Effect of Mineral Dissolution/Precipitation and CO₂ Exsolution on CO₂ transport in Geological Carbon Storage

Geological carbon sequestration (GCS) in deep saline aquifers is an effective means for storing carbon dioxide to address global climate change. As the time after injection increases, the safety of storage increases as the CO₂ transforms from a separate phase to CO₂(aq) and HCO₃^- by dissolution and then to carbonates by mineral dissolution. However, subsequent depressurization could lead to dissolved CO₂(aq) escaping from the formation water and creating a new separate phase which may reduce the GCS system safety. The mineral dissolution and the CO₂ exsolution and mineral precipitation during depressurization change the morphology, porosity, and permeability of the porous rock medium, which then affects the two-phase flow of the CO₂ and formation water. A better understanding of these effects on the CO₂-water two-phase flow will improve predictions of the long-term CO₂ storage reliability, especially the impact of depressurization on the long-term stability. In this Account, we summarize our recent work on the effect of CO₂ exsolution and mineral dissolution/precipitation on CO₂ transport in GCS reservoirs. We place emphasis on understanding the behavior and transformation of the carbon components in the reservoir, including CO₂(sc/g), CO₂(aq), HCO₃^-, and carbonate minerals (calcite and dolomite), highlight their transport and mobility by coupled geochemical and two-phase flow processes, and consider the implications of these transport mechanisms on estimates of the long-term safety of GCS. We describe experimental and numerical pore- and core-scale methods used in our lab in conjunction with industrial and international partners to investigate these effects. Experimental results show how mineral dissolution affects permeability, capillary pressure, and relative permeability, which are important phenomena affecting the input parameters for reservoir flow modeling. The porosity and the absolute permeability increase when CO₂ dissolved water is continuously injected through the core. The MRI results indicate dissolution of the carbonates during the experiments since the porosity has been increased after the core-flooding experiments. The mineral dissolution changes the pore structure by enlarging the throat diameters and decreasing the pore specific surface areas, resulting in lower CO₂/water capillary pressures and changes in the relative permeability. When the reservoir pressure decreases, the CO₂ exsolution occurs due to the reduction of solubility. The CO₂ bubbles preferentially grow toward the larger pores instead of toward the throats or the finer pores during the depressurization. After exsolution, the exsolved CO₂ phase shows low mobility due to the highly dispersed pore-scale morphology, and the well dispersed small bubbles tend to merge without interface contact driven by the Ostwald ripening mechanism. During depressurization, the dissolved carbonate could also precipitate as a result of increasing pH. There is increasing formation water flow resistance and low mobility of the CO₂ in the presence of CO₂ exsolution and carbonate precipitation. These effects produce a self-sealing mechanism that may reduce unfavorable CO₂ migration even in the presence of sudden reservoir depressurization.