7 CO2 storage

In the public domain, there are only few information available on the effect of varying the CO2 injection rate in underground reservoirs, because of the flexible operation of the power plants.

Nowadays, this subject is becoming more and more important because underground storage may be used as an intermediate storage that smoothes the variability of the CO2 flowrate from the power plant, while delivering a constant CO2 stream to end-users, like depleted oil fields (for EOR) or other industrial processes.

In general, it can be stated that the implications of varying injection rates are site-specific, as they depend on the type of storage formation (saline aquifers, depleted oil and gas fields, salt caverns), the reservoir and seal characteristics (dimension, shape, porosity, permeability, salinity, etc.).

To investigate the effects of variable CO2 supply, storage modelling would be required to simulate the variable injection of CO2 in a reservoir, because no specific flexibility has been required to the existing storage applications.

Evaluations have been made on the CO2 migration in the reservoir and the pressure built-up as a function of the distance from the injection wells. Preliminary results show that the extent of CO2 migration in the reservoir is not dependent on the injection rate variability or the extent of confinement of the storage reservoir.

Near the injection well, pressure build-up increases with time, steadily in the case of constant injection, but periodically in case of variable injection, as the pressure buildup in the reservoir increases with the amount of CO2 injected, and the trend of reservoir pressure variation is directly proportional to that of the CO2 injection rate.

It has to be noted that for cases of variable injection near the injection well, the pressure variation cycles amplitude decrease as injection proceeds with time. This is related to the compressibility of CO2, which causes the system to be more flexible as more CO2 is injected with time. However, it is possible to maintain both pressure and temperature within certain limits if the mass flow is reduced by closing off some of the well injectors.

In addition, the periodic variations of reservoir pressure due to periodic variations of CO2 injection rate fades away as the distance from the injection well increases, increasing steadily with time with the amount of stored CO2.

However, further investigations are required to assess in detail the storage ‘flexible’ operation. In particular, the effects of the injection rate variation on how the gas occupy the pore space and consequently on the reservoir capacity, injection and withdrawal maximum rate.