BGS has been reacting carbon dioxide (CO2) with rock material in a laboratory experiment designed to simulate the fate of CO2 injected into deep saline aquifers underground. Using a specialised pressure vessel, the Big Rig, CO2 is injected into a column containing synthetic sandstone and a saline interstitial fluid, maintained conditions at 130°C, 300 bar pressure for 108 days (3.5 months), and recorded the changes in fluid chemistry and mineral reactions over time.
Measurements showed a predictable fall in fluid pH during the experiment. Changes in the concentrations of dissolved major ions were consistent with dissolution of calcite during the initial stages. Examination of the minerals along the column length showed complete loss of calcite and partial dissolution of feldspar at the beginning and minor precipitation of calcite towards the end. No other changes in original minerals were observed.
Deep saline aquifers form potential storage reservoirs for CO2 which can be trapped as:
During the early stages of storage, physical trapping is likely to be the most important mechanism. Over time, hydrodynamic trapping and eventually mineral trapping will make increasing contributions to the long-term containment of CO2.
Computer models are used routinely to predict the long-term fate of CO2 stored underground. The models need validation to increase confidence in their predictive capability: use of experimental test cases such as this help to provide supporting validation.
A titanium column was packed with fresh synthetic minerals as summarised in the table below. The column was held within a large heated pressure vessel (the Big Rig) and confining pressure maintained throughout the experiment. Reactant fluid was equilibrated with CO2 at the experimental temperature and pressure before being displaced by the pressurising CO2. CO2 pressure and fluid flow were also controlled during the experiment.
Samples of the reactant fluid were depressurised and prepared for chemical analysis. Sub-samples were analysed immediately for pH and bicarbonate (HCO3-). Other sub-samples were analysed subsequently for major and trace cations by inductively-coupled plasma optical emission spectroscopy (ICP-OES) and anions by ion chromatography (IC).
On completion of the experiment, the column was recovered from the pressure vessel, the rock material sectioned in several places and examined for mineral changes. Solids were prepared and examined by scanning electron microscope (SEM). Mineral compositions were obtained using an energy-dispersive X-ray microanalysis system (EDXA) and quantitative x-ray diffraction (XRD) analysis.
Experimental run conditions
Temperature 130°C, pressure 300 bar
|Total duration of experiment 3.5 months|
Synthetic formation fluid
Synthetic mineral composition
|pH||7.1|| (at 20°C, 1 bar)
||Mineral phase||weight %|
|K||1.35 x 10-5||3.46 x10-10||Calcite||5|
|HCO3-||1.91||1.59 x10-4||Total weight (g)||1298|
|Initial porosity (%)||44.1|
The experiment showed evidence for reaction of carbonate and, to a lesser extent, silicate minerals, with resultant changes in fluid chemistry and porosity. The results provide supporting data to validate geochemical models of long-term storage of CO2 underground.
The project was funded by the European Commission (EC) and Natural Environment Research Council (NERC) via CO2GeoNet (EC project SES6-CT-2004-502816) and CRIUS (NERC grant NE/F002645/1).
Please contact Keith Bateman for further information