Anorthite Dissolution under Conditions Relevant to Subsurface CO2 Injection: Effects of Na+, Ca2+, and Al3

New insights into the dolomitization and dissolution mechanisms of dolomite-calcite (104)/(110) crystal boundary: An implication to geologic carbon sequestration process

Geologic carbon sequestration (GCS) is a promising strategy to reduce the harm of CO2 due to the rapidly increased fossil fuel combustion. Dolomitization and dissolution processes of deeply buried carbonate reservoirs significantly impact the potential of GCS. However, previous investigations mainly focus on the macroscopic batch experiments, the mechanisms at atomic level are still unclear especially for crystal boundary, but urgently required. Herein, the GCS potential and the effects of boundary dissolution on calcite and dolomite were investigated based on both analytical and simulation methods such as molecular dynamics simulation (MDS) and density functional theory (DFT) calculations, to deeply unveil the mechanisms of dolomitization and formation of intergranular secondary pores from the atomic perspective. The morphology results indicated that the dissolution of calcite and dolomite in carbonic acid solution started via the edges and corners. In addition, the simulated results showed that the carbon sequestration potential presented an order in dolomite (PMg50%) > PMg40% > PMg30% > PMg20% > PMg10% > calcite by dolomitization due to the reduced bulk volume but increased lattice stress. Furthermore, both electrons transfer and diffusion coefficients results suggested that the (104)/(110) boundary was preferentially dissolved as compared to the (104) and (110) planes, indicating that crystal boundary was beneficial to the formation of pores for the oil and gas storage, but harmful to the stability of long-term GCS. Therefore, this study, for the first time, provides new insights into uncovering the mechanisms of the GCS process in depth, from an atomic level focusing on the crystal boundary, thereby promoting the understand of the long-term evolution for both calcite and dolomite in deep reservoirs.

Effects of Na+ and K+ Exchange in Interlayers on Biotite Dissolution under High-Temperature and High-CO2-Pressure Conditions

Cations in formation brine can affect CO₂-induced dissolution of minerals during geologic CO₂ sequestration (GCS), affecting the GCS performance. This study investigated the dissolution of biotite with 0-4 M Na⁺ and 0-10 mM K⁺ under high temperature and high CO₂ pressure (i.e., 95 °C and 100 bar CO₂). At <0.5 M Na⁺ concentration, Na⁺ replaced K⁺ in the biotite interlayer and enhanced the biotite dissolution. In >0.5 M Na⁺, however, the enhancing effect of Na⁺ was mitigated by an inhibition caused by competing sorption between Na⁺ and protons. With 0.5 M Na⁺ concentration, coexisting K⁺ significantly inhibited the biotite dissolution with high sensitivity at even lower K⁺ concentrations, such as 0.1-0.5 mM. In this study, we also reported the dissolution of Na-treated biotite, mimicking biotite naturally equilibrated with Na⁺-abundant brine. Na-treated biotite dissolved faster than natural K-containing biotite, and during the dissolution, it transformed to vermiculite. Aqueous Na⁺ inhibited the dissolution of Na-treated biotite by suppressing the release of interlayer Na⁺, and aqueous K⁺ inhibited the dissolution of Na-treated biotite by replacing the interlayer Na⁺. These findings contribute to better understanding of biotite dissolution in the presence of potassium-containing clay-swelling inhibitors and different salinities at GCS sites.

Nanoscale Chemical Processes Affecting Storage Capacities and Seals during Geologic CO2 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.