Nanoscale Chemical Processes Affecting Storage Capacities and Seals during Geologic CO 2 Sequestration.

Geologic CO2 sequestration (GCS) is a promising strategy to mitigate anthropogenic CO2 emission to the atmosphere. Suitable geologic storage sites should have a porous reservoir rock zone where injected CO2 can displace brine and be stored in pores, and an impermeable zone on top of reservoir rocks to hinder upward movement of buoyant CO2 . The injection wells (steel casings encased in concrete) pass through these geologic zones and lead CO2 to the desired zones. In subsurface environments, CO2 is reactive as both a supercritical (sc) phase and aqueous (aq) species. Its nanoscale chemical reactions with geomedia and wellbores are closely related to the safety and efficiency of CO2 storage. For example, the injection pressure is determined by the wettability and permeability of geomedia, which can be sensitive to nanoscale mineral-fluid interactions; the sealing safety of the injection sites is affected by the opening and closing of fractures in caprocks and the alteration of wellbore integrity caused by nanoscale chemical reactions; and the time scale for CO2 mineralization is also largely dependent on the chemical reactivities of the reservoir rocks. Therefore, nanoscale chemical processes can influence the hydrogeological and mechanical properties of geomedia, such as their wettability, permeability, mechanical strength, and fracturing. This Account reviews our group's work on nanoscale chemical reactions and their qualitative impacts on seal integrity and storage capacity at GCS sites from four points of view. First, studies on dissolution of feldspar, an important reservoir rock constituent, and subsequent secondary mineral precipitation are discussed, focusing on the effects of feldspar crystallography, cations, and sulfate anions. Second, interfacial reactions between caprock and brine are introduced using model clay minerals, with focuses on the effects of water chemistries (salinity and organic ligands) and water content on mineral dissolution and surface morphology changes. Third, the hydrogeological responses (using wettability alteration as an example) of clay minerals to chemical reactions are discussed, which connects the nanoscale findings to the transport and capillary trapping of CO2 in the reservoirs. Fourth, the interplay between chemical and mechanical alterations of geomedia, using wellbore cement as a model geomedium, is examined, which provides helpful insights into wellbore and caprock integrities and CO2 mineralization. Combining these four aspects, our group has answered questions related to nanoscale chemical reactions in subsurface GCS sites regarding the types of reactions and the property alterations of reservoirs and caprocks. Ultimately, the findings can shed light on the influences of nanoscale chemical reactions on storage capacities and seals during geologic CO2 sequestration.