New Paper Highlights Differences In Underground CO2 Storage Reservoirs

New Paper Highlights Differences In Underground CO2 Storage Reservoirs - Carbon Herald

A new paper published in Geosciences examines the differences between saline aquifers and depleted gas fields as CO2 storage sites. 

Since 1996, scientists have been exploring storing captured carbon dioxide in deep underground formations called saline aquifers as a way to fight climate change. 

Now, depleted gas fields, once used for natural gas extraction, are also being considered for large-scale CO2 storage projects. While both offer potential solutions, they have key differences.

One major difference lies in available information. Depleted gas fields have been extensively studied during hydrocarbon exploration and production. 

This provides a much clearer picture of the reservoir rock, its sealing caprock, internal structure, and how fluids flow within it. Saline aquifers, on the other hand, haven’t been as thoroughly explored.

Another difference is pressure. Depleted gas fields are often at lower pressure than their natural state before CO2 injection begins. 

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This might require starting with a gas form of CO2 instead of a denser liquid form typically used in saline aquifers. However, the pressure difference in depleted fields might also allow for faster initial injection rates.

The way CO2 gets trapped is also distinct. In saline aquifers, buoyancy plays a key role. The injected CO2 rises due to its lighter weight, with some dissolving in the brine over time.

Depleted gas fields, however, might see CO2 form a sinking pool that pushes down on the remaining methane gas, creating a cushion.

Storage capacity is another factor. Saline aquifers have limited space for CO2 due to buoyancy and viscosity differences with the brine, leading to lower storage efficiency (around 2-20%). 

Depleted gas fields can hold much more (up to 80%) because the existing, lower-pressure methane is easily displaced.

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Chemical reactions are also a concern. Saline aquifers, containing a lot of water, pose a higher risk of CO2 reacting with salt minerals, forming scale. This is less likely in depleted gas fields with less water.

Leakage risks differ as well. Depleted gas fields have more potential leakage points due to the higher number of wells drilled for exploration and production. 

Additionally, their pressure history and changes in rock properties due to depletion can create unique leakage risks.

In conclusion, even though both saline aquifers and depleted gas fields aim for the same goal – permanent CO2 storage to reduce greenhouse gas emissions – they require different approaches due to their inherent geological and operational characteristics. 

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