Fluid-rock interactions

Figure 1. Calcite crystal with locally etched labradorite behind; extensive dissolution of calcite is visible along crystal boundaries

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.

Background

Deep saline aquifers form potential storage reservoirs for CO2 which can be trapped as:

  • 'free' CO2, most likely as a supercritical phase (physical trapping)
  • CO2 dissolved in interstitial water (hydrodynamic trapping)
  • carbon precipitated in carbonate phases such as calcite (mineral trapping)

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.

The experiment details

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
Column 100 cm long, flow rate 5 mL/hr

Total duration of experiment 3.5 months

Synthetic formation fluid

Synthetic mineral composition

pH 7.1 (at 20°C, 1 bar)
Mineral phase weight %
         
  mg/kg mol/dm3 Quartz 70
Na 11500 5.00 x10-1 Labradorite 20
K 1.35 x 10-5 3.46 x10-10 Calcite 5
Ca 48.5 1.21 x10-3 Muscovite 5
Cl 17800 5.02 x10-1    
HCO3- 1.91 1.59 x10-4 Total weight (g) 1298
Initial porosity (%) 44.1

Fluid chemical changes

  • initial pH was 7.1. After equilibration with CO2 at the experimental conditions, pH (20°C, 1 bar) was 6.5 and remained constant at 6.1±0.1
  • under in-situ conditions (130°C, 300 bar pressure), pH was measured as 4.7
  • bicarbonate (HCO3-) concentrations (Figure 2) decreased in the early part of the experiment but stabilised after about 1000 hours
  • calcium (Ca) increased in the initial fluids but stabilised after about 700 hours
  • sodium (Na) and and chlorine (Cl) concentrations changed little throughout the experiment
  • silicon (Si) concentration increased in the first 200 hours, then decreased sharply and stabilised after about 700 hours. The initial fluctuation may have been due to dissolution of fine particles produced during sample preparation

Mineral changes

Figure 2: Change in pH and alkalinity with time in the experiment
  • investigation showed complete removal of calcite from a section 10—30 cm into the column. Original calcite was observed only after about 30 cm from the column inlet
  • labradorite (feldspar) showed signs of dissolution (Figure 3) at the beginning of the column, but remained unaltered from 18 cm onwards
  • none of the other original minerals showed evidence of modification
  • limited evidence suggested that kaolinite and/or aluminium hydroxide formed in the early part of the column
  • EDXA analysis suggested the formation of secondary calcite towards the end of the column

Implications

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.

Funding

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).

Contact

Please contact Keith Bateman for further information