Geochemical interactions between supercritical CO2 and the Utsira Formation: an experimental study

This report describes work undertaken at the British Geological Survey (BGS) that forms part of the international SACS (Saline Aquifer CO2 Storage) project. The SACS project aims to monitor and predict the behaviour of injected CO2 in the Utsira Sand reservoir at the Sleipner field in the northern North Sea, using methods that include; time-lapse geophysics, modelling its subsurface distribution and migration, and simulating likely chemical interactions with the host rock. This report describes a laboratory experimental study aimed at providing geochemical data to help constrain geochemical modelling activities, and to further our understanding of the longer-term fate of CO2 injected into the Utsira formation. The experimental study was undertaken in the Hydrothermal Laboratory of the BGS, where a range of measurements were made using actual Utsira sand core material and synthetic Utsira porewater. The experimental conditions chosen were mainly 37°C and 10 MPa (in-situ temperature and pressure in the Utsira formation at Sleipner), though some experiments were run at 70°C and 10 MPa to enhance the rates of reaction. Experiment durations ranged from one week to two years. Experiments were pressurised with either nitrogen or carbon dioxide. The former provided a ‘non reacting’ reference point from which to compare the more reactive experiments containing CO2. However, they also helped provide confidence in the baseline conditions within the Utsira formation prior to CO2 injection. The CO2 experiments provided direct information on how CO2 reacted with the Utsira sand and its porewater. Most of the observed reactions are deduced from fluid chemical changes and involve dissolution of a carbonate phase (probably shell fragments), a significant proportion of which appear to have dissolved over a 2 year period under in-situ conditions (37°C, 10 MPa). However, sand porosity is approximately 40%, and as carbonates are a minor component of the whole rock (3.9% of the total rock volume), significant dissolution is unlikely to change overall porosity by a large degree. Dissolution of silicate minerals was a much slower process, and was still ongoing (though at a reduced rate) after 2 years of reaction. There was no direct evidence for the formation of appreciable quantities of secondary precipitates. Overall however, observed CO2-water-rock reactions have resulted in relatively little dissolution of the Utsira sand. From a geochemical standpoint, and just considering the host rock, the results of this study indicate that the Utsira sand would appear to be a suitable host for injected CO2.