Reactive geochemical transport simulation to study geochemical trapping of carbon dioxide: the Tokyo Bay model

Abstract

  Two dimensional numerical simulation was performed using reactive geochemical transport code TOUGHREACT based on the geology of the Tokyo Bay area to understand long-term geochemical evolution of aqueous and mineral phases after CO₂ injection and to elucidate key issues affecting CO₂ mineral trapping. The injected CO₂ migrates upward in the reservoir (the Umegase Formation) towards the cap rock (the Kokumoto Formation) driven by buoyant force. In the CO₂ plume, water pH drops from 7.8 (initial) to 4.9 by dissolution and dissociation of CO₂. The acidified water containing CO₂ and related species induces mineral dissolution and promotes complexing of dissolved ions, which increase the water density and cause its downward movement to the bottom of the reservoir. This process gives rise to convective mixing which further promotes CO₂ solubility trapping. In the peripheral zone of the CO₂ plume, the acidified water comes in contact with intact reservoir water, giving rise to the precipitation of calcite (and siderite, in lesser amount). The precipitation of calcite and other carbonates might form a low permeability barrier surrounding the CO₂ plume, and is expected to increase the storage security with time.
  Among newly formed carbonates, calcite becomes the most dominant in a long period after injection cease. Dawsonite, which is occasionally considered to be important in trapping CO₂, is found to have limited stability that requires high CO₂ partial pressure at the conditions of saline aquifer storage. Our study also finds that the formation of dawsonite is in a close relation to the dissolution of plagioclase, and that its precipitation is strongly suppressed by the formation of kaolinite. After reviewing rates of dissolution of carbonates and feldspars, we conclude that the precipitation of dawsonite is controlled by plagioclase dissolution kinetics which determines the supply of Al as a major constituent of dawsonite. Plagioclase is a dominant clastic mineral in sandstones in geologically young formations of the Japanese Islands. Its dissolution could be a key issue in geochemical modeling of CO₂ aquifer storage.