Natural Analogues for the Geological Storage of CO₂

Publication Overview

This is the final report of the Natural Analogues for the Storage of CO₂ in the Geological Environment (NASCENT) project. The Nascent project has studied natural accumulations of carbon dioxide (CO₂ – an important greenhouse gas, thought to be responsible for climate change) as analogues of the geological storage of anthropogenic CO₂ emissions.Before large-scale underground CO₂ storage can take place, it will be necessary to demonstrate that the processes are well understood, risks to the environment and human populations are low, and environmental disturbances can be minimised. One way of demonstrating that CO₂ can remain trapped underground for geologically significant timescales is to provide evidence from existing naturally occurring accumulations. These accumulations occur in a variety of geological environments and many can be demonstrated to have retained CO₂ for periods longer than those being considered for CO₂ storage.

Publication Summary

For geological storage, understanding the long-term effects of CO₂ on a reservoir is important for several reasons. In certain circumstances, CO₂ may dissolve in the reservoir pore water and react with minerals within the reservoir, which could ultimately lead to long-term trapping through precipitation of carbonate minerals. Our ability to model the geochemical and geomechanical processes that occur in the reservoir, that could influence its long-term storage performance, can be tested by modelling natural analogues of geological storage. In addition, information on how CO₂ might migrate from the initial storage reservoir through fractures, and about processes that could occur in fractures in limestones, has been obtained in this study.

To address these issues, several geological sites were studied. The sites were selected because relevant supporting background data were already available and, for the geochemical modelling, data could be obtained from rocks, waters and gases. The availability of these samples enabled detailed models of past behaviour to be created, which could then be compared with the geochemical and geomechanical modelling.

The sites chosen, therefore, were the natural CO₂ fields in the Florina Basin, northern Greece and the Montmiral field in the Southeast Basin in France, plus the Sleipner natural gas field on the southern edge of the Viking Graben in the North Sea. The geomechanical modelling work was led by TNO with collaboration and analytical support from Statoil and IGME. The geochemical work was led by BRGM with collaboration from BGS and IGME.

At Florina, the quantitative impact of geochemical reactions is minor, due to the replacement of one mineral (siderite) with another (Fe oxide, probably goethite). Geochemical modelling indicates that the system is far from equilibrium and, as observed in petrographic examination of cores from the Florina CO₂ accumulation, CO₂ is not being precipitated as a carbonate mineral.

Montmiral is one of several small CO₂ gas accumulations in the Southeast Basin of France. There are also numerous CO₂-rich springs and gas vents throughout the region, some of which are exploited as mineral waters. The CO₂ is considered to be of deep crustal or mantle origin. In order to determine the CO₂-water-rock interactions within the reservoir, it was necessary to reconstitute the original brine composition, which has been evolving to increasing salinity during the lifetime of the CO₂ production. This temporal variability is not related to changes in water chemistry, but due to changes in the respective volumes of discharged brine and CO₂-H2O gas mixture. The brine is derived from an evaporated Triassic seawater that underwent dilution by meteoric water before, or at the same time as, the CO₂ invasion. The brine composition indicates that the CO₂-water-rock system is not at equilibrium. Diagenetic studies suggest that introduction of CO₂ into this particular reservoir caused dissolution of feldspar, and a slight increase in reservoir porosity. Modelling suggests that dissolution of feldspar leads to some precipitation of kaolinite, carbonates and chalcedony (though the latter was not observed in the reservoir rock). For the model to achieve the porosity changes observed, it had to be assumed that the system was open, i.e. the reservoir has been flushed with fresh CO₂-charged waters. Even after contact times of at least hundreds of thousands of years, feldspars are still present in both cases. This illustrates that reaction kinetics, based on short term kinetic data derived from the literature, can be much slower than expected. Reservoir temperature is an important parameter when assessing the storage capacity of a reservoir, with reaction rates potentially increasing by orders of magnitude where high temperatures prevail. However, to accurately constrain the kinetic rates of the geochemical reactions more detailed information on the reservoir evolution is required. No evidence was found of extensive mineral trapping of CO₂ through precipitation of carbonates.