A review of natural CO ₂ accumulations in Europe as analogues for geological sequestration

Abstract

Natural geological accumulations of carbon dioxide occur widely throughout Europe, often close to population centres. Some of these CO2 deposits leak, whereas others are sealed. Understanding these deposits is critical for selecting and designing underground storage sites for anthropogenic CO2. To provide confidence that the potential risks of geological CO2 storage are understood, geologists are required to predict how CO2 may behave once stored underground. Natural CO2 accumulations provide a unique opportunity to study long-term geochemical and geomechanical processes that may occur following geological storage of anthropogenic CO2. In addition, natural CO2 springs and gas vents can provide information on the mechanisms of gas migration and the potential effects of CO2 leakage to the surface. This paper provides a description of some natural, European CO2 occurrences.CO2 accumulations occur in many basins across Europe. In addition, volcanic areas and seismically active areas allow CO2-rich fluids to migrate to the near surface. Many of these occur in areas that have been populated for hundreds and thousands of years.Stratigraphic traps have allowed CO2 to accumulate below evaporite, limestone and mudstone caprocks. Comparisons between reservoir sandstone and equivalent nearby sandstones that contain no CO2 indicate that reservoir sandstones may experience increased secondary porosity development through feldspar dissolution. Where fracture reactivation allows CO2-rich fluids to migrate, limited self-sealing may take place through calcite precipitation. Gas migration experiments indicate that, due to geochemical interactions, fine-grained seals would be able to trap smaller volumes of CO2 compared to, for example CH4. In natural systems most leakage from depth occurs along fractures and is typically extremely localized on a metre-scale.

Chemical–mechanical coupling observed for depleted oil reservoirs subjected to long-term CO2-exposure – A case study of the Werkendam natural CO2 analogue field

“…Studies investigating CO 2 /brine/rock interactions occurring in natural CO 2 -rich siliciclastic reservoirs showed that the majority of reactions involve dissolution of feldspars and micas, and the precipitation of clays (e.g. illite, kaolinite) and carbonates (Baines and Worden, 2004;May, 2005;Moore et al, 2005;Pearce et al, 2004;Watson et al, 2004). Though CO 2 mineralisation in the form of carbonate minerals is argued to be a major sink for CO 2 , detailed geochemical studies found that carbonate precipitation from CO 2 -rich fluids is rather limited (Gilfillan et al, 2009).…”

CO2 retention values in enhanced oil recovery

“…Without regulations or environmental incentives, however, maximization of the amount of oil produced by a given volume of injected CO 2 is the alternative of choice for industry because the operators receive no income or credit for leaving CO 2 in the reservoir. Like hydrocarbons, CO 2 can be found in natural reservoirs (Pearce et al, 1996 and2004). In addition to economic incentives, the CO 2 needs to be of anthropogenic origin for the CO 2 -EOR to have some positive environmental impact.…”

Can CO 2 hydrate assist in the underground storage of carbon dioxide?

“…Indeed, many natural CO 2 fields have been discovered that are far older than this (e.g. Baines & Worden 2004b;Czernichowski-Lauriol et al 1996a;Pearce et al 1996Pearce et al , 2004Zheng et al 2001). For example, the CO 2 in the natural carbon dioxide field at Pisgah Anticline in Central Mississippi, USA is thought to have originated from thermal metamorphism of Jurassic carbonates by the Jackson Dome igneous intrusion during late Cretaceous times (Studlick et al 1990), which ended some 65 million years ago.…”