Carbon capture, utilisation and storage (CCUS) has recently received a lot of attention in the news and mainstream media, but what does this technology actually do and what role might it play in the journey towards NetZero?
CCUS technologies, in the broadest terms, capture carbon dioxide (CO2) for either reuse or storage in deep geological formations.
Capture
It is both interesting and pertinent to here mention natural carbon sinks; various natural processes absorb and sequester (store) carbon (e.g., forests, soils, wetlands, mangroves and the weathering of minerals). The capacity of these natural processes to sequester carbon can be anthropogenically enhanced (e.g., USGS LandCarbon), however, the following account refers entirely to industrial carbon capture.
New carbon capture technologies are continuously being developed, aiming to increase both cost and energy efficiencies. A broad distinction is made between industrial separation technologies and direct air capture. Within industrial carbon capture, there exists ‘post-combustion’, ‘pre-combustion’ and ‘oxy-fuel combustion’ capture.
Post-combustion and oxyfuel technologies are easily retrofitted to existing plants, whereas pre-combustion technology is better suited to being incorporated into newly built plants.
The above carbon capture processes employ various technologies to physically separate the CO2:
Direct air capture technologies remove CO2 directly from the atmosphere. Similar technologies are employed in direct air capture as in industrial carbon capture (e.g., absorption and adsorption), however, this process is complicated due to the much lower concentration of CO2 in the atmosphere compared to that of exhaust emissions from industrial processes.
Utilisation
Reuse of carbon dioxide can assign economic value to the gas as a product, examples include conventional uses such as the carbonation of fizzy drinks, enhancing greenhouse crop production and for use in enhanced oil recovery. Enhanced oil recovery (EOR) describes a practise in which CO2 is injected into oil fields and aids the recovery of the remaining oil in place; CO2-EOR can extend the lifetime of oil fields. The role CO2 plays in EOR is dependent upon the reservoir pressure and the oil density. In higher pressure reservoirs with low density oil, the CO2 is observed to be in the oil, the resultant fluid mixture has a lower viscosity and thus is more easily produced from the reservoir. Where the reservoir pressure is lower and oil density higher, injected CO2 is immiscible in oil (doesn't form a homogeneous liquid), but instead is observed to cause swelling of the oil which reduces its density and consequently enhances recovery. A portion of the CO2 injected in EOR remains trapped in the subsurface; evidently this is beneficial from an environmental perspective and is the aim of underground carbon sequestration.
Storage
An alternative to repurposing waste carbon dioxide, is long term geological storage. Carbon dioxide can be stored over geological time periods in deep underground formations. The aim of this process is to ‘permanently’ trap the CO2 underground via various trapping mechanisms.
The two primary types of geological stores are depleted hydrocarbon reservoirs and saline aquifers. A benefit of using a depleted hydrocarbon reservoir as a CO2 storage site is the existing data and knowledge of the reservoir and caprock properties. However, saline aquifers do not suffer from the pervasive drilling history that hydrocarbon reservoirs have experienced, compromising the seal integrity of the caprock. Other, less common CO2 storage scenarios include injection into un-mineable coal beds, salt caverns and basalt. Similar to a conventional hydrocarbon play, a CO2 store requires a reservoir, caprock and some sort of trapping structure.
Trapping mechanisms are vital for the long-term storage of carbon dioxide. Initially, the buoyancy of the injected CO2 plume causes it to rise and is trapped by the structural or stratigraphic trap it has been injected into. As the plume migrates, the 'trail' of the plume becomes residually trapped; some of the CO2 becomes trapped in the pores of the reservoir by capillary forces; this phenomenon actually ensures that even in the case of a catastrophic leak, only a portion of the carbon dioxide injected can come out, that which is not residually trapped in the reservoir. On a longer timescale, injected CO2 dissolves in the pore waters of the reservoir, this carbon dioxide-saturated fluid is denser than ambient reservoir fluid and thus sinks, increasing storage security. Eventually, chemical reactions result in mineral trapping of the CO2 through the formation of carbonate minerals. As the graph indicates, CO2 storage actually becomes more secure over time.
To maximise storage efficiency, CO2 is generally stored under supercritical conditions (neither a gas or a liquid, but significantly compressed and allows more CO2 to ‘fit’ into the reservoir). As a generalisation, this is approximately 800m below the earth’s surface, however this will vary dependent on the temperature gradient and the local geology of the storage site. Whilst 800m is generally the minimum injection depth to ensure supercritical CO2, most active carbon dioxide sequestration projects inject at depths much deeper than this.
The Future
Carbon capture and storage has experienced a turbulent history within the UK in terms of funding and support, however, once again there is growing interest in the technology. Harnessing CCS technologies can enable the continued consumption of fossil-fuel derived energy whilst keeping on track for meeting climate change targets such as NetZero. Employing CCS technologies within the UK oil and gas industry holds the potential to extend the life of North Sea hydrocarbon fields whilst limiting the release of harmful emissions.