CO2 Adhesion At the Hydrated Mineral Interface Could Greatly Reduce the Risk of Leakage From Geologic Carbon Sequestration Sites

Mineral surfaces in the subsurface are typically water wetting but under certain conditions can strongly adhere CO₂, a largely nonpolar molecule, at the interface. This adhesion has been observed on mica surfaces in the past but it has not been described in the literature. The goal of this work was to understand how adhesion could impact transport processes of interest such as capillary trapping and residual saturation. In particular, we sought to understand how the chemistry in the aqueous phase along with the surface chemistry and morphology of the solid mineral, would impact the prevalence of adhesion. Both static captive drop contact angle measurements for discrete bubbles and advancing/receding contact angle experiments were carried out. Adhesion was observed for a large number of the bubbles on highly polished trioctahedral phlogopite mica and to a lesser extent on amorphous silica and calcite surfaces. These surfaces were the most polished of all the minerals tested here. Measurements on illite, kaolinite, and quartz showed little to no adhesion, but ultra smooth samples of these minerals were not tested. The results represent the aggregate understanding developed by observing thousands of bubbles on dozens of replicates on various representative mineral surfaces. Adhered bubbles typically exhibited distorted ternary phase lines suggesting that some degree of contact line pinning was taking place. The adhered bubbles were not easily removed from the surface even under conditions of high shear in the aqueous phase, which stands in stark contrast to most CO₂ bubbles on the mineral surface, which move around readily. Adhesion was found to depend on two factors, aqueous chemistry and surface roughness. Roughness measurements were made of all the mineral samples and those that could be cleaved along basal planes to create atomically smooth surfaces were the most likely to exhibit adhesion. On phologopite, for example, up to 80% of bubbles would adhere on a given experiment. Those surfaces with a roughness on the order of 10s of nms exhibited the most adhesion. The chemistry of the water layer existing between the CO₂ and the mineral was also of great importance in determining the prevalence of adhesion. Increases in both ionic strength and partial pressure of CO₂ increased adhesion of CO₂ bubbles on the surface. The presence of strong acid or strong base in the aqueous phase (0.1 M of HCl or NaOH) eliminated the incidence of adhesion. In advancing/receding contact angle measurements, adhesion could increase the contact angle by a factor of 3 and drive a transition from nonwetting to wetting. The underlying mechanism driving  adhesion is still being developed but the results have important implications for a variety of transport processes in sandstones and other porous media in which large areas of mineral surfaces are exposed to multiple phases of brine and CO₂. These results could be of particular interest in characterizing leakage processes wherein subtle buoyancy driven forces will be greatly impacted by wettability characteristics in formations overlying target repositories. These results could also be of great interest in a variety of contexts beyond geologic sequestration including enhanced oil recovery and buoyancy driven leakage of gases from hydraulically fractured formations.