Fact box – Aquifers

Sub-surface sedimentary strata have different properties. Some consist mostly of sand and are easily permeable by water, oil and gas. Strata dominated by clays or shales are more or less tight and impermeable to liquids. A great deal of seismic data have been acquired and many wells drilled in the mature parts of the NCS, and the extent of the strata is well known. Production data and downhole pressure metering reveal how the liquids flow. An aquifer is a volume of rock where water can flow without much hindrance. In the North Sea, aquifers can be present at many levels down through the rock column and may extend across several geological formations.

Improved understanding of aquifers is increasingly important in mature areas. If the aquifer in contact with a field is strong, water flows easily into the reservoir as the hydrocarbons are produced. The pressure drop is then neither as quick nor as large as when the aquifer is weak. This is significant for planning water injection and for understanding how producing fields influence each other. The aquifer is therefore included in the simulation models, but often only schematically modelled. If CO2 is to be stored, the operator is required to construct simulation models which can predict pressure buildup and how the CO2 will move in the aquifer.