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.