Formation and Stability of Heterogeneous Organo-Ionic Surface on Carbonates

ABSTRACT: Many geological processes from oil recovery to underground CO2 storage are affected by natural molecules adsorbed on rock surfaces. Yet, geochemical models tend to overlook their formation and stability, let alone existence. With a suite of analytical techniques, we address this missing-link and describe fundamental mechanisms for (i) the deposition of surface-active molecules in complex brines and oils on underground minerals and (ii) the desorption of heterogeneous sorbents and its dependence on aqueous composition. First, we show that organic and inorganic constituents of both formation water and crude oil form an organo-ionic surface layer on calcite. Primary modifiers are revealed as aqueous and nonaqueous polyaromatic molecules with polar and metal-binding functional groups and solubility characteristics of asphaltenes. Formed via pi-stacking and ionic and hydrogen bonding interactions, the heterogeneous organo-ionic layer establishes a physical barrier between the mineral and ambient atmosphere/fluid, impacting the dissolution and wettability of rocks. Second, we investigate desorption of the organo-ionic layer in various brines under flow and static conditions. With chromatographic and spectroscopic methods (including Raman and sum-frequency generation), we show that the release of adsorbed material from carbonate surfaces encompasses key coupled reactions: (i) dissolution of brine-soluble asphaltenes, leading to relative interfacial enrichment of bulky brine-insoluble asphaltenes, (ii) nanoscale orientational changes of surface asphaltene assemblies, carbonate ions, and water molecules at the brine-rock interface, and (iii) dissolution and surface reconstruction of the carbonate mineral. Through these reactions, the low-salinity effect is uncovered as a two-stage desorption process: the initial release or selective extraction of water-soluble sorbents and subsequent delamination of residual water-insoluble asphaltenes from the dissolving mineral surface. Illuminating the surface reactions of geological minerals, we conclude that surface passivation by heterogeneous organo-ionic matter is not only ubiquitous in nature but also a key regulator of the interfacial chemistry, reactivity and wettability of underground rocks.

Automated summary

This article introduces that many geological processes are affected by natural molecules adsorbed on rock surfaces, such as oil recovery and underground CO2 storage. However, geochemical models often overlook their formation and stability. With a suite of analytical techniques, the researchers reveal this missing link and describe the mechanisms for the deposition and desorption of these molecules on rock surfaces.