Storage

A potential storage site for CO2 geological sequestration must have adequate structural stability, volume, porosity and permeability: stored CO2 must remain isolated from the atmosphere and from surface water bodies (lakes, rivers, oceans) and groundwater (aquifers exploited by man).

Many types of formations are suitable to store CO2 for several tens/hundreds of thousands years. The CO2 can be injected under supercritical conditions (a fact which occurs at about 800 m depth below the surface) in porous formations containing fluids (deep saline aquifers) and in depleted hydrocarbon reservoirs. The CO2 in this state is 500-600 times more dense than on the surface and can penetrate into the pores between grains of rock with the diffusivity of a gas. Once injected, a series of processes increases the security of its storage. Lighter than fluid formation, CO2 tends to migrate upwards, until the layer of impermeable rock that seals the reservoir. CO2 dissolving in the formation water makes it heavier and creates a downward flow that reverses the initial trend. In the longer period CO2 begins interacting with the rock, precipitates and forms new minerals.
The third option for CO2 storage is about unmineable coal seams. CO2 is injected into a suitable seam in an underground coal reserve. The CO2 is absorbed by the coal, which releases the eventual methane content, which can be recovered. The value of such recovery frequently compensates the costs of storage; however, this option is the one with the lowest potential.

Evaluations on sedimentary basins and on the known hydrocarbons deposits estimate the following theoretical storage capacity in the world: 40 Gt of CO2 in coal deposits, 920 Gt of CO2 in depleted hydrocarbon reservoirs, 400-10000 Gt of CO2 in saline aquifers, corresponding respectively to 2%, 45% and 20-500 % of emissions until 2050.

Industrial pilot projects for geological storage launched in the world, and tens years of experience in the USA on the use of CO2 to recover more oil from reservoirs with EOR technology (Enhanced Oil Recovery), have demonstrated the effectiveness of deep monitoring techniques to control CO2 migration in reservoir and of modeling techniques to predict the evolution in medium/long periods.