“Geological storage of CO2 is the mirror image of oil and gas production. Instead of drilling wells into the earth to extract oil and gas, wells are drilled to inject CO2” – Howard Herzog, Senior, Research Engineer, MIT.
Geological storage of CO2
To effectively address climate change risks, man-made carbon dioxide (CO2) emissions need to be kept out of the atmosphere. For CO2 emissions from a wide variety of industries, this can be achieved very effectively by permanently storing CO2 deep in the ground in suitable geological storage formations.
Suitable storage sites include depleted oil or gas reservoirs and deep saline formations (porous rocks filled with highly saline water), or operating oil and gas fields, where the injected CO2 may increase the amount of hydrocarbons recovered, while storing CO2. Depleted oil and gas reservoirs have already been extensively studied and have geological and hydrodynamic assessments readily available. This allows for fast assessment of CO2 storage. Deep saline formations offer the largest potential carbon dioxide storage capacity worldwide.
The Oil and Gas Climate Initiative’s CO2 Storage Resource Catalogue is a publicly available global repository of independent geological CO2 storage assessments based on the Society of Petroleum Engineers CO2 Storage Resources Management System (SRMS). Even based on early iterations, the analysis shows that the amount of practically accessible geological storage capacity globally for carbon dioxide on and offshore is enough to meet Paris goals and across all maturity classes, there is a total of 12,300 GT. This means that even under the most stringent emission reduction targets for the rest of this century, storage capacity is ample and will not be the limiting factor for large-scale CCUS deployment.
Gases, such as methane and carbon dioxide, accumulate naturally in the subsurface. The gases get trapped by structures and impermeable cap rocks and remains trapped for millions of years. This principle of storing large volumes of gas in porous rocks/reservoirs with impermeable cap rocks/seals has been used for decades for the engineered storage of natural gas and carbon dioxide.
For example, at the end 2017, there were 671 underground natural gas storage facilities in operation in the world. The global working gas capacity for natural gas currently is about 417 billion cubic metres, which equates to approx. 231 Gt. In comparison, the amount of currently stored man-made CO2 is about 200 Mt, and steadily growing.
Chemistry of storage
When CO2 is injected into a geological reservoir, it displaces the water in the pore spaces of the rock. The same geological forces that kept the original fluid in place will also secure the CO2. To ensure the integrity of a CO2 storage formation, it should meet four main criteria:
- The reservoir rock must be both porous (have pore spaces where CO2 can reside) and permeable (have links between pore spaces allowing CO2 to permeate through the rock).
- An impermeable caprock must provide a seal over the formation. Thick impermeable layers, consisting mainly of clay or salt, commonly make excellent caprocks.
- The reservoir ideally needs to be thick and continuous over a large area, enabling storage of large volumes of CO2.
- The reservoir must be at a depth of 800 metres or more. Pressure at these depths ensures that CO2 remains in a dense liquid-like state (supercritical). Supercritical CO2 has ‘gas-like’ flow behaviour (low viscosity) to effectively fill the pore space in the geological storage reservoir. At this highly compressed state, CO2 is much denser and has decreased its volume 277 times, therefore making geological storage very effective. Storage reservoirs have typical depths of one to three kilometres.
Once injected, CO2 should not escape from the reservoir under any circumstance. Four key mechanisms work together to ensure CO2 is permanently retained underground structural trapping, residual trapping, solubility (also known as dissolution) trapping and mineral trapping. These four mechanisms come into effect at different time periods in the total lifespan of the CO2 mitigation process. Having multiple mechanisms at work ensures that the CO2 is permanently and securely trapped and helps to optimise the use of a CO2 storage site. Click here for more information on the chemistry of storage and the trapping mechanisms at work.