Carbon capture and storage: A lot of eggs in a potentially leaky basket
As the global surface temperature keeps rising, carbon capture and storage (CCS) is gaining ever more attention as a possible way to achieve deep reductions in atmospheric CO2. And you might not know that the Intergovernmental Panel on Climate Change (IPCC)’s latest report on global warming already includes an annual 12 gigatons of carbon dioxide (CO2) reduction from bioenergy with CCS (BECCS) by 2100 as one of the strategies to limit global warming to 1.5 degrees Celsius in its ‘middle of the road’ scenario. Introducing such a large amount of BECCS means not only sectoral transformations, including in transport but also potentially energy-system reform.
Where bioenergy is concerned, we previously explored how much energy could actually be available from biomass and how much of that could potentially be available for transport. With CCS, CO2 is captured and stored in a way that keeps it isolated from the atmosphere; this keeps that CO2 from warming the climate and that’s a good thing. But succeeding with CCS will require clearing a number of hurdles, including capture efficiency, finding suitable storage sites, and financing the high costs involved. But perhaps CO2 leakage from the storage site is the most worrisome of all the potential risks.
Let’s unpack why. Although there are multiple ways to store carbon, geological storage has been practiced the most, and at a large scale, this typically takes the form of CO2 injection for enhanced oil recovery (CO2-EOR). CO2-EOR has been around since the 1970s and is often used by the oil industry to extract residual oil. The way it works is that CO2, usually in the form of a supercritical fluid at a very high temperature and pressure, is injected into an oil reservoir, usually to a depth of 1 kilometer or more. The injected CO2 helps push out the residual oil, which improves oil extraction rates, and at the same time, most of the injected CO2 stays in the reservoir. The minority of CO2 that re-emerges mixed with the oil is separated and reinjected again, and thus little CO2 escapes to the atmosphere during the EOR process.
The problem for storage is that the CO2 pumped underground can later escape through multiple channels, including geological features such as fractures in the rock. Luckily researchers expect this kind of leakage to be low, generally less than 1% of stored CO2 leaking over a 1,000-year period.
A bigger problem lies in leakage through wells. Both active wells and abandoned, idle wells can be pathways of CO2 leakage, and well leakage can take the form of either continuous leakage or well blowouts. Continuous leakage is a slow leakage that happens as a result of improper well construction, such as failures in casing or cement degradation. Well blowouts are abrupt incidents when the pressure control fails.
Based on historical information, researchers estimated an average of 7.5% of wells may experience continuous leakage, at about 150 metric tons of CO2 per year for active wells and 300 metric tons of CO2 per year for abandoned wells. The estimates for leakage through abandoned wells are higher because these wells generally don’t have frequent monitoring systems in place and thus leaks might not be addressed very quickly.
Although well blowout is rare, it can result in rapid leakage. The worst historical blowout in the United States to date was in 1982 at the Sheep Mountain CO2 Dome in Colorado, and between 7,000 and 11,000 tons of CO2 was released each day for about 7 days. Additionally, a modeling study calculates that the maximum hypothetical CO2 leakage through a single well could be as high as 20,000 tons of CO2 per day. Another study modeled a middle scenario assuming nearly 0.7 million metric tons of CO2 leakage per blowout with a frequency of 1 blowout per 50,000 wells per year. Hypothetically, if all of the 1 million active oil and gas wells and 3.2 million abandoned wells in the United States were used for CCS and leaked at the middle frequency and rate, the annual CO2 leakage from blowouts could be nearly 60 million metric tons. Regarding continuous leakage, in the United States, there could be about 80 million metric tons of CO2 per year, using the above-mentioned average leakage frequency and rate. Together this is nearly 3% of the U.S.’s total CO2 emissions, or 8% of the U.S.’s transportation CO2 emissions, in 2017. While it’s unlikely that all of the wells would be used, even if only one-quarter of them were used, that creates the potential for approximately 35 million metric tons of CO2 emissions from both forms of leakage. Thus, it’s clear that the risk of generating emissions from a strategy designed to reduce them is far from negligible.
This emphasizes the importance of appropriate regulation of storage, particularly concerning monitoring and remediation planning. One good example is the California Air Resources Board, which has developed a comprehensive protocol for CCS projects covered under its Low Carbon Fuel Standard program. This protocol has explicit requirements for well integrity and emissions monitoring and also detailed plans for well abandonment and remedial response plans for emergencies. Any greenhouse gas reduction policy incentivizing CCS would do well to consider similar approaches to make sure the intended reduction goals can indeed be reached.
It also makes sense to be cautious about relying on CCS as any kind of silver bullet to address climate change. The amount of BECCS in the IPCC’s climate strategy is roughly one-third of current global CO2 emissions. That’s putting a lot of eggs into a potentially leaky basket. Given this, it might be most prudent to focus the strongest incentives on strategies that could effectively lead to transport decarbonization, including efficiency, electric vehicles, and sustainable low-carbon fuels.