Masao Sorai

Dissolution kinetics of anorthite in a supercritical CO2–water system

Abstract The Gibbs free energy change, ΔGr, dependence of the anorthite dissolution rate in a supercritical CO2-water system was measured as part of a geochemical assessment of CO2 geological sequestration. Being bounded at ΔGr crit, the critical ΔGr required for an opening of the etch pit at a screw dislocation, it has been accepted that mineral dissolution follows etch pit formation assisted by dislocations below ΔGr crit, whereas the horizontal step retreats without the etch pit above ΔGrcrit. The experiment described herein, however, revealed that another mode of dissolution occurs more distant from equilibrium by spontaneous formation of the etch pit over the entire surface, as observed on calcite. The dissolution rate is higher by more than one order of magnitude than that in the dislocation-assisted mode. Therefore, including the rate gap at ΔGrcrit, a nonlinear curve with three steps instead of a sigmoidal curve is proposed for the ΔGr dependence of the anorthite dissolution rate. Extremely slow rates were observed depending on observed points for the same ΔGr condition. Although the reason for such a rate difference remains unknown, it is likely related to the defect density on the crystal surface. It is possible that initial spreading of the dissolved surface attributable to the etch pit formation assisted by defects provides some trigger for subsequent explosive etch pit formation. These findings suggest that the initial transient process can strongly influence the kinetics of geochemical reactions that occur during CO2 geological sequestration.

Experimental study of sealing performance:2. Effects of particle size distribution on threshold pressure of sintered compacts

In connection with an assessment of the sealing performance of a caprock such as mudstone for geologic CO2 sequestration, we prepared sintered compacts comprising two different-sized silica beads with various particle size ratios and mixing ratios. Then we examined the correlation between permeability k and threshold pressure Pcth of the samples. Specifically, Pcth was measured in supercritical CO2-water system under conditions of 1000 m depth (10 MPa and 40°C). A series of studies was conducted to identify factors affecting data variability on the Pcth-k correlation chart and to ascertain the variation ranges of both parameters. This paper corresponds to a second step of work, following previous measurements obtained from monodisperse samples. The samples k showed good correlation with both the mixing ratio and mean area diameter, whereas Pcth with great variation was correlated with neither parameter. In this context, results suggest that Pcth, which was controlled by local structures within samples, could not be predicted simply from the pore diameter distribution. Regarding the Pcth-k correlation, monodisperse samples are scattered around the closest-packing line. In contrast, bidisperse samples are shown mainly below the closest-packing line. Moreover, the slope of a fitting line on the Pcth-k double logarithmic chart was lowered from −0.62 for monodisperse particles to −0.43 for bidisperse particles. Nevertheless, the low slope on mudstone cannot be explained solely by the effect of the particle size distribution. Therefore, the contribution of the particle configuration is expected to be greater as a factor affecting the internal structure of mudstone.

Field Reaction Experiments of Carbonate Minerals in Spring Waters: Natural Analogue of Geologic CO2 Storage

To diminish the uncertainty of the mineral trapping rate during geologic CO2 storage, the growth rate of carbonate minerals was measured in CO2-containing spring waters, which can be regarded as a natural analogue of geologic CO2 storage. The authors’ approach, using nanoscale analysis of seed crystal surfaces after immersion into spring waters, enables rapid and accurate measurement of mineral reaction rates. The results show that calcite growth rates in spring waters were lower by 1–3 orders than the values given in a database of laboratory experiment results. We verified the traditional paradigm that Mg2+ controls carbonate reaction kinetics. An increase of the Mg/Ca ratio to around 5 by adding Mg2+ to spring waters markedly reduced the calcite growth rate. However, even if effects of Mg2+ and flow rate are considered, we were unable to explain satisfactorily the difference of the calcite growth rates between those of spring waters and laboratory experiments. Therefore, other factors might also be related to the slow growth rate in nature. The present results, including the fact such that neither dolomite nor magnesite was formed even at the high Mg/Ca ratio, are expected to provide an important constraint to overestimation of the mineral trapping rate.