Carbon Capture and Storage, or CCS, is a process that will allow industries that emit carbon dioxide (CO2) to both maintain their business plans and reduce CO2 emissions by using special equipment to remove the CO2 from their facilities’ exhaust before it reaches the air. Once captured, the CO2 can be transported by pipeline to an underground storage area for safe and permanent storage. The same type of solid layers of rock that have kept oil and natural gas locked underground for millions of years can safely store CO2.
Proper storage areas are underground geologic formations, such as depleted oil and natural gas reservoirs, saline deposits and unmineable coal seams, with four special characteristics. The first is that part of the rock formation is porous because the pores provide the space in which the CO2 is stored. The second is permeability, meaning the porous rock is arranged in layers that allow the CO2 to be injected and move through the rock formation. The third is the existence of a cap rock, a solid, impermeable layer of rock that acts as a seal trapping the CO2 underground. Finally, the rock formation must be deep enough to have multiple layers of rock above it and to be subject to enough pressure to keep the CO2 stored within the pores of the rock.
CO2 is compressed from a gas into an almost liquid state before being injected through a well into the rock formation. Once underground, the CO2 remains trapped in one of four ways. Some of the CO2 will remain in the pore space, while other parts will float up through the pore space and stay trapped under the cap rock. A large portion of the CO2 will dissolve into the brine water that is also trapped in the pore space. Over time, some of the dissolved CO2 will react with the minerals in the rock and solidify.
Once stored, the storage sites are constantly monitored for any potential geologic changes. ExxonMobil plans to use a three-tier system of technologies to monitor sites at the atmospheric, near-surface and deep sub-surface levels. These include optical laser sensors, pressure-monitoring transponders and 4D seismicity technology.
How permanent storage works
- Compared to drilling for natural gas, instead of drilling through rock to let the trapped gas out, carbon dioxide (CO2) is injected thousands of feet underground into thick, impermeable rocks that have kept oil, natural gas and naturally occurring CO2 underground for millions of years.
- Potential CO2 storage sites are carefully selected only after undergoing rigorous analysis to ensure they are geologically suitable. This analysis helps mitigate the risk of the CO2 migrating to other formations or to the atmosphere. The CO2 is also stored thousands of feet underground, well below any sources of drinking water, or under the sea floor.
- Industry has been safely injecting CO2 around the world for decades, successfully demonstrating the capability to safely and permanently store CO2.
- Naturally occurring underground deposits of CO2 have stayed locked deep underground for millions of years.
- Once stored, the storage sites will be constantly monitored. A combination of technologies is planned to detect any potential leaks at the atmospheric, near-surface and deep sub-surface levels, including optical laser sensors, pressure-monitoring transponders and 4D seismicity technology.
CO2 storage safety in action
- Just like the U.S. DOT and PHMSA regulated pipelines, the U.S. Environmental Protection Agency (EPA) regulates the underground storage of carbon dioxide (CO2). In some cases, EPA has delegated its regulatory powers to state agencies.
- Government regulations require extensive study of the underground geology to demonstrate a site is safe for CO2 storage.
- Underground storage sites must meet federal construction, operations, monitoring and recordkeeping requirements.