Brine formations under oil reservoirs ideal for CO2 storage

Layers of rock containing brine (saltwater) and found below oil reservoirs are the best storage sites for CO₂, according to a recent study.

Capturing CO₂ emissions from major point sources and injecting the greenhouse gas deep underground in layers of rock for permanent storage is seen as an attractive option by many for reversing the trend of increasing levels of atmospheric CO₂. Injecting CO₂ into the ground is not a new idea. Oil companies have been successfully using this technique to help extract oil since the 1970s and much expertise already exists, as well as a limited transport infrastructure in the United States.

The researchers investigated the theoretical behaviour of CO₂ in saline geological formations below and in contact with oil bearing formations and used the calculations in a case study of oil reservoirs in the south-western United States.

The main finding of the study was that if an oil reservoir lies above the CO₂ injection zone (the brine formation), it acts as a physical barrier to CO₂, trapping it in place and preventing it from moving upwards. In addition, oil reservoirs are themselves trapped and sealed in.

How effectively the CO₂ is trapped relates to the specific conditions of pressure, temperature and salinity (degree of saltiness) found in different geological formations in which the CO₂ is stored. Changing pressure and temperature will change the density of the CO₂ with important implications for storage.

The researchers recommend that CO₂ is injected when it is 'supercritical' - when it shows properties of both a fluid and a gas, because the density of the CO₂ in this state closely matches that of the surrounding brine. This occurs at approximately 800 kilograms per cubic metre. Supercritical CO₂ is less buoyant, which reduces the risk of it rising, as well as reducing its volume and thus storage space required.

The injected CO₂ will also dissolve in both the brine and oil. However, the potential to trap CO₂ in oil is over 30 times greater than trapping CO₂ in brine formations, further preventing leakage of the CO₂ to the surface.

CO₂ solubility typically increases with reservoir pressure trapping more CO₂ at higher pressures. To ensure that injected CO₂ exists as a supercritical phase, the right amount of pressure and temperature are needed in the field. Ideally, reservoir fluid pressures would range from 15 to 25 MPa (15-25 x 106 pascals of pressure) with temperature between 35 to 50°C. Technology for enhancing pressure has existed for many years; water flooding is one example. Under these conditions, an oil reservoir may effectively act as a seal that prevents the CO₂ from migrating towards the surface. With the existing infrastructure at suitable oil fields, abandoned wells could be converted and used as monitoring wells relatively easily.

Source: Han, W.S., McPherson, B. (2009). Optimizing geologic CO₂ sequestration by injection in deep saline formations below oil reservoirs. Energy Conversion and Management. 50:2570-2582.

Contact: wshan@egi.utah.edu