The study of the wettability of carbonate rocks is essential for a correct comprehension of fluid transport in the subsurface, affecting for example energy recovery or storage initiatives. Probing molecular interactions at the mineral-water interface also contributes to the fundamental understanding of the physical and chemical mechanisms behind the complex concept of hygroscopicity of a mineral surface.<br/><br/>Carbonate rocks are of special interest since they are abundant in the earth crust and are formed in a large variety of environments. They present a complex structure, heterogeneous composition – potentially including organic matter – and textures made of (bio)minerals, which results in a surface chemistry with an inherent heterogeneity down to the nanoscale. Therefore, the scale of observation as well as the organic-mineral interactions are potentially important parameters when describing the surface chemistry properties of these types of rock.<br/><br/>We probed the surface properties of carbonate rocks using Atomic Force Microscopy (AFM) and Atomic Force Spectroscopy (AFS), exploring multiple spatial scales, from the mm to the nm. We measured the topographical features of two carbonate rocks from different environments and with different content of skeletal and non-skeletal particles. Furthermore, we determined their adhesion properties in liquid environments across scales to evaluate the spatial variability of carbonate surface chemistry. We also performed AFS experiments on both rocks using tips functionalized with different chemical groups to directly measured the organic-mineral interactions. We found homogeneous hydrophobic areas at scales smaller than 5 µm. The origin of this hydrophobicity is linked to the type of rock, but also to the biogenic vs. non-biogenic origin of the mineral grains, the former exhibiting a larger hydrophobic behavior than the latter. Overall, this AFM-based study presents a nanoscale approach to unravel the role that organic moieties and biominerals play on the control of wettability properties of rock-water interfaces.

Carbonate rocks wettability across scales: Elucidating the role of biominerals

ACS Spring 2022

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