Understanding CCUS: Carbon capture, utilization and storage

Source: Global CCS institute 

The injection and storage of CO2 has been working safely and effectively for 45 years. In fact, With abundant underground storage resources at our disposal, storage remains the easiest and most logical CO2 mitigation solution. There are many similar geological systems throughout the world that are capable of retaining centuries’ worth of CO2 captured from industrial processes. 

Although geologic storage of gases occurs naturally and has been used safely by industry for many decades, it remains a challenge to describe this process to the public. Fortunately, there are many locations globally that have formations with these characteristics; most are in vast geological features called sedimentary basins. Almost all oil and gas production is associated with sedimentary basins, and the types of geologic formations that trap oil and gas (and also naturally occurring CO2) are similar to those that make good CO2 storage reservoirs.

HOW DOES GEOLOGICAL STORAGE OF CO2 WORK? 

Geological storage involves injecting CO2 captured from industrial processes into rock formations deep underground, thereby permanently removing it from the atmosphere. Typically, the following geologic characteristics are associated with effective storage sites: 

• Rock formations have enough millimeter-sized voids, or pores, to provide the capacity to store the CO2 

• Pores in the rock are sufficiently connected, a feature called permeability, to accept the amount of CO2 at the rate it is injected, allowing the CO2 to move and spread out within the formation 

• An extensive cap rock or barrier at the top of the formation to contain the CO2 permanently.

The storage overview figure shows the different types of storage options available. 

1. Deep saline formations refer to any saline waterbearing formation (the water can range from slightly brackish to many times the concentration of seawater, but is usually non-potable). The saline formation is sealed by a caprock for permanent storage. 

2. EOR (Enhanced oil recovery), which involves injecting CO2 to increase oil production from mature oil fields. 

3. Depleted oil or gas fields that are no longer economic for oil or gas production, but have established trapping and storage characteristics.

HOW IS CO2 INJECTED UNDERGROUND AND WHY DOES IT STAY THERE? 

Once captured, the CO2 is compressed into a fluid almost as dense as water and pumped down through a well into a porous geological formation. The pores in underground formations are initially filled with a fluid – either oil, gas, or salty water. 

Whilst a majority of existing CCS facilities utilise storage associated with EOR, future deployment of CCS will increasingly require storage in deep saline aquifers, which have wider geographical distribution and larger theoretical storage resources in comparison to oil and gas reservoirs. Because injected CO2 is slightly more buoyant than the salty water that co-exists within the storage formation, a portion of the CO2 will migrate to the top of the formation and become structurally trapped beneath the impermeable cap rock that acts as a seal. In most natural systems, there are numerous barriers between the reservoir and the surface. 

Some of the trapped CO2 will slowly start to dissolve into the saline water and become trapped indefinitely (called solution trapping); another portion may become trapped in tiny pore spaces (referred to as residual trapping). The ultimate trapping process involves dissolved CO2 reacting with the reservoir rocks to form a new mineral. This process, called mineral trapping, may be relatively quick or very slow, but it effectively locks the CO2 into a solid mineral permanently.

HOW DO WE KNOW THAT IT WORKS?

Over 200 million tonnes of anthropogenic CO2 has been successfully injected underground. Accumulated experience of CO2 injection worldwide over several decades has proven there are no technical barriers preventing the implementation of storage. Over 40 sites have or are presently safely and securely injecting man-made CO2 underground, mainly for EOR or explicitly for dedicated geological storage. Additional experience is also gained from industrial analogues such as waste water or natural-gas storage. A variety of monitoring technologies have been successfully deployed, demonstrating our ability to measure, monitor and verify injected CO2 in the subsurface. Monitoring of a CO2 storage site occurs over its entire lifecycle from pre-injection to operations to post-injection; it enables the progress of CO2 injection to be measured and provides assurance that storage is developing as expected. Operational and research experience over several decades demonstrates that injected CO2 can be monitored to confirm its containment.

HOW MUCH CO2 CAN BE STORED UNDERGROUND?

Many people assume that one of the biggest challenges impeding the acceleration of CCS facilities is limited underground CO2 storage resources. The reality is, there is more underground storage resource than is actually needed to meet climate targets. In fact, a large proportion of the world’s key CO2 storage locations have now been vigorously assessed and almost every high-emitting nation has demonstrated substantial underground storage resources. As an example, there is between 2,000 and 20,000 billion tonnes of storage resources in North America alone. Countries including China, Canada, Norway, Australia, US and the UK all boast significant storage availability, and other countries such as Japan, India, Brazil and South Africa have also proven their storage capability.

Side note on Carbon Dioxide Enhanced Oil Recovery

By far the most extensive use of CO2 in the United States is for enhanced oil recovery. Carbon dioxide enhanced oil recovery (CO2 EOR) is a technique used to recover oil, typically from mature fields that have ceased being productive through traditional primary and secondary recovery methods. Primary and secondary recovery methods typically leave two-thirds of the original oil in place (OOIP). To put the potential of EOR into perspective, of the total of 600 billion barrels of oil that have been discovered in the United States, approximately 400 billion barrels are unrecoverable by conventional methods. Half of that unrecoverable oil (200 billion barrels) is at reasonable depths at which EOR may be applicable.

CO2 EOR is an established technique in the United States, and is the only oil recovery technique that has shown any growth since the 1980s. In fact, CO2 EOR now accounts for over 5% of the Nation's oil production. It can extend the productive life of an existing oilfield by several decades, and it can lead to recovery of millions of barrels of additional oil.

The basic principle behind CO2 EOR is the mutual solubility of crude oil and CO2 in the temperature and pressure conditions of a geologic reservoir. Given the right conditions, injected CO2 is able to dissolve and displace oil residue that is trapped in rock pores (like a solvent is able to displace grease from a dirty bicycle chain). In a typical CO2 flood operation, a pipeline delivers CO2 to the oilfield, where it is directed to injection wells. These wells are strategically placed to optimize the areal sweep of the CO2 through the reservoir. As the injected CO2 moves through pore spaces in the rock, it encounters residual crude oil. The crude oil mixes with the CO2, decreases oil viscosity, pressurizes it, and mobilizes it, forming a concentrated oil bank that is swept to producing wells. In this way, oil and gas companies are able to gain access to oil that would otherwise be left in the ground.

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