Pore-scale observations of CO2 thermodynamic properties and dissolution trapping in porous media

Abstract

Carbon geological storage (CGS) presents a promising approach to mitigate climate change. Partially free CO₂ is trapped in tiny pores and gradually dissolves into formation fluid. However, thermodynamic properties of CO₂ remain inadequately understood, which strongly influence activation possibility of these free CO₂. In this study, we innovatively mapped CO₂ evolution and redistribution at micro scale by using X-ray CT. CO₂ growth or shrinkage was determined by the threshold equal to pore volume. Bellow the threshold, bubbles experienced thermodynamically unstable shrink, following a power-law distribution in its specific interfacial area. Otherwise, bubble growth was governed by diffusion between gas and liquid phases, unlike conversely bubble coalescence. Larger bubbles expand at the expense of smaller ones through Ostwald ripening. The surface energy of CO₂ bubbles during Ostwald ripening in porous media was firstly quantified. The whole system tended to reduce surface energy and larger bubbles have lower surface energy. In additional, high curvature interfaces prefer to rupture and growth. It was also found that CO₂ bubble disappears in about 5.87 h at laboratory scales, which was 23.79 times greater than that estimated by the traditional LSW equation. This longer period allowed CO₂ to redistribute within porous media, diminishing solubility and mineral trapping efficiency, thereby increasing the risk of long-term storage leakage.