Storage of CO₂ in depleted hydrocarbon reservoirs in low-permeability 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 m and covers an area of 14 km². 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. Some of the promising geological structures for underground CO₂ storage are depleted or nearly depleted oil and gas reservoirs. Most Danish hydrocarbon reservoirs are composed of chalk carbonate characterized by very low matrix permeabilities, which may cause concern that the injectivity is prohibitively low. 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. The advantage of selecting a gas field for CO₂ storage rather than an oil field is that the top seal integrity will not be challenged to the same extent, because the pressure difference across the seal upon CO₂ flooding will be comparable to the initial difference prior to reservoir depletion.