Carbon Storage is the isolation of gases responsible of the greenhouse effect from the atmosphere.
The goal is to reverse the atmosphere pollution by injecting captured Dioxyde (CO2) underground in reservoirs for long-term storage or to be re-used in a close future.
Once injected underground, the Dioxyde (CO2) can either:
- Deteriorate over a long period of time
- Participate to Enhanced Oil Recovery Operations (EOR)
- Be later transformed in plastics, fuel, concrete.
Carbon Capture relies on reservoirs to store gas onshore or offshore. Different options are already used and available, representing around 500 years of storage capacity given today CO2 emissions.
The different types of reservoirs are:
- Natural Cave
- Former Oil & Gas field
- Coal mines
- Saline Formation
- Saline Aquifer
- Oil field in operation
To note, Oil fields in operation injecting CO2 underground for EOR may not be considered as Carbon Storage. In fact, the CO2 injected is stored only for a limited amount of time as the gas is once again emitted when the oil is burnt.
Each type of reservoirs present different characteristics, yet the geological sequestration relies on different common steps of storage which happen over time, four of them can be listed:
- Structural storage
First, the CO2 is injected underground in its gas state or in aqueous solution by one well. At the well exit, the CO2 moves up and is trapped by a impermeable layer of rocks just above the reservoir.
Structural storage is the preliminary mechanism of carbon capture.
- Residual storage
Then as presented in the picture above, rock pores trap CO2 molecules when the gas move in the reservoir. The residual storage depends on the density of the rock pores, which are simple microscopic holes between rocks.
- Dissolution storage
On the medium term, the salty water flowing through the reservoir will absorb part of the CO2. This CO2, by dissolving in the liquid will make the water more dense which will fall to the bottom of the reservoir, creating more storage capacity in the reservoir.
- Mineral storage
After a very long period of time, the Carbon Dioxyde will bind chemically with its surroundings rocks and become a rock itself. Making the Carbon Storage irreversible.
In the case of empty rock reservoirs, the injection of the CO2 is rather simple as it can be achieved under its gas state.
In all the other cases, which represent the vast majority, the injection is made by a mix of CO2 and water to have a greater mobility. In fact the CO2 if injected alone will turn into a supercritical phase due to the temperature and pressure underground with limited mobility in the reservoir.
To occupy all the reservoir space, the CO2 is therefore injected with brine at the ratio of 1kg of brine for 1kg of CO2. Thus the mixture of brine/CO2 is able to flow easily in all the reservoir and store more gas.
A second well can be used as a loop, lower in the reservoir, to extract only the brine, later re-use solution for injecting more CO2.
The main risk behind Carbon Storage is the leakage of CO2. Yet several studies have evaluated the risk of leakage at 1% of reservoirs capacity over a period of 1000 years.
In fact, as the storage mechanisms evolve over time from structural to mineral, the CO2 becomes less and less mobile, therefore the longer CO2 is stored the lower the risk of any leakage.
The second half of 2019 has experienced an acceleration in the number of projects for Carbon Storage. Acceleration which has gained even more momentum in 2020 mainly due to the fact only Carbon Capture and Storage projects can offset CO2 emissions for now.
And of course, you can count on Project Smart Explorer to guide you on these projects: www.projectsmartexplorer.com
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