Molecular-scale origins of wettability at petroleum–brine–carbonate interfaces

Abstract

Wettability control of carbonates is a central concept for enhanced petroleum recovery, but a mechanistic understanding of the associated molecular-scale chemical processes remains unclear. We directly probe the interface of calcium carbonate (calcite) with natural petroleum oil, synthetic petroleum analogues, and aqueous brines to understand the molecular scale behavior at this interface. The calcite–petroleum interface structure is similar whether or not calcite was previously exposed to an aqueous brine, and is characterized by an adsorbed interfacial layer, significant structural changes within the calcite surface, and increased surface roughness. No evidence for an often-assumed thin-brine wetting layer at the calcite–petroleum interface is observed. These features differ from those observed at the calcite–brine interface, and for parallel measurements using model synthetic petroleum mixtures (consisting of representative components, including dodecane, toluene, and asphaltene). Changes to the interface after petroleum displacement by aqueous brines are also discussed.

Systematic observations of enhanced oil recovery and associated changes at carbonate-brine and carbonate-petroleum interfaces

“…5 aii; previously reported in Ref. 20 ) are indistinguishable from those of the calcite-petroleum interface without prior exposure to any brine (red circles, Fig. 5 aii).…”

“…5 bii) has XR signals that are broadly similar to the calcite-brine interface (Figs. 5 aii) and characterized by small interfacial displacements (as seen previously 20 ). The calcite-petroleum interface after displacement of the TW2 brine (magenta triangles, Figs.…”

“…5 bii). The calcite-petroleum interface is very different from that seen in brines 20 . The XR signals from the calcite-petroleum interface (red circles, Fig.…”

“…5 bii) than the calcite-petroleum without prior exposure to the TW2 brine. We have previously discussed how the magnitude of structural displacements in the calcite surface can be thought of as a measure to probe the chemical interactions between calcite and a fluid 20 . Therefore, the smaller subsurface displacements observed in Fig.…”

“…As described previously 20 , caution is needed in interpreting the optimized structural models. The model implicitly assumes that the interfacial structure is laterally uniform (i.e., having the same molecular-scale structure within each surface unit cell) and that it can be described by a unique substrate structural distortion, a few adsorbed species at the calcite surface, along with a bulk-like fluid above the surface.…”