Distribution of saline formations in Germany (click to enlarge)Tightness, porous volume and the permeability of the rock are crucial when choosing suitable CO₂ reservoir sites. The storage reservoir must be able to accommodate the captured CO₂ reliably and permanently. In Germany, depleted gas and oil fields as well as deep, saltwater-bearing sandstone layers, so-called saline formations, found at a depth of over 1,000 m especially in Northern Germany are suited for this purpose.

Owing to their physical and chemical properties, they have effective retention mechanisms which are ideal for long-term CO₂ storage. In this environment, the CO₂ forms bonds with other substances: the pH value of the water drops when the gas dissolves. The water turns acidic and is able to dissolve salts from the surrounding porous rock. These in turn react with the dissolved CO₂, creating mineral compounds such as calcite, for instance. Mostly colourless or white, calcite is one of the most important rock-building minerals.

The structure of the underground reservoir formation also ensures safe storage: sealing layers of overlying rock above the reservoir provide additional optimum retention mechanisms. Nature itself demonstrates that CO₂ reservoirs can be tight over millions of years: Natural gas as well as natural CO₂ reservoirs work exactly the same way.

f.l.t.r: Thin section CO2 reservoir rock; scanning electron microscope: CO2 sandstone; scanning electron microscope: CO2 sandstone unsuitable