Olivine dissolution rates in wet CO₂ match or exceed aqueous rates

Abstract

Carbon dioxide injection into subsurface basalt, where it mineralises into carbonates, enables permanent emissions removal. Its scalability is limited by slow aqueous dissolution rates of metal silicate minerals, primarily olivine. Aqueous dissolution rate laws are well-established and apply to projects where CO₂ is dissolved in water before injection underground. An alternative approach reduces water consumption by directly injecting CO₂. However, mineralisation kinetics with CO₂-rich fluids are uncertain. Past observations are mostly qualitative and inconsistently report higher and lower reactivity compared to aqueous conditions. Here we quantify olivine dissolution rates using a novel plug-flow reactor setup in which capillarity is used to saturate a forsteritic olivine grain pack with CO₂, complemented by X-ray imaging. Co-injection of water and liquid CO₂ at elevated capillary pressure resulted in dissolution rates matching or exceeding aqueous rates, with rate enhancement of up to a factor of 1.1–3.76 depending on how the normalisation to surface area is treated. Conversely, limiting water mobility inhibited rates and is likely an explanation for past observations. Reactivity with liquid CO₂ is governed by the mobility of interfacial water films rather than total water content. Carbon mineralisation with the direct injection of CO₂ will have comparable or faster mineralisation as aqueous injection while requiring far less water.