The following information can be found in the CO₂ Capture Project’s CCS In Depth brochure. Please visit www.co2captureproject.com to download a copy.

Site selection

The oil and gas industry has decades of experience understanding and assessing sites kilometres deep underground. The latest technology to map oil and gas fields is now being used to assess sites suitable for CO₂ storage. The most effective way to ensure permanent safe storage is to choose sites of sufficient depth (typically deeper than 800 metres) with adequate capacity and an overlying sealing system to ensure containment of fluids.

Capture

Over 90% of the CO₂ produced by fossil fuels at large fixed installations can be captured and prevented from reaching the atmosphere. Three main technology types – pre-combustion, post-combustion and oxy-firing – are available, allowing CO₂ to be captured from industrial processes such as power generation, oil refining and cement manufacture.

Pre-combustion capture

Pre-combustion capture involves partial combustion of CO₂ to produce hydrogen and CO₂. Hydrogen combustion produces no CO₂ emissions, with water vapour being the main by-product. The component parts of pre-combustion technology exist today at commercial scale; the challenge now is to integrate these in a power application.

Post-combustion capture

In post-combustion capture, the CO₂ is removed after combustion of the fossil fuel. CO₂ is captured from exhaust gases and other large point sources. Post-combustion can be installed on both new and existing power plants – of vital importance given that the average power plant operates for 40 years. The challenge around post-combustion is scale-up of the technology to commercial scale in a power application, as well as integration.

Oxy-firing capture

Oxy-firing involves burning fuel in pure oxygen instead of air. This results in an exhaust gas made up largely of CO₂ and water) which is ready to be dried and compressed for storage

Transport

Today CO₂ is transported by truck, ship or pipeline. However, to transport the large amounts of CO₂ from power plant emissions, pipelines are the only practical solution. This pipeline transportation process is well understood as CO₂ pipelines have been used since the 1970s, transporting large volumes of CO₂ to oil fields for enhanced oil recovery (EOR). For example, US pipeline infrastructure has the capacity to safely and reliably carry 50 million tons of CO₂ a year.

Injection

The oil and gas industry has years of experience injecting CO₂ underground into geological formations, a process used to enhance oil recovery (EOR). Millions of tonnes of CO₂ are injected annually under regulations which protect local communities and the environment. As oil and gas has become more difficult to access, the industry has rapidly developed precise drilling practices to meet the challenge. This technology is being deployed to securely store CO₂.

Trapping mechanisms

Oil and gas have remained underground for millions of years. The same natural conditions allow injected CO₂ to be stored securely. Once CO₂ is injected deep underground (typically more than 800 metres) it is absorbed and then trapped in minute pores or spaces in the rock structure. Impermeable caprock acts as a final seal to ensure safe storage for millions of years.

There are four main storage mechanisms:

Structural trapping – At the storage site the CO₂ is injected under pressure deep down into the ground until it reaches the geological storage formation. The rocks of the storage formation are like a rigid sponge; they are both porous and permeable. Fluid CO₂ tends to rise towards the top of the formation until it reaches an impermeable layer of rock overlaying the storage site. This layer, known as the caprock, securely traps the CO₂ in the storage formation. Structural trapping is the same mechanism that has kept oil and gas securely stored under the ground for millions of years.

Residual trapping – Another natural process further traps the CO₂. As the injected CO₂ moves up through the geological storage site towards the caprock some is left behind, trapped in the microscopic pore spaces of the rock. This process is similar to air becoming trapped in a sponge.

Dissolution and mineral trapping – Two additional mechanisms also trap CO₂. Over time the CO₂ stored in a geological formation will begin to dissolve in the surrounding salty water. The salty water combined with the CO₂ becomes heavier and sinks towards the bottom of the formation over time. This is known as dissolution storage. Mineral storage occurs when the CO₂ held within the storage site binds chemically and permanently with the surrounding rock.

Depleted hydrocarbon reservoirs, such as oil and gas fields, are highly suited to such geological storage of CO₂. Other potential storage sites are saline formations – permeable rock formations, which contain salty waters in their pore spaces – and unminable coal beds. According to the Intergovernmental Panel on Climate Change (IPCC), such geological formations could provide storage space for at least 2000Gt (billion metric tonnes) of CO₂.