Abstract
In a saturated solution, submicrometer calcite (CaCO3) crystals recrystallize and coarsen to minimize surface area. The thermodynamic driving force is described by the Gibbs-Thomson equation, but the rates and mechanism are poorly understood. Calcite grain coarsening has many implications in industry and nature, but the specific focus of this research project was to understand how small, biogenic calcite particles in chalk have resisted grain coarsening for over 60 million years in saturated reservoir fluids.
A new method was developed to produce pure calcite powder that has submicrometer grain diameter: The small particle size was similar to coccolith elements in chalk. Calcite was aged in saturated solutions for up to 261 days at temperatures up to 200 °C. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and BET surface area data showed fundamental insight into grain coarsening – small grains coarsen by aggregation at high temperatures, followed by Ostwald ripening. Alginate, a model for the acidic polysaccharides produced by coccolithiphores, inhibited coarsening at a steady rate. A Pseudomonas aeruginosa biofilm preserved particles for at least 60
days before a threshold was overcome, possibly because the biofilm was degraded. Chalk particles were preserved even after oxidation treatments to the surface and temperatures up to 200 °C.
Overall results suggest that impurities in chalk particles prevent grain coarsening because pure calcite particles of the same size coarsen at reservoir conditions. However, oxidation of the chalk surface is not enough to induce coarsening, so the impurities are likely inside the particles
A new method was developed to produce pure calcite powder that has submicrometer grain diameter: The small particle size was similar to coccolith elements in chalk. Calcite was aged in saturated solutions for up to 261 days at temperatures up to 200 °C. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and BET surface area data showed fundamental insight into grain coarsening – small grains coarsen by aggregation at high temperatures, followed by Ostwald ripening. Alginate, a model for the acidic polysaccharides produced by coccolithiphores, inhibited coarsening at a steady rate. A Pseudomonas aeruginosa biofilm preserved particles for at least 60
days before a threshold was overcome, possibly because the biofilm was degraded. Chalk particles were preserved even after oxidation treatments to the surface and temperatures up to 200 °C.
Overall results suggest that impurities in chalk particles prevent grain coarsening because pure calcite particles of the same size coarsen at reservoir conditions. However, oxidation of the chalk surface is not enough to induce coarsening, so the impurities are likely inside the particles
| Original language | English |
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| Publisher | Department of Chemistry, Faculty of Science, University of Copenhagen |
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| Number of pages | 115 |
| ISBN (Print) | 978-87-91963-28-5 |
| Publication status | Published - 2013 |