Geochemical interactions between supercritical CO2 and the Utsira caprock. I : introduction to fluid-rock interaction experiments

This report describes work that will be 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. Most of the work detailed in this report forms part of the TFE- (Total Fina Elf) extension of the SACS2 project (here described as SACS-TFE, but also known as SACS2-addendum). This extension will be of relatively short duration, and is planned to link the longer-term (and much larger) SACS2 and CO2STORE (effectively SACS3) projects. This report aims to provide a brief description of the laboratory experimental techniques that will be applied at the British Geological Survey to investigate and quantify geochemical reactions between CO2 and the Utsira caprock. The aim of the work under the SACS-TFE project will be to initiate a series of long-term experiments, and conduct some preliminary sampling. These experiments will be continued into the CO2STORE project, whose programme will also involve a series of shorter-term experiments. Although the experiments have not yet been started (at the time of writing this report [early September 2002]), it is hoped that this report will provide useful preliminary information to other researchers working on the SACS project. Later reports will detail the results of the experiments. The experimental study is being undertaken in the Hydrothermal Laboratory of the BGS, where various measurements are being taken. The techniques that will be used are based upon those used in previous CO2 projects (e.g. during the JouleII CO2 storage project [Holloway, 1996; Czernichowski-Lauriol et al., 1996], and during the SACS project [Rochelle et al., 2002]). Use of similar techniques will hopefully allow for better inter-comparison between the various studies. The experiments will utilise actual caprock core material from the Sleipner field, together with synthetic formation waters based upon measured compositions of porewaters squeezed from samples of the same core. The experimental conditions chosen for the investigation will concentrate upon those representative of in-situ conditions within the caprock (30°C, 8 MPa [80 bar]). However, it is possible that more elevated temperatures (e.g. 70°C) may be used in the future if it is necessary to increase reaction rates. Experiment durations are planned to range up to 2-3 years. Experiments will be pressurised with either nitrogen or carbon dioxide. The former will provide a ‘non reacting’ reference point from which to compare the possibly more reactive experiments containing CO2. However, it is hoped that they will also help to provide increased confidence in the baseline conditions within the Utsira caprock prior to CO2 injection into the Utsira Formation.