Experimental study of well cement carbonation under geological storage conditions

Abstract

Batch experiments were carried out to examine the carbonation reaction of neat cement and two cement-fly ash admixtures (poz mix and lightweight) in 0.5 M NaCl solution at 53°C and 10 MPa CO2. The reactions were monitored for 3, 7, 14, 28, and 84 days. The major cation contents of the solutions were determined. Detailed characterization of cement samples was performed using SEM&EDS and EPMA. Density, porosity and air permeability were also determined in cement samples. Complete carbonation was achieved within 28 days in poz mix and 7 days in lightweight samples, while carbonation in neat cement is limited to about 200 microns after 84 days of reaction. A carbonation zone and a porous transition zone were observed in partially carbonated samples. These reaction zones can be delineated by the elemental profiles, which show Na enrichment and Cl depletion in carbonated cement. The carbonation zone is dominated by calcium aluminosilicates. Micro-analytical techniques show that some of these phases are compositionally similar to zeolites. Calcium carbonate formed in the carbonation zone and outgrowth of well-formed crystals was also observed. Near the surface, a region leached of calcium carbonate developed. Carbonation causes very little change in both the porosity and permeability in the neat cement sample. An increase in the porosity was measured in fully carbonated poz mix cement; however, the permeability remained essentially unchanged. For the lightweight cement, the permeability increased from 0.16 to 1.1 mD after 84 days of reaction, suggesting that carbonation results in the degradation of lightweight cement under the experimental conditions. The integrity of lightweight cement could be compromised if it is in contact with a considerable volume of CO2-rich brine that is not previously equilibrated with carbonate minerals.