Storage of CO2 in depleted hydrocarbon reservoirs in lowpermeability chalk

Depleted oil and gas reservoirs are candidates for underground CO2 storage. The Roar field is a high-porosity, low-permeability chalk carbonate reservoir in the Danish North Sea, and a detailed reservoir model has been used for a simulation study of the influencing factors during CO2 injection. Roar is an anticlinal structure, induced through tectonic uplift. The reservoir is located at a depth of 2000 in and covers an area of 14 km(2). The reservoir rock is chalk with porosities in the range of 35 - 45% and matrix permeability between 0.01 and 10 mD. There are only a few significant natural fractures. The accumulation consists of free gas and a thin oil zone. The gas contains condensate. The ECLIPSE reservoir simulator is used to predict the state of the reservoir after 16 years of production and to model the subsequent injection Of CO2 over a period of 30 years. Various CO2 injection scenarios have been simulated and a number of factors and parameters are identified which have a considerable influence on the field injectivity and simulated results.