The success of carbon dioxide removal (CDR) as a climate solution depends on the ability to safely and permanently store CO2 (Matter, 2016). To achieve a significant drawdown of CO2, both carbon capture and storage (CCS) and direct air capture (DAC) require sequestration infrastructure to ensure that captured carbon stays out of the atmosphere. According to the EPA, CCS technologies can bury up to 90% of power plant emissions (Hardcastle, 2016). Geologic sequestration has proven itself to be the safest and most enduring way to keep carbon out of the atmosphere. As reviewed above CF has demonstrated that through a process of mineralization, they can permanently sequester up to 95% of captured carbon. Sequestered carbon can be permanently locked in basaltic rock formation and other underground locations including depleted oil wells and deep saline aquifers. CE’s founder and Acting Chief Scientist David Keith thinks saline formations may be the best long-term large-scale locations for storing carbon (Andrews, 2018).
To keep temperatures from exceeding the upper threshold limit of 2 degrees C above pre-industrial norms we will need to keep atmospheric carbon levels below 450 ppm which translates to the removal and storage of 120–160 GtCO2. (Mac Dowell, Fennell, Shah & Maitland, 2017). Finely ground olivine-rich rock can absorb up to its weight in CO2 and basalt and volcanic rock can absorb CO2 equivalent to approximately 20% of its weight. Phil Renforth, associate professor at Heriot-Watt University in the UK, says as much as 10 Gt of rock mining and grinding per year is feasible by 2100 (Kramer, 2020).
Estimates of the total US CO2 storage capacity are estimated to be between 2,618 and 21,978 billion metric tons (U.S. Department of Energy, 2015). China can sequester 600 years’ worth of its current emissions (Global CCS Institute, 2018). One study estimated the total global underground storage capacity for CO2 to be around 10,450 to 33,153 Gt. At current global yearly emission rates, this would equate to around three centuries of storage capacity (Smith, 2016; Budinis et al, 2016). In 2017 the theoretical global CO2 storage capacity was estimated to be significantly more than enough (Mac Dowell, Fennell, Shah & Maitland, 2017). According to Friedmann, the total amount of CO2 that can be sequestered is easily 20 trillion to 30 trillion tons and Columbia University geochemist Peter Kelemen says there is enough accessible mantle rock to permanently capture hundreds of trillions of tons of CO2 (Kramer, 2020). Another study found that the pore space in sedimentary rocks around the globe far exceeds all the CO2 that must be removed from the atmosphere (Folger, 2018).
Only a very small percentage of CDR technologies are currently being sequestered underground. As of 2019 there are only five dedicated geological CO2 storage locations operational worldwide (Kramer, 2020). The longest-running sequestration operation is the North Sea’s Sleipner gas field, where 1 million tons of CO2 have been sequestered since 1996 with no leakage (Global CCS Institute, 2018). The world’s largest dedicated geological storage site is Chevron’s liquefied natural gas project in Western Australia which the company says can sequester up to 4 million tons of CO2 each year. The only dedicated geological storage site in the US is the Illinois ethanol demonstration plant (Kramer, 2020).
Well-selected and managed underground storage is not likely to leak. According to an IPCC report more than 99% of the sequestered CO2 will remain sequestered for at least 1,000 years (IPCC, 2005). A 2018 modeling study found that there’s a 50-50 chance that 98% of the sequestered CO2 will stay in the subsurface for 10,000 years. Even under less than optimal conditions, more than three-quarters (78%) of the sequestered CO2 would remain underground (Alcalde et al, 2018).
There are a couple of significant reasons why underground sequestration is better than other NCS approaches. The first is that there are limits to the total amount of CO2 that can be sequestered by most NCS approaches. The second is the duration of sequestration. As explained by Mac Dowell, geological sequestration is permanent whereas locking CO2 in trees and vegetation is inherently leaky because it has a limited lifespan and can be released by fires or floods (Kramer, 2020). Despite the evidence, some remain concerned about leakage and other risks.
According to Daniel Sanchez, an engineer at the University of California, Berkeley, who studies CO2 removal methods, it will cost between $1 million and $33 million to drill a well capable of injecting 1 megaton of CO2 annually. Assuming such a well has a 20-year lifetime that translates to less than $1 per ton of CO2. He estimates the total cost of storage, including operation, maintenance, monitoring, and verification, at around $5/ton (Kramer, 2020). A report by the Congressional Research Service says that the long-term average cost of CO2 transport and storage should stay below the level of approximately $12–$15/ton in North America, due largely to the abundant capacity offered by deep saline formations (Folger, 2018). Some estimates run as high as $50/ton (Kramer, 2020).
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