Mineral Interfaces and Oil Recovery: A Microscopic View on Surface Reconstruction, Organic Modification, and Wettability Alteration of Carbonates

Abstract

While it is generally known that aging protocols have an important impact on the interaction between crude oil (CRO), brines, and mineral surfaces, the microscopic consequences of the various steps of aging have hardly been described. In this study, we characterize the properties of fluids and carbonate mineral surfaces throughout a series of equilibration steps at 95 °C and correlate these microscopic observations with macroscopic contact angle measurements. Chemical equilibration of CRO (eqCRO) and FW (eqFW) leads to transfer of organic molecules from the former to the latter, causing also a pH change in the eqFW. Confocal Raman microscopy, atomic force microscopy, and infrared spectroscopy are used to reveal how consecutive aging of calcite in eqFW and eqCRO induces: first, in eqFW, considerable surface reconstruction and precipitation of mineral particles with colocalized organic species, and second, upon exposure to eqCRO, the formation of a second adlayer primarily composed of polyaromatic hydrocarbon-rich particles. Our results show how these interconnected microscopic chemical and topographical surface modifications give rise to more "oil wetting" contact angles after the two-step aging procedure.

Artificial Diagenesis of Carbonates: Temperature-Dependent Inorganic and Organic Modifications in Reservoir Mimetic Fluids

“…Within petroleum reservoirs, rock structures are invariably associated with aqueous fluids termed as "connate water," "interstitial water," or "FW." Serving as the medium for mineral diagenesis, connate waters are typically enriched with diverse ionic species such as Na þ , Ca 2þ , Mg 2þ , Cl À , HCO 3 À , and SO 4 2À in (metastable) equilibrium with mineral phases, while organic species extracted from CRO are also present (Helgeson et al 1993;Eftekhardadkhah et al 2016;Rao et al 2020). To elucidate the inorganic modifications of carbonates by connate waters, Iceland spar crystals were aged in synthetic FW at either RT, 60 C, or 95 C for 16 hours.…”

“…S4; Plummer and Busenberg 1982;Dai et al 2017) as well as the rich ionic composition of FW (Table 1). Such topographical modifications are likely to influence mineral wettability, considering that surface roughness can singly affect contact angles estimated in oil/brine/ calcite systems (Rao et al 2020;Sari et al 2020).…”

“…FW/CRO Equilibration. To mimic the equilibration of aqueous fluids and CRO at elevated temperatures within reservoirs, a previously described protocol was followed (Rao et al 2020). In brief, equal parts of CRO and FW were equilibrated at 95 C for 48 hours.…”

“…Synthetic replication of reservoir-relevant oil/brine/mineral systems can facilitate the understanding of the in-situ chemical and topographical properties of minerals (Chen et al 2017;Rao et al 2020). For this, certain key factors require attention, such as the equilibration of CRO and reservoir brines at geological scales as well as the diagenetic alterations of minerals at elevated temperature and pressure.…”

“…However, these aging protocols vary substantially between different laboratories, and their usefulness and reliability are generally only assessed by macroscopic tests. Their consequences for the microscopic structure and composition of the mineral surfaces are hardly ever characterized in detail, in sharp contrast to the fact that these microscopic properties are widely believed to control wettability (Chen et al 2017;Rao et al 2020). To achieve a consistent fundamental picture and also an understanding of waterflooding and more advanced IOR techniques in carbonate reservoirs, it is therefore essential to achieve a thorough microscopic picture of the processes controlling the properties of the oil/brine/mineral interfaces under realistic reservoir conditions.…”