De-risking CO₂ storage in chalk formations

Carbon Storage in geological media has been widely considered as a solution to reduce the environmental footprints of anthropogenic emissions of CO₂ from point sources. The depleted hydrocarbon chalk fields in the Danish North Sea are potential candidates for carbon storage1 , as they comprise a significant pore volume and many are approaching the final stages of production lifetime. To develop cost effective and insignificant risk CO₂ storage solutions in depleted hydrocarbon chalk reservoirs and to unlock the main storage potential in Denmark a new multidisciplinary CO₂ storage research program² has been initiated at the Danish Offshore Technology Centre. The scope for CO₂ storage in carbonate reservoirs is potentially large as carbonate reservoirs, including limestones, dolomites and chalks, comprise one of the most prevalent types of hydrocarbon reservoir worldwide. However, the geochemical response of chalk and other fractured and layered carbonate rocks to CO₂ and the geomechanical consequences must be investigated carefully to de-risk a storage scenario. The research program includes modelling studies covering a wide range of coupled simulations including chemical reactions and transport phenomena at different length scales. In addition, the program comprises laboratory experiments on core flooding at representative reservoir temperature and pressure condition, geochemical analyses and geomechanical tests under similar in-situ conditions. The chemical processes that can take place at the surface of the rock in contact with natural formation water and dissolved CO₂ can lead to potential reservoir changes, both upon injection and over time. CO₂ may be injected as supercritical CO₂ in which case solvation of the gas will take place upon contact with the formation water in place, or it may be injected in the form of carbonated water which will lead to a direct acidification of the water in contact with the rock. The geochemical reactions between carbonate rock and dissolved CO₂ depend on the thermodynamic conditions, the salinity of the water, pressure and can potentially affect significant dissolution of the rock matrix either in the near well bore areas or in the wider matrix, or in both. Other factors that can affect the safety and efficiency of storage are the presence of residual hydrocarbons, specific lithography and local water chemistry. The consequences of dissolution can be amongst other things general geomechanical weakening, subsidence, leakage, permeability changes and diversions of flow. The dissolution can take place rapidly upon injection or more slowly during storage. The studies included in the research program address both short and long-term responses of chalk to CO₂ injection considering the relevant physical and chemical processes as well as longer term storage test in the laboratory. The aim is to identify the conditions under which CO₂ storage in chalk is feasible and sufficiently safe. In this paper we first highlight the main challenges and enablers for CO₂ storage in chalk fields in terms of the storage mechanisms, injectivity, risk, containment, and longevity. Secondly, we describe the progress of the established research program towards determining if chalk is suitable as a long-term storage reservoir for CO₂.