Carbon capture, utilisation and storage

A critical tool in the climate energy toolbox

Carbon, capture utilisation and storage (CCUS) is one of the only technology solutions that can significantly reduce emissions from coal and gas power generation and deliver the deep emissions reductions needed across key industrial processes such as steel, cement and chemicals manufacturing, all of which will remain vital building blocks of modern society.

CO2 storage

How is CO2 stored underground?

CO2 storage involves the injection of captured CO2 from various sources into deep underground geological reservoirs of porous rock (the reservoir) overlain by a layer an impermeable layer of rocks (seal), which prevent the upward migration of CO2 beyond the storage complex. There are several types of geological formations which are suitable for CO2 storage mainly saline aquifers and depleted oil and gas reservoirs. Depleted oil and gas reservoirs are porous rock formations containing either mainly crude oil or gas that has been physically held in stratigraphic or structural traps for millions of years. Saline aquifers are layers of porous and permeable rocks saturated with salty water (brine), which are fairly widespread in both onshore and offshore sedimentary basins.

Projects such as the Sleipner CCS project in Norway have been demonstrating safe and secure storage of CO2 for more than 20 years. There are now four projects undertaking dedicated geological storage at large-scale and one in construction.

Global CO2 storage resources are considered to be well in excess of likely future requirements, even under very ambitious scenarios. However, in many regions significant further assessment work is required to convert theoretical storage capacity into “bankable” storage.

Source: Element Energy (2010), One North Sea (

What happens to the injected CO2?

At ambient conditions, CO2 is a gas. However it becomes supercritical when injected underground due to the associated pressure, meaning it becomes dense like a liquid but expands like a gas, filling pore space. The reservoir must also be at depths greater than 800 m that can retain the CO2 in a dense phase.

Source: adapted from Queensland Storage Atlas, 2009

Once CO2 reaches the target reservoir, the injected CO2 will flow through the reservoir driven by the injection pressure. Due to its low density compared to other reservoir fluids (water, oil and/or gas), the CO2 will then rise by buoyancy to the base of the seal where it is physically trapped (structural trapping) preventing it from migrating to the surface. While the CO2 migrates through the reservoir, the injected CO2 will dissolve in the water. The more contact CO2 has with water, the more effective solubility trapping is.

Some CO2 droplets can become trapped in individual or groups of pores (residual trapping). Finally, CO2 that has been dissolved in brine can react with the reservoir rocks to form carbonate minerals (mineral trapping). The nature and the type of the trapping mechanisms, which vary within and across the life of a site and depend on the geological conditions, will need to be well understood for reliable and effective CO2 storage.

Leakage risk

Appropriately selected geological storage sites will safely store injected CO2 permanently, which is defined as more than one thousand years. Computer simulation of storage reservoir dynamics uses modelling to assess the mechanisms that control the behaviour of injected CO2 underground. CO2 storage site monitoring allows for the ongoing observation of the performance of the storage site and the early detection of warning signs of unexpected behaviour. Monitoring is essential not only in observing CO2 behaviour, but also in calibrating and validating predictive models.

CO2-EOR: a pathway for large-scale CCUS deployment

CO2-EOR has played a significant role in the early deployment of CCUS with three-quarters of operating CCUS projects involving enhanced oil recovery, and almost all located in North America. The revenue from the sale of the captured CO2 for EOR has been important in securing investment in CCUS facilities. It is expected that CO2-EOR will continue to play a major role in supporting early CCUS deployment not only in the US but also in the Middle East, China and other regions.

Schematic diagram of anthropogenic CO2-EOR

CO2-EOR is a well-established commercial technology which has been carried out since the 1970s, mainly in the US. CO2-EOR consists of injecting CO2 into depleted oil reservoirs to enhance the recovery of additional crude oil that remains trapped in the reservoirs by increasing flow between oil, gas and rock.

During this process, most of the CO2 mixes with the oil before being produced to surface where the CO2 is separated and re-injected a short time later. However, some portion of the CO2 remains inherently trapped, over time, in the oil reservoir. CO2-EOR sites can therefore provide excellent geologic reservoirs with known injectivity, capacity, and a demonstrated seal providing good geological containment for CO2.

Schematic CO2-EOR processes

Historically, CO2-EOR operations have focused on optimizing oil production. Transforming current practices to co-optimise CO2 storage and oil extraction “an EOR+ approach” - could not only deliver cost effective emissions reductions, it can help build large-scale CCUS infrastructure whilst producing lower carbon footprint oil.

Conventional EOR+	59.99630741
Advanced EOR+	239.9852296
Maximum Storage EOR+	359.9778444
		"text":"Global storage potential for different CO2-EOR practices"
			"text":"billion tonnes of CO2"
		"valueSuffix":" billion tonnes"