CO2 地中貯留における灰長石溶解プロセスの速度論的考察

Abstract

The Gibbs free energy change, ΔGr, dependence of the anorthite dissolution rate under a supercritical CO2-water system was measured as part of a long-term assessment of CO2 geological sequestration. The surface observation on a nanoscale described herein showed that the ΔGr dependency follows to the rate function with three steps (i.e., the horizontal step retreats without the etch pit, the etch pit formation assisted by dislocations, and the spontaneous formation of the etch pit) instead of transient state theory and a traditional sigmoidal curve, where the rate increases abruptly more distant from equilibrium by spontaneous formation of the etch pit over the entire surface. Such a variation of the dissolution rate can affect largely not only the change in porosity or permeability of reservoir rocks and the resultant flow property of formation water, but also the precipitation rate of secondary minerals. Our preliminary numerical simulation revealed that the difference of the rate function form according to ΔGr dependence produces the time gap more than two orders on both the dissolution of anorthite and the precipitation of secondary minerals. On the other hand, extremely slow rates were observed depending on surface conditions for the same ΔGr condition. Therefore, understanding of transient processes before a steady state is also necessary for evaluation of the mineral dissolution rate under natural conditions including CO2 geological sequestration.