Thus, as with interfacial tension measurements, pressure and temperature play a key role and the contact angle measurements should be carried out at appropriate conditions. The chemical and topographical composition of the mineral surface should also be considered.
In the short (3 min) video below, you will hear Prof. Sandro da Rocha talking about CO2 sequestration.